Bottle Rockets 2015

Mass should be between 200 and 240 grams

http://www.tclauset.org/21_BtlRockets/BTL.html

The objective of this event is to get your rocket to stay up in the air as long as possible. There are 3 basic factors that will determine your time: Height, the recovery system, and the weather. Colder weather will have a negative effect on your rocket's performance. Try to waterproof your rocket. In combination, these will get you the time desired. One strategy is to go all out on one of these by either having a rocket that goes almost out of sight but has a weak recovery or having a rocket with a great recovery system that doesn't go that high. This will get you some success but to get the great times you will need a good mixture.

Height- The most common misconception is that the lighter a rocket is, the higher it will go. This is not fully true. To achieve maximum height you must have maximum stability. Maximum stability occurs when the center of gravity is ahead of the center of pressure. To find the center of gravity simply balance your rocket on a finger. The center of pressure is the point where you rocket would be balanced vertically if held in a strong wind. Achieving stability often means adding weight to the end of the nose cone. Again a good way of finding stability is by launching your rocket with different amounts of weight put in different places. My suggestion is adding balls of clay one at a time until your height starts to decrease. This make it easy to find the optimum balance of your rocket. You will also want to have fins on your rocket, but the shape and size of the fins do not appear to have a large influence on height as long as there are even fins.

http://txsnapper.eezway.org/powerdeployment/

http://waterrocket.uh-lab.de/backglide.htm http://txsnapper.eezway.org/waterrocketguy/backslider.html

These are the 2 rockets I was testing. Both consist of two Robinson coupled 0.5 liter Fanta bottles of 65 mm diameter; these are made of 0.3 mm PET. The top bottle got a guppied nose. The fins are taped on modular skirts.

Left: BackMax 1 (26th August 2001)4 PET fins, taped on a modular slightly tapered skirt. Mass 122 g, Length=47.5 cm, CG=28 cm from nose tip.Fin size is 42 mm x 52 mm.

Right: BackMax 2  (20th October 2001)3 laminated paper fins, taped on a modular cylindrical skirt.Mass 116g, Length 48 cm, CLA= approx. 26 cm from nose tip, CG=27.8 cm from nose tip, BCP > CG.Fins have 61 mm x 38 mm max. dimensions.

Click here for a larger image (50 KB).

CG=Center of GravityCLA= Center of Lateral Area

BCP=Barrowman Center of Pressureexplanations here.

BackMax 1: Optimization method: adding weight

Optimization: the rocket was repeatedly shot with air at only 2 bar pressure. Small weights were added (first 2, then another 2, ...), making sure that after each weight increase the weight distribution stayed symmetrical. I used flat nuts and lamp rods - that's what I had with me. When the rocket descended in the desired backgliding way, I increased the pressure to 4 and 6, later 8 bars. With increasing pressure, small weight increases were necessary. Too much weight at the tail must be avoided: instable ascent would be the sure consequence.

BackMax 2: Laminated fins with cut lines

To ease the optimization process, I made fins with printed-on square lines. This helps maintaining identical fin size while cutting the fins to a smaller size.

Click here to see how to make these Laminated Cardboard Fins.

::. Fins : MakingCreating a water rocket fins out of PETE plastic, that are durable and aerodynamic.

First, I would like to thank Robert Youens for the inspiration on these fins. He made his out of paper but I'm making these out of PETE. It's more durable as they crash straight through trees on their return trip to Earth. Check out his excellent guide on this page.

This page will be describing how I create four, structured, fins out of a single PETE 2 liter bottle. These fins are very durable, and aerodynamic in shape. It takes a while to build them but the results are well worth it. You'll need one 2 liter bottle, a knife, scissors, a 12" ruler, a Sharpie brand pen, and some packaging tape. Please read all instructions before cutting your plastic. I really mean it. Read through all of this mumbo jumbo so you don't mess up. A bottle is a terrible thing to waste.

Here we are with our victim. I've got all of my supplies together and we're ready to operate. Don't worry. The bottle won't feel any pain during the operation.

I've taken my knife and started a slit and then switched to scissors to cut out the "flat" portion of the 2 liter bottle. The top and bottom of the bottle is scrap. You'll end up with a rectangular piece of PETE bottle that we're goint to turn into our four fins.

Here's the top and bottom of the bottles. You could keep them and use them for various other purposes such as in parachute deployment or as a funnel for creating FTC moulds. The only limitation is your imagination. Now, back to creating our fins.

I fold over our sheet of plastic long ways and then cut it into two equal sized pieces. It might actually work better if you just mark the half way point with the Sharpie, draw a straight line between them and cut it with the scissors.

Here's another view. I've folded the piece in half and I'm preparing to cut it in half.

Here, I'm cutting the plastic that I've folded in half. You could just mark it and cut it.

I've got two approximately equal pieces of the PETE plastic. I'm now folding over one of the pieces and I'm going to make two fins out of this piece.

I'm using the ruler to press down on the plastic to get a good crease in the pastic. It's not easy to do actually as some good pressure is required. I also mark the plastic with the sharpie half way through the plastic. The sheet is 5.5 inches wide so I can create two 2.75 inch fins from this.

I have drawn all of my lines. Notice that I have marked 0.5 inches in from the fold of the plastic. I'm going to use my scissors to cut the fin. Notice that the "short" end of the fin is the side with the fold to it. Don't mark the plastic with the short end of the fin being the open side. It helps to get a good hold on the open side while cutting. The plastic will want to move on the upper level more than the lower level of plastic as you are cutting. Holding it tight helps prevent this and gets you two good semetrical pieces. Don't ask me how I know this, I just know it, all right!?!?!

We're now going to use pieces of the packaging tape to tape the leading and trailing edges of the fin. The ruler helps use get a good crease of the plastic and keep the edges together as we're taping.

With careful application of the tape, you won't get many air bubbles in the tape. Tape one side then flip the fin over and fold in the tape onto the other side of the fin.

This fin is fully taped. It's hard to see since it's blurry but it's a fully taped fin. The hardest part is over.

Here's another look. You can see the packaging tape in this picture on the leading and trailing edges.

In this picture, I've marked a spot 0.5 inches up from the open end of the fin. We'll use those marks to fold open the flanges.

I press the ruler down along the marks and fold up one side of the flange. I then remove the ruler and then fold the plastic over some more. Once the fold is begun, it's easy to fold it over and get a good flange.

Here's our fin almost done. The flanges are folded out on each side and will make a good base to glue to the rocket.

These fins will have structure to them. To get the fin into a more aerodynamic shape, cut a slit half way down the flange. Repeat on the exact opposide side. Once we do this, the fin can flex open.

Here you can see that the fin can now flex open to take our spar that goes inside.

I've taken some of the scrap pieces of plastic and I've cut them down and made them thinner. We'll use them as the spar on the inside of the fin. I've also flipped over the fold so the plastic flares to the outside making a "V" shape.

I slip in the spar into the fin and it holds open the fin. This fin is done. I now make three more of them from the other pieces of plastic. Make sure to keep all of the fins symetrical in size and diameter.

Here's another view of the fin sitting on the table.

You can see the shape of the fin. Narrow at the tip, wide at the base. These fins should really help the bottle fly straight.

Here are a picture of all of the fins, all taken from one 2 liter bottle.

Nose cone and rocket diameter affect dragThe amount of air resistance that opposes a rocket’s motion depends mainly on the shape of the nose cone, the diameter of the rocket and the speed of the rocket.

The first point that meets the air is the nose cone at the front end of the rocket. If the speed of a rocket is less than the speed of sound (1200 km/h in air at sea level), the best shape of a nose cone is a rounded curve. At supersonic speeds (faster than the speed of sound), the best shape is a narrower and sharper point.

Rockets with a larger diameter have more drag because there is more air being pushed out of the way. Drag depends on the cross-sectional area of the object pushing through the air. Making a rocket as narrow as possible is the best way to reduce drag.

The speed of a rocket through the air similarly increases drag. As speed doubles, drag increases four times as much.

Fins control direction and stabilityThe stability of a rocket is its ability to keep flying through the air pointing in the right direction without wobbling or tumbling.

Fins are used on smaller rockets to provide this stability and control direction. It works in the same way as placing feathers at the tail of an arrow. The greater drag on the feathers keeps the tail of the arrow at the back so that the point of the arrow travels straight into the wind.

To understand how to place fins and how large to make them, it is important to understand about centre of mass and centre of pressure.

Centre of massThe center of mass of an object is the point at which all of the mass of an object can be thought to be concentrated.

To find the center of mass of a rigid object such as a water bottle rocket, balance the rocket on your finger so that the rocket is horizontal. The center of mass is a point directly above your finger.

The center of mass can be moved closer to the nose cone end of a rocket by adding some mass near the nose cone. This will increase stability.

https://www.youtube.com/watch?v=b9AfA-85osM

Centre of pressureThe single point at which all of the aerodynamic forces are concentrated is called the center of pressure.

To find the approximate position of the center of pressure, draw an outline of the rocket on a piece of paper. The center of the area of the outline shape is approximately the center of pressure.

Image: Centre of pressure

For a rocket to be stable, the center of pressure needs to be closer to the tail end than the center of mass. If the center of pressure is at the same position as the center of mass, the rocket will tumble. Stability increases as the distance between the center of mass and the center of pressure increases.

Placing fins at the tail end of a rocket moves the center of pressure closer towards the tail end and increases stability. However, this also increases drag, so there is an optimal size for fins so that the rocket has enough stability without having too much drag.

114 Materials

2-liter soft drink bottle (1 per team) Styrofoam food trays Posterboard, cardboard Masking tape Low-temperature glue guns and glue 1- to 2-inch piece of 1/2” PVC pipe 4X4X1-inch board (per team) and small screw and washer 4 ounces of clay Eye protection Plastic grocery sacks or thin fabric scraps String Sandpaper or emery boards Art supplies Water rocket launcher (see page 109) Bicycle pump or small compressor

National Science Content Standards Physical Science • Position and motion of objects • Motions and forces Science and Technology

• Abilities of technological design National Mathematics Content Standards

• Geometry

• Measurement National Mathematics Process Standards

• Connections Rocket Activity

Water Rocket Construction Objective Student teams will construct water rockets and successfully launch them. Description Using plastic soft drink bottles, cardboard or Styrofoam food trays, tape, and glue, small teams of students design and construct rockets. A simple assembly stand assists them in gluing fins on their rockets, and a nose cone is mounted on the top. A small lump of modeling clay is inserted into the nose cone to enhance the rocket’s stability in flight. The rocket is launched with a special launcher. The plans for the launcher are found in the Water Rocket Launcher activity. 115

constructing water rockets through launch and reporting. Student teams form rocket companies and compete for government contracts. The procedures that follow here should be used for the construction phase of Project X-51. BackgroundA water rocket is a chamber, usually a 2-liter soft drink bottle, partially filled with water. Air is forced inside with a pump. When the rocket is released, the pressurized air forces water out the nozzle (pour spout). The bottle launches itself in the opposite direction. The bottle usually has a nose cone for streamlining and fins for stability. Water rockets are easily capable of 100-meter-high flights, but advanced hobbyists have combined bottles and staged bottles for flights over 300 meters high. Water bottle rockets are ideal for teaching Newton’s laws of motion. The launch of the rocket easily demonstrates Newton’s third law. Students can see the water shooting out of the nozzle (action) and see the rocket streak into the sky (reaction). Students can also experiment with different pressure levels inside the chamber and different amounts of water. The rocket will not fly very high if it is filled only with air. The air will quickly rush out during the launch, but its mass is very low. Consequently, the thrust produced is also low (Newton’s second law). By placing water in the bottle, the air has to force the water out first before it can leave the bottle. The water increases the mass expelled by the rocket, thereby increasing the thrust. Like all rockets, the flight performance of water bottle rockets is strongly influenced by the rocket’s design and the care taken in its construction. Beveling the leading and trailing

edges of fins allows them to slice through the air more cleanly. Straight-mounted fins produce little friction or drag with the air. A small amount of ballast weight inside the nose cone helps balance the rocket. This moves the center of mass of the rocket forward while still leaving a large fin surface area at the rear. In flight, the Pre-cut the PVC segments. The cuts can be slanted to streamline them. A saw or PVC cutter is used for cutting. The segments act as launch lugs to guide the rocket up the launch rod during the first moments of the rocket’s skyward climb. Be sure to use low-temperature glue guns. High-temperature guns will melt the plastic bottle. A small dish of ice water in a central location is helpful for students who get hot glue on their fingers. Immersing the fingers will immediately chill the glue. Do not put bowls of water near the guns themselves because the guns use electricity for heating, and shorting could occur if they get wet. ManagementBegin collecting 2-liter soft drink bottles a few weeks before the activity. Save the caps, too. Rinse the bottles and remove the labels. There will be some glue adhesive remaining on the bottle. Goo remover can be used to clean it off, but it tends to smear the surface. Construct assembly stands out of small blocks of wood. Attach a bottle cap to the middle of each board with a small screw and a washer through the cap. When students begin constructing their rockets, they screw the bottle neck into the cap, and the board below will hold the rocket upright for gluing. The blocks also make a convenient way of storing the rockets upright when not being worked on. Launch lug with slanted cuts. 116

The Assembly Stand supports the rocket while it is being constructed.Launch Lug

7. Have teams glue launch lugs to the side of the rocket midway up the body of the rocket and position it midway between two fins.8. Challenge teams to think up a way to add a parachute to their rockets for soft landings. Plastic grocery bags or lightweight fabric scraps can be cut to make parachutes and strings can be used to attach them. The nose cone must remain in place until the rocket reaches the top of its flight; then it should open and release the parachute.

Trim fin edges with sandpaper to give them knife-blade shapes to slice through the air.rocket design acts like a weather vane, with the nose cone pointed up and the fins down. Procedure1. Set up a supply station with materials such

as Styrofoam food trays, posterboard, tape, sandpaper, and art supplies.

2. Set up a gluing station with several heated low-temperature glue guns and extra glue sticks.

3. Divide students into teams for constructing rockets. If using Project X-51, describe the project to them and explain its objectives. Discuss construction techniques for their rockets. Give each team an assembly stand and a 2-liter soft drink bottle. Project X-51 requires teams to keep track of the materials they used. Even if they are not doing the project, it is still good for teams to account for the materials used.

4. Show teams how to use the glue guns and point out the cold water dish in case glue gets on fingers. Students should wear Eye protection when gluing.

5. Describe how fins can be smoothed with sandpaper to slice through the air with little drag.

6. Remind teams to add clay to the inside of their nose cones. 117

Do a swing test

http://txsnapper.eezway.org/waterrocketguy/paperfin.html Excellent details.

http://www.siouxbsa.org/pubs/c/98_waterbottlerocket.pdf

http://scioly.org/wiki/index.php/Bottle_Rocket

https://www.youtube.com/watch?v=jTkBHuV1i_I

https://www.youtube.com/watch?v=gO-903DGbw0

https://www.youtube.com/watch?v=6X8abVd8GRM

There are three points along a rocket that are important in calculation of flight stability:

Center of Gravity (CG):  the point at which the rocket balances.

Center of Lateral Area (CLA):  the point along the rocket where, if you were to attach a pivot and then hold the rocket in the wind by that pivot, the wind forces on either side of the CLA are equal, so the rocket wouldn't point either into or away from the wind: it would be perpendicularly "wind balanced."    http://u.hornstein.bei.t-online.de/wr_cla_calculator.htm

Center of Pressure (CP): a point that is often referred to as the aerodynamic center. This is the point where the aerodynamic forces acting on the rocket in front of this point is equal to the forces acting behind this point during normal flight. The only way to truly calculate this point is through the use of a wind tunnel. Generally CP is and CLA are very close on rockets that are shorter and wider. The CP is usually located far behind the CLA on long skinny rockets. To stabilize a rocket the CG is typically placed 1 or more rocket diameters ahead of the CP.

Variables you can adjust to increase stability during ascent.

A. Add mass to the front (moves CG forward)B. Decrease mass in the back (moves CG forward)C. Increase fin size (moves CP & CLA backward)D. Increase number of fins (moves CP backward with little effect on CLA)E. Move fins further back (moves CP and CLA backward)F. Lengthen rocket (moves CP back in relationship to CLA)G. Add helical twist to fins (moves CP back and induces inertial stability)H. Check Body Alignment (Cone, Extension, Pressure Vessel)

I. Check Fin alignmentJ. Make sure mass is center through the cross section of the rocket Discuss basics of passive deployment

Passive deployment depends on a parachute escaping from its containment area at or near apogee and opening rather quickly without the aid of timers or kick out devices. To achieve this, a rocket must have initial stability but must have some design characteristic to make it deploy its parachute. Some different designs that have worked in the past include:.

A. Rocket falls slowly through apogee and tall cone separates from the rocket deploying parachute.B. Rocket falls backward at apogee and short cone separates from rocket deploying parachute.C. Rocket falls backward at apogee and drone chute on front of rocket pulls out a main chute.D. Rocket falls backward at apogee and goes into a horizontal glide and uses no chute.E. Spinning rocket destabilizes at apogee as spin slows down. Cone & chute are freed.

Making a rocket move slowly or backward through apogee.Rocket can be designed so that they will have initial flight stability but will fall backward or move slowly through apogee. The angle that they fall backward can be controlled through design. As early as 1938 Robert Goddard observed a peculiar phenomenon. During a test flight of his model L-16, a tall, slender rocket, problems were encountered and the rocket fell from apogee horizontally. The phenomenon was used in rocket designs over the years but went virtually unexplained until 2000 when Robert & Peter Always published a research and development project for the National Association of Rocketry. (http://members.aol.com/petealway/srrg.htm)  They determined that a backward slide from apogee no longer need be an accidental quirk, but can be a deliberate design feature. By designing a rocket so that the CG is located between the CLA and the CP, a backward movement can be expected after achieving apogee. Through experimentation, I have found that movement through apogee can be slowed down, by applying the same principles.These changes can be attempted but may destabilize your rocket.A. Decrease mass in the front (moves CG backward)B. Increase mass in the back (moves CG backward)C. Move Fins further forward (moves CLA & CP forward)These changes will not decrease stability but will increase the likelihood of slowed apogee or backslide since the CG is more likely to fall between the CLA and CP.

D. Decrease fin size & increase the number of fins (moves CLA forward & moves CP back)G. Lengthen Rocket  (moves CLA forward & moves CP back)

http://txsnapper.eezway.org/txsnapper/stability.html

http://www.pack903ba.org/events/make-water-bottle-rocket

A little science about balancing your rocket…

A rocket that flies straight through the air is said to be stable. A rocket that veers off course or tumbles is said to be unstable. Whether a rocket is stable or unstable depends upon its design.

Hang your rocket from a string to find the center of balance.

All rockets have two “centers.” The first is the center of mass. This is a point about which the rocket balances. The picture to the right shows a rocket suspended from a string. The rocket is hanging horizontal. That means that it is balanced. The string is positioned exactly beneath the rocket’s center of mass. (This rocket looks like it should really hang with its tail section downward. What you can’t see in the picture is a mass of clay placed in the rocket’s nose cone. This gives the left side as much mass as the right side. Hence, the rocket balances.)

The center of mass is important to a rocket. If the rocket is unstable, it will tumble around the center of mass in flight the way a stick tumbles when you toss it.

The other “center” of a rocket is the center of pressure. This is a point in the shape of the rocket where half of the surface area of the rocket is on one side and half on the other. The center of pressure is different from the center of mass in that its position is not affected by what is inside the rocket. It is only based on the rocket’s shape.

Air strikes the surface of the rocket as the rocket moves. You know what this is like. If you stick your arm outside a car window when it is moving, you feel pressure from the air striking your arm. The center of pressure of a rocket is the middle point. Half of the total pressure on the rocket is on one side of the point and half on the other.

Depending upon the design of the rocket, the center of mass and the center of pressure can be in different places. When the center of mass is in front of the center of pressure (towards the nose end), the rocket is stable. When the center of pressure is towards the front, the rocket is unstable.

A simple way to accomplish stability is to place fins at the rear of the rocket and place extra mass in the nose.

You can verify your design results by conducting a swing test. Balance the rocket again with the string. Use a couple of pieces of masking tape to hold the string loop in position. Stand in a clear area and slowly start the rocket swinging in a circle. If the rocket is really stable, it will swing with its nose forward and the tail to the back.

In flight, the rocket will try to tumble around its center of mass. If the center of pressure is properly placed, the rocket will fly straight instead. More air pressure will be exerted on the lower end of the rocket than on the upper end. This keeps the lower end down and the nose pointed up!

Alright, there you have it. Now you have your own Water Bottle Rocket! See you on launch day!

As a model rocket flies through the air, aerodynamic forces act on all parts of the rocket. In the same way that the weight of all the rocket components acts through the center of gravity cg, the aerodynamic forces act through a single point called the center of pressure cp. How do you determine the location of the center of pressure?

Calculating cp

You can calculate the center of pressure. But, in general, this is a complicated procedure requiring the use of calculus. The aerodynamic forces are the result of pressure variations around the surface of the rocket. In general, you must determine the integral of the pressure times the unit normal, times the area, times the distance from a reference line. Then divide by the integral of the pressure times the unit normal, times the area. Lot's of work! For a model rocket, there are some simplifying assumptions that we can use to make this task much easier. Model rockets are fairly symmetric about the axis of the rocket. This allows us to reduce the full three dimensional problem to a simple, two dimensional cut through the axis of the rocket. For model rockets, the magnitude of the pressure variation is quite small. If we assume that the pressure is nearly constant, finding the average location of the pressure times the area distribution reduces to finding just the average location of the projected area distribution.

Simplified Calculation of cp

The figure shows a simplified version of the calculation procedure that you can use to calculate the cp of a model rocket. We assume that we already know the projected area and location, relative to some reference location, of each of the major parts of the rocket: the nose, body tube, and fins. The projected area A of the rocket is the sum of the projected area a of the components.

A = a(nose) + a(tube) + a(fins)

Since the center of pressure is an average location of the projected area, we can say that the area of the whole rocket times the location of the center of pressure cp is equal to the sum of the projected area of each component times the distance d of that component from the reference location.

A * cp = [a * d](nose) + [a * d](tube) + [a * d](fins)

The "location" of each component is the distance of each component's center of pressure from the reference line. So you must calculate or determine the center of pressure of each of the components. For example, the projected area of the body tube is a rectangle. The center of pressure is on the axis, half way between the end planes.

Mechanically determining cp

For a model rocket, there is a simple mechanical way to determine the center of pressure for each component or for the entire rocket. Make a two dimensional tracing of the shape of the component, or rocket, on a piece of cardboard and cut out the shape. Hang the cut out shape by a string, and determine the point at which it balances. This is just like balancing a pencil with a string! The point at which the component, or rocket, is balanced is the center of pressure. You obviously could not use this procedure for a very large rocket like the Space Shuttle. But it works quite well for a model.

http://polyplex.org/rockets/other/rocket_bob/FinJig/