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AUTHOR Vogt, Gregory L.TITLE Rockets: A Teaching Guide for am Elementary Science

Unit on Rocketry.INSTITUTION National Aeronautics and Space Administration,

Washington, D.C.REPORT NO NASA-PED-112PUB DATE Nay 91NOTE 3Sp.PUB TYPE Guides - Classroom Use - Teaching Guides (For

Teacher) (052)

EDRS PRICE NFO1/PCO2 Plus Postage.DESCRIPTORS Aerospace Education; Elementary Education;

*Elementary School Science; Enrichment Activities;Instructional Materials; Learning Modules; *Mechanics(Physics); *Science Activities; Science Education;Science History; *Science Instruction; *ScienceMaterials; Units of Study

IDENTIFIERS Newton Laws of Motion; Rocket Propellants; *Rockets;Space Shuttle

ABSTRACTUtilizing simple and inexpensive equipment,

elementary and middle school science teachers can conductinteresting, exciting, and productive units on rockets, the oldestform of self-contained vehicles in existence. This teaching guidecontains the f011owing: (1) a brief history of experimentation andresearch on rockets and rocket propulsion from ancient Chinesewarfare to modern space exploration; (2) the mechanical principles ofrockets in terms of Newton's Laws of Motion; (3) a discussion of thescience of practical rocketry in terms of rocket engine design, typesof propellants, engine thrust control, flight path stability andcontrol, and weight versus payload considerations; (4) 10 hands-onclassroom activities which include required materials and tools,procedures, and typical classroom discussion topics relating to eachactivity; and (5) a list of commercial suppliers for model rocketryresources. The activities are effective for teaching the science ofrocketry, utilizing readily available materials such as soda cans,pencils, balloons, and straws. Further, these activi_tes aresuggested as an introduction to a follow-up activity of building andlaunching commercially-supplied model rockets. (PR)

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Acknowledgments

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This publication was produced as a part of thecontinuing educational activities of the Na-tional Aeronautics and Space Administrationthrough the Aerospace Education ServicesProject. The following deserve thanks for theirassistance:

Dr. Robert W. BrownDirectorNASA Educational Affairs DivisionNN; A HeadquartersWashington, DC

Mr. Frank OwensDeputy DirectorNASA Educational Affairs DivisionNASA HeadquartersWashington, DC

Dr. Eddie AndersonBranch ChiefElementary and Secondary ProgramsNASA Educational Affairs DivisionNASA HeadquartersWashington, DC

Mr. Larry B. BilbroughEducational Programs OfficerNASA Educational Affairs DivisionNASA HeadquartersWashington, DC

Ms. Deborah V. GallawayEducational Programs OfficerNASA Educational Affairs DivisionNASA HeadquartersWashington, DC

Dr. Kenneth E. Wigginsand the project staff of theAerospace Education Services ProjectOklahoma State Universitywho developed many of the activity ideas.

Table of Contents

iv

Introduction vBrief History of Rockets 1

Rocket Principles 6Practical Rocketry 10Activities 17&sources 32

5

Introduction

6

R ockets are the oldest form of self-contained vehicles inexistence. Forenmners to the rocket were in use more

than two thousand years ago. Over a long and exciting histoty,rockets have evolved into mighty vehicles capable of launchinga kpaceenfl that can travel out into the galaxy beyond oursolar system. Few experiences can compare with the excite-ment and thrill of watching a tocket-powered %Thiele such asthe Space Shuttle thunder into space.

Children are interested in rockets. Rockets arc exciting.They thunder, roar, and shoot flames. They carry machinesand humans off the Earth and out into space (the only kind oftransportation now able to accomplish that). It is only naturalthat dreams of rocket flight to distant worlds fire the imagina-tion of children.

With some simple and inexpensive materials, you canconduct an exciting and productive science unit about rocketsfor children even if you don't know much about rockcayourself. The many activities contained in this teaching guideemphasize hands-on involvement. Background informationabout the history of rockets and basic science explaining whyrockets work sill make you an l'expert."

The science unit that follows is based on Isaac Newton'slaws of motion. These laws explain why rockets work and howto make them more efficient. The unit also indudes basicct.ncepts of rocket control and descriptions of different kinds ofrockets.

Brief History ofRockets

Hero Engine

rr oday's rockets arc remarkable collections of humaningenuity. NASA's Space Shuttle and the Soviet

Union's Buran (shuttle) are among the most complexflying machines ever invented. They stand upright on alaunch pad, lift off as rockets, orbit Earth as spacecraft,and mturn to Earth as gliding airplanes. The SpaceShuttle and the Bum are true space ships. In a fewyears they will be joined by other spaceships. TheEuropean Space Agency is building the Hermes andJapan is building the HOPE. Still later will comeaerospace planes that will take off from runways asairplanes, fly into space, and return as airplanes. TheUnited States, Eumpean Space Agency, United King-dom, and Japan are all working on vehicles for the eariytwenty-first cemury. All these new and future spaceshipshave their mots in the science and technology of thepast. They arc natural outgrowths of literally thousandsof years of experimentation and research on rockets androcket propulsion.

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One of the first devices to employ the principlesessential to rocket flight successfully was a

wooden bird. In the writings of Aulus Gellius,a Roman, there is a story of a Gneek namedArchytas who lived in the city of Tarentum,

1, now a part of southern Italy. Somewherer el, around the year 400 BC, Archytas mystifiedand amused the citizens of Tarentum by

flying a pigeon made of wood. The bird was- suspended on wires and propelled along by

escaping steam. The pigeon operated on theaction-reaction principle that was not to be stated as a

scientific law until thc seventeenth century.

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About three hundred years after the pigeon, anotherGreek, Hero of Alexandria, invented a similar

rocketlike device called an arolipile. lt, too,used steam as a propulsive gas. Hero mounteda sphere on top of a water kettle. A fire below

the kettle turned the water into steam, and thcgas traveled through pipes to the sphere. Two

I.-shaped tubes on opposite sides of thc sphereallowed the gas to escape, and in so doing thrust the

sphere, rotating it.

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_A". Just when the first true rockets appeared is unclear.Stories of early rocket-like devices appear sporadicallythrough the historical records of various cultures.Perhaps the first true rockets were created by

accident. In the first century A.D., the Chinesewere reported to have had a simple form ofgunpowder made from saltpeter, sulfur, andcharcoal dust. It was used mostly for fireworks in

religious and other festive celebrations. Bam-boo tubes were filled with the mixture andtossed into fires to create explosions during

religious festivals. It is possible that some of those tubesfailed to explode anci instead skittered out of the fires,propelled by the gases and sparks produced by theburning gunpowder.

ft is certain that the Chinese began to experiment withthe gunpowder-filled tubes. At some point, bambootubes were attached to arrows and launched with bows.Soon it was discovered that these gunpowder tubescould launch themselves just by the power producedfrom the escaping gas. The true rocket was born.

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Chinese Fire-Arrows

True rockets were probably launched beginning in theyear 1232. At this time, the Chinese and the Mongolswere at war with each other. During the battle of Kai-Kens, the Chinese repelled the Mongol invaders by abarrage of "arrows of flying fire." These fire arrows werea simple form of solid-propellant rocket. A tube, cappedat one end, was filled with gunixwder. The other endwas left open and the tube was attached to a long stick.When the powder was ignited, the rapid burning of thepowder produced fire, smoke, and gas that escaped outthe open end and produced a thrust. The stick acted asa simple guidance system that kept the rocket headed inone general direction as it flew through thc air. It is notclear how effective these arrows of flying fire were asweapons of destruction but their psychological effects onthe Mongols must have been formidable.

2

Following the battle of Kai-Kens, the Mongols pro-duced their own rockets and may have been revonsiblefor the spread of rockets to Europe. All through thethirteenth to the fifteenth centuries there were reports ofmany rocket experiments. In England, a monk namedRoger Bacon improved gunpowder to increase the rangeof rockets. In France, Jean Froissart found that flightscould be more accurate by launching rockets throughtubes. Froissart's idea was the forenmner of the modernbazooka. Joanes de Fontana of Italy designed a surface-running rocket-powered torpedo for setting enemy shipson fire.

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Surface-Running Torpedo

By thc sixteenth century rockets were no longer used asweapons of war, though they were still used for fireworksdisplays. A German fireworks maker, Johann Schmidlap,invented the "step rocket," a multi-staged vehicle forlifting fireworks to higher altitudes. A large sky rocketcarried a smaller sky rocket as a payload. When the largerocket burned out, the smaller one continued to a higheraltitude before showering the sky with glowing cinders.Schmidlap's idea is basic to all rockets today that go intoouter space.

Nearly all uses of rockets up to this time were for warfareor fireworks, hut there is an interesting old Chineselegend that reported the use of rockets as a means oftransportation. With the help of many assistants, a low-ranking Chinese official named Wan-Hu assembled a

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rocket-powered flying chair. Attached to the chair weretwo large kites, and fixed to the kites were forty-sevenfur-arrow rockets.

On the day of the flight, Aran-Hu sat on the chair andgave the command to lif ht the rockets. Forty-sevenrocket assistants, each armed with torches, rushedforward to light the fuses. There was a tremendous roaraccompanied by billowing douds of smoke. When thesmoke cleared, Wan-Hu and his flying chair were gone.No one knows for sure what happened to Wan-Hu, butit is probable that if the event really did take place, Wan-Hu and his chair were blown to pieces. Fire-arrows wereas apt to explode as to fly.

Rocketry Becomes a ScienceDuring the :atter part of the seventeenth century, thescientific foundations fiar modern rocketry were laid bythe great English scientist Sir Isaac Newton (1642-E17). Newton organized his understanding of physicalmotion into three scientific laws. The laws explain howrockets work and why they arc able to work in thcvacuum of outer space. (Newton's three laws of motionwill be explained in detail later.)

Newton's laws soon began to have a practical impact onthe design of rockets. About 1720, a Dutch professor,Willem Gravesande, built model cars propelled by jets ofsteam. Rocket experimenters in Germany and Russiabegan working with rockets with a weight of more than45 kg. Some of these rockets were so powerful that theirescaping exhaust flames bored deep holes in the groundeven before lift-off.

During the cnd of the eighteenth century and early intothe nineteenth, rockets experienced a brief revival as aweapon of war. Thc success of Indian rocket barragesagainst the British in 1792 and again in 1799 caught theinterest of an artillery expert, C.olonel William Congreve.Congreve designed rockets for use by the British mili-tary.

The Congreve rockets were highly successful in battle.Used by British ships to pound Fort McHenry in theWar of 1812, they inspired Francis Scott Key to write"the rockets' red glare," in his poem that later becameThe Star-Sparigkd Banner.

Even with Congreve's work, the accuracy of rockets hadnot improved significantly from the early days. Thedevastating nature of war rockets was not their accuracyor power, but their numbers. During a typical siege,thousands of them might be fired at the enemy. All overthe world, rocket researchers experimented with ways toimprove accuracy. An Englishman, William Hale,developed a technique called spin stabilization. In this

method, the escaping exhaust gases struck small vanes atthe bottom of the rocket, causing it to spin much as abullet does in flight. Variations of the principle are stillused today.

Rockets continued to be used with success in battles allover the European continent. However, in a war withPrussia, the Austrian rocket brigades met their matchagainst newly designed artillery pieces. Breech-loadingcannon with rifled barrels and exploding warheads werefar more effective weapons of war than the best rockets.Once again, roc'Acts were relegated to peacetime uses.

Modern Rocketry BeginsIn 1898, a Russian schoolteacher, Konstantin Tsiol-kovsky, (1857-1935) proposed the idea of space explora-tion by rocket. In a report he publish:A' in 1903,Tsiolkovsky suggested the use of liquiti propellants forrockets to achieve greater range. Tsiolkovsky stated thatthe speed and range of a rocket were limited only by theexhaust velocity of escaping gases. For his ideas, carefulresearch, and great vision, Tsiolkovsky has been calledthe father of modern astronautics.

Early in the twentieth century, an American, Robert H.Goddard, (1882-1945) conducted practical experimentsin rocketry. He had become interested in a way ofachieving higher altitudes than was possible for lighter-than-air balloons. He published a pamphlet in 1919

Tsiolkovsky Rocket Designs

3

entitled, A Method ((Reaching Femme Altitudes. It wasa mathematical analysis of what is today called themeteorological sounding rocket.

In his pamphlet, Goddard reached several conclusionsimportant to rocketry. From his tests, he stated that arocket operates with greater efficiency in a vacuum thanin air. At the time, most people mistakenly believed thatair was needed for a rocket to push against and a NewTurk Timer newspaper editorial of the day mockedGoddard's lack of the "bagic physics ladled out daily inour high schools." Goddard also stated that multistage(or step) rockets were the answer to achieving highaldtudes and that the velocity needed to escape theearth's gravity could be achieved in this way.

Goddard's earliest experiments were with solid-propel-lant rockets. In 1915, he began to try various types ofsolid fuels and to measure the exhaust velocities of thcburning gases.

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4

While working on solid-propellant rockets, Goddardbecame convinced that a rocket could bc propelled moreeffidently by liquid fuel. No one had ever kilt asuccessful liquid-propellant rocket before. It was a muchmore difficult task than building solid-propellant rockets.Fuel and oxygen tanks, turbines, and combustionchambers would be needed. In vim of the difficulties,Goddard achieved the first successful flight with a liquid-propellant rocket on March 16, 1926. Fueled by liquidoxygen and gasoline, the nxket flew kir only two and ahalf seconds, climbed 12.5 m, and landed 56.1 m awayin a cabbage patch. By today's standards, the flight wasunimpressive. But like the fust powered airplane flightby thc Wright brothers in 1903, Goddard's gasolinerocket was the foreninner of a new era in rocket flight.Goddard, for his achievements, has been called the fatherof modem rocketry.

Goddard's experiments in liquid-propellant rocketscontinued for many years. Hs rockets became biggerand flew higher. He developed a gyroscope system forflight control and a payload compartment for scientificinsuuments. Parachute recovery systems were employedto return rockets and instruments safely.

A third great space pioneer, Hermann Oberth, (1894-1989) of Germany, published a book in 1923 aboutrocket travel into outer spaa. His writings were impor-tant. Because of them, many small rocket societiessprang up around the world. In Germany, the formationofone such society, the Verein fur Raumschiffahrt(Society for Space Travel), led to the development of thcV-2 rocket, which was used against London duringWorld War II. In 1937, thousands of engineers andscientists, including Oberth, assembled in Peenemundeon the shores of the Baltic Sca. There the T11041 ad-vanced rocket of its time would bc built and flown underthe directorship of Wernher von Braun.

The V-2 rocket (in Germany called the A-4) was smallby comparison to today's rockets. It achieved its greatthrust by burning a mixture of liquid oxygen and alcoholat a rate of about one ton every seven seconds. Oncelaunched, the V-2 was a formidable weapon that coulddevastate whole city blocks.

Fortunately for London and the Allied forces, the V-2came too late in the war to change its outcome. Never-theless, by war's end, German rocket scientists andengineers had already laid plans for advanced missilescapable of spanning the Atlantic Ocean and landing inthe United States. The range of one of these multistagevehicles with a winged upper stage would have been6440 to 8050 km. It could have carried a warkad ofonly about about 20 kg, would have glided at lowaltitude and subsonic speed, and would have had aninacurracy of at least 32.2 or 48.3 km.

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With the fall of Germany, many unused V-2 rockets andcomponents were captured by the Allies. Many Germanrocket scientists came to the United States. Others wentto the Soviet Union, The German scientists, includingWernher von Braun, were amazed at the progressGoddard had made.

Both the United States and the Soviet Union realizedthe potential of rocketry as a military weapon and begana variety of experimental programs. At first, the UnitedStates began a program with high-altitude atmosphericsounding rockets, one of Goddard's ideas from manyyears before. Later, a variety of medium- and long-rangeintercontinental ballistic missiles were developed. Thesebecame the starting point of the U.S. space program.Missiles such as the Redstone, Atlas, and Titan wouldeventually launch astronauts into Earth orbit.

On October 4, 1957, the world was stunned by thenews of an Earth-orbiting artificial satellite launched bythe Soviet Union. Called Sputnik I, the satellite was thefirst ruccessful entry in a race for space between the twosuperpower nations. Less than a month later, the Sovietsfollowed with the launch of a satellite carrying a dognamed Laika on board. Laika survived in space for sevendays before being put to sleep before the oxyben supplyran out.

A few months after the first Sputnik, the Umted Statesfollowed the Soviet Union with a satellite of its own.&plow I was launched by the U.S. Army on January 31,1958. In October of that year, the United Statesformally organized its space program by creating theNational Aeronautics and Space Administration (NASA).NASA became a civilian agency with the goal of peacefulexploration of space for the benefit of all humankind.

Soon, many people and machines were being launchedinto space. Astronauts orbited the Earth and landed onthc Moon. Robot spacecraft traveled to the planets.Space was suddenly opened up to exploration andcommercial exploitation. Satellites enabled scientists toinvestigate our world, forecast the weather, and tocommunicate instantaneously around the globe. As thedemand for more and larger payloads increased, newerand bigger rockets had to be built.

At present, 18 nations working alone and in consortiahave rockets capable of orbiting satellites around Earth.Two nations, the United States and the Soviet Union,have manned orbital launch capability and Japan, China,and the 13 member nations of the F.uropean SpaceAgency expect to orbit astronauts before the year 2005.

Since the earliest days of discovery and experimentation,rockets have evolved from simple gunpowder devicesinto giant vehicles capable of traveling into outer space.Rockets have opened the universe to direct explorationby humankind.

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Rocket Principles

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Balloon

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Arocket in its simplest form is a chamber enclosing agas under pressure. A small opening at one end of

the chamber allows the gas to escape, and in so doingprovides a thiust that propels the rocket in the oppositedirection. A good example of this is a balloon. Airinside a balloon is compressed by the balloon's rubberwalls. The air pushes back so that the inward andoutward pressing forces are balanced. When the nozeeis released, air escapes and propels the balloon in a rocketflight. The balloon's flight is highly erratic because thereare no structures to stabilize it.

When we think of rockets, we rarely think of balloons.Instead, our attention is drawn to the giant vehicles thatcarry satellites into orbit and spacecraft to the Moon andplanets. Nevertheless, there is a strong similarity be-tween the two. The only significant difference is the waythe pressurized gas is produced. With space rockets, thegas is produced by burning propellants that can be solidor liquid in form or a combination of the two.

One of the interesting facts about the historical develop-ment of rockets is that while rockets and rocket-powereddevices have been in usc kir more than two thousandyears, it has been only in the past three hundred yearsthat rocket experimenters have had a scientific basis forunderstanding how they work.

The science of rocketry was first recorded withpublishing of a book in 1687 by the great Enghsiscientist Sir Isaac Newton. His book, entitledPhilosophiac Naturalis Principia Mathematica, describedphysical principles in nature. Today, Newton's work isusually just called the Principia.

In the Principia, Newton stated three important scien-tific principks that govern the motion of ail objects,whether on Earth or in space. Knowing these principles,now called Newton's laws of motion, rocketeers havebeen able to construct the modern giant rockets of todaysuch as the Saturn V and the Space Shuttle. Here now,in simple form, are Newton's laws of motion.

I . Objects at rest will stay at rest, and objects in Motionwill stay in motion in a straight line unless actedupon by an unbalanced force.

2. Force is equal to mass times acceleration.

3. For every action there is always an oppositt: and equalreaction.

As will be explained shortly, all three laws are reallysimpk statements of how things move. But with them,precise determinations of rocket performance can bemade.

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Newton's First LawThis law of motion is just an obvious statement of fact,but to !mow what it means, it is necessary to understandthe terms PIA, nosion, and unbalanced force.

Rest and motion can be thought au apposite. Rest isthe state of an object when it is not changing position inrelation to its surroundings. If you are sitting still :n achair, you can be said to be at west. This term, however,is relative. Your chair may actually be one ofmany seats on a speeding airplane. Theimportant thing to remember here is thatyou arc not moving is relation to yourimmediate nirramsdivr. If rest were definedas a total absence of motion, it would notexist in nature. Even if you were sitting inyour chair at home, you would still bemoving, because your chair is actually sittingon the surface of a spinning planet that isorbiting a star, and the star is moving thtougha rotating galaxy that is, itself, moving through theuniverse. While sitting "still," you arc, in fact,traveling at a speed of hundreds of miles per second.

Motion is also a relative term. All matter in the universeis moving all the time, but in the first law, motion meanschanging position in relation to surroundings. A ball isat rest if it is sitting on the ground. Thc ball is inmotion if it is rolling. Then it is changing its position inrelation to its surroundings. Whcn you are sitting an achair in an airplane, you arc at rest, but if you get up andwalk down the aisle, you arc in motion. A rocketblasting off the launch pad changes from a state of restto a state of motion.

The third term important to understanding this law isunbalanced force. If you hold a ball in your hand andkeep it still, the ball is at rest. All the time the ball isheld there though, it is being acted upon by forces. Theforce of gravity is pulling the ball downward, while yourhand is pushing against the ball to hold it up. Thc forcesacting on the ba.11 am balanced. Let the ball go, or moveyour hand upward, and the forces become unbalanced.The ball then changes from a state of rest to a state ofmotion.

In rocket flight, forces become balanced and unbalancedall the time. A rocket on the launch pad is balanced.The surface of the pad pushes the rocket up while gravitytries to pull it down. As the engines arc ignited, thethrust from the rocket unbalances the forces, and therocket travels upward. Later, when the rocket runs outof fuel, it slows down, stops at the highest point of itsflight, then falls back to the Earth.

3

Ball at Rest

Objects in space also react to forces. A spacecraftmoving through the solar system is in constant motion.Thc spacecraft will travel in a straight line if the forces onit arc in balance. This happens only when the spacecraftis very far from any large gravity source such as the Earthor the other planets and their moons. If the spacecraftcomes near a large body in space, the gravity of thatbody will unbalance the forces and curve the path of thespacecraft. This happens, in particular, when a spacecraftis sent by a rocket on a path that is parallel to the Earth'ssurface. If the rocket shoots the spacecraft Fast enough,the spacecraft will orbit thc Earth. As long as an unbal-

anced force, (friction or the firing of a rocket engine indie opposite direction from its movement) does not stopthe spacecraft, it will orbit the Earth forever.

Now that the three major terms of this first law havebeen explained, it is possible to testate this law. If anobject, such as a rocket, is at test, it takes an unbalancedforce to make it move. If the object is already moving, ittakes an unbalanced force to stop it or to change itsdirection or speed.

Newton's Third LawFor the time being, we will skip the second law and godirectly to the third. This law states that every action hasan equal and opposit: reaction. If you have ever steppedoff a small boat that has not been properly tied to a pier,you will laiow exactly what this law means.

A rocket can lift off from a launch pad only when itexpels gas out of its engine. The rocket pushes on thegas, and the gas, in turn, pushes on the rocket. Thewhole process is very similar to riding a skateboard.Imagine that a skateboard and rider are in a state of rest(not moving). The rider steps off the skateboard. In thethird law, the stepping off is called an aetion. Theskateboard responds to that action by traveling somedistance in the opposite direction. The skateboard'smotion is called a mac**. When the distance traveledby the rider and the skateboard ..xe compared, it wouldappear that thc skateboard has had a much greaterreaction than the action of the rider. This is not thecase. The reason the skateboard has traveled farther isthat it weighs less than the rider.

With rockets, the action is the expelling of gas out of theengine. The reaction is the movement of the rocket inthe opposite direction. To enable a rocket to lift offfrom the launch pad, the action, or thrust, from theengine must he greater than the weight of the rocket. Inspace, however, even tiny thrusts will cause the rocket tochange direction.

One of the most commonly asked questions aboutrockets is how they can work in space where there is noair to oppo... iheir movement. The answer to thisquestion comes from the third law. Imagine the skate-

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8

board again. On the ground, the only part air plays inthe motions of the rider and the skateboard is to slowthem down. Moving through the air causes friction, oras scientists call it, drag. The surrounding air imped.the action-reaction.

As a result rockets actually work better in space than theydo in air. As the exhaust gas leaves the rocket engine, itmust push away the surrounding air; this uses up someof the energy of the rocket. In space, the exhaust gasescan escape freely.

Newton's Second LawThis law of motion is essentially a statement of a mathe-matical equation. The three parts of the equation aremass (m), acceleration (a), and force (f). Using letters tosymbolize each part, the equation can be written asfollows:

f ma

By using simple algebra, wc can also write the equationtwo other ways:

Action

3 =

m =a

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The first version of tne equation is the one most com-monly referred to when talking Woout Newton's secondlaw. It reads: force equals mass times acceleration.

Force in the equation can be thought of as the thrilst ofthe rocket. Mass in the equation is the amount of rocketfuel being burned and converted into gas that expandsand then escapes from the rocket. Acceleration is therate at which the icas escapes. Inside the rocket, the gasdoes not really move, but as it leaves the engine it picksup speed.

The second law of motion is especially useful whendesigning efficient rockets. To enable a rocket to climbinto !ow Earth orbit or escape the Earth's gravitationalpull, it is necessary to achieve velocities, or speeds, inexcess of 213,000 km per hour. The speed necessary toescape Earth's gravity, 4E1,250 km per hour, is called'escape velocity." Attaining escape velocity requires therocket engine to achieve the greatest action forcepossible in the shortest time. In other words, the engine

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must burn a large amount of fuel and push the resultinggas out of the engine as rapidly as possible. Ways ofdoing this will be described in the next chapter.

Newton's second law of motion can be restated in thefollowing way: The greater the mass of rocket fuelburned and the faster the gas produced can escape theengine, the greater the thrust of the rocket.

Putiing Newton's Laws of Motion TogetherAn unbalanced force must bc exerted fix a rocket to liftoff from a launch pad or for a craft in space to changespeed or direction (first law). The amount of thrust(force) produced by a rocket engine will be determinedby the mass of rocket fuel that is burned and how fastthe gas escapes the rocket (second law). The reaction, ormotion, of the rocket is equal to and in an oppositedirection from the action, or thrust, from the engine(third law).

9

Practical Rocketry

10

fr he first rockets ever built, the fire-arrows of theChinese, were not very reliable. Many just

exploded on launching. Others flew on erratic coursesand landed in the wrong place. Being a rocketeer in thedays of the fire-arrows must have been an exciting but ahighly dangerous activity.

Today, rockets are much mote reliable. They fly onprecise courses and are capable of going fast enough toescape the gravitational pull of the Earth. Modernrockets are also more efficient because we have a firmunderstanding of the scientific principles behind rock-etry. This has led us to develop a wide variety of ad-vanced rocket hardware and to devise new propellantsthat can be used for longer trips and more efficienttakeoffs.

Rocket Engines and Their PropellantsMost rockets today operate with either solid or liquidpropellants. The word "propellant" does not mean:iimply fuel, as you might think, it means both fuel andoxidizer. The fuel is the chemical the rocket burns, butfor burning to take place, an oxidizer (oxygen) must bepresent. Rockets do not have the luxury that jet planeshave. Jet engines draw oxygen into their engines fromthe surrounding air. Rocket engines, which operatemostly in space where there is no air, must carry theiroxygen with them.

Solid rocket propellants, which are dry to the touch,contain both the fuel and oxidizer in the chemical itself.Usually the fuel is a mixture of hydrogen compoundsand carbon. The oxidizer is made up of oxygen com-pounds. Liquid propellants, which are often gases thathave bccn chilled until they turn into liquids, are kept inseparate containers, one for the fuel and the other forthe oxidizer. Then, when the engine fires, the fuel andoxidizer are mixed together in the engine.

A solid-propellant rocket has the simplest form ofengine. It has a nozzle, a case, insulation, a propdlant,and an igniter. The case of the engine is usually arelatively thin metal that is lined with insulation to keepthe propellant from burning through. The propellantitself is packed inside the insulation layer.

Many solid-propellant rocket engines feature a hollowcore that runs through the propellant. Rockets that donot have the hollow core must be ignited at the lowerend of the propellants and burning proceeds graduallyfrom one end of the rocket to the other. In all cases,only the surface of the propellant burns. However, toget higher thrust, the hollow core is used. This increasesthe surface of the propellants available for burning. Thepropellants burn from the inside out at a much higherrate, and the gases produced escape the engine at much

higher speeds. This gives a greater thrust. Somepropellant cores am star shaped to increase the burningsurface even more.

To fire the solid pmpellants, many kinds of igniters canbe used. Fire-arrows ws.re ignited by fuses, but some-times these ipited too quickly and burned the rocke-teer. A far safer and more reliable form of ignition isone that employs electricity. Au electric avant,coming through wires from some distance away, heatsup a special wire inside the rocket. The wire raises thetemperature of the propellant it contacts to the com-bustion point.

Many igniters arc more advanced than the hot-wiredevice. Some arc encased in a chemical that ignitesfirst, which then ignites the propellants. Other igniters,especially those for large rockets, arc rocket enginesthemselves. The small engine inside the hollow coreblasts a stream of flames and hot gas down flom thetop of the core and ignites the entire surface area of thepropellants in a fraction of a second.

The nozzle in a solid-propellant engine is an opening atthe back of the rocket that permits the hot expandinggases to escape. The narrow part of the nozzle is thethroat. Just beyond the throat is the exit cone.

The purpose of the nozzle is to increase the accekra-tion of the gases as they leave the rocket and therebymaximize the thrust. It does this by cutting down theopening through which the gases can escape. To sechow this works, you can experiment with a garden hosethat has a spray nozzle attachment. This kind of nozzledoes not have an exit cone, but that does not matter in

the experiment. The important point about the nozzleis that the size of the opening can be varied.

Start with the opening at its widest point. Watch howfar the water squirts and feel the thrust produced bythe departing water. Now reduce thediameter of theopening, and again note the distance the water squirtsand feel the thrust. Rocket nozzles work the samc way.

As with the inside of the rocket case, insulation is

needed to protect the nozzle from the hot gases. Theusual insulation is one that gradually erodes as the gaspasses through it. Small pieces of the insulation getvery hot and break away from the nozzle. As they arcblown away, heat is carried away with them.

The other main kind of rocket engine is one that usesliquid propellants. This is a much more complicatedengine, as is evidenced by the fact that solid rocketengines were used for at least seven hundred yearsbefore the first successful liquid engine was tested.

BellNorth)

Throat

Solid Propellant Rocket

Liquid propellants have separate storage tanksonc for

the fuel and one for the oxidizer. They also have pumps,a combustion chamber, and a nozzle.

., fuel of a liquid engine is usually kerosene or liquidhydrogen; the oxidizer is usually liquid oxygen. Theyarc combined inside a cavity called the combustionchamber. Here the propellants burn and build up tohigh temperatures and pressures, and the expanding gasescapes through the nozzle at the lower end. To get the

11

NOZ210

CombustionChamber

Fins

Liquid Propellant Rocket

most power from the propellants, they must be mixed ascompletely as po.ssible. Small nozzle injectors on theroof of the chamber spray and mix the propellants at theN3nIc time. Because the chamber operates under highpressures, the propellants need to be forced inside thechamber. Powerful, ligiitweight turbine pumps betweenthe propellant tanks and combustion chambers do thisjob.

With any rocket, and especially with hquid-propellantrockets, weight is an important factor. In general, thehosier the rocket, the more the thrust needed to get it

12

off tin ground. Because of the pumps and fuel lines,liquid engines am much hcavier than solid engines.

Onc especially good method of reducing the weight ofliquid engines is to make the exit cone of the nozzle outof very lightweight metals. However, the extremely hot,fast-moving gases that pass through the cone wouldquickly melt thin metal. Themfore, a cooling system isneeded. A highly effective though complex coolingsystem that is used with some liquid engines takesadvantage of the low temperature of liquid hydrogen.Hydrogen becomes a liquid when it is chilled to minus423q (-2SPC). Before injecting the hydrogen into th:combustion chamber, it is first circulated through smalltubes that lace the walls of the exit cont. In a cutawayview, the exit cone wall looks like the edge of corrugatedcardboard. The hydrogen in the tubes absorbs theexcess heat entering the cone walls and prevents it frommelting the walls away. It also makes the hydrogenmore energetic because of the heat it picks up. We callthis kind of cooling system retnerativs

Engine Thrust ControlControlling the thrust of an engine is very important tolaunching payloads (cargoes) into orbit. Too muchthrust or thrust at the vaong time an cause a satellite tobe placed in the wrong orbit or set too far out into spaceto be useful. Too little thrust can cause the satellite tofall back to the Earth.

Liquid-propellant engines control the thrust by varyingthe amount of propellant that enters the combustionchamber. A computer in the rocket's guidance systemdetermines the amount of thrust that is needed andcontrols the propellant flow rate. On more complicatoriflights, such as going to the Moon, the engines must bestarted and stopped several times. Liquid engines dothis by simply starting or stopping the flow of propel-lants into the combustion chamber.

Solid-propellant rockets are not as easy to control asliquid rockets. Once started, the propellants burn untilthey arc gone. They are very difficult to stop or slowdown part way into the burn. Sometimes fire extin-guishers are built into thc engine to stop the rocket inflight. But using them is a tricky procedure and doesn'talways work. Some solid-fuel engines have hatches ontheir sides that can be cut loose by remote control torelease the chamber pressure and terminate thrust.

The burn rate of solid propellants is carefully planned inadvance. The hollow core running the length of thepropellants can be made into a stai shape. At first, thereis a very large surface available for burning, but as thepoints of the star burn away, the surface area is reduced.For a time, less of the propellant bums, and this reduces

thrust. The Space Shuttle uses this technique to reducevibrations early in its flight into orbit.

CAUTION: Although most rockets used by govern-ments and research organizations arc very reliable, thereis still great danger associated with the building andfiring of rocket engines. Individuals interested inrocketry should never attempt to build their ownengines. Even the simplest-loobing rocket engines arcvery complex. Cast-wall bursting strength, propellantpacking density, nozzle design, and propellant chemistryam all design problems beyond the scope of mostamateurs. Many home-built rocket engines haveexploded in the faces of their builders with tragicconsequences.

Stability and Control SystemsBuikiing an efficient rocket engine is only pan of theproblem in producing a successful rocket. The rocketmust also be stable in (telt. A stable rocket is one thatflies in a smooth, uniform direction. An unstable rocketEa along an erratic path, sometimes tumbling orchanging direction. Unstable rockets are dangerousbecause it is not possible to predict where they will go.They may even turn upside down and suddenly headback directly to the launch pad.

Making a rocket that is stable requires some form ofcontrol system. Controls can bc either active or passive.The difference between these and how they work will beexplained later. It is first important to understand whatmakes a rocket stable or unstable.

All matter, regardless of size, mass, or shape, has a pointinside called the senor glows (CM). The center ofmass is the exact spot where all of the mass of that objectis perfectly balanced. You can easily find the center ofmass of an object such as a ruler by balancing the objecton your finger. If the material used to make the ruler isof uniform thickness and density, the center of massshould be at the halfway point between one end of thestick and the other. If the ruler were made of wood, anda heavy nail were driven into one of its encfs, the centerof mass would no longer bc in the middle. The balancepoint would then be rtarcr the end with zhe nail.

The center of mass is imponant in rocket flight becauseit is around this point that an unstable rocket tumbles.As a matter of fact, any object in flight tends to tumble.Throw a stick, and it tumbles end over end. Throw aball, and it spins in flight. The act of spinning ortumbling is a way of becoming stabilized in flight. AFrisbee will go where you want it to only if you throw itwith a deliberate spin. Try throwing a Frisbee withoutspinning it. If you succeed, you will sec that the Frisbeeflies in an erratic path and falls far short of its mark.

In ffight, spinning or tumbling takes place around one ormore of three axes within the object. They are calledrag pitch, and yaw. The point where all three of theseaxes intersect is the center of mass. For rocket flight, thepitch and yaw axes are the most important because anymovement in either of these two directions can cause therocket to go off course. The roll axis is the least impor-tant because movement along this axis will not affect theflight path. In fact, a rolling motion will help stabilizethe rocket in the same way a properly passed football isstabilized by rolling (spiraling) it in flight. Although apoorly paced football may still fly to its mark even if ittumbles rather than rolls, a rocket will not. The action-reaction energy of a football pass will be completelyexpended by the thmwer the moment the ball leaves thehand. With rockets, thrust from the engine is still beingproduced while the rocket is in flight. Unstable motionsabout the pitch and yaw axes will cause the rocket toleave the planned course. To prevent this, a controlsystem is needed to prevent or at least minimize unstablemotions.

In addition to center of mass, there is another importantcenter inside the rocket that affects in flight. This is thecenter of pressure (CP). The center of presure existsonly when air is flowing past the moving rocket. Thisflowing air, rubbing and pushing against the outersurface of thc rocket, can cause it to begin movingaround one of its three axes. Think for a moment of aweather vane. A weather vane is an arrowlike stick that ismounted on a rooftop and is used Aur telling winddirection. The arrow is attached to a vertical rod thatacts as a pivot point. The arrow is balanced so that thecenter of mass is right at the pivot point. When thewind blows, the arrow turns, and the head of the arrowpoints into the oncoming wind. The tail of the arrowpoints in the downwind direction.

The mason that the weather vane arrow points into thewind is that thc tail of the arrow has a much largersurface area than the arrowhead. The following airimparts a greater force to the tail than the head, andtherefore the tail is pushed away. There is a point in thearrow where there is exactly thc same surface arca on mu:side of the point as on the other. This spot is called thecenter of pressure. The center of pressure is not in the

Center of Pressure Center of Mass

A 1113

same place as the center of mass. If it were, then neitherend of the anvw would bc favored by the wind and thearrow would not point. The center of pressure isbetween the center of mass and the tail end of the arrow.This means that the tail end has morz surface area thanthe head end.

It is extremely important that the center of pressure in arocket be located toward the tail and the center of massbc located toward the nose. If they are in the same placeor vety near each other, then the rocket will be unstablein flight. The rocket will then try to rotate about thecenter of mass in the pitch and yaw axes, producing adangerous ituation. With thc center of pryssure locatedin the right place, the rocket will remain stable.

Control systems for rockets keep a rocket stable in flightand steer it. Small rockets usually require only a stabiliz-ing control system, but large rockets, such as the onesthat launch satellites into orbit, require a system that notonly stabilizes the rocket but also enables it to changecourse while in flight.

Controls on rockets can either bc active or panive.Passive controls arc fixed devices that keep rocketsstabilized by their very presence on the rocket's exterior.Active controls can bc moved while the rocket is in flightto stabilize and steer the craft.

Thc simplest of all passive controls is a stick. TheChinese fire-arrows were simple rockets mounted on theends of sticks. The stick kept the center of pressurebehind the center of mass. In spite of this, fire-arrowswere notoriously inaccurate. Before the center ofpressure could take effect, air had to be flowing past therocket. While still on the ground and immobile, thearrow might lurch and fire the wrong way.

Years later, the accuracy of fire-arrows was improvedconsiderably by mounting them in a trough aimed in thcproper direction. The trough guided the arrow in theright direction until it was moving fast enough to bestable on its own.

As will be explained in the next section, the weight ofthe rocket is a uitical factor in performance and range.The fire-arrow stick added too much dead weight to therocket, and therefore limited its range considerably.

An important improvement in rocketry came with thereplacement of sticks by clusters of lightweight finsmounted around the lower end near the nozzle. Fins

14

could be madc out of lightweight materials and bestreamlined in shape. They gave rockets a dartlikeappearance. The large surface area of the fins easily keptthe center of pressure behind the center of mass. Someexperimenters even bent the lower tips of the fins in apinwheel fashion to promote rapid spinning in flight.With these "spin fins," rockets become much morestable in flight. But this design also produces more dragand limits the rocket's range.

With the start of modern rocketry in the twentiethcentury, new ways were sought to improve rocketstability and at the same time reduce overall rocketweight. The answer to this was the development ofactive controls. Active control systems included vanes,tilting fins, canards, gimbaled nozzles, vernier rockets,fuel injection, and attitude-control rockets. Tilting finsand canards arc quite similar to each other in appear-ance. The only real difference between them is theirlocation on the rockets. Canards arc mounted on thefront end of the rocket while the tilting fins are at thcrear. In flight, the fins and canards tilt like rudders todeflect the air flow and cause the rocket to change

Rocket Changes Direction

Movable Fins

20

course. Motion sensors on the rocket detect unpbnmddirectional changes, and corrections can bc made byslight tilting of the fins and canards. The advantage ofthese two devices is size and weight. They arc smallerand lighter and produce less drag than the large fins.

Other active control systems can eliminate fins andcanards altogether. By tilting the angle at which theexhaust gas leaves the rocket engine, course changes canbe made in flight. Several techniques can be used forchanging exhaust direction.

Vanes arc small finlike devices that are placed inside theexhaust of the rocket engine. Tilting the vanes deflectsthe exhaust, and, by action-reaction, the rocket respondsby pointing the opposite %%Inf.

Another method for changing the exhaust direction is togimbal the nozzle. A gimbaled nozzle is one that is ableto sway while exhaust gases are passing through it. By

Gimbaled Nozzle

tilting the engine nozzle in the proper direction, therocket responds by changing course.

Vernier rockets can also be usfd to change direction.These are small rockets mounted on the outside of theL .-ge engine. When needed they fire, producing thedesired course change.

In space, only by spinning the rocket along the roll axisor by using active controls involving the engine exhaustcan the rocket be stabilized or have its directionchanged. Without air, fins and canards have nothing towork upon. (Science fiction movius showing 3 inspace with wings and fins arc !erg on fiction and shoTton science.) Tire most common kinds of active controlused in space arc attitude-control rockets. Small dustersof engines are mounted all around the vehicle. By firingthe right combination of these small rockets, the vehiclecan be turned in any direction. M soon as they arcaimed properly, the main engines fire, sending the rocketoff in the new direction.

WeightThere is another important factor affecting the perform-ance of a rocket. The weight of a rocket can makc thedifference between a successful flight and just wallowingaround on thc launch pad. As a basic principle of rocketflight, it can be said that for a rocket to leave theground, the engine must produce a thrust that is greaterthan the total weight of the vehicle. It is obvious that arocket with a lot of unnecessary weight will not be asefficient as one that is trimmed to just the bare essentials.

For an ideal rocket, the total weight of the vehicleshould be distributed following this general formula:Of the total mass, 91 parent should be propellants; 3percent should be tanks, engines, fins, ctc.; and 6percent can be the payload. Payloads may bc satellites,astronauts, or spacecraft to other planets or moons.

In determining the effectiveness of a rocket design,rocketeers strak in terms of mass fraction (MF). Themass of the propellants of the rocket divided by the totalmass of the rocket gives MaSS fraction:

MFmass of propellants

total mass

The mass fraction of the ideal rocket given above is 0.91.From the mass fraction formula one might think that anMF of 1.0 is perfect, but then the entire rocket would benothing more than a lump of propellants that wouldsimply ignite into a fireball. The larger the MF number,the less payload the rocket can carry; the smaller the MF

21 15

number, the less its range becomes. An MF number of031 is a good balance between payload-carrying capabil-ity and range.

Large rockets, able to carry a spacecraft into space, haveserious weight problems. To reach space and properorbital %elocities, a great deal of propellant is needed,and therefore the tanks, engines, and associated hard-ware become larger. Up to a point, tugger rockets flyfarther than smaller rockets, but when they become toolarge their structures weigh them down tor much, andthe mass fraction is reduced to an impossible number.A solution to the problem of giant rockets weighing too

16

much can be credited to the sixteenth-century fireworksmaker Johann Schmidlap. Schmidlap attached smallrockets to the top of big ones. When the large rocketwas exhausted, the rocket casing was dropped behindand the remaining rocket fired. Much higher altitudeswere achieved by this method.

The rockets used by Schmidlap were called step rockets.Today this technique of building a rocket is calledmagiffg. Thanks to staging, it has become possible notonly to reach outer space but the moon and otherplanets, too.

Activities The following activities are useful forteaching about the science of rocketry.

They make use of simple and inexpensivematerials. It is suggested that these activitiesbe used as an introduction to a follow-upactivity of building and launching commercialmodel rockets.

17

ClassroomActivities

Soda Pop Can Hero EngineSubject RocketryTopic Newton's Laws of MotionDescription: Water streaming through holes in thebottom of a suspended soda pop can causes the can torotate.Contributed by: Tom Clausen, Kennedy Space CenterExplorations Station

Materials and Tools:Empty soda pop can with the opener lever intactNail or icr pickFishing lineBucket or tub of water

Method:1. Lay the can on its side and using the nail or ice

pick carefully punch four equally spaced smallholes just above and around the bottom rim.Then, before removing the punching tool fromeach hole, push the tool to the right (parallel tothe rim) so that the hole is slanted in thatdirection.

2. Bend the can's opener lever straight up and tic ashort kngth of fishing line to it.

3. Immerse the can in water until it is filled. Pullthe can out by the fishing line. Water streamswill start the can spinning.

Discussion:The soda pop can Hero engine Ls an excellent demon-stration of Newton's bws of motion. The can rotatesbecause a force is exerted by the flowing water (firstlaw). The rate of natation will vary with differentnumbers of holes and difkrent diameters of holes inthe can (second law). Try two holes and try a can withlarge holm versus a can with small holes. The canrotates in the opposite direction from the direction ofthe water streams (third law).

For more information about Hero engincs, refer to thc'Hero Engine* activity.

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ClassroomActivities

Rocket PinwheelSubject R.ocketryTopic Action-Rcaction PrincipleDiscription: Construct a balloon-powered pinwheel.Contributed by: John Harts& ld, Lewis ResearchCenter

Matuials:Wooden penal with an eraser on one end&night pinRound party balloonFlexible soda strawPlastic tape

Method:1. Inflate the balloon to stretch it out a bit.2. Slip the nozzle cnd of the balloon over the end of the

straw farthest away from the bend. Usc a short pieceof plastic tape to seal the balloon to the straw. Theballoon should inflate when you blow through thestraw.

3. Bend the opposite end of the straw at a right angle.4. Lay the straw and balloon on an outstretched finger

so that it balances. Push the pin through the straw atthe balance point and then continue pushing the pininto the eraser of the pencil and finally into the wood

S. Spin the straw a few times to loosen up the hole thepin has made.

6. Inflate the balloon and let go of the straw.

Discussion:The balloon-powered pinwheel spins because of theaction-reaction principle described in Newton's third lawof motion. Stated simply, the law says every act:on isaccompanied by an opposite and equal reaction. In thiscaw, the balloon produces an action by squeezing on theair inside causing it to rush out the straw. The air,traveling around the bend in the straw, imparts areaction force at a right angle to the straw. The result isthat the balloon and straw spin around thc pin.

19

ClassroomActivities

Rocket CarSubject RocketryTopic Newton's Third Law of MotionDescription: A small car is propelled by the action-reaction force generated by a balloon.Contributed by: Gregory Vogt, Oklahoma StateUniversity

Materials and Tools:4 PinsStyrofoam meat trayCellophane tapeFlexi-strawScissorsDrawing compassMarker penSmall party bailoonRuler

Procedure:1. Using the ruler, marker, and drawing compass,

draw a rectangle about 7.5 by 18 cm and fourcircles 7.5 cm ia diameter on the flat surface ofthe meat tray. Cut out each piece.

2. Push one pin into the center of each circle andthen into the edge of the rectangle as shownin the picture. The pins become axle, forthe wheels. Do not push the pins in snuglybecause the wheels have to rotate freely. It isalight if the wheels wobble.

3. Inflate the balloon a few times to stretch it out abit. Slip the nozzle over the end of the flexi-strawnearest the bend. Secure the nozzle to the strawwith tape and seal it tight so that the balloon canbe inflated by blowing through the straw.

4. Tape the straw to the car as shown in the picture.5. Inflate the balloon and pinch the straw to hold in

the air. Set the car on a smooth surface and releasethe straw.

20

Discussion:The rocket car ispropelled along thefloor according to theptinciple stated in IsaacNewton's third law ofmotion. "For everyaction there is an oppositeand ova! reaction." Becausethe balloon is attached to thecar, the car is pulled along bythe balloon.

/ "\

ClassroomActivities

Paper RocketsSubject: RocketryTopic: StabilityDescription: Small flying rockets made out of paperand propelled with air blown through a straw.

Tape

Materials:Scrap bond paperCellophane tapeScissonShaspened fat pencilMilkshake straw (slightly thinner than pencil)

Procedure:1. Cut a narrow rectangular snip of paper about

13 cm long and roll it tightly around the fat pencil.Tape the cylinder and remove it from the pencil.

2. Cut points into one end of the cylinder to make acone and slip it back onto the pencil.

3. Slide the cone end onto the pencil tip and squeezeand tape it together to seal the end and kirm anose cone (the pencil point provides support fortaping). An alternative is just to fold over one cndof the tube and seal it with tape.

4. Remove the cylinder from the pencil and gentlyblow into the open end to check for leaks. If aireasily escapes, use more tape to seal the leaks.

S. Cut out two SCIS of fins using the pattern on thispage and fold according to the diagram. Tape thefins near the open cnd of the cylinder. The tabsmake taping easy.

0 7

Flying the Paper Rocket:Slip the straw into the rocket's opening. Point therocket in a safe direction, sharply blow through thestraw. The rocket will shoot away. Be careful not toaim the rocket toward anyone because the rocketcluld poke an eye.

Discussion:Paper rockets demonstrate how rockets fly through thcatmosphere and the importance of having fins forcontrol. For experimental purposes, try building arocket with no fins and one with the fins in the front tosee how they will fly. Practice flying the rockets on aballistic trajectory towards a target. Also try making arocket with wings so that it will glide.

Fold Down

Fin Shape

21

ClassroomActivities

Water RocketsSubject; RocketsTopic: Newton's second law of motionDescription: Varying the amount of water and pressurein water rockets afkcts thc distance they travel.Contributed by: Gregory Vogt, Oklahoma StateUniversity

Materials and Tools:2 Water rockets and pumps (available for a few dollarseach from toy stores)WaterSmall wooden stakes, small flags, or other materials to

serve as markers

Procedure:1. Take the water rockets, pumps, water supply, and

markers to an outside location such as a dear, grassyplaying field.

2. Attach both rockets to their pumps as shown on thepackage they came in. Pump one rocket 10 times.Pump the second rocket 20 times. Have twostudents point the rockets across the field in adirection in which no one is standing. The rocketsshould bc held next to each other at exactly thesame height above the ground and aimed upward atabout a 45 degree angle. Count backwards from 3and have the students release the rockets. Mark thedistance each rocket flcw.

3. Pour water into one rocket so that it fills up to the30.48 meter line. Do not pour water into thc otherrocket. Attach both rockets to their pumps. Pumpboth rockets 20 times. Again aim the rockets acrossthe field as before and release them simultaneously.Mark the distance each rocket flew. (Caution: Therocket with the water will expel the water from itschamber and may spray a student. lithe studentstands to the side for the release, thc water sprayshould miss the student.)

22

4. Try other combinations for simultaneous firings ofthe rockets, such as a small amount of water in oneand a larger amount of water in the other, or equalamounts of water but one pumped differently. Resure to change only one variable at a time (i.e. varyonly the water or only the pumping).

Discussion:Isaac Newton's second law of motion states thatforce is

equal to mas times arcekration. In rocket terms, itineans that the thrust of z rocket is equal to the amountof mass (fire, smoke or steam, and gas) expelled by therocket engine times how fast it is expelled. Designers ofpowerful rockets try to maximize both mass and accel-eration to get the greatest thrust possible because thrustis a product of the two. In the 1960s, designers of theSaturn V rocket built first-stage engines that burnedkerosene and liquid oxygen. The two propellants wereinjected into each of the five engines of the first stage byhigh-speed turbines. Kerosene was chosen as thc fuelbecause it is a very dense liquid when compared to otherrocket fuels. During operation, the tive engines con-sumed 15 tons of propellants per second. The combina-tion of dense fuel and high-speed combustion produceda liftoff thrust of 7.5 million pounds.

Water nxkets can demonstrate Newton's second lawwhere one varies the pressure inside the rocket and theamount of water present. In the first test, neither rocketwent very far because the air inside did not have muchmass. The rocket that was pumped more did travelfarther because the air in that rocket was under greaterpressure and it escaped the rocket at a higher speed(acceleration). When water was added to one of therockets, the effect of mass was demonstrated. Before theair could leave the water rocket, the water had to beexpelled. Water has a much greater mass than air and itcontributed to a much greater thrust. The rocket withwater flew much farther than the rocket filled only withair. By varying the amount of water and air in the rocketand measuring how far the rockets travel, stud- .ts cansec that the thrust of the rocket is dependent c *he

mass being expelled and how fast it is being expt -d.Thrust is greatest when mass and acceleration aregreatest.

23

ClassroomActivities

Balloon StagingSubject RocketryTopic: Rocket stafOngDescription: Two inflated balloons simulate a multi-stage rocket launch as they slide along a fishing line onthe thrust produced by escaping air.

4. Take the balloons to one end of the fishing lineand tapc each balloon to a straw. The balloonsshould bc pointed along the length of the fishingline.

5. If you wisk, do a rocket countdown and release thesecond balloon you inflated. The escaping gas willpropel both balloons along the fishing line. Whenthe first balloon released runs out of air, it willrelease thc other balloon to continue the trip.

Materials and Tools:2 Long party balloonsNylon monofilamcnt fishing line (any weight)2 Plastic straws (milkshake size)Styrofoam coffee cupMasking tapeScissors

Procedure:I. Thread the fishing line through the two straws.

Stretch the fishing line snugly across a room andsecure its ends. Make sure the line is just highenough for people to pass safely underneath.

2. Cut the coffee cup in half so that the lip of the cupforms a continuous ring.

3. Loosen the balloons by preinflating them. Inflatethe first balloon about three-fourths full of air andsqueeze its nozzle tight. Pull the nozzle throughthe ring. While someone assists you, inflate thesecond balloon. The front end of the secondballoon should extend through the ring a shortdistance. As the second balloon inflates, it will pressagainst the nozzle of the irst balloon and take overthc job of holding it shut. It may take a bit ofpractice to achieve this,

24

Discussion:Traveling into outer space takes enormous amounts ofenergy. Much of that energy is used to lift rocketpropellants that will bc used for later phases of therocket's flight. To eliminate the technological prob-lems and cost of building giant one-piece rockets toreach outer space, NASA, as well as all other space-faring nations of the world have chosen to use a rockettechnique that was invented by sixteenth-centuryfireworks maker Johann Schmidlap. To reach higheraltitudes with his aerial displays, Schmidlap attachedsmaller rockets to the top of larger ones. When thelarger rockets were exhausted, the smaller rocketsclimbed to even higher altitudes. Schmidlap called hisinvention a "step rocket."

Today's space rockets make use of Schrnidlap's invention through "multistaging." A large first-stage rocketcarries the smaller upper stages for the first minute ortwo of flight. When the first stage if exhausted, it isreleased to return to the Earth. In doing so, the upperstages are much more efficient and are able to reachmuch higher altitudes than they would have been ableto do simply because they do not have to carry theexpired engines and empty propellant tanks that makeup the first stage. Space rockets are often designedwith three or four stages that each nrc in turn to send apayloaci into orbit.

36

ClassroomActivities

Newton CartSubject: RocketryTopic: Newton's laws of motionDescription: A wooden block is thrown by a slingshot-like device from a cart and produces a thrust that causesthe cart to move in the opposite direction.

Materials And Tools:1 Wooden block about 1020x2.5 cm1 Wooden block about 73xx2.5 cm3 - 7.62 cm No. 10 wood scntwe ;round head)12 Round pencils or short lengths of similar

dowel rods3 Rubber bandsCotton string (several feet)Matches6 Lead fishing sinkers (about 14 gm each)Drill and bitViceScrewdriver

Procedure:1. Screw the three screws in the large wood block as

shown in the figure.2. Hold the short piece of wood with a vice and drill

two holes large enough to drop two sinkers in each,3. Tie several small loops of string (the same size).

4. Place one string loop over a rubber band and thenplace the ends of the rubber band over the twoscrews on one end of the large wooden block. Pullthe rubber band back like a slingshot and slip thestring over the third screw to hold the rubber band

stretched.5. Arrange the pencils or dowel rods in a row like

railroad ties on a level table top. Set the large block

on one end of the row so that the single screwpoints to the middle of the row. Slip the small block(without sinkers) into the rubber bands.

6. Light 4 match and ignite the ends of the stringhanging down from the loop. When the stringburns through, the rubber band will throw the blockoff the cart and the cart will roll in the other direc-tion. Note how far the cart travels along the tabletop.

7. Reset the equipment and add a second rubber band.Again, light the string and note how far the carttravels.

8. Reset the equipment and try again with 3 rubberbands. Try again with onc rubber knd and twosinkers, 4 sirkers, etc,

3125

Discussion:The Newton ,:art provides an excellent demonstrationof Isaac Newton's three laws of motion. The laws arestr.cd in general urrns below

First I ^w:

Second Law:

Third Law:

An object at rest willremain at rest or anobject in motion willremain in motion in astraight line unless actedupon by an unbalancedforce.Force equals mass timesacceleration.For every action there isan opposite and equal .

reaction.

The Newton cart remains at rest on the tabletop untilan unbalanced force (from the expelled block) causes itto move. A rocket remains on the launch pad until thethrust from the engines propels it upward.

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The amount of thrust on the cart is determined by themass of the block and how fast it is thrown off. This caneasily be seen in how far the cart traveled. The cart witha single rubber band and no sinkers did not travel veryfar. With two and three rubber bands, the block wasthrown off much faster (acceleration). With sinkersadded to the block, the block had more mass and thiscaused the cart to travel farther than when the block hadno sinkers at all. In rockets, the propellants provide themass and the rocket engine burns the propellants anddirects the gases produced in the proper direction.Rockets can increase their thrust by burning pmpellantsfaster (mass) and by expelling the gases produced out .Athe engine faster (acceleration).

Throwing the block from the cart is an "action" and thecart's movement is a "neaction." The action and reac-tion are in opposite directions. Rockets produce action,or thrust, by the escape of gases out of their engines.The rocket's movement in the opposite direction is itsreaction.

ClassroomActivities

Pencil RocketsSubiecb SPacc FlightTopic RocketsDescription: Rockets, with pencils for bodies, arclaunched with a rubber-band-powered launch platform.

Materials and Tools:2 Pieces of wood 7.$101012.5 cm2 Cup hooksI Wooden spring clothespin1 Small wood savwI Screw eye2 Metal angle irons and screws4 Feet of heavy stringIron baling wireSeveral rubber bandsSeven) wooden pencilsSeveral pencil cap erasersCellophane or masking tapeHeavy paperSawWood filcDrill 0.5 cm diameterPliers

Figure 1

Procedure: Launch Platform1. Join the two pieces of wood as shown in the dia-

gram to form the launch platform. Use a metalangle iron on each side to strengthen the structure.

2. Screw in the cup hooks and screw eye into the woodin the places indicated in Figure 1.

3. Disassemble the clothespin, and file the "jaw" of onewood piece square as shown in Figure 2. Drill a holein this piece and two holes in the other piece asshown.

4. Drill a hole through the upright oiece of the launchplatform as shown and screw th 1,othespin to theupright piece so that the lower holes in the pin lineup with the hole in the upright. Reassemble theclothespin.

S.

to.

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Tie a big knot in one end of the string and feed itthrough thc clothespin as shown in Figure 1,through the upright piece of the platform, and thenthrough the screw eye. When the free end of thestring is pulled, the string won't slip out of the hole,and the clothespin will open. The clothespin hasbecome a rocket hold-down and release device.Loop four rubber bands together and loop theirends on the cup hooks. The launch platform is nowcomplete.

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Hole for wow

CI Figure 2\String

Rocket1. Take a short piece of baling wire and wrap it around

the eraser end of the pencil about 2.6 cm from theend. Use pliers to twist the wire tightly so that it"bites" into the wood a bit. Next, bend the twistedends into a hook as shown in Figure 3.

2. Take a sharp knife and cut a notch in the other endof the pencil as shown in Figure 3.

3. Cut out small paper rocket fins and tape them to thepencil just above the notch.

4. Place an eraser cap over thc upper end of the rocket.This blunts the nose to make the rocket safer if ithits something. The rocket is now complete.

Launching Pencil Rockets1. Choose a wide-open area to launch the rockets.2. Spread open the jaw of thc clothespin and place the

notched end of the rocket in the jaws. Close thejaws and gently pull the pencil upward to insure therocket is secure. lithe rocket does not fit, changethe shape of the notch slightly.

3. Pull the rubber bandsdown and k)op them over thewire hook. Be sure not to look down over therocket as you do this in case the rocket is prema-turely released.

4. Stand at the other end of the launcher and step onthe wood to provide additional support.

5. Make sure no one except yourself is standing next tothe launch pad. Count down from 10 and pull thestring. Step out of the way from the rocket as it fliesabout 22.79 meters up in the air, gracefully turnsupside down, and returns to Earth.

6. The rocket's terminal altitude can be adjusted byincreasing or decreasing the tension on the rubberhands.

2t4

DiscussionLike RDbert Goddard's first liquid-fizel rocket in 1926,the pencil rocket gets its upward thnist from its nosearea rather than its tail. Regardless, the rocket's fins stillprovide stability, guiding thc rocket upward for asmooth flight. If a steady wind is blowing during flight,the fins will steer the rocket toward the wind in a processcalled *weather cocking." Adam controls steer NASArockets during flight to prevent weatha cocking and toaim them on the right trajectory. Active controls includedlting nozzles and various forms of fins and vanes.

3 4

Figure 3

ClassroomActivities

Space Shuttle ModelSubject: Manned Space FlightTopic Model BuildingDescription: Constnict a model of the Space Shuttleorbiter from scnp materials.

Materials and Tools:2-Liter plastic soda pop bottle2 Egg canons6-oz Paper cupMasking tapeNewsPaPerGlue for papier-macheWhite ghteScissors

Procedures:wing shape

1. Cut two wings from the top of an cgg anon asshown in the diagrams. Tape the wings, as shown,to the bottle.

2. Cut out an "egg well" from the carton and tape tothc bottom of the cup to round off the flat surface.Tape the cup over the neck of the bottle. If theneck is too long to permit a good fit, take a sharpknife and trim it off a bit.

3. Cut out a vertical tail for the model from the eggcarton and tape it onto the bottle.

4. Cover the model with papier-mache. Narrow stripsof newspaper arc easiest to work with. Let thepapier-mache dry and add additional layers forstrength.

S. Cut three egg wells to make engines for the orbiter.Cover each well with papier-mache and let it dry.

6. When the body of the orbiter and the engines aredry, glue the engines to the tail end of the model asshown.

7. Paint the model and add decals, stars, and otherdecorations when dry.

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ClassroomActivities

Hero EngineSubject: Aeronautics, RocketryTopic WeightlessnessDesciption: Plans for constructing a working model ofaHero engine.Contributed by: Gregory Vogt, Oldahoma StateUniversity

Materials and Tools:Copper toilet tank floatThumb screw, 0.635 cmBrass tube, 30.48 cm, .476 in diameter

(from hobby shops)SolderFishing line and swivelIce pick or drillMetal filePropane Torch

Procedure1. File the middle of the brass tube until a notch is

produced. Do not file the tube in half.

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2. Using the ice pick or drill, bore two small holes onopposite sides of the float at its middle. The holesshould be just large enough to pass the tubestraight through the float.

3. With the tube positioned so that equal lengthsprotrude through the float, heat the contact pointsof the float and tube with the propane torch.Touch the cnd of the solder to the heated area sothat it melts and seals both joints.

4. Drill a sow access hole thtough the threadedconnector at the top of the float to make filling withsvater easier. (Water can enter the compItted enginethrough the tubes.)

5. Using the torch again, heat the protruding tubaabout 2.54 cm from each end. iffith pliers, carefullybend thc tube tips in opporite directions. Bendslowly so that the tubes do not crimp.

6. Drill a small hok through the flat pan of the thumbscrew for attaching the fish line and swivel. Twistthe thumb screw into the threaded connector of thefloat in Step 4 and attach the line and swivel.

Procedure: Using the Hero Engine

1. Place a few tablespoons of water into the float. Theprecise amount is not important. The float can befilled through the top if you drill an access hole orthrough the tubes by partially immersing the enginein a bowl of watcr with one tube submerged and theother out of the water.

2. Suspend the engine and heat it at its bottom withthe torch. In a minute or two, the engine shouldbegin spinning. Be carefid not to operate the enginetoo long because it probably will not be balancedand may wobble iolently. If it begins to wobble,remove the heat.

Caution: This demonstration should be done byadults only. Bye protection is advised. Be sureto confirm that the tubes arc not obstructed inany way before heating. Test them by blowingthrough one like a straw. If air flows out theother tube, the engine is safe to use.

Discussion:The Hero engine was invented by Hero (also calledHeron) of Alexandria sometime in the first century BC.His engine was a sphere with two L-shaped tubesconnected to a water-filled kettle heated from below.Steam produced by boiling water caused the sphere tospin. Remarkably, the Hero engine was considered anovelty and, reportedly, no attempt to harness its powerwas made at that time.

The principle behind the engine is simple. Steam fromthe boiling water inside the float pressurizes the float.The steam rapidly escapes through the tubes producingan action-reaction force causing the float to spin. Theaction-reaction principle of the Hero engine is the samethat is used to propel airplanes and rockets.

For an interesting follow-up demonstration, make asecond engine using a larger diameter brass tube.Compare the rotation of the two engines. The escapingsteam from the larger-tube engine will have a loweracceleration and will not spin as fast (New-tones secondlaw of motion).

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Resources1990 Model RocketryManufacturers Directory(Provided by the National Amociation of Rocketry,1311 Edgewood Drive, Dept. JR, Altoona, Wi 54720)

Estes Industries1295 H StreetPenrose, CO 81240Estes Industries is a leader in the model rocketindustry. Estes carries a complete line of modelrocket kits, motors, launchers, computer programs,and educational materials.

Apogee Components11111 Greenbrier RoadMinnetonka, MN 55343Apogee Components is your source for high technology competition parts. Apogee's componentsinclude phenolic body tubes, injection molded nosecones, and "waferglass" fin stock.

Spectrum Video ProductionsBox 3698Ontario, CA 91761Spectrum Video Productions offers a complete lineof NASA films in the VHS format. Film blies indudeMercury, Gemini, and Apollo missions, as well ascoverage of many of the Space Shuttle missions.

U.S. RocketsBox 1242Claremont, CA 91711U.S. Rockets offers kits and parts for the high-powerrocket enthusiast.

Model Aviation FuelsR.D. 6, Box 172Clarke Summit, PA 18411Model Aviation Pads offers interesting kits to themodel-making entilusiast. They offer body tubes,nose cones, and fin materials for the D-F-F-Gpowered models.

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MARS2240 9th StreetNational City, CA 92050MARS offers specialized tubes for the modeler.Their tubes are bright silver for easy tracking.

Space FrontiersBox 6488Newport News, VA 23606Space Frontiers magazine offers modelers interestingfacts on space and rocketry.

Bellevilk Wholesale Hobby1827 North Charles StreetBelleville, IL 62211Belleville Wholesale Hobby offers the modelerwholesale prices on the complete line of Estesproducts.

Flight Systems, Inc.9300 East 68th StreetRaytown, MO 64133Flight Srtems, Inc., offers a complete line of kitsand motors in D-G powered model rockets.

ACME RocketsBox 28283 Dept. AS27Tempe, AZ 85285If you are interested in building the kits of yesterday,then ACME Rockets is your resource for findingout-of-production model rocket kits.

East Coast Rockets408 Lark DriveMount Laurel, NJ 08054East Coast Rockets offers a line of kits for the A-Dclass model rocket flyer, as well as a line of parts andaccessories.

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