THIOKOL PROPULSION

BASICSROCKET

ROCKET BASICSA Guide to Solid Propellant Rocketry

For additional information about Thiokol Propulsion please contact:

Thiokol PropulsionP.O. Box 707Brigham City, Utah84302-0707

(435) 863-3511

THIOKOL PROPULSIONAn Alcoa Business

Background and Applications .......3

What Makes a Rocket Operate? .......8

Defining Performance .....10

Modern Rocket Designs .....11

Cases .....14

Ignition .....18

Nozzles .....20

Flight Direction Control .....25

Propellants .....29

INDEX

1

“For every action there is an equal and opposite reaction.”

Sir Isaac Newton The Third Law of Motion

This booklet was first produced by Thiokol nearly thirtyyears ago, in the early 1970s. In that time, many changesin materials and processes have occurred. These changeshave had a significant impact from initial design and testingto manufacturing the final product. New materials andprocesses allow Thiokol’s scientists and engineers greatercontrol - improving quality, reliability and performance.

Certain statements in this booklet are obviously dated. Less efficient designs give way to those that prove to bemore efficient. Through all of these changes, the basic principles of physics remain constant and the informationpresented here remains valid.

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Rockets have been around in one form oranother since at least the 13th century.Precisely who invented them is uncertain,although it is likely that an ancient Chinesescientist or philosopher happened upon theprinciple when he observed the violent exit ofexhaust products from a jar or tub in whichhe was mixing black powder. Or, he mayhave accidentally caused a vessel of somesort to fly into the air by lighting powderbeneath it.

And, it probably was a Chinese who devel-oped the first rocket, because they are gener-ally credited with the discovery of black pow-der and the first use of “fire arrows” in battleduring the siege of Kaifung-fu (Peiping) in1232 A.D.

In its most elementary form (the fireworksrocket used throughout the world in displaysand celebrations), the rocket consists of fourmain components.

1. A charge of propellant to develop the propulsive force, which is termed thrust.

2. A hollow tube or chamber within which thepropellant is burned.

3. An igniter with which to start combustionof the propellant.

4. A nozzle or outlet through which gases ofcombustion are exhausted.

Precisely what caused a rocket to be pro-pelled from one place to another probablymade little difference to the Chinese, Arabs,and others who produced them for use in bat-tle through the earliest years of recordedtime. Prior to about 1500 A.D., all black pow-der was rather slow burning and thereforesuitable for use as a rocket propellant.

Improvements in recipes used to manufac-ture powder after the invention of the gun,however, produced a powder that burned toofast for use in rockets. After this date, lazygun powder, i.e., gun powder whose rate ofburning has been reduced by the addition ofextra charcoal, was used. In 1591 theGerman author Johann Schmidlap describedrocket manufacture in great detail.

Background and Applications

HISTORY

Chamber

Propellant

Nozzle (Outlet)

Igniter

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4

One of the first written attempts to explainwhat causes the rocket to be propelledthrough the air was published in the year1540 by an Italian, Vanoccio Biringuccio, inhis book “De La Pirotechnia.” He attributedthe propulsive force to a “strong wind,” thedevelopment of which he described thus:

“One part of fire takes up as much space asten parts of air, and one part of air takes upthe space of ten parts of water, and one partof water as much as ten parts of earth. Nowsulphur is earth, consisting of the four ele-mentary principles, and when the sulphurconducts the fire into the driest part of thepowder, fire, and air increase. . . the otherelements also gird themselves for battle witheach other and the rage of battle is changedby their heat and moisture into a strongwind.”

Biringuccio’s description of the burning, gasexhausting phenomenon was correct enough,despite its nontechnical language. But itdidn’t explain why a strong wind, which wasblowing downward should cause the rocket torise upward. It was nearly a century andone-half later that Sir Isaac Newton, theEnglish mathematician, scientist, and philoso-pher, developed his Third Law of Motion toexplain what occurs.

The Third Law of Motion states that “for everyaction there is an equal and opposite reac-tion,” which in itself doesn’t really explain howa rocket can be propelled upward by exhaustof particles of matter through a pot pointing inthe opposite direction.

This phenomenon can be best illustrated byusing a balloon or other airtight vessel as anexample. What compressed air or othergases are contained in a balloon press equal-ly on all parts of the chamber, so the balloonremains at rest. If a leak should develop,however, some of the gas is allowed to

escape, thus creating an action - the move-ment of gas particles at a velocity greaterthan that of the surrounding air. The remain-ing gas within the balloon still pressesagainst the constraining parts of the wallequally; however, release of pressure throughthe leak causes a strong pressure unbalancedirectly opposite the leak. Consequently, thepressure opposite the leak is not counteract-ed, and the balloon moves away from it.

A simple illustration of Newton’s law and whatcauses the rocket to move upward as thegases are exhausted downward can bedrawn by imagining a grasshopper sitting ona piece of driftwood which is floating in aquiet lake.

Assume that the weight of both the grasshop-per and his perch are precisely the same. Ifthe grasshopper decides to “abandon ship”and jumps from the wood to the shore, a dis-tance of four inches, discounting any dragwhich might be imposed on the wood, thewood will move in the opposite direction fourinches. Because of their equal weight, boththe grasshopper and his erstwhile perch willmove at the same speed. Had both movedoutward along the balance of a scale, the bal-ance would not have moved at all because ofthe perfect balance of the two weights inopposite directions.

What Causes Rocket Movement?

Air pressureequal to allparts of theballoon.

Air escapes,creating anequal, butopposite reac-tion and theballoon movesaway.

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The movement of the rocket is very much likethat of the piece of driftwood away from thegrasshopper, the gases being exhaustedthrough the nozzle providing the power,which in our example was provided by thegrasshopper’s legs, causing the driftwood(rocket) to be kicked the opposite way.

From its first introduction as an artilleryweapon by the Chinese, the solid propellantrocket was used primarily as a weapon ofwar until the post World War II era.

Introduced to the Mideast and European areaby the Arabs, the war rocket was a primaryinstrument of both land and sea warfare atvarious times throughout history. It fell intodisuse for nearly two centuries following the

invention of handguns, mortars, siege gunsand cannons. Interest in war rockets was revived during theearly 1800’s and they were developed andused extensively during what is known to mil-itary historians as the “Congreve Period.”This interest was the direct result of theheavy damage suffered by British troops in

military action against the native ruler, HydarAli, Prince of Mysore, India. The British suf-fered severe losses from bombardment withwar rockets from a distance of 1 to 1 1⁄2miles, especially at Seringapatam, in 1792and 1799.

Colonel William Congreve read about the bat-tles and began his experiments with largeskyrockets in 1801 or 1802. He obtained theuse of the laboratories and firing ranges atthe Royal Laboratory at Woolwich for devel-opmental work. His rocket developmentswere first put to use in the siege of Boulogne,in 1805 by the British Army. Their useagainst Fort McHenry during the War of 1812was the event which inspired Francis ScottKey in writing the words “the rockets redglare, the bombs bursting in air. . . “ in theStar Spangled Banner.

Congreve’s rocket, with a long stick trailing toprovide stability, gave way during the mid-19th century to a stickless variety invented byBritish inventor William Hale. His contribution

to rocketry was the impartation of spin to sta-bilize the missile through insertion of threeslightly inclined metal vanes in the nozzle.

The rocket principle was adopted about 1821by Captain Scoresby of the whaling ship,Fane, for propelling harpoons, but was laterreplaced with the harpoon gun that providedgreatly improved accuracy. In areas whereextreme accuracy is not critical, however, therocket has served seamen well for propellinglines for rigging breeches, buoys, and forhurling small anchors ashore so that lifeboat-size vessels can negotiate a heavy surf.

An underwater war rocket patterned after apopular fireworks projectile of the early 18thcentury was developed following the acciden-tal demonstration of its explosive power in1734. During a celebration on the Thames,an underwater fireworks rocket rose up under

ACTION: Grasshopper jumps toshore

REACTION: Driftwood is pushedin opposite direction

APPLICATIONS: Ancient and Modern(as of 1960s and 1970s)

Congreve and His Developments

Other Applications and Experiments

the hull of a barge and the strong final pow-der charge exploded, causing the barge tosink and its passengers to be thrown into thewater.

The submarine mine, based on the underwa-ter explosive power principle, was developedby the Russians during the Crimean War andby Confederate engineer officers during theAmerican Civil War.

The first experiment with an underwater tor-pedo carrying an explosive warhead wasconducted by its inventor, U.S. Army MajorHunt, near Red Hook in New York Harbor in1862. Although never used as a naval orcoastal defense weapon by any country, therocket torpedo was the forerunner of thepresent-day naval torpedoes powered byelectrical motors or steam turbine engines.

The French were the only combatants to userockets operationally during World War I.Incendiary rockets were first used against aGerman Zeppelin (gas-filled heavier-than-aircraft) during an air- raid on the Bar-le-Ducrailroad yards in 1916.

Later, the French also used the first air-launched rockets, again, against zeppelins.Nieuport biplanes were outfitted with racks forfour rockets on each wing strut and their useagainst captive balloons was credited withforcing the Germans to discontinue use ofthis type of observation aircraft.

All major combatants in World War II, exceptthe Italians, developed war rockets. Some ofthe most widely used and best known were:the American Bazooka antitank missile,Calliope 4.5 inch battery rocket, and antisub-marine Projector Mark 10 (known as theHedgehog); the German Nebelwerfer 15 and21 centimeter high explosive and SchweresWurfgerat 21 centimeter incendiary rockets;and the British 3 and 5 inch rockets.

A German development, the V-2 rocket bombwith which they bombarded London, was themodel upon which much of the early postwardevelopment of intermediate and interconti-nental range ballistic missiles was based byboth the American and Russian governments.

The nonstorable nature and extended fuelingtime of most liquid fueled rockets, however,led researchers to look for a method of man-ufacturing propellant for solid fueled missileswhich would be storable, instantly ready forlaunch, and provide more constant thrustthan that produced by granular “gunpowder”types of propellant which had been used tothat time.

America’s first synthetic rubber, a polysulfidebase material discovered by Dr. J.C. Patrick,in 1928 provided the basis for modern solidpropellant development. Used as a binder inwhich metallic fuel and chemicals oxidizingagents are suspended, the viscous materialcan be cured to a firm rubbery consistencyafter it has been cast into the rocket motorcase. Demonstration by Thiokol ChemicalCorporation in 1949 of the principle of inter-nally burning material bonded to the caseproved the applicability of solid propellants inlarge rocket motors. This bonding principlehas since been applied successfully to suchoperational ballistic and guided missile sys-

World War I

World War II

Postwar Developments

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tems such as Minuteman, Sergeant, Falcon,Genie, Polaris and Poseidon. Smaller rocketsalso have been used extensively in scientificresearch programs aimed at uncovering thesecrets of the Earth’s upper atmosphere andouter space.

Unlike systems such as the modern turbojet,pulse jet and ducted aircraft engines, thesolid fueled rocket carries its oxygen supplyaboard as an integral part of the propellantand consequently can operate efficiently inthe rarefied air or the stratosphere andbeyond and in the airless vacuum of space.Because of this fact, small solid rocketmotors have been used with great reliabilityto decelerate spacecraft returning from tripsto the Moon and for other space applications.

Solid motors measuring as much as 260inches in diameter and developing as muchas 3.5 million pounds of thrust were devel-oped for possible use as space boosters dur-ing the mid-1960’s. Motors of 120 inches indiameter and approximately 125 feet longhave served as zero stage boosters for theTitan IIIC and 65 inch diameter motors for the

Delta launch vehicles to provide thrust aug-mentation needed to loft heavy payloads assuch as communications and surveillancesatellites into orbit.

Large solid propellant boosters up to 156inches in diameter are being considered foruse as reusable booster stages for the SpaceShuttle which is to be developed in 1975. As man moves out into the farther reaches ofouter space, controllable solid propellantrocket motors, which can be called upon asneeded to provide thrust for mid-course cor-rections will play an increasing role.

Update: Thiokol has been supplying solidpropellant rocket motors for NASA’s spaceprogram since its inception. Thiokol statictested a 156-in, 13 feet diameter, solid rocketmotor in 1964. In 1974 Thiokol was awardedthe contract to develop the twin solid propel-lant booster motors for NASA’s Space Shuttlefleet. Today, two 126 feet long, 12 feet indiameter, solid rocket motors filled with onemillion pounds of propellant each fly on everySpace Shuttle flight.

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What Makes a Rocket Operate?

Basically, a rocket motor is the simplest formof energy conversion device. Matter in thesolid or liquid state is burned, producing hotgases. The gases are then accumulatedwithin the combustion chamber until enoughpressure builds up to force a part of them outan exhaust port. The movement of gasesthrough the exhaust port results in the con-version of heat energy into kinetic energy, orenergy of motion.

In the modern solid propellant rocket motor,finely ground metals or chemicals that canserve as fuels are mixed with other chemi-cals, which contain oxygen (oxidizers). Theproper proportions of fuel and oxidizer arethen suspended in a binder material that canbe cured to a solid state much like the rubberin a tire.

The most simple form of a rocket motor con-sists of a pressure combustion chamber con-taining the fuel (propellant), and a port (noz-zle) through which the hot gases areexhausted. Some means of igniting themotor must be provided, either as a part ofthe motor or as an external ancillary device.The laws of motion developed in the 17th

century by Sir Isaac Newton and the lawsdescribing gaseous behavior developed byRobert Boyle and Jacques Charles describethe underlying principles which govern rocketmotor propulsion.

Newton’s first law, that of inertia states that:in the absence of contrary forces, the speedand direction of an object’s movement willremain constant. Thus, if a rock could bethrown with enough force to overcome theeffects of gravity and wind resistance, itwould continue to move in a straight line atthe same speed as it has attained at releaseby the thrower. Likewise, until some force isapplied, the object will remain at rest wherelast placed or originally formed.

As applied to rocketry, the law of inertia tellsus that the force generated by the thrust ofthe rocket motor must be great enough to liftthe missile’s total weight from the launch site,or it will not fly. The forces of gravity and airresistance, among others, must be taken intoaccount in determining both the requiredthrust and computing its trajectory (flightpath) to the ultimate target.

Energy of Heat => Movement of Gases => Energy of Motion

Propellant

Combustion Chamber Nozzle

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The second law defines acceleration as: theratio of the applied force and the inertial (rest)mass of the object. Thus, a body that is sub-ject to forces moves at a speed which is pro-portional to the amount of force applied. Asan example, a golf ball moves away from thetee much faster when struck with a club thanwhen kicked because much more force canbe applied with a swinging club than with thetoe of a shoe.

In rocketry, the greater the amount of thrustdeveloped by the rocket motor in relation tothe mass of the total vehicle, the faster themissile will move through the air. If enoughthrust is developed, the speed can be built up(usually in stages) until the vehicle canescape the pull of the Earth’s gravity andmove into outer space.

Newton’s third law, the law of action andreaction, states that: for every force exertedby one mass on another, there is an equaland opposite reaction exerted by the second

mass on the first. In the rocket, the expulsionof combustion products (gases) through thenozzle produces a reactive force on the rock-et in the opposite direction which causes it tobe propelled.

Boyle’s law states that reducing the volumeof a container within which a gas is heldcauses its pressure to increase in direct pro-portion. An example is the old fashion tirepump. If air is prevented from escaping andthe plunger is pushed down, as the volume ofthe chamber grows smaller, the pressurerises so that a plug inserted into the holethrough which it normally is exhausted willeventually be forced out. An additional ele-ment of the law is that when gas is com-pressed, its temperature decreases.

Charles noted that if the pressure of a fixedweight of gas is kept constant while the heatis increased, the gas expands by an amountproportional to the change in temperature.

No force applied to plunger, low air pressurein chamber.

Force of plunger decreases air volume,pressure increases.

Volume of chamber decreases,plug is eventually forced out.

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Solid propellant motor designers employ anumber of important parameters (a variableor an arbitrary constant) to define the per-formance of the propellants used and themotors which power rocket propelled vehi-cles. Since most of these terms will be usedthroughout the discussions which follow, briefdefinitions of the more common ones are pre-sented below.

The first and most common term used inrocketry is thrust, which is a measure of thetotal force delivered by a rocket motor foreach second of operation. Essentially, thrustis the product of mass times acceleration. In actual calculations, of course, gravity, pres-sure of the surrounding medium, and otherconsiderations must be taken into account.

After the thrust developed by the rocket hasbeen determined, this value is used to com-pute another important parameter, specificimpulse (Isp), which provides a comparativeindex to measure the number of pounds ofthrust each pound of propellant will produce.Expressed as pound force seconds perpound mass, Isp is referred to in the rocketengineer’s shorthand language as seconds of impulse.

To compute specific impulse, the thrust isdivided by the mass flow, or weight of the gasflowing through the nozzle throat per second.

Another important term is total impulse, whichis abbreviated It. Computing this value is asimple matter of multiplying the motor’s thrustby its period of operation in seconds.

Just as specific impulse is the basic chemicalparameter for propellant, so is mass fractiona basic parameter for motor design. A highmass fraction indicates that more of themotor mass is propellant than is involved inthe case, nozzle, and other componentswhich do not produce thrust. A high massfraction can be achieved by optimizing motorand structural design for minimum weight andor by using more dense propellants, whichthen require smaller combustion chambers.The mass fraction may be defined as theratio of the propellant mass to the initial ortotal mass of the motor when completelyready for operation.

Defining Performance

The thrust (total force)of a football player ismuch like that of arocket. The forcegenerated is a prod-uct of player ’s weight(mass) times player’srate of acceleration.

Solid fuel rocket motor with a lowmass fraction.

Solid fuel rocket motor with a highmass fraction.

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The rocket motor in its simplest form consistsof a combustion chamber which serves as apressure vessel, a port through which the hotgases can be exhausted, and an igniter to ini-tiate burning. The modern solid fuel rocketrequires a number of specialized terms,defined below (size can vary from a fewinches in diameter to more than 21 feet). Motor Case- Metal or fiberglass reinforced plastic vessel inwhich propellant is cast, and which serves asthe combustion-pressure chamber.

Skirt -A motor case wall extension provided as apoint for joining motors in multistage rocketvehicles.

Insulation -Material used to protect motor componentsfrom the extreme heat of combustion devel-oped during the motor’s operation, and fromheat produced by friction on the case exteriorduring its movement through air.

Propellant Release Boot -A sheet of rubber installed at the aft end ofthe propellant grain during casting to permitshrinkage of the cured propellant grain as itcools and thus prevent strain (deformation)with consequent cracking.

Grain -The propellant charge. In composite solidpropellants it consists of a fuel (usually metal-lic) and a chemical oxidizing agent (a sub-stance which, under suitable conditions, com-bines with fuel elements to release the maxi-mum possible energy) blended together andmixed into a binder, or polymer. The mixtureis cast (poured) into the motor case andcured to a hard rubbery state at an elevated

temperature. In double-base propellants, itconsists generally of cotton (cellulose) com-bined with nitric acid to form nitrocellulose(guncotton), which in turn is combined withnitroglycerin, another fuel oxidizer. In the dou-ble-base propellant, the nitrocellulose servesas the binder, and the nitroglycerin causes itto gel.

Inhibitor -A coating of noncombustible material appliedor attached to specified areas of a solid pro-pellant grain to prevent combustion takingplace on that surface, to control the evolutionof gases and prevent combustion in specificareas of the grain.

Gas Port Area - (gas cavity) A cavity formed in the casting processthrough the center of the propellant grain toprovide an increase in the surface area overwhich the flames can burn to produce hotgases and channel them to the nozzle. Thecavities may be formed in a variety of shapesto provide larger or smaller burning areasand thus more or less hot gases as motoroperation progresses, depending upon theamount of thrust which is desired at varioustimes in the operation.

In some motors, no central perforation isformed and only the end of the grain burns.By maintaining constant burning area andthus constant thrust, this type of grain config-uration provides a neutral (level) thrust-timehistory.

Central perforations can be designed so thatthe burning area, and therefore the gas evo-lution rate, the chamber pressure, and thethrust increase with burning time. This typeof rocket motor operational history is referredto as providing progressive burning charac-teristics. If the grain design produced adecrease in these values, the rocket is said

Modern Rocket Designs(modern as of 1960s and 1970s)

Definition of Terms

11

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to have regressive burning characteristics.

Some idea of the wide variety of grain config-urations, especially with regard to gas portshapes, is provided in the illustration below.

The cross sectional configurations showngenerally would be expected to producethrust-time histories of the type indicated bythe letter designations below each drawing.However, many factors must be consideredand such parameters as the length/diameterratio of the motor can affect operation enoughto provide a neutral trace from any of theseconfigurations.

Nozzle -An exhaust port in the motor case aft endshaped to control the flow of gases and toconvert the chemical energy released in com-bustion into kinetic energy, or energy ofmotion.

Nozzle Throat -The smallest internal diameter of the nozzleassembly. In the convergent-divergentDeLaval type of nozzle normally used, theinlet converges to the throat, then flares outin the nozzle expansion area.

Approach or Convergent Section -That portion of the nozzle upstream of thethroat in which the flow area is reduced andvolume of the exhaust gases is reduced asthey are channeled to the throat.

Throat -The point at which exhaust gases are passedthrough the minimum flow area and reach thespeed of sound before expanding in the exit

cone to supersonic speed.

Exit Cone -Also called the expansion section, this part ofthe nozzle may be contoured or conical, andis designed to provide rapid but controlledexpansion of the exhaust gases passingthrough the throat on their way to the exitplane.

Exit Plane -The extreme edge of the exit cone, where thestatic pressure of the gases should beapproximately equal to the atmosphere pres-sure at the design operating altitude.

Raceway -A metal or plastic covering inside of which thecontrol and electrical system wiring orhydraulic leads are placed.

Igniter -A device to initiate propellant burning, usuallyconsisting of a small pyrotechnic devicewhich produces sufficient heat and flame tocause the surface of the propellant grain toignite.

Thrust Termination Port -A port provided in the rocket motor case tovent combustion gases so that rocket opera-tion can be terminated. The port usually isprovided for in the head end of the motor sothat gas flow is effectively diverted from thenozzle. The port is formed by firing a shapedcharge of explosive placed against the out-side of the forward end of the motor.Designs of the many components that makeup the modern solid propellant rocket motorrequire consideration of many factors, includ-ing: size and weight of the total vehicle, themission to be accomplished, materials avail-able, cost limitations, and the various state-of-the-art techniques to be employed.

P (progressive), N (neutral) , R (regressive)P N R

Exit Cone

ThroatIgniter

Thrust Termination Port

Raceway Exit Plane

Nozzle

Convergent Section

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This next section discusses materials anddesign techniques used in developing com-ponents for propulsion systems for ballisticand guided missiles powered by solid propel-lant motors.

The case of a solid propellant rocket motormust serve two purposes:

1. A container for the solid propellant fuel.

2. A vented pressure chamber in which thefuel is burned to provide forward thrust whenexpanded into the exit cone of the nozzle.

Because of this dual role, development of anoptimum case design involves providing forthe interplay of the thermodynamic (chemicalheat and energy) and mechanical forces(bending, pressure, etc.), which will act uponit during operation. The designer also mustconsider factors affecting overall performanceof the vehicle, compatibility with other vehiclesystems and components and cost. Depending upon the size and complexity ofthe vehicle for which the motor is beingdeveloped, the case may or may not incorpo-rate all of the features illustrated.

Cases

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The external configuration (shape, form) ofthe case usually is selected as the optimum(best) envelope for maximizing performanceor minimizing cost when all independent vehi-cle and case design variables have beenevaluated. The final case configuration maybe determined, however, on the basis of:payload, limitations imposed by interstageconnecting structures, available tooling,transportation size or weight limitations, han-dling equipment, launch facilities, use of aspecified grain design, or auxiliary equip-ment. The size and shape constraints forsmall motors performing specialized functionson larger vehicles (such as retro fire duty onApollo command modules) usually aredefined by the vehicle system and imposeddirectly on the small motor configuration.

In selecting the materials from which themotor case will be constructed, a thoroughevaluation must be conducted of the types of loads to which it will be subjected duringmanufacture, handling, storage, and theoperation of the ballistic or guided missile,sounding rocket vehicle, spacecraft, artificialsatellite, or other vehicle of which it will be a part.

Representative types of loads may include:

1. Internal pressure from hot gases producedby the motor during propellant combustion.

2. Thrust produced by the motor during oper-ation, as well as the load produced by lowerstage motors in a multiple stage missile.

3. Buckling, aerodynamic bending, andground handling loads developed during handling, storage, and flight.

4. The effects on material strengths of heatfrom both inside and outside the motor,should there be an insulation failure oruneven burning.

Today’s (1960s, 1970s) rocket designer has avariety of materials from which to choose indeveloping the optimum design to meet mis-sion objectives. Most commonly used arethe ferrous alloys, including conventionalquench and temper (hardened by heatingthen plunging into liquid to cool suddenly)steels and nickel precipitation-hardening alloysteels, nonferrous titanium alloys, aluminumalloys, and fiberglass reinforced plastics.

Cylindrical Conical

Spherical EllipticalOblate Sphereoid

15

The most efficient material to withstand theoverall loading conditions at the highest tem-perature expected during the missile’s usefullife is selected. Critical case loading may beimposed by internal pressure or bucklingforces, depending upon mission applicationand the location of the case in the vehicle. In single stage vehicles and the upper stagesof multistage vehicles, internal pressure usu-ally imposes the critical loading condition.Intermediate stages can be critical in internalpressure, buckling resistance, or stiffness.First stages usually are pressure critical;however, buckling and bending loads arehigh and may sometimes be critical.

The design engineer must consider how thewelding, forming, or shaping operations towhich materials will be subjected will affecttheir tensile strength (resistance to breakingwhen subjected to stretching force), fracturetoughness (resistance to complete separationby cracking), and other mechanical and structural properties.

The size of the motor case may place arestriction on use of certain types of metalbecause heat facilities are not available tohandle the extremely large sizes. Availabilityof odd or nonstandard sheet sizes in certaintypes of metal may influence the number andlocation of welds required to fabricate thecase.

Several sources of heat may have an effecton the motor, including: aerodynamic heatingcaused by air friction along the walls andattachments; conductive heating from propel-lant combustion; both radiant and conductiveheating during handling, assembly, andcheckout on the launch pad and during flight;and sterilization for planetary exploration, ifpart of a space vehicle.

All possible harmful environments and chemi-cals which may be encountered during thelife of the motor case must be considered,beginning with production of structuralmaterials and continuing through final serv-

ice use. These may include corrosive (grad-ual chemical alteration at normal tempera-tures) influences, electromagnetic and ele-mentary particle radiation, meteoroid impact,and other hazardous environments.

Conventional quench and temper steels havebeen used extensively as structural materialsfor cases, and a great deal of information isavailable on their properties and reaction tovarious fabrication processes.

Alloys containing nickel and cobalt, and other steels which harden when cold-rolled,rubbed, or struck with a hammer require special care in fabrication to prevent reduc-tion of the ultimate strength (the strength atwhich destructive failure will occur), but theirspecial properties can prove useful for special applications.

Fairly recent developments (1960s and1970s) are the maraging (age hardening)steels containing various percentages of steelwhich develop extremely high strengths whenaged at 850 to 950 F for a predeterminedtime. This aging results in formation of amolecular structure within the steel whichgives it good forming and forging characteris-tics; good stability during heat treatment;good retention of carbon, which imparts sur-face hardiness.

Because of these properties, the maragingsteels are the most likely candidates for usein production of large motors for space boost-er and Space Shuttle applications.

Metal Considerations

Metals Commonly Used

Other Harmful Environments

Thermal Environmental Considerations

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Titanium alloys have been used mainlybecause they provide extremely high strengthto weight ratios, which makes possibleincreases in vehicle payload carrying or operational range capabilities; however,titanium alloys provide less resistance tobuckling. Although not generally susceptibleto corrosion (gradual chemical alteration atnormal temperatures) they may developstress corrosion (loss of strength, hardness,and other mechanical properties) under certain conditions.

Although aluminum alloys are not generallyused in motor cases for space vehicles, theycan be used to advantage for small casesand cases where corrosion may be a specificdesign problem. Stress corrosion may be aprime consideration when aluminum alloysare used as case structural material.

In their continuing search for ways toincrease the mass fraction of the total vehicle(ratio of propellant mass to initial total mass)discussed earlier, rocket design engineersturned their attention to reinforced plasticstructures in the mid-1950’s. Today (1960sand 1970s), the best known missile systemusing motors employing this material in allstages is the Poseidon Fleet Missile. Thethird stage motor of the Minuteman ICBM

also uses a fiberglass reinforced plastic case.Because of the importance of physical andchemical properties for determining whatstrength materials must be used, how theywill react to atmospheric and imposed envi-ronments, etc., engineers and chemists whodesign rocket motors naturally would prefer tohave absolutely accurate values with which towork. Experience has shown them, however,that they may use simplified assumptionswhich, although they do not describe condi-tions precisely, provide solutions which give

an estimate of the motor’s operational char-acteristics which is accurate within three percent or less.

In developing a preliminary design, the motoris divided into two sections, the chamber(combustion) and the nozzle (expansion),and conditions are assumed to have stabi-lized to the operating level following ignition.

The first of the assumptions is that combus-tion takes place within an adiabatic chamber(the rocket motor case); for example, there isneither gain nor loss of heat. In the solid pro-pellant motor, the insulating effect of theunburned propellant in the chamber reducesthe loss of heat to the chamber walls; conse-quently, the assumption is justifiable.

The chamber also is assumed to be isobaric;in other words, pressure is assumed to holdconstant, with no changes occurring through-out operation. Under, “steady state” operat-ing conditions, which means that all factorsare in balance, very minor fluctuations inpressure are measurable, so again theassumption is justifiable.

The combustion gases in the chamber areassumed to act as “ideal gases;” that is, theybehave exactly in accordance with gas laws(presented in the section on “What Makes theRocket Operate?”), and any condensedphases (liquid portions) produced areassumed to have negligible volume.

The final chamber assumption is that thevelocity of the gas stream in the chamber isnegligible. Although there is some slightmovement of exhaust gases in a solid propel-lant combustion chamber due to preliminaryacceleration as the gases are forced to thenozzle entrance, they move quite slowly, themajor driving force being compression result-ing from the heat and continued addition ofnew combustion gases.

Reinforced Plastics

Design Techniques

17

Nearly everyone has watched television coverage of the countdown for America’sApollo Moon shots. Most of us probablyhave been amazed at how seemingly long ittakes for the Saturn vehicle to lift off from thepad after the countdown has reached zeroand the rocket engines have ignited. A ballof flame and smoke forms almost instantlyafter we hear the word ignition. Then timeseems to stand still while we wait for theliftoff. In reality, no more than a few secondselapse before sufficient thrust builds up tostart the rocket on its flight.

To the casual observer, igniting the solid propellant motor may appear as simple asflipping a switch to turn on the electric lightsat home. To the chemist, however, these are an almost limitless series of actions andinteractions with which he or she mustbe concerned.

In general terms this is what takes place after the ignition switch is activated. The pro-pellant is ignited using a heat or fire produc-ing instrument; hot gases are produced; thegases are exhausted through the nozzle aspressure builds in the combustion chamber;and thrust is produced to propel the rocket.In this way, the chemical energy of combus-tion is converted to kinetic energy, or theenergy of motion.

Most solid propellant rocket motors are initiated with a pyrotechnic device (squib)which consists of a cartridge containing combustible powders in contact with an electrical resistance wire. Initiation ofburning on the squib develops extremely hotflames, which spread to the solid propellantgrain or pyrotechnic charge of the main ignit-

er. Igniters are usually identified by theirshape, case type, or type of active materialthey contain.

The four main types of igniters are:

1. BasketConstructed of woven wire, perforated sheetmetal, or plastic in which the active materialis contained.

2. JellyrollConsisting of a pyrotechnic coating rein-forced by a flexible sheet of plastic or clothand rolled into a tube of several layers.

3. Can typeUsually consists of a metal, plastic, or papercontainer which is filled with an active pow-dered material.

4. PyrogenDefines an ignition device in which combus-tion of the igniter material takes place withina closed chamber and the combustion prod-ucts are exhausted into the rocket motor. (Asmall rocket motor is used to ignite a largerocket motor).

Solid propellants also can be ignited by afluid (hypergolic) which reacts with the solidpropellant ingredients to cause spontaneouscombustion.

Ignition

18

Produceshot gases

Propellantignited

Ignitionactivated

THRUST

Exhaustedunderpressure

Fire

Ignition Sequence

Typical Pyrogen Igniter

Propellant

PelletsSquib

S&A Device

Hot Exhaust

Safety and armingdevice activates squib

Squib then ignitesexplosive tablets

Flame frompellets ignitespropellant

Hot exhaust fromigniter dischargesinto motor andstarts burn

19

The hot, energetic gases which are releasedby combustion of solid rocket propellant represent a reservoir of potential energywhich must be converted to kinetic (motion)energy if it is to accomplish the work of propelling the rocket from the launch pad toits intended destination. Unless the energy isefficiently applied to the work, much or all ofits potential will be wasted as it dissipatesinto the surrounding atmosphere in the formof heat.

The nozzle is the device by which the internalenergy of the exhaust gases is converted intokinetic energy, thereby producing thrust(force). For any given propellant system, thenozzle acts as a metering device to controlthe rate of gas flow, thus creating a pre-dictable amount of thrust over a programmedtime period. The energy conversion isaccomplished by causing the gas moleculesto accelerate to extremely high velocities asthey leave the motor.

Solid propellant combustion produces a massof hot gases equal to the mass of the solidmaterial being burned. These hot gases fillthe combustion chamber soon after ignition,and the pressure quickly reaches the operat-ing (steady state) level, at which a constant

amount is produced. Just how the nozzlecan cause the gas particles to accelerate byapparently restricting their free flow by provid-ing a smaller exit cross sectional area thanthat of the chamber is not readily apparent.

What actually happens is this: The hot gasesdeveloped by propellant combustion are par-tially entrapped by the motor aft closure andthe convergent section of the nozzle, thuscausing them to compress. A curious fact ofgas behavior is that under compression, if anescape route is provided, pressure drops aspotential (pressure) energy is converted tokinetic energy (velocity).

Rocket engineers measure the speed atwhich gas particles move in relation to the“local speed of sound” (sonic velocity). Theterm “local speed of sound” does not refer tothe 760 mph value at sea level that we asso-ciate with sonic velocity in aircraft. Instead, it refers to the speed at which sound travelsthrough combustion gases under the temper-ature and pressure conditions existing at aspecific point in the gases. At a critical pres-sure, gas flow escapes through the nozzlethroat at acoustic (sonic) velocity, and a bal-ance is reached between gas propagationand exhaust.

NOZZLES

Exit area restrictedby aft closure andnozzle, gases escapeat supersonic velocity

Exit area is restrictedby aft closure, gasesat acoustic velocity.

No restriction of the exitarea, gases escape atsubsonic velocity

20

Since the gases within the rocket motor’scombustion chamber move at a speed lessthan the local acoustic velocity, they are saidto be flowing subsonically. It is the functionof the nozzle to cause them to accelerate ata controlled expansion so that the maximumpossible kinetic energy is developed from thepotential energy produced by combustion.

Approximately 65 to 75 percent of total vehi-cle thrust is developed by acceleration of thecombustion products to sonic velocity at thenozzle throat; the remainder is developed byacceleration to supersonic velocities as thegases expand in the divergent (expansioncone) section.

Thus, without the divergent nozzle section tocontrol and direct the expansion of gasesalready moving at sonic velocity, one-quarterto one-third of the propulsive thrust generatedmight be wasted as the particles dispersedinto the surrounding medium.

All nozzles currently used on solid propellantrocket motors are of the converging-diverging(DeLaval) type, which is made in two basic

configurations, the external and submerged.

A nozzle type known as the “spike” configura-tion has been under development for sometime, but to date has found application mainlyon experimental motors as a means of controlling thrust magnitude and or chamberpressure to provide stop restart capability,and for missile applications requiring controlof the amounts of thrust to be delivered at different times during the rocket’s flight. Theexternal configuration is the basic classicalconvergent-divergent design which is mount-ed entirely external to the motor. In the submerged configuration, the nozzle entry,throat, and part or all of the expansion core is cantilevered or extended into the motorchamber.

The submerged design is the more complexof the two, because: both the inner and outersurfaces of the submerged portion areexposed to the hot gases; and the sub-merged section must structurally withstandexternal pressure forces in addition to theforces developed by gas flow along its inner surfaces.

External Submerged Spike

Nozzle Types

21

In addition, there are two basic exit cone con-figurations, conical and contoured. The pur-pose of contouring the cone is to direct theflow of the exhaust stream so that its forcewill be concentrated as directly as possiblealong the longitudinal centerline of the noz-zle, thus minimizing flare or divergence of theexhaust stream and concentrating thrust asmuch as possible.

The materials used in the fabrication of solidpropellant rocket motor nozzles can be divided generally into five classes: structuralmaterials; adhesives; sealants and greases;thermal insulators; and ablative or erodiblematerials.

Structural materials are applied generallyaccording to the maximum operating temper-ature to which they will be exposed.

Up to 500 °F, the most used materials arealuminum alloys and fiberglass-resin compos-ites, both of which have high-strength-to-weight ratios, are light in weight, easily fabri-cated, have good corrosion resistance, andare reasonable in cost. High strength steelsare used when major considerations are highstrength in thin sections, or operation at thehigher end of the temperature range.

Between 500° and 1,900 °F, the higher tem-perature iron, iron-nickel, nickel, cobalt, andiron-nickel-cobalt chromium base super alloysare used.

Above 1,900°F, alloys of refractory metals(capable of resisting high heat without crack-ing, melting or crumbling) such as molybde-num, columbium, tantalum, and tungsten pro-vide high strength to approximately 4,500°F.

Above 4,500 °F, about the only structuralmaterials available are graphite and pyrolyticgraphite.

Adhesives play an important role in joiningthe various dissimilar materials of which thenozzle is made to assure the soundness ofjoints and for sealing spaces between materi-als to prevent gas leakage.

Sealants such as zinc chromate putty and sil-icone grease may be used to plug cracks. O-rings of rubber are used where more posi-tive seals are required.

Conical Exit Cone

Contoured Exit Cone

Nozzle Materials

22

Thermal insulators are used between thenozzle ablative (erodible) and structuralmaterials for protection of the latter from thehigh temperature exhaust gases. They mayalso be used externally for protection fromaerodynamic heating. The most commonnozzle insulating materials are composites ofasbestos fibers and phenolic resins, andcoatings of ceramic refractory materials, principally zirconium dioxide.

Ablative materials are used to withstand acombination of erosion, fusion, (liquefying ormelting together by heat), corrosion, ordecomposition, leading to progressive degradation (decomposition by stages), andor loss of material as a direct consequence of exposure to the hot propellant gas flow inthe nozzle.

Erosion resistant materials are used in thenozzle throat, since it is subjected to the mostsevere environment. The most commonmaterials are graphite and pyrolytic graphite.The stronger and more heat resistant pyrolyt-ic graphite has high strength, high thermalconductivity (capacity to transmit heat), andlow expansion.

If graphite or pyrolytic graphite cannot beused advantageously because of nozzle sizeor the extremely erosive nature of theexhaust products, the nozzle throat insertmay be a refractory metal or an alloy, such as forged, arc cast tungsten, or tungsten-molybdenum alloy. If high erosion can betolerated as in extremely large nozzles withlarge throat areas, reinforced plastics may beused, such as refractory fiber reinforced plastic composite materials.

More erosion may be tolerated in the exitcone, since it does not exercise control overgas flow, so fiberglass reinforced phenolicresins containing silica, graphite, or carbon to increase erosion resistance are used inthis area.

Composite materials are under developmentwhich may be used to fabricate the wholenozzle for certain applications. Known vari-ously as carbon-carbon composites, fibrousgraphite, prepyrolyzed composites, graphite-graphite composites, graphite composites,and prechars, they will be formed primarily bypressure molding. All are characterized bytwo features: 1) all have been processed at a

Cross Section of a Typical Nozzle

Erosion Resistant Material

Sealants

Ablative Materials

Adhesives

O-Ring Seal

Composite Materials

Thermal Insulators

Structual Shell

23

temperature high enough so that all charablematerial has charred and converted to carbonor graphite; and 2) they contain a graphite orcarbon fabric, fiber, filament, or felt reinforce-ment bonded together, with carbon orgraphite as the bonding material.

Several fabrication methods are employed inthe manufacture of nozzles, the types andnumber depending primarily upon the typesof reinforced plastic components that are tobe used as ablative liners and ceramics ormetallic components that are to be employedin those areas such as the throat where littleor no erosion can be permitted to occur.

Ablative materials used in the exit cone usually are fabricated by wrapping fiberglasstape over a metal form called a mandrel, sothat the grain of the finished unit is orientedto provide the required erosion resistance.Designers have found that grain directionalorientation has an effect on the amount of thematerial that will be eroded away by theexhaust gases during rocket operation. After winding, the tape is cured, machinedas necessary, and assembled with other

components using adhesives and sealants as required.

Throat materials may be forged, cast, orpressure molded.

Structural shells of metal are generallymachined from cast billets of aluminum orother alloys.

Nozzle Fabrication

24

Unless some means of guidance is provided,the flight of the rocket is subject to unpro-grammed variations in direction that can becaused by wind currents or other naturalforces. Without guidance, the rocket mustfollow a ballistic flight path or trajectory; i.e.,its path will be similar to that of a bullet, rock,or other hurled object.

A rocket’s flight path must be planned inthree dimensions, length, width, and height,so corrective maneuvers must be possible inall three planes. The simplest way of visual-izing the path flown by a rocket is to think ofits being propelled down the center of a tube.Each time the rocket’s flight deviates from thebounds of its allotted air space and touchesthe sides of the tube, a correction is made tomove it back toward the tube’s center, nomatter whether it must be guided up or down,left to right.

Maintaining alignment is accomplished bydirectional corrections in the pitch plane forup or down and in the yaw plane for left orright. Correct alignment of these axes inrelation to the reference plane on which theflight path is based, is accomplished by rotat-ing the vehicle clockwise or counterclockwise

around its longitudinal (nose to tail) centerlineusing the roll control.

Most air-to-air missiles, air-to-ground mis-siles, and ground-to-air missiles are guidedthrough the use of movable airfoils, usuallysimilar to the control surfaces (ailerons) onairplane wings and tails. Movable airfoils canbe used for many rocket propelled vehiclesbecause they operate within the Earth’satmosphere and entirely, or very largely, atspeeds at which such control surfaces areeffective. Certain types of rocket vehicles,however, cannot depend entirely upon airfoilsfor control, usually because they operate inthe extreme upper atmosphere or in space,where airfoils are not effective, or becausetheir launching speed is too low for airfoils tobe effective. Most ballistic missiles and spacevehicles fall into this latter category.

To make directional or attitude correctionswithout the use of airfoils, the thrust deliveredby the propulsion system must be deflected(vectored) to cause the rocket to pivot aroundits center of mass (center of gravity) like alever or fulcrum around its pivot point.

Flight Direction Control

Types of Attitude Control Systems

Flight directional control for up ordown is referred to as “pitch”

Flight directionalcontrol for left ofright is referredto as “yaw”

25

Thrust vector control (TVC) systems can bedivided generally into those which change the direction of thrust flow within the fixednozzle and those which change it with themovable nozzle.

A primary approach to rocket motor TVC, andone of the more highly developed because of its long usage, is the diversion of exhaustgas through nozzle movement. The morecommon movable configurations are the single omniaxial (all directional) nozzle, multiple omniaxial nozzles, and multiplehinged nozzles.

Movable nozzles provide directional controlby changing the direction of motor thrust,usually through movement of the exit cone ornozzle throat, or both. Movable nozzles movein a single plane (Stage I Minuteman) or multiple planes (Poseidon).

Single plane nozzles usually are used inseries of four so that pitch, yaw, and roll control can be achieved. When pitch or yawmotion is required, both pitch or yaw nozzlesare moved in the same direction. Roll control usually is achieved with themotion of one nozzle, or with two acting inopposite directions.

Omniaxis nozzles are capable of moving inany pitch or yaw plane with the use of specialload carrying and sealing mechanisms.Since an omniaxis nozzle can provide bothpitch and yaw control, only one nozzle isneeded, and small secondary thrusters areused for roll control.

The two most important features of movablenozzles are the sealing and load carryingsystems between the stationary and movableportions of the nozzle.

The sealing system must allow movement ofthe nozzle while restraining motor pressure.Sealing systems used have included rubberO-rings, bellows, diaphragms, metal rein-forced elastomeric (rubber-like polymer)joints, and various combinations.

Movable Nozzles

Changing direction is affected by movingthe nozzle along a single axis plane.Some nozzles move many directions(omniaxial).

View from below amovable nozzle

Both yaw nozzles are turned right. . . rocket moves right

O-rings form a positiveseal around the nozzlethroat, preventing theescape of exhaustgases and loss ofmotor pressure

O-ring Seal System

26

The load carrying system also must allowmotion between the fixed and movable sec-tions of the nozzle while preventing excessivedeflections in the movable portion. Themotion must be relatively free so that exces-sive forces are not required to obtain motion.Systems which are currently in use includethe gimbal ring (universal joint type system),flexible joint (metal and elastomer laminae),and fluid type bearings. The most importantfunction of these systems is providing mobili-ty as well as support.

The two primary methods of producing lateralthrust with a fixed nozzle include mechanicalinterference (MITVC) and secondary injection(SITVC) thrust vector control.

MITVC involves changing the direction of thesupersonic gases at the nozzle exit plane byinserting a heat resistant body into theexhaust stream to deflect it. Devices used inthis method include jet vanes, jetavators, andjet tabs.

Jet vanes are small. Heat resistant ruddersmounted near the aft end of the nozzle exitcone, which are rotated to deflect the hotexhaust gases, thereby producing a sideforce in the desired direction. Each of the fourwedge-shaped jet vanes in the typicalarrangement is securely attached to an actu-ator shaft. The vane in the typical arrange-ment is securely attached to an actuatorshaft. The vanes, positioned by signals fromthe guidance system to the vane actuator,can be employed to obtain pitch, yaw, or rollcontrol. Mounted to a single nozzle, theopposing pairs of vanes can be operatedtogether to provide pitch or yaw movements;all four operated together can provide rollcontrol.

Designed to reduce drag losses associatedwith jet vanes, the jetavator consists of amovable surface attached to the end of theexit cone. The movable surface may be acomplete spherical internally contoured ringor several segments. When actuated, itrotates into the exhaust stream, deflecting itas desired to provide the necessary thrustvector. With jetavators, pitch and yaw controlcan be obtained, but not roll control.Jetavators are currently used on the Bomarcbooster for TVC.

Load Carrying Systems

Fluid Bearing Load Carrying System

Fixed Nozzles

The fluid bearing sep-arates fixed sectionsfrom movable sectionsof the nozzle

Movement

Mechanical Interference TVC

Jet Vanes

Exhaust gases flow pastvane. When activatedinto flow, gases arediverted by the rudder.

27

The jet tab system vectors the thrust byinsertion of a blunt body (tab) into the super-sonic exhaust stream just aft of the nozzleexit plane. Insertion of the tab induces ashock wave and a localized induced pressurearea just upstream of the tab, thus producinga side force perpendicular to the axial thrustvector. The thrust vector is controlled by vary-ing the ratio of the jet tab blockage area tothe nozzle exit cross sectional area.

Secondary injection TVC is accomplished byinjecting fluid (liquid or gas) into the mainexhaust stream of the rocket motor throughports in the expansion section of the nozzle.

The total side force produced by secondaryinjection consists generally of two parts: (1)the force caused by the momentum of theinjected fluid; and (2) the imbalance of pres-sure induced by the injection reacting on anarea of the nozzle surface perpendicular to

the nozzle centerline.

Because there are several factors which influ-ence the production of total side force duringsecondary injection into a supersonic nozzle,the location and angle of injection must bedetermined through experimentation andapplication of previously developed data.

Power for actuating the control systems isprovided by hydraulic mechanical andelectromechanical means, with electroniccomponents especially designed for theapplication providing the required signals forprecise positioning of components.

Jet Tab

Jetavator Diverting Exhaust

nsertion of the tab creates increased pres-sure within the exit cone. The diverted forceacts to vector the exhaust thrust.

Secondary Injection TVC

SITVC

The thrust is deflected by the force of theinjected fluid and by the imbalance ofpressure created in the nozzle.

Types of Control Systems

28

The dictionary defines a propellant as a sub-stance that causes an object to move or tosustain motion. Thus, rocket propellants existfor only one purpose, to produce the thrustrequired to cause a rocket to move fromlaunch site to intended target.

Thrust, as noted earlier, results from the pro-duction of hot gases during the combustion(burning) of the propellant and acceleration ofthose gases to supersonic velocity through aport or nozzle.

The internal combustion engine which pow-ers automobiles, airplanes, and other vehi-cles obtains oxygen from the air, which ismixed with the fuel in the carburetor in properproportions so that combustion can takeplace in the cylinders. Combustion of the pro-pellant in a rocket motor cannot breathe air toobtain oxygen. Consequently, it must carry itsown oxygen supply in addition to fuel supply.Separate fuel and oxygen supplies are car-ried in tanks aboard liquid rockets and thesematerials are mixed in the combustion cham-ber. In the solid rockets with which we areprimarily interested, however, both fuel andoxidizer supplies must be contained withinthe propellent charge.

To assure the release of maximum usableenergy during propellant combustion, fueland oxidizer materials are chosen carefully,and the exact proportions of each which arerequired for combustion are determined byanalysis and experimentation.

The ideal, or best possible, solid propellanthas as many of the following characteristicsas possible.

1. High level chemical energy release to produce high combustion temperatures and develop maximum thrust from eachpound of propellant (specific impulse) for high performance.

2. Low molecular weight of combustion products (the lower their molecular weight,the greater the acceleration that can beapplied to the gases).

3. Stability (resistance to chemical and physical change) over a long period of time.

4. Maximum density so that the greatestamount of energy possible can be packedinto every unit of case volume.

5. Resistance to the effects of atmosphericconditions, such as humidity, heat, cold, etc.

Propellants

Liquid rocket:Separate oxygen supply neededfor combustion.

Flame

Oxygen

Fuel

Flame

Fuel andOxygen

Solid rocket: Both fuel and oxygenare contained in propellant charge.

29

6. Resistance to accidental ignition from hightemperature or impact.

7. Maximum possible physical strength towithstand the effects of forces imposed byheat and pressure during motor operation.

8. Very small change in volume with eachdegree of change in temperature, preferablymatching that of the case material, to minimize stresses and strains in the two structures.

9. As nearly chemically non-reactive (inert)as possible ensuring storage and operation.

10. Easily produced and with desirable fabri-cation properties, such as adequate fluidityduring casting; easy control of processessuch as curing; and minimum volume change(shrinkage) following casting or molding.

11. Relative insensitivity of performancecharacteristics and fabrication techniques toimpurities or small processing variations.

12. Predictable physical properties andcombustion characteristics (burning rate) not affected appreciably by a wide range ofstorage and operating temperatures.

13. Smokeless exhaust gas to avoid deposi-tion of smoke particles at operational loca-tions and detection in military usage.

14. Readily bondable to metal parts, theapplication of inhibitors, and to different production techniques; and amenable to useof simple igniter.

15. Nonluminous, noncorrosive and nontoxic exhaust.

16. Simple method of preparation, not requir-ing complex chemical plant installation.

17. Grain should be opaque to radiation toprevent ignition at locations other than burn-

ing surface.

18. Able to withstand repeated extreme tem-perature variations prior to operation withoutphysical or chemical deterioration.

19. Raw materials cheap, safe, and easy tohandle and transport.

20. Burns at steady, predictable rate atmotor operating conditions.

Solid propellants form mono propellant(mono = single) systems in which the oxidizerand fuel components are combined in asingle mixture with a liquid material whichholds them in suspension. This compositionis then cast (poured) into the case, where it iscured to a solid state.

Propellant specialists divide solid propellantsinto two classifications - double-base andcomposite. These classifications refer to thephysical and chemical characteristics of thepropellants, as well as to the types of materi-als used in their manufacture.

The double-base ( homogeneous) typepropellants use nitrocellulose (guncotton) andan energetic plasticizer to cause it to dissolveand then harden into a solid form. Each molecule of this final material contains thefuel and oxygen required to sustain combus-tion (burning). The chemicals used in themanufacture of this type of propellant areunstable, usually nitrocellulose and or nitro-glycerin. Each of these materials containsthe necessary fuel and oxygen in its individ-ual molecules to burn or explode withoutcoming into contact with any other material.

Composite propellants are composed of sep-arate fuel and oxidizer materials, neither ofwhich will burn satisfactorily without the other.Mixed together and combined with a liquid

Solid Propellant Types

30

31

which later is cured to solid form, they forma compound in which fuel and oxidizer areadjacent to each other in close enough con-tact to assure efficient combustion.

Composite propellants generally are com-posed of finely ground chemical oxidizers and inorganic fuels (made from noncarbon-containing material) which are suspended ina binder of rubber-like material which alsoserves as a fuel. Most composite propellantsalso may use a variety of chemicals toincrease or decrease the burning rate (tocontrol hot gas production rate), provide bet-ter physical properties than are obtainablewith the basic binder, or to regulate chemicalreactions for better control during the manu-facturing process.

The nitrocellulose used in the manufacture ofdouble-base propellants, is prepared by com-bining cotton (cellulose fuel) with nitric acid(an oxidizer), which produces the singlechemical nitrocellulose. Addition of nitroglyc-erin (a shock sensitive high explosive) to thenitrocellulose causes a physical reactionwhich partially dissolves the latter and causesit to swell, then to gel or solidify the nitroglyc-erin. In the process, the nitroglycerin is madeless sensitive to shock. Certain other chemi-cals may be added to control the speed ofgelling (solidification), speed or slow theburning rate, and improve physical character-istics or other properties of the propellant.

Each molecule contains both fuel and oxidiz-ers. Chemical reactions solidify the propel-lant and additional materials improve itsphysical properties.

During storage, nitrocellulose decomposesslowly but steadily by releasing oxides of

nitrogen. The rate of decomposition is accel-erated by the presence of these oxides.Certain materials called “stabilizers” can becombined with the oxides to remove them,thus slowing the rate of decomposition andconsiderably extending the useful life of thepropellant.

Materials used to improve the mechanicalproperties and extrusion (pressure forming)characteristics of double-based propellantsare known as plasticizers. They may beeither explosive or nonexplosive materials.

Because of the tremendous heat developedduring combustion, high temperature radia-tion may be transmitted deep within the pro-pellant unless darkening agents such as car-bon black or lampblack are added to makethe penetration impossible. This process prevents below-surface ignition, which couldcause uncontrolled burning with an undesir-able increase in chamber pressure.

Other chemicals may be added to improvethe burning rate of other performance charac-teristics of the double-base propellant. Theseare known as ballistic agents.

Still other additives may be used to reducethe temperature of combustion gases as ameans of controlling chamber pressure, or to reduce the hygroscopicity (moistureabsorbing characteristics) of the propellant. And finally, various liquid ingredients may be added to the basic materials to serve as solvents for the purpose of speeding orimproving the reduction of the componentsto liquid form.

The separate fuel and oxidizer componentsused in the production of composite propel-lants must be mixed together in the properproportions to assure complete combustion,since neither will sustain combustion without

Composite Propellants

Double-Base (Homogeneous)

Double-Base Propellants

the other. The chemical oxidizers and inor-ganic fuels are ground to fine, power-likestates, mixed together, then added to a liquidmaterial (binder) which holds them in suspen-sion while it cures to a solid state. The finalcomposition also may include a variety ofchemicals to increase or decrease the burn-ing rate, provide better physical propertiesthan are obtainable with the basic binder, orimprove processing quantities.

Both natural and man-made materials havebeen used as binder-fuels for solid propellantgrains, including asphalt, and synthetic liquidprepolymers. All of the organic prepolymershave rubbery properties following cure andform a strong matrix (binding structure) withinwhich the inorganic fuels and chemical oxi-dizers are solidly bound.

The most commonly used liquid prepolymerscure up much like rubber. An excellentexample is the curing of the white of an egg,which solidifies and becomes rubbery whenheat is applied. The egg white also acts as abinder in a cake, holding the other ingredi-ents together through development of a long,chain like molecular structure.

The prepolymers used as binders in compos-ite propellants are initially liquid, so that thefuel and oxidizer can be blended in moreeasily prior to the start of polymerization(cure). The polymers cure to an irreversiblesolid form by cross linking (formation of longchain-like molecules linked together).

The first material used as a binder for solidpropellants was asphalt, which was used inthe first jet assisted take-off (JATO) units foraircraft near the end of World War II. Asphaltoccurs naturally and is a by-product of thedistillation of certain crude oils. It has severaldisadvantages for rocket applications, howev-er, including: formation of thick black smokecombustion, a melting point near that of

water (212 F), and brittleness at low tempera-tures unless mixed with oils.

Elastomers are the rubber-like natural andsynthetic materials which have found thewidest application in the modern (50’s - 70’sera) solid rocket propellants. The first materi-al used as a binder was a liquid polysulfideprepolymer, first produced commercially as asolid elastomer in 1932 by Thiokol ChemicalCorporation. Its application in “GALCIT” solidpropellant by the Guggenheim AeronauticalLaboratories of the California Institute ofTechnology marked the beginning of themodern era in solid propellant, since it madepossible production of large (up to 260 inchdiameter) solid rocket motors.

Other liquid prepolymers which have beenand are being used in large rocket motorsinclude polybutadiene-acrylic and acid-acrylonitrile prepolymer, and carboxylterminated polybutadiene. Each of thesematerials provides reproducible physicalcharacteristics, and all have excellent agingand chemical resistance characteristics.

In order to obtain specific properties, severalspecial ingredients may be added to compos-ite propellants, including: oxidants, antioxi-dants, and curing and burning rate catalysts.

In other instances, an increase in the burningrate may be desirable, and this can beobtained by adding chemical agents knownas burning rate catalysts which cause anincrease in the combustion reactions.

A number of inorganic chemicals can be usedto supply the oxygen required to support thecombustion of metallic and other fuels in solid propellants. The amount of oxygen pro-

Binders

Additives

Oxidizers

vided by each oxidizer depends upon itsmolecular structure, and certain performancecharacteristics may restrict applicability tovarious degrees.

In general, the perchlorates: potassium,ammonium, lithium, sodium, and nitronium -have more oxygen in their structure than dothe nitrates: ammonium, potassium, and sodium, although availability of oxygen is notthe only consideration when choosing the oxidizer to be used.

Some of the perchlorates produce hydrogenchloride, a highly corrosive gas which formshydrochloric acid when mixed with water, andother chlorine compounds. The exhaustgases of these oxidizers not only are toxic,but highly corrosive to many materials. Withthe exception of ammonium and nitroniumperchlorates, all form a dense smoky exhaustbecause potassium perchlorate and sodiumchloride are white powders. Ammonium and potassium perchlorate are only slightlysoluble in water, so they can be used in pro-pellants which are exposed to moisture.

The oxidizing potential of the perchlorates isgenerally high, and because of this fact theyare often found in propellants of high specificimpulse. All perchlorate oxidizers are poten-tial explosives, but use of high purity material,special crystal processing techniques, andcareful handling make processing of highenergy propellants using these oxidizers pos-sible. Nitronium perchlorate, the most power-ful of the perchlorates, is very sensitive andcan be detonated readily.

Of the three nitrates of interest, two (potassi-um and sodium nitrate) produce undesirablesmoke because of the solids formed in thecombustion products. Ammonium nitrate,widely used as a fertilizer, has the big advan-tage of producing a smokeless, relativelynontoxic exhaust. With the lowest oxidizingpotential of all the materials used, however,ammonium nitrate is suitable mainly for low

performance, low burning rate applications.

Powdered aluminum is the metallic fuel mostwidely used in solid rocket propellants. Theaddition of approximately 15 percent alu-minum powder to solid propellants seems tohelp in three ways: (1) by causing anincrease in combustion temperature and thus an increase in thrust produced perpound of propellant (specific impulse); (2) by increasing the density of the propel-lant; and (3) by exerting a dampening effecton unstable burning which might develop atmotor operating pressures and temperatures.

Beryllium causes not only an increased combustion temperature, but also produces lower molecular weight exhaust products;consequently, it can increase the theoreticalspecific weight impulse by 5 to 10 percent.The reaction products containing berylliumare extremely toxic, so special safety precau-tions are required.

Since metals lighter in weight than aluminum(lithium, beryllium, sodium, and magnesium)tend to be expensive, dangerous to handle inpowder form, and highly active chemically,aluminum has been adopted as the work-horse metallic fuel in solid propellants.

Metallic Fuels

Solid propellant technology is a continuouslychanging field in which even higher perform-ance is sought and attained. Many of tomor-row’s materials have yet to be developed ortested.

Update: The last two sentences reflect thefuture as well as the past. Thiokol is continu-ously developing new materials to improve itsproducts and meet the needs of the future.

Thiokol PropulsionP.O. Box 707Brigham City, Utah84302-0707

(435) 863-3511

What’s in the future?