pulso jet engines

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Copyright 2003 Bruce Simpson. http://aardvark.co.nz/pjet

The enthusiast’s guide to the design, construction andoperation of

Pulsejet EnginesBy Bruce Simpson

An incomplete preliminary draft (6 November, 2003)

Copyright Notice:

This book is copyright 2003 to Bruce Simpson and all commercial rights are reserved.

However, electronic copies of this book may be freely redistributed for non-commercialpersonal use under the following terms:

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Anyone discovering a violation of these terms is requested to contact the author through thewebpage at http://aardvark.co.nz/contact/

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ForewordModern jet engines, like the ones found on large passenger aircraft or military fighters areincredibly complex and expensive to make.

Built from thousands of individual parts, many of which are made from exotic alloys liketitanium and Inconel, these engines are a masterpiece of modern engineering.

But what if I was to tell you that there is at least one type of jet engine that has been aroundfor almost 100 years, can be built out of plain old steel using simple tools, and in some caseshas no moving parts at all?

Well it’s true and I am, of course, talking about the pulsejet.

In this book I’ll do my best to explain how these engines work, how to build them, how toimprove on the basic designs and how they can be used to power all manner of vehicles frommodel airplanes to gokarts.

In an attempt to make the information contained in this book accessible to the widest range ofpeople, I’ve taken a few liberties in explaining some of the more complex concepts. I’m surethere will be physicists, engineers and mathematicians who will throw up their arms in disgustwhen they look at how I’ve presented some of this information.

My justification for this is that the majority of readers are probably not equipped nor interestedin wading through pages of complex mathematical formulas in order to understand someaspect of a pulsejet’s operation. In such cases, I believe, it’s better to replace all thiscomplexity with a simple analogy or basic calculation that hopefully anyone can follow.

Another are of contention is the very explanation of how a pulsejet works.

Although there is some consensus on the basic mechanisms behind the operation of a pulsejet,much of the detail is a topic of hot debate and disagreement. Wherever possible I’ve tried topresent all sides to an argument but obviously I favor my own opinions which are based onyears of empirical observation and experimentation.

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Why is this book free?When I first considered writing a book about pulsejets I had considered going theconventional way of having it printed, bound and distributed through regular channels such asAmazon.com.

However, given that the market for such a book is rather small and I would be unlikely toattract the interest a traditional publisher or recover the costs of printing and publishing such atome myself, I decided to make it freely available in electronic form over the Internet.

Of course I’d love to see some kind of return for the many hours of research, writing andediting that have gone into this book so I invite readers to make a donation.

If you think this book is worth something, just send me a donation equal to the value you feelyou’ve received from reading it.

I don’t expect to get rich – or even cover my costs but if enough people cough up a fewdollars here and there I’ll probably keep releasing new updated copies with more information,more plans and new ideas.

If you’d like to make a donation of any size, just contact me through the online form athttp://aardvark.co.nz/contact and I’ll provide you with a list of options for sending yourmoney.

Please feel free to redistribute this book in its electronic form but do pay attention to the termsand conditions listed on the front page. I want to make sure that it remains free to downloadand avoid having unscrupulous rogues auctioning copies on eBay or otherwise trying to makemoney from something that should never be sold.

Of course if you’re a print-media publisher who’d like to distribute printed copies of this bookthen please contact me.

And, if you’re one of those who have purchased my pulsejet CDROM then please, considerthis book an additional gift – you don’t need to make a donation.

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A Note About The AuthorWho is Bruce Simpson?

Well I’m a middle-aged guy who has always had a stronginterest in technology and things that go bang. From anearly age you’d find me out in the garage playing with mychemistry set, building all manner of weird and wonderfuldevices from old discarded radios, or just reading booksabout science.

Since the age of about seven, I’ve also been an avidbuilder of model airplanes, mostly of my own design.Over the years I’ve created all manner of odd-ball flyingcreations including flying wings, flying saucers, flyinglawnmowers, flying carpets, and many others.

It was only natural therefore that eventually myfascination with things that go bang, chemistry, physicsand aerodynamics would collide and produce a stronginterest in jet engine technology.

It was also inevitable that, rather than focus on currently fashionable small gas turbinetechnology, I’d instead concentrate my efforts on the almost forgotten pulsejet.

Over the past couple of years I’ve built dozens of different pulsejet designs, mainly to test myown ideas. As a result of this experimentation I’ve developed a reputation for being at theleading edge of this almost forgotten technology and have come up with a number ofinnovations such as the blast ring and a novel fuel-injection system that significantly extendsthe valve life of small pulsejet engines.

For about a year I was actively involved in the commercial manufacture of several of mypulsejet designs but unfortunately I rapidly found myself extremely embarrassed at beingunable to keep up with the unexpected demand. As a result, I have sold the manufacturingrights for these engines and am again focused on pure research and development in this area,working on several projects including some commissioned by clients in the aerospace anddefense industries.

I am also performing some design work on a new generation of ultra-low-cost high-speedpulsejet-powered UAVs designed for reconnaissance and other applications.

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Contents:How do pulsejets work?The world’s simplest pulsejetPulsejets for modelsHow to design a pulsejetComparing intake valving systemsMaking reed valves lastConstructional techniquesPowering things with pulsejets (UPDATED)Schmidt’s contributionsFuel systemsIgnition systemsHow to start a pulsejetValveless pulsejets (UPDATED)Improving pulsejet performanceA simple guide to anodizingMaking reed valves with electrochemical etchingNewton’s third lawThe Reynolds effectThe Bernoulli effectThe Coanda effectPlans (NEW)Wacky ideas (NEW)To be completed

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How Do Pulsejets Work?The honest answer to this question is that nobody’s really 100 percent sure of all themechanisms that drive a typical pulsejet engine.

While most of the basic principles of operation are understood fairly well, there are many smalldetails that are still the subject of debate amongst engineers and experimenters to this very day.

It is safe to say however, that the primary effect behind the function of a pulsejet is the factthat gases are compressible and tend to act like a spring.

This “springiness” is crucial to the way a pulsejet draws in a fresh mixture of air and fuel thenexpels the hot burning gasses that are generated when that fuel is ignited.

The Kadenacy EffectThe effect of this springiness has been labeled the Kadenacy effect and here’s how it works:

Take a regular 12 inch rule and lay it over the edge of a table so that just two or three inchesare held firmly against that table.

Pull the free end down an inch or so and release it.

Did you see what happened?

The ruler, acting like a simple spring, quickly flicked back, away from your hand – but it didn’tstop once it became fully straightened – it continued to move and actually bent the other wayvery briefly.

This flexing back and forth probably continued for a second or so withthe magnitude of each swing being slightly smaller than the previousone.

Now something very similar happens when we take a sealed containerand fill it with a compressed gas such as air.

If we suddenly release that pressure by popping thecork, the compressed air will rush out but (andhere’s the surprising bit) – even once the pressure inside falls to match thepressure outside, the air will continue to flow out.

It’s pretty easy to see that this will cause thepressure inside the container to fall below the

pressure outside – and then the gas will flow back inwards. This cycleof increasing and decreasing pressure will repeat a number of times,decreasing in magnitude each time.

That’s the Kadenacy effect in action driven by the springiness of air.

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For a practical demonstration, find an empty bottle that has an opening about the size of yourfinger or thumb.

Wet your finger or thumb and slide it into the neck of the bottle, allowing the air to escape asyou do.

Now remove your finger quickly.

Hear that sound? That’s the air rapidly bouncing in and out – just like the springy rulervibrated when you let the free end go.

See how easy this jet engine stuff is?

Now that we’ve seen how Kadeancy works it’s time to explain how it becomes the drivingforce behind a pulsejet engine.

The Pulsejet Engine Operating CycleLet’s assume that a mixture of finely atomized fuel and air has just been ignited inside ourpulsejet.

The rapidly burning fuel generates gases such as carbon dioxide, carbon monoxide and water-vapor. These gases take up far more room than the air and fuel alone did so pressure build upinside the engine.

The heat generated by the combustion causes those gases to expand so the pressure isincreased even more.

Our pulsejet has become a container filled with pressurized gases – just like the one describedpreviously.

Those gases now rush out the opening at the end of the engine’s tailpipe and in doing so, theycreate thrust which pushes the engine (and whatever it’s attached to) in the other direction.

It was Benjamin Newton who first described this effect when he said “for each and everyaction there is an equal and opposite reaction.”

A fraction of a second after the air/fuel is ignited and the hot gases have started flowing outthe tailpipe, the pressure inside the engine has dropped to match that of the outside air.However, thanks to the Kadenacy effect, the gases keep flowing down the tailpipe and a partialvacuum is created inside the engine.

At this point, two very important things start happening.

Firstly, the valves at the front of the engine open. They’re pushed open because the air outsidethe engine is at a higher pressure than that inside the engine. This pressure difference pusheson the valve and causes them to move aside, thus allowing fresh air and fuel to enter.

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At the same time, those hot burning gases that were travelling down the tailpipe stop for aninstant then start travelling back towards the front of the engine – driven by the air outsidewhich is at a higher pressure than that inside.

You can see that at this point, we have fresh air and fuel coming in the front and still-burninggases coming back down the tailpipe.

Can you guess what happens when the two meet?

That’s right – the whole cycle starts all over again when the flames and hot gases from thetailpipe ignite the highly flammable mixture of air and fuel that has been sucked in through thevalves in the front.

And of course, as soon as that fuel ignites, the pressures generated push the valves at the frontof the engine closed, leaving the hot gases only one way to go – out the back.

When a pulsejet is running, this whole process is repeated many times per second and it is thisrepeated blasting of hot gases out the tailpipe that gives the engine its characteristic noisy bark.

Wasn’t that simple?

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The World’s Simplest PulsejetHere’s a chance to make your own ultra-simple pulsejet using nothing more than a hammer, ascrewdriver, and a few readily available materials.

This simple design was first dreamt up by one of the grandfathers of pulsejet engines, aDutchman by the name of Francois Henri Reynst back in the first part of last century.

Although this pulsejet won’t hurt your ears or produce massive amounts of thrust, it’s still agood idea to include a few warnings at this point.

SafetyPulsejets use explosive mixtures of air and fuel to create power. They also produce lots ofburning hot gases when running.

For these reasons, you should never attempt to run a pulsejet (not even this very simple one)indoors or near anything that could catch fire.

Also be aware that because a pulsejet generates pressurized gases, there’s always a small riskthat part of the engine could fly off and strike anyone standing nearby. This is particularly trueif you’re using a glass jar in the following experiment. The glass can (and will eventually) crackand break due to the heat and pressures involved.

Here are some basic rules that will help keep you safe:

1. Always wear eye protection2. Keep a safe distance from a pulsejet when it is running.3. Keep a garden hose and/or bucket of water nearby at all times4. Use hearing protection

Now on with the fun.

MaterialsIn order to build our simple demonstration pulsejet you’ll need to find the following items:

1. A small jar with a screw-top metal lid (75mm or 3” diameter)2. A screwdriver or nail.3. Some methanol or model airplane fuel

Here’s how we go about building our engine.

With the nail or screwdriver, make a hole in the center of the removable metal lid. You canthen enlarge this hole to around a half-inch (13 millimeters) in diameter.

Now pour some methanol or model airplane fuel into the bottom of the jar to a depth ofabout a quarter inch (5 millimeters).

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Replace the lid and swirl the liquid around in the bottom of the jar a few times.

Remember that you need to run this little engine well away from anything that could catch fireand one suggestion I have is to dig a small hole in the ground so that the jar can be inserted,leaving the lid slightly above ground-level. This will protect you and reduce the fire risk if thejar should crack.

Now bring a lighted match or flame near the hole in the jar’s lid – keeping your face and handswell away from the area directly above that hole – because a large flame may come shootingout with a whooshing noise that can give you a bit of a fright.

It is highly recommended that you wear eye-protection and at least a long-sleeved cotton shirtto protect yourself when performing this experiment.

If you’re lucky, your simple little “jam jar” engine should start puffing away – producing aseries of little pulses of hot gas and perhaps a gentle purring noise.

IMPORTANT: do not let this simple jam-jar engine run for more than 5-6 seconds at a timeor the glass will crack, possibly spilling burning fuel. You can stop it by covering the hole inthe lid with a small piece of wood or even a suitably sized coin.

How Does It Work?Now the more observant reader will have noticed that this pulsejet has no valves – so howdoes it work?

The answer is simple – when you ignited the air/fuel mixture that was originally in the jar itburnt, expelling that large jet of flame and making that whooshing noise.

Because the hot gases rush out of the jar with great speed, the pressure inside the jar quicklydrops below normal atmospheric pressure and a weak vacuum is created – just as described inthe previous chapter.

When this happens, a fresh gulp of air is sucked in through the hole in the lid and that air thenmixes with highly flamable methanol vapor that is rising from the pool of fuel still sitting in thebottom of the jar. The jar once again contains a combustible mixture of air and fuel – but howdoes it ignite? After all, we had to use a match to ignite it the first time didn’t we?

Well, also inside the jar are some remnants of the hot gases generated from the lastcombustion cycle. Eventually the hot gases and the air/fuel mixture run into each other –whereupon ignition occurs, pushing its hot gases out the hole –starting the whole cycle all overagain.

How’s that? We’re not even a quarter way through this book and you’ve already built yourown pulsejet engine!

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Pulsejets for ModelsForty or fifty years ago there were a number of manufacturers producing small pulsejet enginesdesigned specifically for use on model aircraft.

Most of these engines are very similar in design and construction, consisting of a lightweightstainless-steel body and tailpipe, with a machined aluminum head and thin spring-steel valves.

Quite a few of these engines were made in Eastern-bloc countries and at least one was madeby the OS model engine company in Japan.

However, perhaps the most recognized model pulsejet of all time is the Dynajet.

The DynajetThousands of avid model airplane enthusiasts have owned, seen or lusted after this icon of thepulsejet era.

The Dynajet was so popular, and so many were sold, that it rapidly became the benchmarkagainst which all other small pulsejets were compared.

Its simple design and lightweight construction made it perfect for use on model airplanes andwell suited to the U-control speed models of the era.

Even today the Dynajet is a popular item on auction website such as eBay, often producingbids of several hundred dollars or more.

This picture is of an earlymodel Dynajet that had thespark plug located right at therear of the combustionchamber and didn’t have an anodized head. In later models the sparkplug was moved forwardand the aluminum head was anodized a rich red color which resulted in the engine beingknown as the Dynajet Red-head.

The body of these engines is made from two pressed stainless steel shells that are welded alongan upright seam. This technique results in a nice smooth contour between the combustionchamber and the tailpipe, which probably helps the performance somewhat.

This picture shows the front of a later-modelDynajet with a more deeply finned valve-headsection. You can clearly see the effects of thered anodizing on the aluminum and the moreforward location of the sparkplug.

The company that used to make theseengines, Dynafog, is still in business but nowfocuses on industrial fogging machines whichuse the pulsejet principle to atomize and spray insecticide and other chemicals.

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Another company, Bailey Machine Service, is still making a clone of the Dynajet engine whichit sells for about US$250 although the supply is said to be somewhat erratic.

Many of the other pulsejet designs you’ll see from this era are very similar to the Dynajet insize, dimensions and performance, although there are still plenty of weird and wonderfulvariations on the basic theme.

The OS pulsejetThis engine was manufactured in the 1950s and 1960s by the Japanese company OS.

There were apparentlytwo slightly differentversions of this engine,one being a little larger and more powerful than the other but they were both obviously verysimilar to the Dynajet in both form and function.

However, unlike the Dynajet which used a single machined piece of aluminum for its valve-head and venturi, the OS unit consists of two separateparts which screw together.

There was also a myriad of other pulsejet designsmanufactured about the same time and sold under araft of different names such as TigerJet, etc.

An even wider range of designs and ideas werepublished as plans for a generation of enthusiastswho, in a post-war era, were eager to build one ofthese magical “jet engines” for themselves.

As a result, many magazines such as Popular Science and Popular Mechanics were littered withadvertisements for such plans.

It is unlikely that many of those who purchased these plans ever managed to construct aworking engine and at least a few of the designs were so fatally flawed that the publisher wasobviously just trying to cash in on a craze.

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How to design a pulsejetOne of the more interesting and more readable texts on pulsejet design and theory was writtenby a C.E. Tharratt while he was a staff scientist at the Chrysler Space Division in the late1950s.

Titled “The Propulsive Duct”, this paper condensed much of the known pulsejet theory into afew simple formulas and constants.

Using these calculations Tharratt claimed that “we are in the surprising position of beingable to determine the dimensions of a duct capable of developing a given thrust literally onthe back of an envelope and without knowing anything about its gas dynamics!”

Sounds great doesn’t it?

Unfortunately, while Tharrat’s formulas have stood the test of time and his understanding ofthe mechanisms behind the pulsejet (or “propulsive duct” as he called it) remains valid, when itcomes to designing a powerful, reliable pulsejet engine, the devil is in the detail.

However, here is the simple formula that Tharratt proposed to be the core of pulsejet design(note: Tharratt’s papers and constants are presented in the imperial measurement system sothat’s what I’ve used in this chapter. I’ll update with metric versions in the next release of thisbook):

V/L = 0.00316F

Where:V = engine volume (cu ft)L = effective acoustic length of the engine (ft)F = thrust (lb)

The validity of this formula has been verified against a wide number of different and provenpulsejet designs including the Argus V1 and Dynajet.

Let’s take a look at what this formula actually means in terms of the way that the dimensionsof a pulsejet affect its power output.

We can see that if we kept the volume of the engine (V) constant but increased its effectiveacoustic length (L) then the power would reduce. Note that in order to do this, the diameter(and cross-sectional area) of the engine would need to be reduced – so it would appear thatthere is a definite relationship between cross-sectional area and power.

Now, if we keep the length (L) constant but increase the volume (V), the power wouldincrease. To accomplish this however, we’d have to increase the diameter (and cross-sectionalarea) of the engine. This confirms that relationship between cross-sectional area and poweroutput.

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If we manipulate that simple formula a little more, we come up with a new formula containinga very interesting constant:

F = 2.2A

Where:F = thrust (lbs)A = mean cross-sectional area (square inches)

Let’s just make an important point here – this 2.2lbs/sq-in constant is derived from a formulathat includes the engine’s total volume as a factor. This is why it’s not just the cross-sectionalarea of the tailpipe that is important (as many would have you believe), but the mean (average)cross-sectional area of the entire engine along its total length.

However, it should be added that Tharratt didn’t expect engines built to his formulas to have ahuge bulbous combustion area at the front so don’t expect that such a feature will significantlyincrease an engine’s performance over a straight pipe.

So now we can plug in our required power output of 10lbs thrust and get this:

10=2.2A

which simplifies to:

A=10/2.2A = 4.545 square inches

From this we can calculate the mean diameter of our engine as follows:

D = 2√(4.545/π)D = 2.4 inches

Now we need to decide on a length for the engine. Remember that making the engine longeror shorter won’t actually increase or decrease its power – only a change to the cross-sectionalarea will do that.

However, the length does have a bearing on the frequency at which our engine will operate.

The only reference I can find from Tharratt in respect to the suggested length of a pulsejetengine is that it be at least eight times the mean diameter.

This length to diameter ratio is usually expressed as L/D and Tharratt notes that “withgeometrical ratios of L/D < 7 the development problems become particularly challenging”and that in an engine with an L/D < 7 “combustion with chemical fuels is difficult tosustain, let alone develop maximum thrust”

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It makes sense therefore, to use an L/D somewhat greater than 7 and it’s been my experiencethat a figure of about 14 is a good place to start for relatively small engines like this.

By way of comparison, the Dynajet has an L/D of 15 and the Argus V1 has an L/D of 9.6.As a rule of thumb, the smaller the engine, the higher the LD needs to be in order to getreliable operation and good power levels.

So now we can calculate the length of our engine as follows:

L = 14DL = 14 x 2.4L = 33.6 inches

Okay, so now we know that to create a 10lb-thrust pulsejet engine we’ll need a piece of pipethat is 33.6 inches long and 2.4 inches in diameter -- but wait, there’s more!

Another key formula presented by Tharratt was one for calculating the valve area for an engineof a given size and power:

Valve area = 0.23 x mean cross-sectional areaOr

Valve area = 0.1045F sq in

It is interesting to note that this 0.23 (or 23 percent) figure contrasts sharply with thatproposed by other pulsejet “experts” of the era who suggest a figure of 0.4-0.5 is better.

Let’s use Tharratt’s formula and constant to work out the size of the valve area we need forour 10lb-thrust engine.

Using the first formula we get:

Valve area = 0.23 x mean cross-sectional areaValve area = 0.23 x 4.545Valve area = 1.045 sq in

Using the second formual we get:

Valve area = 0.1045FValve area = 0.1045 x 10Valve area = 1.045 square inches

Yep, we get the same answer both times so we now know that the effective valved area for ourl0lbs-thrust engine is just over 1 square inch.

Now Tharratt’s formulas assume that the intake is an open hole, with no losses due to thepresence of valves. Unfortunately, the presence of spring-steel reed valves will significantlyimpact the flow of gas so we need to take those losses into account when designing our intakevalving system.

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The precise efficiency of a valving system depends on many factors and I suggest you read thechapter on intake valving for more information – but in the case of our little design, let’s use asimple petal valve and assume that it is just 50 percent efficient.

To get the actual area of the valve required to achieve an “effective” area of 1.045 squareinches at 50 percent efficiency we simply divide by 0.5 to get a figure of 2.090.

So let’s see how our pulsejet is looking like so far:

Thrust 10lbsLength (from valves to end of tailpipe) 33.6 inchesMean diameter 2.4 inchesTotal valve area (assuming 50% efficiency) 2.090 square inches

So what about those valves then?

A petal valve system consists of a ring of holes over which a spring steel valve, consisting of amatching number of petals, is laid.

If we were to use 10 holes, spaced at 36 degree intervals, each hole would need to have an areaof:

2.090/10 = 0.209 square inches

which requires a diameter of 0.516 inches.

Alternatively, if we used a ring of 12 holes spaced at 30 degree intervals, each hole would needto have an area of:

2.090/12 = 0.174 square inches

which requires a diameter of 0.470 inches

It’s been my experience that a half-inch hole is the upper limit for petal valve holes. Once yougo beyond this size the pressure on the valves themselves cause them to be bent so that theybegin to dish into the hole and this adversely affects their operation. For this reason, we’ll gowith the 12-hole option.

You’ve probably already noticed that most small pulsejets have a much larger diameter sectionat the front, from where they funnel down to a narrower tailpipe. Many people mistakenlyassume that this bulbous front is a combustion chamber, designed to contain the burningair/fuel mixture. While it may be true that much of the air/fuel is burnt inside this bulbousfront section, the reason small engines are shaped this way is actually quite different and we’llsee why when we do the next set of calculations.

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We’ve decided to use a petal valve system consisting of 12 holes, each of 0.47 inches indiameter. If we assume that the holes will be placed in a ring around the edge of the pipe, andthat we allow a certain amount of space between the holes so that there’s room for the spring-steel valves to rest, then we have a problem.

Lets assume we need a ¼ inch gap between the holes – that means the total circumference of acircle drawn through the center of each hole will be:

NumOfHoles x diameter + NumOfGaps x GapSize

And when we plug in our numbers we get:

12 x 0.47 + 11 x 0.25 = 8.39 inches

That represents a circle with a diameter of:8.39/π = 2.67 inches

What’s more, that figure is the diameter of a circle that runs through the center of each hole sowe need to add two times the radius of the holes to get the size of a circle that will run aroundthe outer edge of the ring of holes.

2.67 + 0.47 = 3.14 inches

Clearly, the absolute minimum diameter of our valve system (3.14 inches) is larger than thecalculated diameter of our engine’s pipe (2.4 inches)

This disparity grows even further when we build in a bit of extra space so that the valves don’tscrape against the side of the engine when they swing open and closed. It’s been myexperience that you should allow a space between the outer edge of the valve holes and theside of the engine which represents an area equal to the total area of our valve holes.

Or in other words, we need to leave 2.090 square inches of space around our ring of holes.That can be calculated as follows:

The area of a single circle covering our ring of valve holes would be:

πR2 or 3.1415 x 1.57 x 1.57 = 7.74 square inches

Add 2 .090 square inches to get the size of the larger circle and we get 9.830 square inches

From this we can calculate the diameter needed to provide that extra 2.090 square inches ofspace around the edge of the valve-ring:

Diameter = 2√(9.830/π)Diameter = 3.53 inches

Is your head sore from all these calculations yet? Don’t worry, we’re nearly done.

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Now that we know the diameter of our “combustion chamber” we need to calculate how longthis section of the engine should be.

Since Tharratt didn’t use petal valves, he didn’t need this bulbous front section so has noadvice (that I can find) for calculating this dimension.

However, we can look to the work performed by Schmidt (the guy who designed the Argus V1engine) and my own empirical research that indicates the following:

During the intake phase of a pulsejet’s operation it will draw in a fresh charge of air equalto 15%-20% of the total engine volume.

I see no reason why we shouldn’t design the engine so that this front section is just largeenough to hold this fresh charge of air/fuel. In that case we need to do some morecalculations to determine its length:

If our engine were just a straight pipe of 33.6 x 2.4 inches then it would have a volume of152.7 cubic inches. Such an engine would suck in 152.7 x 0.2 = 30.54 cubic inches of airduring each cycle – so our front section needs to have a volume of 30.54 cubic inches.

We’ve already calculated the area of this section as being 9.83 square inches we can calculatethe required length as follows:

30.54 / 9.83 = 3.11 inches.

Of course more alert readers will notice that this 30.54 cubic inches no longer represents 20percent of the engine’s total volume. This is because by making this front section widerwithout reducing the overall length of the engine, we’ve actually increased its total volume byan additional 16.4 cubic inches.

The total volume of our engine is now nearer 169 cubic inches so the 30.54 cubic inch frontsection only represents 18 percent of the total volume – but this difference is so small as to beunimportant.

The only thing remaining now is to join the front section of the engine to the tailpipe using acone with a diameter of 3.53 inches at one end and 2.4 inches at the other.

How long (ie: what angle) should this cone be?

Well if we look at the Dynajet we can see that it’s hardly a cone at all – more of an abrupttransition. By comparison, the Argus V1 engine uses a very long, shallow angled cone to jointhe two sections. So how do we decide which is best?

Let’s look at the effects that the angle of this cone might have on an engine’s operation.

Coming up with a suitable angle for this cone requires balancing a number of factors. Toexamine them, let’s look at extreme examples:

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1. If we simply used a flat plate to join the two sections of the engine then the hot exhaustgases would have a rather torturous path to follow. Some of those gases would have totravel around two 90 degree bends to get from the combustion chamber to the tailpipe andthat would potentially reduce the speed at which they were able to exit from the engine.Remember, the speed at which the gases leave the engine affects the thrust – we wantthose gases leaving as fast as possible. For this reason, a flat plate is obviously not such agood idea.

However, this configuration is not quite as bad as you might think, after all, the Dynajethas a very steeply angled cone that must constrict the flow of exhaust gases right? The veryfact that it is so hard for the combustion gases to get into the tailpipe means thatimmediately after ignition, pressure will build up inside the combustion chamber as allthose gases try to rush around a tight bend and down the tailpipe.. Those higher pressurescan improve combustion efficiency and actually increase the speed of the gases in thetailpipe. This is called post-ignition confinement (PIC).

It’s also worth noting that in the case of a flat plate, the hot gases that return from theengine's tailpipe and ignite the fresh air-fuel charge may do so far more efficiently. This isdue to an interesting effect that occurs in the way they form a narrow jet that reaches deepinto the chamber rather than a larger diffuse front that ignites the fuel more slowly.

Remember that the faster the fuel burns, the more power our pulsejet will develop becauseit will have less time to expand as it burns – thus producing the higher internal pressuresthat will, in turn, result in higher tailpipe gas velocities.

This diagram shows how ignition differs based on the angle of the cone betweencombustion chamber and tailpipe. Note that in the second diagram, the distance betweenthe hot gases and the engine body is far less than in the first – this is important.

The speed at which the combustion flame-front travels through the fresh air/fuel mixtureis relatively slow (just a few tens of feet per second) in a low-compression engine like thepulsejet. Because of this, the mixture in the second diagram will be burnt far more quicklythan that in the first, since the flame-front will be wider with a much shorter distance totravel.

2. If we used a very long cone that had a shallow taper all the way to the end of the engine (ie:no tailpipe as such) --then it would obviously be much easier for the combustion gases to

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flow out under pressure. However, we’d also be significantly reducing the ability of theengine to create a vacuum after combustion is completed because a much smallerpercentage of the exhaust mass would be travelling at maximum velocity inside the engine.Remember that the “force” exerted by the escaping gases is equal to their mass times thevelocity to which they’re accelerated (F=MA). For a given size of engine engine, the masswill always be the same but the velocity to which those gases are accelerated will dependvery much on the design of the tailpipe. We need plenty of velocity to get the forcerequired to establish a strong Kadenacy effect. In fact, tests conducted by the NACAduring the 1950s indicated that an engine designed with just a long convergent coneinstead of a straight tailpipe was very difficult to get running at all.

Once again it seems that a compromise is in order so we’ll chose an angle of 30 degrees for thesection between the combustion chamber and the tailpipe. This will provide some post-combustion confinement to increase the internal operating pressures while ensuring that theengine still has good internal mass-flow speeds to provide maximum Kadenacy effect.

A 30-degree cone will be a fairly short section – just 1.84 inches long.

So here are the final dimensions of our pulsejet engine, wasn’t that simple?

It should be noted that this is a very basic engine and there are still a few tricks we can use toimprove its performance – but more of this later.

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Comparing Intake Valving SystemsOne of the most critical components of a traditional pulsejet engine is the intake valvingsystem.

The demands placed on the intake valves are amazing.

They have to open and close several hundred times a second while being exposed to thethermal stresses associated with being alternately blasted by searing hot combustion gases andcold incoming air. At the same time, these thin strips of spring steel must resist metal fatigueand fracture resulting from the high mechanical stresses imposed.

What’s more, they have to do all this while providing a 100 percent seal against combustiongases when closed, and allowing the smooth, unimpeded flow of fresh air when open.

To make life even harder, the only power available to open them is the tiny difference inpressure between the outside air and the small vacuum created inside the engine by thekadenacy effect of escaping exhaust gases down the tailpipe. (just a few psi).

It’s no wonder therefore, that no aspect of pulsejet design and construction has caused moresleepless nights, scratched heads and frustration than the valving.

Lets examine the alternatives:

Petal ValvesSmall engines almost always use a petal-valve. These valves offer the following benefits:

1. simplicity. The valve can be etched or cut from a single piece of spring-steel.

2. Low cost. As a side effect of their simplicity, petal valves can also be very economical tomanufacture – especially when you consider that the valve plate consists of a simple pieceof aluminum with a ring of holes drilled in it.

Unfortunately, the petal valve also has a number of disadvantages:

1. poor aerodynamic performance. Since the air passing through a petal valve must negotiatetwo near-90 degree bends on its way into the engine, the efficiency of such a system is notparticularly high.

2. low durability. Because the tips of the petals are directlyexposed to the hot combustion gases, petal valves oftensuffer from premature tip cracking or fracture.

3. High maintenance. Since petal valves are usually made asa single piece, the failure of individual petal requires thereplacement of the entire spring-steel valve.

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Despite their drawbacks, petal valves are generally the best option for small pulsejet engines,although I wouldn’t recommend them for any engine larger than about 20lbs of thrust.

The V or multi-V valveGenerally only seen on larger engines, these valves aregenerally more efficient than petal valves because theyproduce less deflection of the airflow when they’re inan open position.

There are two basic methods of constructing such avalve system – one involves the use of two or more flatmetal plates with holes in them, joined at an angle (45degrees is a good starting point).

The other method of forming a V valve is the one usedin the Argus V1 where a cast or machined spacer withmultiple ribs is used to hold the valves in position and limit their movement as in the diagrambelow:

V valves provide the following benefits:

1. Higher efficiency than a simple petal valve. Since the incoming air has a far straighterpathway into the engine, more air is able to flow for a given size of valve opening whencompared to a petal-valve.

2. Lower maintenance costs. Since the individual spring steel valves in a V-valve system canbe replaced as/when they fail, maintaining the engine becomes a less expensive task and allvalves can be used to the full extent of their lifespan.

3. Scalability. Unlike the petal-valve, a V-valve can be easily scaled to create the required valvearea by simply increasing the length or number of V-valves in the array.

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Of course there are downsides too:

1. Greater complexity. A V-valve generally requires more machining steps and a highercomponent count than a petal-valve setup.

2. Increased expense. As a side effect of this complexity, the production cost for a V-valvesystem is significantly higher than for a petal-valve. This is another reason why most cheapmodel engines don’t use V-valving.

Less commonly used valving systemsPetal and V-valves are not the only systems that have been used on pulsejets but they are byfar the most common.

Perhaps the only other practical valving system for a pulsejet is:

The Rotary ValveThese generally consist of either a spinning disk containing a hole that controls the flow of gasby covering and uncovering a matching hole in the front of the engine, or a spinning butterfly-type valve that alternately blocks and allows the flow of gas.

Rotary valves can be made very robust and thus have the potential to create very reliable, long-lived pulsejets. Unfortunately however, they are fraught with hidden complexities, not theleast of which includes the issue of timing.

In a conventional pulsejet valving system, the valve timing is automatically controlled by thechanging pressure inside the engine. When the internal pressure goes up (because the fuel hasignited) then the valves close. When the pressure falls (due to the Kadenacy effect) then thevalves open). This results in a very simple and quite reliable system that automaticallycompensates for any fluctuations in the engine’s operating frequency or phase.

Rotary valves on the other hand, have no such intrinsic timing control and therefore require avery sophisticated system to control their rotational speed and phase relationship to theengine’s basic operating cycle. This immediately negates the pulsejet’s two single mostendearing qualities – simplicity and low cost.

Research done in the USA during the 1940s cited engines using the rotary valve as offering“very long useful operating periods” along with “good thrust and specific fuelconsumption” but also mentioned the complexity associated with driving such a valve in asynchronous manner.

Never the less, rotary valves are being considered as a viable option for the new generation ofpulse detonation engines (PDEs) currently under development. Since these PDEs alreadyrequire a significant number of ancillary control systems anyway, the overhead of the rotaryvalve adds little to the cost or complexity of these engines.

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Making Reed Valves Last LongerThe thin spring-steel valves normally used to control the flow of air into a pulsejet and stopthe hot combustion gasses from escaping out the front are the most highly stressed part of aconventional pulsejet engine.

The life of the reed valves in most pulsejet designs is measured in minutes rather than hoursand they must be considered a “consumable” part of any conventional pulsejet engine.

It's not hard to understand why these fragile little pieces of metal don't last long. They'reslammed back and forth between the intake and retainer plates with great force, severalhundred times per second. What's more, they're usually exposed to extremely hot combustiongasses

There are three factors that contribute to reed-valve failure:

• heat-damage• impact damage• fatigue due to flexing

A well designed valve system attempts to minimize all these factors so as to provide maximumvalve life but (woudn’t you know it) there are always compromises involved.

Let’s look at the simplest and easiest to solve issue first:

Fatigue due to flexingThis picture shows the effect of valve failure due to metalfatigue brought about by the flexing motion of a petal valve.Note that one of the petals has completely broken near theroot. Close inspection of this valve showed that stress crackswere starting to appear at the root of the other petals. In thiscase, failure was due to a poorly designed valve-retainer whichhad an uneven radius of curvature.

Obviously, reed valves must flex in order to operate properly.The key to avoiding premature failure however, is to limit thisflexing to a bare minimum and try to keep the flex radius aslarge as possible.

To this end, the conventional petal-valve arrangement with a curved valve-retainer is quitegood – providing the curvature of the retainer is of a constant radius.

If the valve retainer doesn’t have an even, large radius curve to it, most of the valve flexingwill be concentrated over a small area near the root of the petal. In a fairly short space of time,the stresses caused by this flexing will cause the spring steel to crack and fracture.

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This diagram shows the right and wrong way todesign the valve retainer for a petal valve system.Note that when a small radius is used near thebase of the valve retainer, most of the reed valveremains unbent so all the stress is concentratedat the root.

There are several ways to make a valve retainerthat has a large, even radius but because it’s somuch easier to make a straight-sided retainer,people often make the mistake of creating whatamounts to a shallow cone instead– withpredictably bad results.

Impact DamageMost small pulsejets run at somewhere between 180 and 250 cycles per second. This meansthat the valves must open and close as often as 15,000 times per minute.

Each time the valves close, they slam into the valve-plate at quite high speed and thereforewith significant force. All the energy that is contained in these fast-moving valves has to gosomewhere – and some of it is absorbed by the valve material itself.

This constant hammering eventually causes minute cracks toform at the tips of the valves after which they begin to frayand small fragments will eventually flake off. If you’rerunning your pulsejet at night, these small fragments can beseen as impressive sparks flying out the tailpipe.

This picture shows a badly frayed petal valve that has certainlyreached the end of its useful life. It is a very good idea toreplace valves long before they get to this state because thesharp, ragged ends will soon damage the comparatively softmaterial of the aluminum valve-plate against which theyimpact.

There are a few techniques that can be used to reduce impact damage to reed valves.

• Reduce the amount of valve travel. If the valves can open too far then they will reach amuch greater velocity when they’re closing and this will increase the forces applied tothem as they impact the valve seat. Of course limiting the valve-opening will also tend toreduce an engine’s power as it means that less air can be drawn in during the intake phase.

• Use a softer material for the valve-plate. Most small pulsejets already use an aluminumalloy for the valve plate so there’s not a lot of room for improvement here. However, theNACA did conduct tests on engines which had a think coating of neoprene on the valve-plates. This was said to almost double valve-life by reducing the impact shockexperienced by the closing valves.

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Note that although it might seem like a good idea to simply increase the amount that a valveoverlaps the hole it covers so that the air trapped between the valve and the plate acts as acushion to soften the impact, this is actually a bad idea.

If the overlap area is too great, the air can’t get out of the way quickly enough and the tips ofthe valves are actually bent backwards by this trapped air. As a result, tip fraying isdramatically increased because any cracks that form grow very quickly due to this additionalstress.

Determining the ideal overlap area is something best done by trial and error. If the overlap istoo great then you’ll get premature fraying and poor engine performance (due to late closingof the valves). If the overlap is too small then the valve plate will be damaged by the highpressure loadings and this will ultimately affect engine performance because the valves will nolonger seal properly.

Heat DamageReed valves are usually made from hardened, tempered spring steel because it’s strong and willreturn to its original position after flexing.

The problem with spring steel is that the hotter it gets, the softer it gets. If it gets too hotthen it will lose much of its strength and some of its springiness.

Unfortunately, the inside of a pulsejet engine is a very hot place and it is the pressuregenerated by extremely hot (1,500 deg C) combustion gasses that actually cause the valves tobe closed.

So why don’t the valves simply get red hot and go all floppy?

Well fortunately, the valves are only exposed to the hot combustion gasses for part of theoperating cycle. During the intake cycle they’re cooled by the incoming charge of fresh air.

In a petal valve setup, the valve retainer also provides a measure of protection from the heatof combustion by shielding most of the valve from direct exposure to the hot gases.

However, the tips of the reed valves will still get hot and, as a result, they will become softerthan the rest of the valve. What’s more, the valve retainer will itself heat up once the engine isrunning and some of this heat will be transferred to the valves when they’re open.

A number of solutions have been proposed to the problem of valve heating but most of themwill reduce an engine’s performance to some degree.

One of the simplest solutions is to place a flame-trap in the engine directly behind the valves.This flame trap consists of little more than a mesh of stainless steel or some similar heat-resistant metal.

It’s well known that a metal mesh with suitably sized holes will not allow a flame to passthrough it – but it will allow air and other gases to do so. This was the principle behind the

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old miner’s safety lamps. Before the days of battery-operated flashlights, miners still neededsome form of illumination while working underground. A naked flame such as that from acandle or oil lamp would pose a very real danger as it risked igniting underground pockets ofexplosive gases such as methane.

By enclosing the flame in a metal mesh, the flame could not extend beyond that mesh so the“safety lamp” could provide light and still be used without risk of sparking an explosion.

Now, the problem with a using a flame-trap mesh in a pulsejet is that in order to be effective,the size of the holes in the mesh must be quite small. As a result, the mesh represents asignificant obstacle to the flow of the incoming fresh air charge. This means less air is drawnin during each intake cycle so less power is produced..

Never the less, a flame-trap mesh is one way of making a relatively low-powered engine thatwill run for far longer between valve-changes than a regular pulsejet.

Over the past two years I’vegiven quite a bit of thought tothe issue of extending valve lifeand have come up with twoideas of my own.

The first is the Blast Ringconcept which works byproviding a physical shieldbetween the hot exhaust gasesand the valve tips.

By blocking the direct path of the combustion flame to the valve-tips, the operatingtemperature of the valves is significantly reduced. However, because the Blast Ring has a verylarge hole in the middle it doesn’t restrict the flow of the incoming charge of fresh air to thesame degree as a flame-trap mesh.

This picture shows my PJ15 design running with aBlast Ring in place.

You’ll notice that the ring itself is glowing red-hot, anindication that it is indeed absorbing much of the heatthat would otherwise be reaching the valves.

However, even this system imposes about a 15%-20%performance penalty on the power levels that wouldotherwise be obtained from an engine.

Another method for reducing the valve heating is to design the engine so that there’s a bufferof cold, dense air between the valves and the combustion gases. The easiest way to do this isto inject the fuel into the combustion chamber some distance from the front.

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In such an engine, the air infront of the injection point willnot contain any fuel thus willnot actually take part in thecombustion process. It willhowever, act as an insulatingbuffer between the hotcombustion gases and thevalves.

Unfortunately, as with the other methods mentioned so far, there’s a performance penaltyassociated with this method.

Since a pulsejet normally only draws in a fresh charge of air equal to about 15-20 percent of itstotal volume, creating a buffer zone which contains no fuel leaves less available for thecombustion process. That means less fuel can be added and, as a result, the engine producesless power.

There are very few free lunches in the world of pulsejet design.

More recently however, I have come up with what appears to be a system that imposes noperformance penalty, yet significantly improves valve-life.

This system works by creating a two-layer valve retainer that iscooled by the incoming fuel.

As you can see in this diagram, the fuel (purple) passes between thetwo thin dished valve-retainer plates before mixing with theincoming air.

As you can see, this method allows virtually all the air in the combustion chamber to be mixedwith fuel and therefore provides good power.

This provides multiple other benefits over the traditional system.

1. The fuel is pre-heated and/or vaporized before it mixes with the incoming air. Thisprovides a much better (and more combustible) mixture than is normally achieved eitherby direct injection or atomization.

2. As it atomizes, the fuel absorbs a tremendous amount of heat from the two dished plates,this cooling them to a much lower temperature than they would otherwise run at.

3. The two disks, with the small gap between them, act as a far more efficient heat shieldthan does the normal one-piece valve retainer.

4. The small gap between the retainer disks tends to absorb some of the hot combustiongases that would otherwise reach the valves.

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Here’s how I’ve implemented this design concept onmy PJ15 engine. The two disks are formed from thin0.020” (0.5mm) stainless steel which is spun to shapeon a lathe.

I’ve actually observed a small power increase afterchanging from a conventional one-piece valveretainer to this new concept and valve-life has beenalmost doubled.

Unfortunately, building a valve system like this takes more time and more time and skill thanthe traditional one-piece valve-retainer – remember what I was saying about free lunches?

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Fuel Systems

Next on the list of critical elements of a pulsejet must surely be the fuel system.

AtomizationSmaller engines such as the Dynajet have traditionally used a very crude form of carburetorthat using the incoming air to create a spray of rather coarsely atomized fuel droplets.

This atomizing process occurs right at the front of the engine when the incoming air is forcedthrough a slight venturi.

An Italian by the name of Bernoulli discovered that the faster air flows, the lower its pressurebecomes. This observation was promptly labeled (wait for it…) the Bernoulli Effect.

The atomizer on these small pulsejets uses a venturi to squeeze the incoming air through anarrowing in the intake. As it squeezes through, it has to speed up. As it speeds up – thepressure drops.

Now, if we stick a pipe carrying some fuel into the middle of this low-pressure area, the fuel isliterally sucked out and turned into a fine spray of droplets.

What could be simpler?

Unfortunately, although this system does work, the magnitude of the low-pressure area createdin the pulsejet’s venturi is quite small and this means that there’s not much energy available tosuck that fuel through.

A Note About Atomization and VaporizationAnother problem with the simple atomizer is that the fuel droplets created tend to be verylarge and therefore do not vaporize particularly well. It should be remembered that liquid fuelsthemselves don’t actually burn – only the vapors that they emit will ignite. In order to obtaingood vaporization, the goal should be to create the smallest possible droplets because thisresults in the largest surface area (from which vapor is emitted) for a given volume of liquid.

Fortunately however, the inside of a pulsejet engine is a very hot place so, despite the fact thatthe simple atomizer does a poor job of converting liquid fuel into a nice fine spray, the highinternal temperatures of the engine greatly assist the conversion of those large droplets of fuelinto vapor.[endnote]

The end result is that most of these small pulsejets are extremely sensitive to just where thefuel tank is placed relative to the atomizer assembly.

If you place the tank too low then the engine won’t have enough “suck” to pull the fuel up tothe atomizer nozzle.

Place the tank too high and gravity will draw the fuel through – effectively flooding the engine.

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What’s worse, even if you do get the engine running nicely, moving the fuel tank up or downby even an inch or two can cause it to stop because the fuel flow is affected.

There are ways to reduce this sensitivity to fuel-head however and perhaps the simplest is touse a pressurized fuel tank.

By delivering the fuel under pressure, the effect of a changing fuel-level is dramaticallyreduced. The big problem is how do we generate this pressure?

One option is to simply pump some compressed gas into the fuel tank then seal it up. Inorder for this to work, the tank should only be filled with fuel to only about 25 percent of itscapacity otherwise the pressure inside will drop significantly as the fuel is drawn off.

Alternatively, the compressed gas can be stored in a separate container and fed into the fueltank through a regulator. This is how the fuel system for the Argus engine that powered theV1 flying bomb was configured and is illustrated in the diagram above.

Rather than rely on a large reservoir of compressed airinside the tank, it is possible to tap into the pressureproduced by the combustion of the pulsejet itself.

This diagram shows how some of that pressure can bedirected into the tank to keep it pressurized. Note thesmall reed valve that stops the pressure from leakingback into the engine during the intake phase.

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In practice, the reed valve should be placed in the pipe that leads from the engine to the tankrather than in the tank itself. Surprisingly, there’s little risk that the hot gases from inside theengine will ignite the fuel in the tank. This system can be used with both atomized andinjected fuel systems.

Another simple way to achieve fuel pressurization is to use something like a small balloon for afuel tank. This configuration is called a “bladder tank”. The elasticity of the balloon willautomatically pressurize its contents – but be aware that some fuels will quickly break downthe rubber from which normal balloons are made and if it goes “pop”, you’ll have a very realfire danger.

Some of those using pulsejets in model airplanes often use these bladder tanks to ensure goodpressurization and reliable fuel feed under varying G-forces. It’s worth noting however, thatthe rubber tank is usually contained inside another leak-proof container such as a plastic sodabottle. This way, if the bladder bursts, the fuel remains contained

Most flyers of pulse-jet powered model airplanes also use a device calleda Cline regulator to ensure not only that the fuel pressure remainsconstant but also to automatically shut off the fuel flow if the enginestops unexpectedly.

You should also be aware that any leak in a fully pressurized fuel systemcan result in large amounts of flammable liquid being dumped onto theground or in the general area of the engine. This is an obvious fire risk. What’s even worse isthat if the engine stops for any reason, the flow of fuel will continue to flood into what is nowa red-hot steel tube. That can result in a very impressive fireball that could also be verydangerous.

InjectionVirtually all engines over 20lbs of thrust use direct fuel injection rather than atomization.

In such a system, the fuel is squirted directly into the engine’s combustion chamber undersome form of pressure.

This makes the engine’s operation far more reliable and adds the additional benefit that byvarying the amount of fuel being injected, the engine’s power can be varied. Yes, a throttleablepulsejet!

The Argus V1 engine used direct injection but, to the best of my knowledge, no attempt wasmade to provide any form of throttle control – not that it would have been of any use on aflying-bomb anyway.

The downside of fuel injection is that you need some method of pressurizing the fuel to forceit into the engine in a fine spray.

There are really only two options – use a fuel pump or pressurize the entire fuel tank.

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The V1-flying bombs used the latter option and the fuel tank was pressurized using the samecompressed-air source which drove the missile’s gyroscopes and other onboard systems.

Most of my injected engines use propane as a fuel because this has the advantage of being self-pressurizing. Your common BBQ tank has around 100psi of pressure in it so you can use thisfor direct injection without the need for a supply of compressed air or a fuel pump.

Using such a system, the pulsejet remains a stand-alone engine that requires no extra bits andpieces to keep it running.

The simplest injection system for a petal-valved enginesimply involves locating a cross-drilled injection nozzledirectly behind the valve-retainer plate.

This nozzle is drilled so that the incoming fuel is sprayedout directly towards the side of the combustionchamber. This ensures optimum mixing with the air and(in the case of liquid fuels) means that any droplets offuel that aren’t vaporized by the incoming air will beinstantly flashed into vapor when they hit the hotcombustion chamber walls.

A more recent innovation I’ve come up with howeverinvolves placing an additional disk behind the valveretainer, separated by just a small space.

By injecting the fuel in thesame radial pattern aswith the previous system

but between the two disks, the fuel is not only vaporized moreeffectively but also serves to cool down the valve retainer disk(and the valves). Building a system like this does however, requireaccess to a lathe in order to turn up the key component which isthis radial injector nozzle.

Using this double-disk setup I’ve been able to double the life of the reed valves used in a petalvalve engine while also slightly increasing the engine’s performance and throttle range.

Timed InjectionOne disadvantage of direct fuel injection is that simple systems such as the one used in theArgus V1 engine tend to spray fuel throughout the engine’s operating cycle.

Fuel will only burn efficiently (or at all) when mixed with exactly the right amount of air. Thiscombustible mixture of air to fuel is referred to as the “stoichiometric ratio” and it variesdepending on the type of fuel being used.

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It makes little sense therefore, to waste fuel by injecting it when there is no incoming air to mixwith it as that fuel will be unable to burn inside the engine thus contributes nothing to thethrust being generated.

Back in 1947, the guys at Princeton University came to this same conclusion and suggestedthat using timed fuel injection would be a way to improve the fuel-efficiency of pulsejetengines.

Now there are two ways in which timed fuel injection could be done: the simple way and thecomplex way.

Given that the simplicity of a pulsejet is its single greatest virtue, I’m all in favor of keeping atimed fuel injection system simple too.

I regularly get email from people who think it would be a good idea to use an electricallydriven fuel injector like the ones used in modern car engines – but I disagree.

In order to make one of these injectors work you’d need a rather complex system that involveda battery to drive the injector, sensors to measure the pressure inside the combustion chamberfor timing, and some electronics to tie the whole thing together.

This setup, although I’m sure it could be made to work, would be costly, complex and offeronly minimal benefits over the system I use to obtain timed fuel injection.

Fortunately it is a simple job to synchronize the injection of fuel into the engine with the intakeof a fresh air charge. This is because the pressure inside the engine falls to below 1atmosphere (14.7psi at sea-level) during the intake phase and rises to as much as twiceatmospheric (30psi+) during combustion and exhaust phases.

A valve placed over the fuel jet is sufficient to provide a degree ofinjection timing and the addition of this mechanism can provide anoticeable improvement in the fuel-efficiency of a large pulsejet.

I’ve experimented with a number of different valved injectors rangingfrom a simple bolt drilled length-wise with a flap of spring-steel over theend like the one illustrated here…

To this carefully machined injector made from stainless steel andnickel-plated steel components I fitted to the 100lbs-thrust engine onmy gokart. I noted a very definite improvement in the fuel-efficiencyof this engine after fitting the timed injector system.

What Fuel is Best?One of the great advantages of pulsejet engines is that they can, at least in theory, be made torun on almost any type of combustible liquid or gas.

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Pulsejets aren’t limited to liquid or gas fuels however – on at least two occasions, coal dust hasbeen used as a fuel. It is rumored that the Germans attempted to run the Argus V1 engine oncoal dust when liquid fuel supplies became almost unobtainable near the end of WW2 andsome of Reynst’s pulsed combustors were designed specifically to use this unusual fuel.

Before you start worrying too much about what is the best fuel, it’s worth citing part of areport published by Princeton University in 1947 that summarized a large amount of theresearch done into pulsejet engines up to that time. It said “the pulsating jet engine ofcontemporary design ran on almost any common fuel with negligible variations inperformance.”

The only caveat the report included was that “principal [sic] differences were in the degreeof body heating and the rapidity of valve destruction.”

They found that even the use of exotic fuels such as nitropropane or nitromethane offeredonly a slight power increase at the expense of doubling an engine’s fuel consumption.

It makes sense therefore to choose your fuel on the basis of whatever’s cheapest or mostconvenient to use.

For most of us however, the choice of fuels is fairly simple and boils down to one of these:

GasolineThis has the advantage that it’s relatively cheap, very easy to obtain, and is pretty clean burning.It’s also quite volatile so atomizes easily to promote easy starting.

Note that, contrary to what you might think, higher-octane gasoline is not going to produceany more power than regular gasoline. In fact (in theory) it may produce slightly less power. Ifyou plan to use gasoline, just use whatever’s cheapest.

Propane (LPG)Thanks to the popularity of gas-fired BBQs, propane has also become quite easy to obtain andsuitable 20lb refillable tanks can be bought for well under $50.

In some countries, propane is even cheaper than gasoline and it burns very cleanly indeed –leaving no smell and very little residue at all. Despite the fact that it’s stored under pressure, itis actually quite a bit safer to use than gasoline because its vapors dissipate very quickly in theopen air.

Since the boiling point of propane is well below normal room temperature, it either comes outof the tank as a gas (thus avoiding the need for vaporization) or, when drawn off as a liquid,instantly boils into a vapor. This makes a propane-powered pulsejet one of the easiest to start.

Note that bigger engines will almost certainly demand to be fed with liquid propane because anaverage BBQ tank simply can’t provide gas at a sufficiently high rate to keep up.

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If you’re planning to use a BBQ tank of propane as a fuel, you’ll have get rid of the regulatorthat is normally used to limit the flow of gas. This regulator reduces the pressure of the gas tojust a few psi, far too low for a pulsejet’s needs.

To give you an idea of just how much gas is needed to run a pulsejet, my own 15-lbs-thrustengines (PJ15) will drink all the propane gas you can feed them – with the regulator removed.The Lockwood valveless engines will drink all the liquid propane you can feed them withoutany regulator in place.

If you try to use propane without removing the regulator then all you’ll get from a pulsejet is afew bangs and pops – it won’t run.

However, you will still need some form of control over the flow of gas into the engine and forthis I recommend buying a cheap propane/air torch – of the type often used for soldering orbrazing.

These torches are available from almost any hardwarestore and cost just $25-$30. Note that depending onthe exact make/model of torch you buy, you mayneed to purchase an additional adapter fitting so thatit can be screwed directly onto a 10lb or 20lb propanetank.

To use a torch like this as the gas-control valve foryour pulsejet, simply unscrew the burner fitting on the end and slide your propane-certifiedplastic fuel pipe over the end, securing it with a small hose-clip.

The gas-flow knob on the torch will now enable you to control the amount of gas that isdelivered to your engine. If you invert your BBQ tank of propane, the torch will still serve as avery simple way to control the flow of liquid propane to larger engines. Very simple, veryinexpensive, and very effective.

Another method of controlling the flow of propaneto your engine is to simply use a device called aneedle-valve. These valves are readily available froma number of sources and, just like the gas-torch, offera very fine degree of control over fuel-flow.

ButaneIt should be noted that although it is also often sold for use on small camp stoves, butane isnot a good substitute for propane. It contains less energy and doesn’t produce as muchpressure as propane at room temperature. In short – don’t waste your time or money trying touse butane as a fuel for pulsejet engines.

White Spirit/Coleman fluidThis is simply a very low octane unleaded form of gasoline which has no fancy anti-knock orcombustion “improvemnet” additives included. It’s actually a better fuel than high-octane

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gasoline for pulsejet use. Many of the early small pulsejet engines such as the Dynajet run beston this fuel.

MethanolThis is my second-favorite pulsejet fuel. It has the advantage that it will burn over a very widerange of rich/lean mixture settings – making an engine less sensitive to fuel head or startingconditions.

It also burns very cleanly with no smelly or oily residue and creating little more than watervapor and some C02 as combustion byproducts.

On the downside, methanol is more expensive than gasoline, your engine will burn more of itfor a given amount of power, and it can be very dangerous if spilled because it burns with analmost invisible flame. Many people have been burnt because they’ve walked straight into amethanol fire without seeing it.

Despite the downsides, I prefer to use methanol for all my aspirated engines because itgenerates a little more power, allows the valves to run cooler, and doesn’t leave my handsstinking of gasoline.

Note that you shouldn’t use pre-mixed model airplane fuel instead of straight methanol.Model airplane fuel contains up to 20% oil that will leave significant deposits inside yourpulsejet and also affects the vaporization of the mixture. It’s also a lot more expensive thanplain old methanol so you’ll be wasting money.

Your local hot-rod or drag-racing club ought to be able to help you find a source of methanolbut if all else fails, try one of the major oil companies like Mobil – they sell me 5-gallon drumsof the stuff when I want it.

Another thing to watch when using methanol as a fuel is that it is very hydroscopic – which isto say that it tends to absorb moisture out of the air. If you leave a can of methanol uncappedthen it may well absorb so much moisture that its combustibility is affected and this can resultin hard-starting.

Also be aware that when you use methanol as a fuel, one of the combustion byproducts iswater (albeit as water vapor). This means that the spring-steel reed valves used in an enginerun on methanol are prone to rusting unless you oil them lightly before storing your engineafter each run.

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Constructional TechniquesOnce you’ve calculated the dimensions for a pulsejet, how do you then go about building one?

Of course It really helps if you’ve got access to a workshop or some basic metalworking toolssuch as a hacksaw, drill, welder, etc. but don’t be put off if your resources are a little moremodest.

You’d be surprised how helpful your local welder or engineer can be when you explain thatyou’re building a jet engine and would be happy to demonstrate it to them when it’s done.

It’s also amazing what you can do with a minimum of tools – if you’ve got enough patience.

The Engine Body/TailpipeMost commercial pulsejets are made from thin stainless steel sheet that is rolled or otherwiseformed into tubes and cones before being welded together.

This results in a durable engine that is light enough to be practical for such uses as poweringmodel airplanes.

These cones and tubes are formed using a device known as a slip-roll which looks rather likean old washing-machine wringer and consists of three rollers that can be adjusted to both gripand curve the metal sheet as it’s wound through.

A hand-operated slip-roll like this one is limited to rolling stainless steel that is no more than1mm thick – and even then it’s damned hard work if you’re rolling a piece the full 600mm longwhich is the maximum this set of rolls can handle.

For larger engines it really pays to find someone who has a set of motorized rolls that canhandle the thicker material and longer lengths you’ll need to use.

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Unfortunately, stainless steel is not only expensive but can also be very difficult to weld whenit’s very thin. For this reason, many enthusiasts prefer to make their engines from cheaper andmore easily worked materials

Providing weight isn’t a problem you can save a lot of time and significantly simplifyconstruction by using mild steel pipe for the body of your engine. The best stuff is exhausttubing which is usually protected from rusting by a thin layer of aluminum on the surface.

This stuff is relatively cheap, available in a widerange of sizes and can be cut and welded easilyusing MIG, arc or oxy-acetylene equipment.Unfortunately it’s also quite heavy, but that’s usuallyunimportant to the eager enthusiast who simplywants to build an engine and get it running ASAP.

Here’s a picture of my gokart with an engine builtfrom exhaust tubing. It only produced just enoughthrust to get the kart moving on a smooth surfacebut it was an interesting experiment none the less.

As you can see, this pipe is quite thick-walled which accounts for its weight and easy weldingcharacteristics.

The Valving SystemExactly how you construct this depends on the type of valving system you plan to use. If youhave access to a lathe then you can easily make a petal-valve system with a nicely turnedaluminum head. If you’re not lucky enough to have a lathe at your disposal, all is not lost.

Instead of making the front of the engine from a single, large piece of aluminum rod, you cancut a circle from a sheet of plate. Even on small engines it pays to use a piece at least 3/8”(8mm) thick for this.

Because aluminum is such a soft metal, you can actually do a pretty good job of cutting a circlefrom a flat sheet by using a jig-saw. You can even use a small coping saw or jeweler’s saw tocut the valve holes (after drilling a starting hole first).

To create a circular valve-plate from a flat piece of aluminum, simply mark out your circleusing a compass then cut it slightly over-sized. It can then be filed down to a precise fit intothe front of your engine.

However, before you cut out the circle, it pays to mark out and drill the valve holes. By doingthis before you cut the circle to shape, you have a larger piece of metal to hold onto whendrilling the valve holes and this makes the job much easier.

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Here’s a valve-plate that was made from flat-sheet,cut to shape with a jigsaw and with valve-holes thatwere cut with a jeweler’s saw. In this case, a lathe wasused to finish the surface of the disk and it was thenanodized to provide a hard, corrosion resistant layerand a pleasing gold color. See the chapter onanodizing for details of how to perform the anodizingprocess.

Note that there’s no reason why the valve-holesshould be circular and a trapezium shape as in thepicture actually allows a greater valve area for a givensize of valve-plate.

Having made the valve plate a snug fit in the front of your engine, you can then drill 4-6 smallholes around the circumference of the pipe so that they also go through into the edge of yourvalve plate. By choosing the correctly sized drill, you can then fit self-tapping screws to holdthe valve-plate firmly in place.

Any leaks in this area can be fixed by the liberal application of some muffler-sealant – the typethat comes in a small tube and is designed to block-up gaps in exhaust systems. This stuff willexpand slightly as the engine heats and seal any small gaps between the valve plate and theengine body.

If you don’t have a lathe then the other option is to build a V-valve system instead of a petalvalve one.

The V-valve can be created from flat pieces of steel as in the diagram below.

[diagram]

Here’s a picture of the V-valve head I used on my exhaust-tubing pulsejet. Note that thisengine uses direct injection and runs on propane.

Making Reed ValvesSince the reed-valves are the heart of a conventional pulsejet engine, it’s important that theyare well made and that the right materials are used.

Most valves are made from high-carbon spring steel of between 0.006” and 0.012” thickness.

If you can’t find a source for this material locally then I suggest you take a look at some of theonline mail-order metal supply companies.

Once you’ve got the right material, the next problem is cutting it to the required shape.

Spring steel sheet is incredibly brittle and will split or crack very easily if you try to cut complexshapes with regular metal snips.

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While it’s simple enough to cut the rectangular shaped valves used in a V-valve system,creating the intricate shape of a petal valve represents more of a challenge.

There are two methods you can use to fabricate a petal valvefrom spring-steel sheet.

The first involves using a Dremel or similar tool fitted with acut-off tool as shown in this picture.

This technique requires a bit of practice and is most suitablefor smaller valves.

The preferred method for making petal valves involves the useof a process known as electrochemical etching.

Full details of how to make reed valves using this process are provided later in this book.

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Powering things with pulsejetsPulsejets have been used to power just about every type of vehicle you can think of.

GokartsMy own simple gokart has been pushed along at various speeds by a simple exhaust-pipe

pulsejet, a much larger 100lbs-thrust pulsejet similarto the Argus V1 design, and a Lockwood valvelessengine.

What I learnt from this is that on a smooth, flatsurface a gokart is going to need at least 30lbs ofthrust to be much fun. If the surface is a littlerougher or if there’s any kind of slope then you’regoing to need at least 50lbs of thrust or more.

Once you get to 100lbs of thrust, riding on a pulsejet-powered gokart becomes a very “interesting”experience and not something recommended for thefaint of heart.

The picture above is that of the pulsejet-powered kart I helped build when I was expert for ateam of Royal Navy Engineers on the popular Scrapheap Challenge TV series.

Model AirplanesBut perhaps the most common type of craft to bepowered by pulsejet engines are model airplanes.

Back in the 1950s, many Dynajet engines were soldfor use on control-line speed models, many of whichreached speeds well over 150mph.

These days there are a growing number of radio controlled models being fitted with pulsejetengines as people are attracted to the simplicity and comparatively low cost of these engineswhen compared to more expensive turbine units.

There are problems associated with the use ofconventional pulsejet engines in model airplanes however– not the least of which is that of providing a constantfuel supply under varying G-forces.

The most effective methods used to achieve this essentialsteady flow of fuel are either the use of a pressurizedbladder tank or the addition of a small electric fuel pumpand the use of direct-injection. See the chapter on fuel systems for more information onbladder tanks.

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Another component finding great favor with aeromodelers is adevice known as the Cline regulator. This is a “demand regulator”that only allows fuel to flow when there’s suction applied to theoutlet connection. The advantage of this for pulsejet flyers is that itavoids an undesirable situation which could see an engine stoprunning unexpectedly but continue to be flooded with fuel from apressurized system.

BoatsYes, believe it or not, pulsejets have been used to power both model and full-sized speedboats.

Back in the late 1950s an enterprising guy bythe name of Bill Pearson fitted two 105-lbs-thrust pulsejet engines to a small speedboatand had some fun.

According to an article published in PopularMechanics, the boat had a top speed of morethan 45 miles per hour but consumed anenormous six gallons of gasoline per mile.

The other problem with this jet-powered boat was the fact that it could be heard from milesaway. Not exactly the type of boat you’d want for a quiet afternoon’s fishing and I pity anyonewho might have tried to use it as a ski-boat.

Here’s another boat powered by a huge Lockwood valveless pulsejet – watch for it on your TVscreens sometime in 2004. No, I didn’t build it or design the hull but I did design the engine.Unfortunately it was not built very well and failed to produce anything like the power levels itshould have but you can still see that it does a good job of chopping up the water behind it.

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HelicoptersBack in the 1950s a number of experimental helicopters were designed that used tip-mountedpulsejets to power the rotors.

None of these ever saw commercial production and they all suffered from a number ofproblems. The arrival of the gas turbine turboshaft engine effectively killed off such ideas butthere are a number of places that will sell you plans for such suicide machines if you have adeath wish.

The NACA even published a research paper on the effects that the powerful centrifugal forcesexperienced by a tip-rotor pulsejet would do to its combustion efficiency and reliability.

Civil AircraftAlthough I’m not aware of any civilian light airplane that has used a pulsejet as its primarysource of power, valveless pulsejets were once used (mainly in Europe) to power high-performance sailplanes. Because these craft are so aerodynamically efficient, only a very smallamount of thrust is required to keep them airborne meaning that pulsejet power was almostpractical despite the high fuel-consumption.

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Schmidt’s ContributionEarlier in this book I used Tharratt’s formulas, mainly because of all the designers, his work isthe most readable and easily applied by those without math and physics degrees.

However, Tharratt wasn’t the only researcher who spent time working with pulsejets and manyothers have come up with all manner of findings in relation to these engines.

Perhaps the most notable of these other pulsejet designers was a German by the name of PaulSchmidt who was responsible for much of the Argus V1 engine.

Schmidt laid a lot of the groundwork from which others such as Tharratt made theirobservations. In fact I’d wager that there has been no other pulsejet engine in history that hasundergone as much prodding, poking and analysis as the Argus.

In the years after WW2 the US NACA invested a hugeamount of time and effort investigating the potential ofpulsejet engines and much of their work centered aroundcopies of the Argus. The US Air Force even commissionedFord to make a run of Argus engines to power a planned1,000 “flying bombs” of their own carrying the designationJB-2. Even the US Navy had its own version of the JB-2which was more commonly known as the “Loon”.

Many of the NACA reports into the design and operation of the Argus have been released intothe public domain and have been included in the appendix of this book or can be downloadedfrom the NACA archives on the Internet.

Like Tharrat, Schmidt was very much enamoured of engines designed around straight orslightly divergent pipes. One such engine, designated the SR500, was little more than analmost straight tube. I say “almost straight” because it was actually a cone that diverged by 2degrees. With a total length of around 11 feet, this divergence meant that the, 18 inch diameterat the front became 22 inches at the rear,

Despite it’s simple shape, the static performance of this engine appears to exceed that of theArgus eventually used to power the V1.

The SR500 was reported to develop a static thrust of some 1,500 lbs which would result in aperformance of some 3.2 lbs thrust per square inch of mean cross-sectional area (some 50%better than Tharrat’s constant). This would make the SR500 very efficient for a conventionalengine. I should not that I’ve not had first-hand access to these reports but have no reason todoubt their veracity.

Schmidt observed an interesting phenomenon during his pulsejet development work – a rapidmode of combustion that appeared to be much faster than regular deflagration. He went on tospend much effort in trying to achieve true detonation but was ultimately unsuccessful.

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It is possible that this rapid combustion phenomenon goes some way to explaining theseemingly extraordinary performance of the SR500.

My own experiments into promoting the simultaneous combustion of the entire air/fuelcharge within a pulsejet have also yielded very promising results. It looks as if Mr Schmidtreally knew how many beans makes two.

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Ignition SystemsOne of the great things about pulsejet engines is that, unlike the engine in your car, oncestarted, they don’t require any regular source of timed ignition to keep running.

However, if there’s one part of pulsejet starting that leaves many people scratching their headsit’s how to create the spark needed to initially ignite the air-fuel mixture during that start-upperiod.

I fit spark plugs to all my engines and have built a simple electronic circuit to create a suitablespark – but if you feel you don’t have the necessary skills to do this there are alternatives.

One method I’ve used before is still electrical but just uses parts you can get from any auto-wrecker or auto-electrical store.

All you need is an auto ignition coil, an auto flasher unit and a 12-volt battery.

Wire the three items together as in this diagram:

The component labeled C1 is a capacitor (or condenser) that stops arcing across the contactsinside the flasher unit. Any auto parts store or wrecker’s yard should be able to supply this butif not, you can use a 0.47 to 1.0 mfd capacitor rated to about 250V from Radio Shack or someother electronic components store.

It’s pretty easy to see that the signal light flasher turns off and on at regular intervals and thison/off current flow causes the coil to create a nice bright spark. There’s not much to gowrong here and there aren’t many mistakes you can make wiring it up so have a go. It mightbe worthwhile adding an off/on switch and a fuse in the lead that goes from the battery to thecoil + connection.

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However, if you’re comfortable working from electronic circuit diagrams you might want tobuild the circuit below.

This simple circuit will drive a regular auto coil and produce a continuous stream of about 5-8sparks per second. By comparison, the previous circuit may only produce one spark everysecond or so.

Another alternative is to dispense with a spark plug all together.

Some people start their pulsejets by simply placing a lighted fireworks sparkler in the tailpipe.I can’t say I’ve actually tried this myself but I don’t see why it wouldn’t work.

Apparently, once the engine starts, the sparkler wire is blown out the back so doesn’t interferewith the engine’s operation.

Of course if you’re working on a new design or have an engine that refuses to start, you couldwaste a lot of money on sparklers using this technique so having an electrically operatedignition system is still a good idea.

One point that is seldom discussed is exactly where do you put the sparkplug?

Once again – it’s compromise time!

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If you put the sparkplug towards the back of the combustion chamber then the engine willprobably start more easily because, by the time the air/fuel mixture reaches this point it will bemore thoroughly mixed and thus more explosive.

However, the the further back you place it, the greater the chance that the protruding plug willinterfere with the passage of the fast-moving exhaust gases.

It’s worth noting that the original Dynajets had their sparkplug right at the back of thecombustion chamber just where it starts to taper down to join with the tailpipe. Later versionssaw the plug moved much further forward.

I tend to stick mine half way along the chamber and have no problems starting, nor do I seeany significantly adverse effect on power output.

And while on the subject of sparkplugs, what type should you use?

Well there are special miniature plugs available from companies like Champion, but I’ve notbeen able to find a source of such plugs at anything like a reasonable price.

So, I tend to use commonly available plugs that are originally designed for use on chainsaws,weed-wackers and the like.

These plugs are much smaller than regular automobile units, but still use the standard 14mm x1.25mm thread.

On a very small engine they may look somewhat out of place but on anything larger than an8lb-thrust unit they’re just fine. Note that you will want to open up the gap to about twice thefactory setting before use. This will ensure easy starting.

I’m sometimes asked whether it’s possible to use a model airplane engine glow-plug instead ofa sparkplug.

To be honest, I haven’t tried this but I’m pretty sure it would only work if you were usingmethanol as a fuel and even then you’d probably find that it was pretty marginal.

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How to start a pulsejetPulsejet engines have a sometimes well-deserved reputation for being very difficult andproblematic to start. Fortunately this doesn't always have to be the case.

In order to start a pulsejet you need three things:

1. fuel2. air3. an ignition source

Not only must you have all three -- but they must also be provided at the correct time and inthe right proportions.

FuelPulsejets can run on a wide range of fuels ranging from LPG/propane through to diesel orkerosene.

For the purposes of small pulsejet engines however, the most common fuel is gasoline of somekind. This can be white-gasoline or low-octane gas from the local pumps. You can use high-octane gasoline but you'll simply be wasting money and possibly getting a little less power atthe same time.

In cold conditions where the air temperature is less than 60 degrees F or around 17 degrees Cyou may find that gasoline isn't sufficiently volatile to ignite reliably when starting an engine. Ifthis is the case then it is recommended that you add some ether -- up to 25 percent. This willsignificantly increase the ease with which the fuel can be ignited.

Cold can also affect engines that are using propane/LPG because the pressure available from atank of this gas reduces quite significantly as the temperature drops.

It's worth mentioning at this stage that there are two methods of delivering the fuel to theengine:

aspirationThis is when the fuel is drawn into the engine through an atomizer by the air which entersthrough the intake. This has the advantage that it is very simple and requires no fuel pump orother ancilliary equipment.

injectionThis involves spraying fuel directly into the combustion chamber where it mixes with air thathas already passed through the valves. This has the advantage that you can throttle the enginesimply by varying the amount of fuel injected -- but it does require the use of a pressurized fuelsystem such as a bladder or pump.

The Air Supply

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Just supplying the engine with fuel is not enough -- you need to force some air into the intakeso as to create an explosive mixture inside the engine.

In the case of an injected engine, an air source such as a leaf-blower or vacuum-cleaner with ablow attachment will likely do the job. You don't need a lot of pressure but you do need areasonable volume of air.

With an aspirated engine you'll need less volume but more pressure. This is because the air hasto draw the fuel up the fuel-line and atomize it into a fine spray before it passes through thevalves into the engine.

An air-gun driven by a compressor is perfect for the job. However, if you don't have suchluxuries available to you a great substitute is to inflate a car tube or tire without a valve in it --using a length of flexible plastic tubing slipped over the valve stem to deliver the air to theengine's intake. The flow of this air can be controlled by kinking the tubing. Slipping a thinnerpiece of pipe into the open end of the tubing will give you a narrower and more easilycontrolled air-jet to spray into the engine. Between starting attempts you can replace the valveand pump the tire up to 40-60 psi using a foot-pump or whatever. This is a great low-cost wayto start your engine at the flying field or away from other sources of compressed air.

The Ignition SourceThe best ignition source is a spark plug mounted in the combustion zone section of the engine– but there are alternatives...

The simplest but least effective ignition source is a naked flame situated at the end of thetailpipe. This could be a gas-torch or a spirit burner but you'll find starting an engine using thismethod to be more difficult than with a sparkplug.

One other technique sometimes used when a sparkplug isn't mounted in the engine itself is aspark-wand. This consists of two wires, separated by an insulator with a spark-gap at the end.It is inserted and energized when starting the engine and quickly withdrawn once it is running.

A third option is to simply place a lighted fireworks sparkler into the engine. This will burinfor about 20 seconds and provide a nice hot ignition source. Try to place the sparker as fartowards the front of the engine as possible. It will be blown out when the engine starts.

Check the chapter in this book on ignition systems for more information and the plans for twoelectrical ignition systems.

Putting It All TogetherHere's the sequence for starting an aspirated pulsejet:

1.Connect the fuel line and tank. Make sure that the fuel level is no more than an inch or so (20mm) from the engine's fuel jet (the hole where the fuel comes out into the engine).

2.Turn on the spark or light your ignition source

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3.Direct a jet of compressed air over the fuel jet so that it creates a fine spray of fuel dropletsthat are then blown towards the engine's valves.

Note that it will take some experimentation, practice,and coordination to get this right and you'll probablyfind that it helps to move the jet of air back and fortha little so as to vary the spray a somewhat.

If your engine has a “flowjector” setup like theDynajet then you won’t be able to vary the angle orposition of your air source very much.

At this stage the engine should at least pop, bang orburb a little, even if it doesn't immediately burst intolife.

If you don't get much activity, try richening the mixture a little (if your engine has thiscapability) by losening the locknut and opening the mixture screw by a quarter of a turn.Repeat this process until things improve. If they don't improve, return the screw to its originalposition and try closing it a quarter turn at a time in case the mixture is already too rich.

The procedure is even simpler for an engine with direct LPG/propane injection.

Thanks to the fact that the fuel is already in the engine's combustion chamber, all we need dois turn on the spark and blow some air into the intake.

If the engine doesn't fire immedately then the gas should be turned up or down until theengine starts. If you still have trouble, try varying the amount of air being blown into theintake.

Once you get the hang of starting a LPG-injected pulsejet its exceptionally easy to do

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Valveless PulsejetsWhile most people marvel at the simplicity of the humble pulsejet with its single moving part,there are designs that are even simpler.

I refer of course, to the incredible valveless pulsejet engine. A device that has absolutely nomoving parts at all!

The design of valveless engines is even more of a black art than the design of regular pulsejets,as witnessed by the amazing diversity of shape and form to be seen amongst the variousvalveless designs.

If you’ve performed the jam-jar experiment detailed elsewhere in this book then you’ve alreadyseen a valveless pulsejet engine in action, otherwise here’s a brief description of how theywork.

Regardless of the shape, form and size of a valveless pulsejet engine, they are all basically acombustion chamber with a pipe or pipes attached. Inevitably, one or more of the pipes ismuch shorter than the other(s).

The short pipe is often referred to as the intake and the longer is referred to as the exhaust.

When the air/fuel in the engine explodes, hot gases rush out both pipes (yes, even the intake)at great speed -- just as they rush out of a conventional pulsejet’s tailpipe.

When Mr Kadenacy pokes his head in to see what’s happening, the rapidly exiting hot gasescreate a partial vacuum inside the engine.

At this stage, the gas-flows reverse and the engine starts sucking back through its exhaust andintake pipes.

Now, since the intake pipe is much shorter than the tailpipe, fresh air is able to be drawn intothe combustion chamber. It is then mixed with fuel (usually directly injected) and we have acombustible mixture all ready to fire.

Meanwhile, gases continue to flow into the other end of the combustion chamber from theexhaust pipe. Since the exhaust pipe is much longer than the intake pipe, these incoming gasescontain no fresh air – just hot reminants of the previous combustion cycle.

As soon as these hot (still burning) gases hit the fresh air/fuel mixture – boom – the wholecycle starts again.

AdvantagesIt’s pretty easy to see that the biggest advantage of a valveless pulsejet engine is the fact that itdoesn’t have any delicate valves to wear out or break. All else being equal, the operating life ofa valveless engine is limited only by the amount of fuel available and the durability of thetubing from which it’s made.

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Without the need for valves, such an engine is also, at least in theory, much easier to make.No holes to drill, no spring steel to etch – just pipes and cones to weld together.

DisadvantagesOh boy, didn’t you just know this wasn’t going to be a free lunch?

That’s right, there’s always a price to pay in the pulsejet world and in return for its improvedsimplicity of construction and operation, the valveless engine suffers from lower levels ofperformance and efficiency.

Why is this?

Well you’ll recall that a little earlier on I mentioned something called “post ignitionconfinement” (PIC) and mentioned that it sometimes had a beneficial effect on an engine’sefficiency.

In the case of a traditional (valved) pulsejet engine, there’s always a reasonable degree of PICbecause before the burning gases can expand, they’ve got to push all that gas in the tailpipe outof the way.

This slug of cold tailpipe gas provides a form of “inertial confinement” that ensures moreefficient combustion occurs in the front part of the engine.

However, when we look at a valveless engine – there are now two (sometimes more) gapingholes in the engine – and in the case of the relatively short intake pipe, there’s not a lot of coldgas there to provide any reasonable level of inertial confinement.

So, when the air/fuel mixture in a valveless engine ignites, the gases are able to escape beforethey can create the same pressure levels as you’d find in a valved engine with its much smallerescape route and the larger inertial mass in its tailpipe.

It is for this reason, plus the problems associated with making a valveless engine into a sensibleshape, that engines like the Dynajet continue to dominate the small pulsejet arena.

Perhaps the only valvless pulsejet that has ever enjoyed any successis the oddly-shaped Lockwood Hiller design.

By bending the engine in half, Lockwood was able to have theintake and exhaust pipes facing in the same direction so that theyboth contributed to the total thrust.

Unfortunately this engine has never lived up to the claims that weremade for it.

Most notably it’s claim to provide an amazingly low (for a pulsejet)fuel consumption figure have been independently verified despite

numerous engines having been built exactly to Lockwood’s own plans.

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Just to prove to myself that this weird engine doesn’t live up to the claims made for it, I builtone that was supposed to produce 75lbs of thrust.

No matter how hard I tried, I could onlycoax between 50 and 55 lbs of thrust outof this beast and it used nearly four timesthe amount of fuel claimed by Lockwood.

Several other people have verified this lackof thrust and thirst for fuel so I think theresults are quite conclusive.

Of course having built such an engine, Ihad to do something with it, so I stuck iton my gokart just for fun. The onlyproblem is that a full BBQ tank ofpropane only lasts about 5-6 minutes.

Here are just a few of the various valveless pulsejet engine designs that have been created overthe years:

The ReynstThis design was neverintended to create propulsivethrust and was intended tosimply be a more efficient wayof burning fuel to create heat.It’s included here mainlybecause it’s a great example ofthe operating principlesbehind pulsejet engines andbecause its inventor (Reynst)was such an important part ofpulsejet history.

The MarconnetDesigned almost 100 years ago, this engine, named after its inventor Frenchman GeorgesMarconnet, is one of the oldest valveless designs. It seems to have relied on having a smallerdiameter intake tube than exhaust tube so that while some of the combustion gases escapedout the front, even more rushed out the rear, producing a thrust imbalance that pushed theengine forwards.

[diagram]

The EscopetteThis was a logical evolution of the Marconnet and a similar engine known as the Resojet, themajor difference being that the Escopette’s intake tube is bent through 180 degrees to point

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towards the rear. By doing this, the gases that are ejected through this tube contribute to thetotal thrust.

[diagram]

The LockwoodThis “bend it in half” theme was taken further by Lockwood except that he bent the exhausttube on his design which meant that the overall length of the engine was reduced by half.

This compact size and the ready availability ofplans has meant that Lockwood’s engine isperhaps the most popular valveless designamongst amateur pulsejet builders.

The full patent application for the Lockwoodengine is included in an appendix at the end ofthis book for those who are interested in theexact theory of its operation.

The ChineseI don’t know how this engine got its name but it’s a simple design that follows the traditionalvalveless design of having a long tailpipe and a short intake tube, both of which face towardsthe rear of the engine.

From all reports, this engine is a rather poor performer, producing barely half the power of aDynajet despite being almost twice the size. Plans for a version of this engine are included inthe appendix of this book.

The ThermojetAlthough it looks rather cool, the Thermojet was reportedly something of a disappointment interms of the amount of power it generated. As you can see, it’s not too much different to theChinese design except that it has two intakes that are parallel to the tailpipe. Note also that thetailpipe has a fairly large flared cone. Some variants of the thermojet were built with as manyas four intake tubes.

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The KentfieldThis is actually just a variation on the basic Lockwood design. However, instead of bending theexhaust or a single intake tube through 180 degrees, this engine uses four intake tubes (muchas I did with my first valveless engine) and relies on the output of these tubes being fed into“flow rectifiers”. These flow-rectifiers are actually just U-bends (not shown in the diagrambelow) that are spaced slightly away from the intake tubes (so that intake air can enter throughthe gap).

Kentifield claims that his engine produces more power per unit of length than a conventionalvalveless engine and a better TSFC (fuel consumption) figure.

Details of the Kentfield engine can be found in a paper published by the AIAA with thereference number AIAA 98-3879. This paper can be purchased from the AIAA’s website atwww.aiaa.org

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Improving Pulsejet PerformanceDesigning a pulsejet that produces more power and uses less fuel was the Holy Grail ofengineers during the period from 1940-1960.

However, as the turbojet, and later the turbofan, continued to provide increasing levels ofpower and performance, no serious effort was applied to improving the humble pulsejet.

This doesn’t mean that the pulsejet is a lost cause though, and in recent years there has beensomething of a renaissance in the design and construction of these engines, driven partly by aglobal community of enthusiasts.

It’s worth noting that as long ago as 1947, engineers working with pulsejets suggested thatthere were significant performance gains yet to be made with this technology.

Fuel ConsumptionThe single biggest problem with pulsejet engines is their horrendous fuel consumption.

Most conventional pulsejets use somewhere between 3.0 and 5.5 pounds of fuel per hour forevery pound of thrust generated.

That means a that even a pulsejet operating at the more economical end of this scale andgenerating 100lbs of thrust would consume 300lbs of fuel per hour. That’s some 50 gallons ofregular gasoline.

By comparison, a modern turbofan engine uses as little as 0.34 pounds of fuel per hour forevery pound of thrust produced. This means a 100lbs-thrust turbofan would use less than sixgallons (just 34lbs) of fuel to accomplish the same task.

I hope you can see now why there are no commercially made pulsejet-powered aircraft around.Not only would such a craft struggle to get into the air under the weight of its enormous fuelburden, but it would also be incredibly expensive to fly.

So how can we go about reducing the fuel consumption of a pulsejet engine?

The single largest problem is that pulsejets have a very low pressure ratio. In plain-speak, thissimply means that it uses fuel very inefficiently because there’s a bloody great hole out the backthat makes it hard for the engine to build up any significant amount of pressure inside.

When fuel is ignited in your car’s engine it has already been compressed to about 1/10th itsnormal volume and 10 times atmospheric pressure. By compressing the air/fuel before it’signited, more energy is obtained from that combustion and greater pressures are generatedinside the engine’s combustion chamber.

A car engine uses a solid aluminum piston to compress the air/fuel mixture and confine itonce combustion has occurred.

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Unfortunately, pulsejets don’t have nice solid pistons to compress and confine the air/fuelmixture prior to, or immediately after ignition.

The only thing compressing and confining our air/fuel mixture is the light, relativelyineffective column of gas in the engine’s tailpipe. Clearly this column of hot gas is going to actlike much of a piston.

This is confirmed by measurements made inside a running pulsejet engine which indicate thatthe fresh air/fuel charge is only compressed by a tiny amount (around 2psi) before it is ignited.

Other measurements indicate that the peak pressure generated inside the engine as the hotburning gases push against the gas column in the tailpipe is a very modest 12-15psi.

Clearly design change that allows an engine to increase the operating pressures that aregenerated inside it will have a positive effect on both power output and fuel consumption.

Some suggestions for obtaining higher operating pressures have included:

1. multi-point ignition to reduce the time taken to ignite the entire air/fuel load2. the use of detonation waves to provide pre-ignition compression and much faster ignition

of the total air/fuel load3. increasing the level of turbulence inside the engine so as to promote faster ignition of the

total air/fuel load

All of these are valid suggestions but none of them are easily achieved without compromisingsome other aspect of the engine’s operation.

Multi-point ignitionThe faster you can burn the entire air/fuel charge inside the engine, the more power will beproduced and the more efficiency will be obtained.

In a perfect engine we’d be trying to obtain what’s known as “constant volume” combustionwhere the air/fuel charge isn’t allowed to expand at all until it’s been entirely consumed. Insuch an engine, the internal pressure generated will often be more than ten times that whichwas present before ignition occurred.

If we have a pulsejet that achieves an internal pre-ignition pressure of just 2psi

The reason for this is simple – the only thing stopping the burning air/fuel from escaping outthe rear of a pulsejet engine is the relatively light column of gas in the tailpipe. Being light, thisgas actually does a very poor job of stopping the air/fuel from

The most promising of these has been the use of a detonation wave to ignite the air/fuel load.It is this concept that has spawned a whole new arm of pulsejet research and the developmentof the pulse detonation engine (PDE).

Unfortunately, PDEs have yet to make it out of the laboratory and into practical applicationanywhere. Although they do offer the promise of massive improvements in efficiency, they

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are complex beasts that require a truckload of support gear in the form of pumps, valveactuators, oxidizer tanks and much more.

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A Simple Guide To AnodizingWhat is Anodizing?

Anodizing a process that creates a very hard, protective, and sometimes decorative finish toaluminum.

Aluminum is a wonderful material and is often used for the valve seat and intake section ofpulsejet engines. Its excellent thermal conductivity and ability to cushion the impact of reedvalves as they slam shut make it a good choice for these components – but its lack of hardnessalso means that it’s easily damaged.

In this picture you can see the effect of hardened reed valves repeatedly slamming into a plainaluminum valve plate. You can see how the repeated impacts have damaged the surface of thevalve seat and this can affect the ability of the valves to seal properly.

By anodizing the valve plate, a thin protective layer of aluminum oxide can be created whichwill significantly reduce the amount of damage that occurs.

This protective layer is extremely hard – almost as hard as diamond in fact – but it is so thinthat it doesn’t adversely affect the valve seat’s ability to absorb the heavy impacts of the valves– thus you get the best of both worlds.

Choosing an Aluminum Alloy

Note that only some aluminum alloys can be anodized successfully. Some grades, such as thecopper-alloys of the 2000 series (2024, etc) which are often considered “high-strength” are notsuitable.

The easiest aluminum to anodize are the “pure” alloys of the 1000 series (1050, 1100, etc) –but these are very weak and not suitable for such things as valve-plates where there aresignificant physical stresses involved.

On the PJ8 and PJ15 engines, I use 6061 grade aluminum tempered to a T-5 level. This alloy isa considered a medium to strong structural alloy and anodizes very well indeed.

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How To Anodize Aluminum

The anodizing process is deceptively simple and uses readily available chemicals. Providingyou exercise reasonable care, it’s also a very safe process with no adverse environmentalimpact.

Materials:To anodize aluminum you’ll need the following materials:

1. Fresh sulfuric acid. This can be obtained from your local automotive battery supplier and isvery cheap. Don’t be tempted to use acid decanted from an old battery – it will becontaminated and produce very poor results.

2. Distilled water. If you don’t have a distiller, this can also be obtained from your localbattery supplier. You’ll need to dilute the acid with 2 parts of water for each part of acid.

3. A plastic tub to hold the acid solution and the items to be anodized. I used a containerdesigned for kitchen use – this had the added advantage that it came with a secure clip-onlid so I can store the acid solution in this container between anodizing sessions.

4. A lead or pure aluminum plate to act as the cathode. The purity of this plate is fairlyimportant – if it’s alloyed with some other metal then there could be leaching into the acidsolution with resultant contamination. Use 1050 or 1100 grade aluminum or lead from aknown source (although some people have reported good results using lead flashing tornfrom old roofing iron).

5. Some aluminum wire or thin rod. A good source of this is aluminum welding wire as usedin MIG welders. Alternatively you can cut a thin strip of aluminum from a sheet of themetal and use that. This will be used to hang the item in the solution and provide anelectrical connection. Remember that this piece of metal will also be anodized and if thereisn’t a firm, watertight connection to the workpiece, the oxide layer created may break thecircuit and stop the current from reaching it.

Remember that at the point where the wire connects to the workpiece, anodizing will notoccur. Make sure that the connection point is not in a position where this lack ofanodizing will cause an unsightly blemish if looks are important.

You’ll also need a source of 12-24V DC electrical power. Since the anodizing process candraw several amps of current, you’ll need something more powerful than a simple wall-warttype of supply. I use a car battery (or two) that I recharge between sessions.

To connect everything up you’ll want to use several clip-leads (lengths of wire with a crocodileclip on each end) and be sure there’s a fuse in the circuit close to the power supply or batteryterminals. If the item being anodized accidentally touches the lead/aluminum plate then ashort-circuit will occur and, without a fuse, this could cause a fire or even an explosion.

Setting up Your Anodizing Tank

Because anodizing uses an acid solution and creates large amounts of potentially explosivehydrogen gas you should set up your equipment in a safe, well-ventilated location where it’snot going to be knocked or in the vicinity of any type of naked flame.

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First, cut and bend your cathode plate (aluminum or lead sheet) so that it runs around theinside edge of your plastic container.

To get an even and “all-over” layer of anodizing, you need to have your item surrounded bythe cathode plate.

Place the item to be anodized into thecontainer and make sure that plenty of space(at least a couple of inches) remains between itand the cathode plate. This is to make surethat you don’t accidentally touch the twotogether and produce a dangerous short-circuit.

The amount of acid solution you need to mixup depends pretty much on the size of theobject you’re planning to anodize and thevolume of your plastic container.

To avoid having a “bald-patch” on the bottomsurface, you need to suspend the item to beanodized in the acid solution so that it’s not touching the bottom of the container so allowextra liquid for this.

Don’t’ forget however, that when you lower your item into the solution, the level will rise. Ifyou are planning to anodize a larger item, the level may rise significantly so factor this in so asto ensure you don’t overflow the container.

Workpiece Preparation

It can’t be emphasized just how important it is that the workpiece is scrupulously clean prior toanodizing.

Any grease, oil or fingerprints will be magnified by the process and produce unsightlyblemishes in the final finish.

To this end, the following steps are useful in ensuring the cleanliness of the workpiece:

1. wash the workpiece in a weak solution of detergent in hot water2. rinse thoroughly with hot water as any detergent residue will also cause problems.3. Dip the workpiece into a warm (120 deg F) bath of 5% sodium hydroxide (lye) solution for

about one minute. This will remove any remaining oil or grease and provide a veryconsistent surface.

4. Rinse thoroughly with hot water as in step 2 (above)

I find that wearing a pair of cheap latex or nitride gloves when performing the above stepsallows the workpiece to be held without leaving further fingerprints on the cleaned surface.

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It’s also advisable in light of the fact that the sodium hydroxide solution is extremely corrosiveto your skin.

Performing The Anodizing

Having set up your anodizing tank and prepared your workpiece, the item to be anodized islowered into the acid so that it is completely immersed.

It is then connected to the positive side ofyour battery or power supply.

The aluminum cathode plate is connected tothe negative side.

Within a minute or so you should see a steadystream of bubbles rising from the workpiece– if not, you have a problem.

The most common reason that the workpiece fails to bubble is because the electricalconnection to it is inadequate.

Once the anodizing process is underway you can go and read a book for 20-60 minutes,depending on the size of your item and the depth of anodizing required.

You can briefly remove the workpiece from the tank from time to time to see how the processis going. Depending on the alloy involved, it will turn a dull gray or very light yellow color asthe oxide layer forms on the surface.

Using the anodizing setup described above, most workpieces will have a health layer of oxideon them within 60 minutes of processing – at which stage they can be removed and rinsedthoroughly in COLD water.

It is important that you continue to avoid direct contact with the surface or contaminationwith any form of dirt, oil or grease.

Dying the Workpiece

One of the great things about anodizing is that it not only creates a tough, hard layer thatprotects the metal from wear but it also allows you to impart a color to it.

The reason you can dye a piece of anodized aluminum is because the oxide layer formedduring the process is quite porous. The millions of tiny oxide crystals act like a sponge andsoak up any dye that has a fine particle structure.

Of course the dying step is optional and if you’d like to simply retain the gray color of plainanodized aluminum you can move right on to the “Fixing” stage.

The choice of dye is critical to successfully imparting a nice rich color to the anodized layer.

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I’ve had great results from the Dylon range of cold water dyes. These are the little plastic potswith an aluminum lid that can be bought from most fabric stores. They’re dirt cheap and onelittle container produces enough dye to last a very long time.

So far, I’ve tried the gold, red and blue colors with good results but others have reported thatthe greens also work well.

To dye your workpiece you should remove it from the anodizing tank, rinse well in COLDwater (it is important that the water is not warm or hot) and then imerse it in the dye solution.I mix one little pot of Dylon to almost a pint of water (500ml) and this concentration worksfine.

Leave it in the dye for about 10 minutes or so to allow the color to get deep into the crystalinestructure of the oxide layer.

Fixing the Anodizing

This is a critical step that physically and chemically changes the structure of the oxide layerproduced by the anodizing process. If you have dyed your workpiece, it effectively locks thedye in place so it won’t leach out.

To perform the fixing you need to hold the anodized (and optionally dyed) workpiece over asource of steam for at least 10 minutes.

I find that a pot of vigorously boiling water on the stove works fine.

Rotate the work so that the steam comes into contact with all the anodized surfaces.

It’s normal for some of the dye to leach out at this stage and color turn the boiling water – butmost of it will stay in place.

After 10 minutes of steaming, the workpiece canbe immersed in the boiling water to finish thejob. Leave it boiling for another 10 minutes.

Once you’ve removed the item from the boilingwater the anodizing is complete.

Adding Luster

You will notice that your newly anodized (andoptionally dyed) piece of aluminum probablylooks quite dull when it dries. This is normal.The anodizing itself produces a rough surfacethat imparts a matte finish rather than a shinyone.

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To make the surface shine and add luster all you have to do is either rub in some oil or polishof some kind.

With some polishes you may get a small amount of the dye coming off on the polishing clothbut it’s nothing to worry about.

The result should be a nice shiny workpiece with a deep luster.

Hard AnodizingThe anodizing process described here is often referred to as “decorative anodizing” becausethe thickness of the layer produced is quite low (about 0.0001 to 0.0005 inches) and thereforeonly provides a limited amount of protection to the underlying metal.

Another process known as “hard anodizing” can produce layers of up to 0.005 inches whichare extremely hard and protective.

Performing hard anodizing is a little more complex than the above process however andinvolves the use of a chilled acid solution (not necessarily sulfuric) and much highervoltages/currents.

While it is possible to perform hard anodizing in a home workshop, there are safety issuesinvolved since the amount of explosive gas produced and the risk of electrical shock are farhigher.

For most purposes, the simple anodizing process described here will provide more thanenough protection.

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Making Reed Valves Using Electro-chemical EtchingThe most common form of reed valve used on small pulsejet engines is the petal-valve. Theyare given this name because they look like the petals of a flower.

Cutting such a shape from thin (usually 0.006”) spring steel can be very problematic.

While scissors and shears work fine for cutting straight lines, it is almost impossible to cut thecurves and thin slits needed to create a petal valve.

Even if one were careful and lucky enough to produce a petal valve using such tools, theresulting valve would almost certainly have areas where the metal was bent or where there wereovercuts that would encourage the rapid formation of cracks and premature failure.

So just how can you create the relatively complex shape of a petal valve using commonlyavailable equipment?

The answer is to use electrochemical etching.

By painting both sides of the reed valve material and then scratching the shape of the valveinto that paint, it becomes possible to etch the exposed metal so that the valve virtually fallsout of the sheet from which it has been made.

Acid EtchingAt first glance it might seem that we could simply drop a suitably painted and scratched sheetof valve material into a container of acid and the exposed metal would be eaten away to do thejob – but that’s not a particularly good idea for several reasons:

1. Acid is corrosive – and few of us have a pint or two of sulfuric, nitric or hydrochloric acidlaying about the workshop.

2. Acid etching has the undesirable effect of rapidly under-cutting any exposed edge. Thismeans that as well as eating directly into the exposed metal, the acid would start etchingaway under the paint. As a result, our finished reed valve would probably end up with verythin and ragged edges that would be prone to burning and splitting.

Electrochemical EtchingBy comparison, the electrochemical etching method requires nothing more than a battery,some wire, a bowl and common household salt in solution with water.

What’s more, electrochemical etching suffers far less from the under-cutting tendenciesassociated with acid etching.

Here are the steps involved in etching a reed valve using this technique:

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PreparationLet me say right now that preparation is everything. If you skip on this step then you will endup regretting it later.

The metal from which your reed valve will be etched must be absolutely clean with no tracesof rust or grease as these will cause the paint to lift and allow etching to occur in all the wrongplaces.

It’s also important that the metal is slightly roughened so that the paint can adhere properly. Aperfectly polished service will give the paint nothing to hold on to and it will come off in bigflakes.

For absolutely the best results you should scrub the reed valve material with a soap-impregnated wire-wool pad. This will remove all traces of grease and (if your arms are up to it)any rust spots.

Now rinse in very hot water, taking care to hold the metal only by the edges – oily fingerprintswill ruin your hard work.

You can now give the metal an acid-etch if you have some dilute sulfuric acid available. This isdone by dipping the bare metal into a very dilute solution (battery acid can be diluted by 4parts of water). Place the metal in the solution and you should see small bubbles start forming.Lift it out at regular intervals and when it’s turned a dull gray color you can rinse it under hotrunning water again.

This acid-etch will provide the best surface for paint to adhere to – but if you don’t have anysulfuric acid then don’t worry – you can give both sides of the metal a light sanding with 1200grade wet-and-dry sandpaper. This will provide a similar surface roughness to help paintadhesion.

PaintingThe type of paint and the manner in which it’s applied will also be a critical factor in thesuccess of the etching operation.

Don’t use a cheap spray-can enamel – it probablywon’t stick well enough, even if you’ve roughenedthe surface.

What’s needed is an automotive undercoat. Thesepaints are designed to be sprayed directly ontobare metal and can be obtained in spray-can formfrom your local paint store or auto accessoriesoutlet. If you can, use white undercoat – it makesthe job of getting a nice even coat much easierthan when using the traditional gray.

Make sure you get an even and thorough coatingof paint on the metal. I actually find it easier to lay the metal on a sheet of newspaper and

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spray it while it’s flat. This avoids creating paint runs if you over-do it slightly. Of course itmeans that you have to wait for one side to dry before you can turn it over and do the other –but automotive primers tend to be very fast-drying anyway.

Once the first coat is dry, give it another thorough all-over coat.

Don’t be tempted to make do with a single coat. Even though it may look as if you’ve coveredall the metal, experience has shown that there will probably be some very tiny pinholes thatwill cause similar holes to appear in your reed valve. A second coat is good insurance againstthis type of thing happening.

Another reason you want at least two coats is because as you near the end of the etchingprocess, it’s only the paint that will hold everything together. Insufficient paint means that thevalve will start breaking away from the rest of the metal prematurely and this can cause thepaint to rip away from a surface you don’t want etched – with disastrous results.

Let the paint dry at least overnight if you can. Even though these paints are fast-drying, thevery bottom layer tends to remain slightly plastic for several hours and this can cause the linesyou scribe later to close-over.

Marking OutNow that you have a nice piece of reed valvematerial, totally covered in a good solid layer ofpaint, you need to scribe the lines that representthe outline of your reed valve.

NOTE: You only scribe one side! The undisturbedlayer of paint on the back-side of the plate willhold everything together as the metal under thescribed lines is etched away.

You can draw and scribe the pattern of yourreed valve directly onto the piece of metalyou’ve prepared – but it’s a better idea to make atemplate that you can trace around.

The reasons for this are obvious: If you make a mistake whiledrawing the pattern onto your painted material then your workto date will have been wasted.

I’ve made a template from 1mm (0.040”) stainless steel and Isimply press this against the prepared reed valve material andscribe around the edges with a sharp modeling knife. Don’tforget to also scribe the hole in the middle!

If you have an existing reed valve in good condition then youcan use that as the template for scribing your pattern.

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When you’ve finished scribing, the shiny steel underneath the paint should be visible at thebottom of the scribe lines.

Check to make sure that all your lines join where they should – a line that doesn’t quite join upwill leave a bridge of metal that will make complicate removal of the valve from the sheet ofprepared metal.

EtchingYou’ll need a plastic or glass bowl or container that islarge enough to fully submerse your valve materialwhile it’s stood on edge. As you can see here, I’veused an old yogurt container but you can grab yourmother or wife’s Tupperware if she’s not looking andit would do just as well.

Now mix up a solution of common table salt andwater. About a tablespoon per pint will do the trick –the strength of the solution isn’t that critical. You’llneed enough to fill your container to the desired level.

Hint: the salt will dissolve more easily if you use warm water.

Now find yourself a piece of stainless (preferred) orregular steel that will act as a cathode plate in thesolution. It should be about the same area as yourblank sheet of reed valve material – although, onceagain, this isn’t too critical.

Next, you’ll need a source of 6-12V DC. This can be alead acid car or motorcycle battery or, if you have one,a variable voltage/current power supply.

Connect that plate to the NEGATIVE terminal of your battery or power supply and place iton once side of your container, immersed in the salt solution.

Place your painted and scribed piece of reed valvematerial in the salt solution on the other side of yourcontainer – making sure that the scribed side faces thecathode plate.

Make absolutely sure that the two pieces of metal can notaccidentally touch together if they move. One good wayto do this is to place a sponge in the middle. This willabsorb the salt solution and allow the current to flow butstops the two plates from meeting.

Connect the reed valve material to the POSITIVE terminal of your power supply. You maywant to include a resistor (8 ohms is about optimum) in this lead to limit the current flow if

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you’re using a small container as I have here. If you don’t have an 10 watt, 8 ohm resistor, youcan use a 10W-20W 12V light bulb instead.

Once you connect things up, you should see bubbles begin to rise from the cathode plate as inthe picture.

At this stage the salt solution will still be clear.

Depending on a number of factors, it may takebetween 10 minutes and an hour to etch yourvalve. Once the process gets underway, arather awful looking green or brown sludge willbegin to form on top of the solution. This isthe iron that has been removed from thescribed lines.

Things will go more quickly and the results willbe better if you give the reed valve plate a bitof a shake now and then. This dislodges thecrud that forms on the scribed lines so that afresh salt solution can reach the bare metal and continue etching.

If you remove the plate from the solution you’ll see that the formerly shiny metal under thescribed lines has turned black.

Eventually the scribed lines will etch right through and when youremove the plate from the solution you’ll see the paint on theback surface exposed.

If you hold the plate up to a lamp at this stage you can see exactlywhere the etching is complete because the light will shine rightthrough as in this picture.

Post-etching StepsOnce you get to this stage you can disconnect allthe wires and carefully push the reed valve out ofthe plate. There will probably still be some areaswhere the etching is not quite complete but themetal at these points will now be so thin that itwill break away very easily.

Don’t worry if the edges seem a little ragged – thisis normal.

Now wash off the paint with suitable thinners.Another benefit of the automotive primer is thatit washes off very easily with lacquer thinners.

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Now you’ll have a new valve – but chances are that the edges will still be ragged as mentionedabove.

In order to avoid premature cracking of the valve it pays to file or sand those ragged edges tomake them smooth(er).

In theory, you can further reduce the risk of cracking by putting the valve in your oven atabout 200 deg F for an hour or so. What this will do is bake out any hydrogen that might haveentered the structure of the reed valve material as a result of the etching. Hydrogen in themolecular structure causes what is known as “hydrogen embrittlement” and that makes steelfar more prone to cracking. For what it’s worth – I don’t bother. Let’s face it – the valves aregoing to get hot enough anyway once you fire up your engine.

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Newton’s Third LawGood old Sir Isaac Newton was a smart guy and his observations formed the basis of whatwe’ve come to know as Newtonian physics.

One of his cornerstone laws of physics (Newton’s Third Law) says that:

for every action there is an equal and opposite reaction

It is this law that explains why a pulsejet (or any jet engine for that matter) creates thrust.

In short, our pulsejet only provides a forward push because it is also pushing hot gas out thetailpipe with equal force.

You can experience the effect of this law by sitting on one of those swivel-type office charsand rapidly twisting your body to the left. Notice how the seat of the char twists to the right?That’s Newton’s third law in action. The force you used to twist your upper body one waycreated an equal and opposite force that twisted the seat (and your bottom) the other.

You can also try sitting on the chair with a heavy weight in your hands. Throw that weightacross the room and you’ll find that you and the chair move off in the opposite direction,courtesy of Mr Newton.

But what goes on inside a pulsejet and how does this equal and opposite force cause our jetengine to move forwards?

To see just how this works, look at the diagram above which represents two containers filledwith pressurized gas. The first box is sealed so the pressure inside is perfectly balanced andthere’s an equal force pushing on all interior surfaces. The result of this is that everything isbalanced and no thrust is created.

If, as in the second diagram, we suddenly remove one side of the box then suddenly thepressure has nothing to push against. Consider this to be the open tailpipe of a pulsejet.

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However, there is still pressure acting on the other interior surfaces of the box but now wehave an imbalance.

The pressure pushing the box upwards is still balanced by the pressure pushing it downwardsbut the pressure pushing the box to the left is no longer balanced by the pressure pushing it tothe right.

As a result, the box in the second diagram will be pushed to the left by a force equal (butopposite in direction) to the force with which the compressed gas blows out through the largehole.

We can actually calculate the thrust that will be produced by summing the force vectors.

If we assume that the box is a one-foot cube and that the pressure inside is 10psi then we cansee that if one side was suddenly removed there’d be 1 square foot (144 square inches) ofunbalanced force pushing on the left hand side of the box. That would produce a thrust equalto 1,440 lbs.

Of course this simple calculation assumes that the pressure inside the box will remain at 10psieven when the side is removed.

If, instead of knocking the whole right-hand side off the box, we simply cut a 1 square inchhole then just 10lbs of thrust would be generated. The other 1,430 lbs would be pressingagainst the part of the side that was still intact and balancing the force pushing on the left-handside.This simple calculation validates Tharrat’s 2.2lbs of thrust per sq in of cross-sectional areaconstant we mentioned in an earlier chapter.

Tests conducted on the Argus V1 engineshowed an average pressure at the end ofthe tailpipe of around 2.2-2.6 lbs persquare inch.

This figure represents the average value of a pressure wave that fluctuated from a maximumpeak of +6.2 psi achieved during the combustion phase, to a minimum of –4.46 psi producedduring the intake phase.

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The Reynolds EffectWay back in the 1880s, a gentleman by the name of Osborne Reynolds spent some timeanalyzing the way in which gas flows.

Amongst other things, he discovered that “size matters” – well at least when it comes topushing gases down pipes or around obstacles it does.

Although I could again bore you with pages of formulas, I won’t. I’ll condense his lifetime’swork into a simple observation:

As you make things smaller, the air appears thicker (more viscous).

What this means is that it’s disproportionately harder to suck or blow air through a thin strawthan it is through a thick one – and that’s quite important in the world of pulsejets.

To all intents and purposes, the air that passes through a small engine like a Dynajet behaves awhole lot differently to the air that passes through a big pulsejet such as the Argus V1 engine.

This also explains why it’s not possible to simply scale down a large pulsejet engine and stillexpect it to run properly, if at all.

To draw an analogy – at Dynajet sizes, air behaves more like maple syrup. It flows but isreluctant to squeeze through holes and tends to stick to the sides of any pipe you pour itthrough.

At Argus V1 sizes, air behaves more like water – flowing quickly with far less tendency to stickto the sides of a pipe.

This explains why the Dynajet needs to have a valved area which is a full 50 percent of itstailpipe area whereas larger engines often perform very well with as little as 20 percent.

It also explains why small pulsejets generally require a larger L/D ratio to run properly. Inorder to overcome the effective viscosity of the air at these smaller sizes, you need a greater(longer) mass of air in the tailpipe to create sufficient Kadenacy effect for proper breathing.

Mr Reynolds has a lot to answer for!

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The Bernoulli EffectThis might sound a bit crazy but a smart … by the name of Bernoulli discovered that the fastera gas moves, the lower its pressure becomes.

Here’s a simple demonstration to show this effect.

Take a glass of water and stand a regular drinking straw in it.

Note that the water inside the straw is at the same level as the water in the glass. This meansthat the air-pressure inside the straw is the same as the air-pressure outside.

Now blow briskly across the top of the straw.

The water will rise up inside the straw and, if you’ve got really good lungs, it might even reachthe top and make a mess all over the table as exits as a fine spray of droplets.

So why did this happen?

Well, as Mr Bernoulli predicted, by increasing the speed of the air directly above the straw, itspressure was reduced. Since the pressure at the top (and inside) the straw was then less thanthe normal atmospheric pressure acting on the water outside the glass, the water rose up.

So what use is this effect in the wonderful world of pulsejets?

Well an understanding of this effect allows us to control the pressure inside an engine bycontrolling the speed at which gas passes through it.

If want more pressure is desired, we just slow down the gases and if we want less pressure wespeed them up.

So why do we want to change the pressures inside a pulsejet?

Well the atomizer that creates a fine stream of fuel dropletsis one example of why this effect is useful

You’ll remember that in our drinking straw experiment, wewere able to convert the liquid water in the glass into aspray of droplets simply by blowing across the top andcreating an area of low pressure.

To atomize the fuel in our pulsejet we need only do thesame thing.

Engines such as the Dynajet use just such a system for atomizing their fuel. By forcing theincoming air through a narrow opening called a venturi, an area of low-pressure is created that

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sucks the fuel through a ring of small holes. When the fuel hits this fast-moving airflow itbreaks up into droplets.

Without the Bernoulli effect a Dynajet wouldn’t run because it couldn’t suck any fuel into theengine.

There’s another area where some believe that the Bernoulli effect plays an important role inpulsejet operation.

Take a look at the shape of the average petal-valved engine again. Note that it has a largerdiameter section at the front and a smaller diameter tailpipe. These two sections are joined bya cone.

Now imagine what happens when the hot exhaust gases are drawn back into the front sectionby the partial vacuum left after combustion. While in the tailpipe, those gases will be travellingrelatively quickly – so they’ll have a low pressure.

As they pass through the cone, into the larger diameter front section of the engine they willslow down and in doing so, their pressure will (thanks to Mr Bernoulli) increase. One of thethings we want for efficient combustion is as much compression of the air/fuel charge aspossible.

Personally I don’t believe that this particular effect has any real bearing on a pulsejet’soperation – an assertion supported by research that shows pulsejets with a completely straightpipe are capable of producing just as much thrust as those which have an enlarged frontsection.

Others have pointed out that the Argus engine has a diffuser section immediately after thevalve-grid. The claim is that this diffuser is a device that acts like a divergent cone andtherefore increases the pressure of the incoming air/fuel mixture by slowing it down.

Perhaps it does have this effect, but my own experiments with intake diffusers indicates thattheir main benefit is that of forcing the air/fuel mixture through a narrow section so thatmixing is more thorough and, as a result, combustion is more rapid.

I also believe that the Argus diffuser also reduces the valve grid’s exposure to hot combustiongases by effectively choking their flow towards the front of the engine.

One only has to look at how the Dynajet (which has absolutely no internal diffuser) producesalmost an identical power/volume output as the Argus (with its internal diffuser) to realise thatthe effect of such a device on an engine’s power output is minimal at best.

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The Coanda EffectSome time during the 1930s, a Romanian by the name of Henri-Marie Coanda observedsomething very interesting.

He spotted that when air (or any other gas) was flowing along a surface that curved away fromthe flow, the gas didn’t just carry on straight ahead but followed the curvature of the surface.

Or, to put it another way, a stream of fluid or gas will tend to hug a convex contour whendirected at a tangent to that surface.

You can see the effect illustrated in this diagram.Instead of blowing straight past the circular surface,the air will stick to its surface and bend through 90degrees.

You can check this out for yourself by turning on atap, so that there's a steady but gentle continuous stream of water flowing. Now bring the backof a spoon into slight contact with the stream and you'll find that the water will no longer fallstraight down but actually stick to the curve of the spoon.

My own experiments indicate that a single curved surface will deflect a flow of room-temperature air by a maximum of about 90 degrees -- after which the flow detaches itself fromthe curve and once again travels at a tangent.

Although the Coanda effect doesn’t play a large roll in traditional pulsejet engines, anunderstanding that it exists and how it affects the flow of gas is something that all buddingpulsejet designers should have.

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PlansThis chapter of the book includes a numberof plans for various engines, including the 55-lbs –thrust enhanced Lockwood Valvelessengine that currently resides on my kart andwhich featured in the popular TV seriesScrapheap Challenge.

Although based on similar designs, thisparticular version has a flared intake cone andslightly different combustion chamberdimensions both of which help to assist inthrottling and easy starting.

This engine should be built from material that is at least 0.7mm in thickness and, if weight isn’ttoo much of an issue, you might find it worthwhile going up to 1.2 or even 1.5mm material.Although thicker material will cost and weigh more, and is harder to form, the result will be amore durable engine that is less likely to suffer from cracking.

This plan is included as a separate PDF file (55lbslh.pdf) and contains all the dimensions youneed to build your own.

One detail often missing in other Lockwood plans you might find on the internet is how thefuel-injection system works.

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I’ve experimented with two methods that work very well.

Both methods involve passing a tube cross-ways through the engine. This tube is cross-drilledwith a number of small (1.5-2mm) holes through which fuel sprays into the engine.

In the first method, the tube is placed in the intake tube, as close as possible to the cone thatjoins that tube to the combustion chamber. If you’re using a tube here it can be made fromplain steel rather than stainless because it gets plenty of cooling from the incoming airflow.

The second method involves running a much longer tube through the engine so that it passesthrough the cone connecting the intake tube to the combustion chamber. This tube ispositioned as close as practical to the combustion chamber end of the cone and will be quite abit longer than the one required for the first method.

In both cases, the tubes should be around 8mm in diameter and the cross-drilled holes shouldpoint towards the sides of the engine, not to the front or rear.

Full details of the second injector system is included in the CDROM Nick Haddock and Iproduced after appearing on the Scrapheap Challenge TV series. This CD also has plans forhis powerful turbocharger-based gas-turbine engine.

The fuel-tube, once installed, is fed with liquid propane which sprays out through the row ofsmall cross-drilled holes and vaporizes into propane gas.

If you want to run this engine on a liquid fuel such as gasoline, methanol, jet A1 or diesel thenyou can install a second fuel tube running at right angles with the propane tube – so that thetwo tubes form an overlapping cross when viewed down the intake tube.

Because these liquid fuels are not as volatile as propane, you will need to start the engine usingpropane and then start the pump that delivers liquid fuel to the second fuel tube. The propanecan then be turned off and the engine will run entirely on liquid fuel.

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Wacky IdeasI regularly get email from people who want to know if their idea will work when applied topulsejets. Here are some of the most common:

1. Water InjectionFull-sized turbojets can gain significant amounts of power by the use of water-injection. Thisworks in two ways.

If water is injected into the intake of the engine, its evaporation cools the incoming air, thisincreasing its density. Since the power of almost all engines depends on the differencebetween the minimum and maximum temperatures involved, cooling the incoming air willraise power.

This method of water-injection was actually used in some WW2 aircraft piston-engines where amixture of water and methanol was used.

The second way to use water injection is to squirt water into the combustion-chamber wherethe extremely high temperatures will cause that water to flash into steam – increasing thepressure in the chamber. One modern jet aircraft that uses this technique is the HawkerHarrier AV8 “jump-jet”. The use of water-injection is essential when this plane needs toperform a vertical take-off or landing while heavily laden with weapons or other externalstores. Unfortunately, since water itself is very heavy, these planes only have sufficient wateron board to allow a minute or two of vertical flight.

But back to pulsejets.

In theory, water injection should provide some additional power when used in a pulsejet but Ihave yet to try this in practice.

A big problem however, is the additional weight and complexity that the water, plumbing andpump would add to an engine that has simplicity as one of its few virtues.

In reality, it might be more practical to use a pulsejet as a flash-steam boiler by wrapping it incopper pipe through which water is pumped. That water would be flashed into steam by theintense heat of combustion then the steam could drive a turbine or piston.

2. Adding a turbine to the pulsejetAlthough this is an idea that was first contemplated many years ago, my own experimentsindicate that it doesn’t work very well at all.

The problem is that pulsejets suck and blow through the same pipe. That means our turbinewill be subjected to alternate forces that first try to rotate it in one direction then in another.Since efficient turbines tend to be rather delicate devices, pounding them hundreds of timesper minute with pulses of hot gas is not likely to create a very reliable machine.

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When I placed a small fan in the exhaust of a pulsejet I was surprised at just how littleinclination it had to spin – even though the engine was putting out good power. Bringing itcloser to the tailpipe actually reduced the spinning as the effect of the “suck” phase becamemore pronounced.

3. Running a pulsejet on hydrogenThey keep telling us that hydrogen is the fuel of the future, mainly because when it burns allyou get is heat and water (usually in the form of steam).

While that’s true, hydrogen does not make a good fuel for pulsejets for several reasons:

Firstly, hydrogen is an incredible gas insomuch as it has an extraordinarily wide stoichiometricrange. In simple terms, that means it will burn equally well when there’s just a little oxygen aswhen there’s a whole lot of oxygen.

If we compare hydrogen to regular gasoline we see that in order for gasoline to burn it mustrepresent no less than 1% and no more than 7.8% of an air/fuel mixture. Any less and there’snot enough fuel to form a flammable vapor, any more and there’s not enough oxygen tosupport combustion.

Hydrogen however, will burn happily when it constitutes anywhere from 4% to 75% of anair/fuel mixture.

This produces an undesirable effect when used in a conventional pulsejet engine.

As you’ve probably already figured out – a pulsejet does not burn its fuel continuously but inshort bursts. The fuel won’t ignite unless just the right amount of air is mixed with it. Duringthe operating cycle of a conventional pulsejet there are times when we have air, fuel andflames in close proximity to each other – but the air/fuel mixture seems to ignite at just theright time.

One of the reasons for this is that the comparatively narrow stoichiometric range of mosthydrocarbon fuels suppresses premature ignition – the fuel won’t ignite until there’s justenough air in the engine.

In the USA, NASA attempted to run a popular model airplane pulsejet (the Dynajet) usinghydrogen as a fuel and they failed to get it to pulse. The hydrogen seemed to burncontinuously because of its wide stoichiometric range.

There are numerous other problems that preclude hydrogen from being a viable pulsejet (orany engine) fuel for now.

It is extremely difficult to store hydrogen safely. In order to store enough of it to be useful ithas to be either compressed to incredibly high pressures, or chemically bound in a matrix ofhighly reactive materials such as metal hydrides.

In the former case there are obvious risks should a pressure vessel fail and the weight of suchcontainers detracts from the overall efficiency of a hydrogen-powered engine package.

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In the latter case, the chemical storage of hydrogen involves the use of very expensive anddangerous chemicals that react violently with common substances such as water.

4. Fitting an afterburnerSorry, conventional afterburners won’t work on pulsejets for several reasons.

Firstly, they are designed for turbojets where the gas-flow from the engine’s exhaust is aconstant flow. Unfortunately pulsejets create a flow that actually reverses briefly during theintake phase of the engine’s operation.

An afterburner is really just a special kind of ramjet and as such it requires that gases flowthrough it at a very high speed. Although the peak speed of gases coming out of a pulsejet isvery high, the average is quite low and the reverse flow would really mess things up, causingthe engine to suck in hot flames and fuel through its tailpipe when it really needs a slug of nicecold air.

However, all is not lost. I’m currently working on a true afterburner for use on pulsejets,although I’m not 100 percent confident that it will work as planned.

5. Building a really small pulsejetIt’s sad but true that the smaller you make a pulsejet, the more difficult it will be to start andthe less power it will produce.

And don’t think that by halving the size of a pulsejet you halve the power because that’s notso. Halving the size of a pulsejet will actually reduce the power by a factor of four. Yes, that’sright, all else being equal, half the size means just a quarter the power.

Another factor that adversely affects small pulsejets is their very low Reynolds numbers (seethe chapter on the Reynolds Effect). As pulsejets become smaller, the air appears to becomethicker – until it becomes just too thick to be sucked and pushed through the engine given thesmall amount of power available to do so.

The smallest practical pulsejet I’ve been able to get running on regular fuels was about 14inches in length and ¾ inches in diameter.

6. Pulsejet-powered helicoptersI shudder everytime I hear of someone who is thinking of building a helicopter powered by apulsejet. Trust me when I say that this is not a good idea if you value your life.

There have only been a very few such craft ever built and, without exception, they were neverflown for any length of time or made into commercially viable products. Valved pulsejets aresimply not reliable enough to bet your life on and valveless engines are difficult to design insuch a way that they are compact and streamlined enough to fit to a helicopter’s rotor-tips.

Lockwood-Hiller did experiment with special valveless engines designed to be built into theblades of a helicopter but this project was dropped – ask yourself why.

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A Little History and a Few Important People

The pulsejet has a rather erratic history, having been worked on by a number of very brightindividuals, each of which has contributed a little more knowledge and innovation.

One of the key players in the evolution of the pulsejet is a Dutchman by the name of FrancoisHenri Reynst who was born in 1909 and whose collective works were published by PergamonPress in 1961 under the title Pulsating Combustion.

Like all good researchers, Reynst drew heavily on the work done by others and the bookdetailing his work and findings is a must-have for anyone who is seriously interested in thetheory of pulsejets.

[to be completed]

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