NASA ~ffi~~ AN EDUCATIONAL PUBLICATION OF THE

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

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Rockel propulSion Transportation systems are dependent upon engines for locomotion. This is true whether the cargo is a pebble to be delivered by a man-powered slingshot, or the Apollo spacecraft to be pro­pelled to the moon by the giant Saturn V launch vehicle.

Somewhere along the engine power development cycle, oxygen is , or has been , required to give the motor life. In the case of the combustion engine, oxygen is, of course, required to make fuel burn . The slingshot is not an exception-its power is supplied by a man, or boy, who breathes oxygen in the air. Here on the earth the supply of that vital ingredient is virtually inexhaustible. The rocket combustion engine differs from other transportation power systems. It must carry its own oxygen to burn fuel because it is designed to operate mostly beyond the ocean of air surrounding earth.

Today, power for rocket engines is provided by chemicals which , when combined, generate gases that produce the force or thrust necessary for flight.

The push which the rocket vehicle receives from gases expanding out of the engine's nozzle is ex­plained in Newton's third law of motion: "For every action there is an equal and opposite reaction ." This can be illustrated by envisioning a skater standing upright with a bowling ball held near his body. He pushes the ball away. The ball goes fly­ing one way and he will go flying the other way. The faster he pushes the ball away, the harder he will be pushed in the opposite direction. All rockets work on this principle.

A B

This law also explains why a rocket engine is as effective in a vacuum as it is in the atmosphere. The forward motion of the rocket is simply the result of expelling gases in the opposite direction . As a matter of fact , a rocket engi ne is more efficient in a vacuum since air gets in the way of the exhaust.

All of our rocket engines are now using chemical fuels to move vehicles beyond the earth's atmos-

A. Thrust is demonstrated by the balloon and rocket as gas gushes out of the nozzles. The action of the engine forces the gases out of the nozzle. B. Reaction to the thrust makes the balloon or rocket move.

phere. There are several other types of rocket propulsion systems-theoretically , far more effi· cient-under development that will probably be used later for long·distance journeys. These in­clude Nuclear, Solar, Arc , Ionic, and Fusion. These power systems all require the use of nuclear power or solar energy for the energy source. By far the most power, or exhaust velocity, cou ld be obtained from nuclear fusion. However, its use is still far in the future .

Meanwhile , although chemical propulsion is not very efficient because of the tremendous amount of fuel requ ired to deliver the cargo , it is presently the best means available.

To move more weight into space and to move it faster, we can either burn more fuel per second , burn the fuel for a longer time, or increase the speed at which the gases flow through the exhaust nozzle. The developers of our space vehicles are working on all of these methods to increase the efficiency of chemically fueled rocket engines.

Most chem ical rockets used today burn hydro­carbons, such as kerosene , which provide specific impulses of about 300 seconds. Specific impulse is a measurement used to compare the energy avail­able from various fuels. It represents thrust force in pounds per second.

Mathematically, the actual force being generated by a rocket engine, or its thrust , can be calculated by Newton 's second law of motion: Force equals propellant mass flow rate times exhaust velocity (italicized words ours). Mass flow rate is primarily a matter of the size of the rocket engine. The ex­haust velocity is limited by the chemical energy, or heat of combustion of the propellants . Exhaust velocity divided by "g" (force of gravity) is called specific impulse. The state it simply, specific im­pulse is the miles-per-ga llon figure for a rocket engine.

More advanced rockets, such as the Centaur

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upper stage and upper stages of the Saturn vehi­cles , use high -energy propellants, like liquid hydro­gen with liquid oxygen , which provide speci fic im­pulses above 400 seconds.

It should be remembered , however, that even the best chemical propellants have a fixed energy per pound , or heat of combustion . So far, our best chemical rockets develop an exhaust veloc ity of about 9000 miles per hour (the speed of accelera­tion of the propellant, not the final speed the vehi­cle attains). To obtain higher exhaust velocit ies for more ambitious space missions, scientists are working to develop more advanced systems, such as nuclear and electric propulsion.

There are two types of chemical rocket engines used widely today. One burns liquid propellants and the second burns a solid propellant.

In a liquid rocket , the fuel and oxidizer are car­ried in separate tanks and burned in the combus­tion chamber of the rocket engine. Hydrocarbons,

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OXIDIZER PUMP--=:::::::::j:~

TURBINE .......--­EXHAUST

COOLING PASSAGES IN NOZZLE - -

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REACTION -By regulating the flow of propellants to the combustion chamber, liquid engines can operate at d ifferent thrust levels.

such as kerosene, are the most common fuels gen­erally used in rocket boosters such as Atlas, Delta , and Saturn. The Titan II vehicle uses a mixture of unsymmetrical dimethyl hydrazine (UDMH) and hydrazine. Common oxidizers are liquid oxygen, in­hibited red fuming nitric acid, and nitrogen tetrox­ide. In recent years, however, the development of high-energy propellant technology has led to the use of higher specific impulse fuels such as liquid hydrogen. And even higher energy is available when liquid hydrogen and liquid fluorine are combined. The latter propellant combination is only in the experimental stage.

Liquid rocket engines are more complex than solid motors. A liquid propulsion system consists of the propellant tanks , fuel and oxidizer pumps, a turbine to drive them , a gas gel'lerater or gas-

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bleed system to drive the turbine, a flow-control system, propellant injectors, and a cooled combus­tion chamber and rocket nozzle.

The simplest way to force propellants into the combustion chamber is to pressurize the propellaht tanks. Another way is to use turbine·driven pumps powered by hot gases obtained in one of several ways: from a gas generator especially for the purpose; by bleeding off propellant heated by the engine; or by tapping off combustion chamber gases. By regulating the flow rate of propellants to the combustion chamber, the liquid engine can operate at different thrust levels.

Solid rocket motors have been used primarily for sounding and military rockets so far. However, technology has been advancing rapidly and solid motors as large as 260 inches in diameter have been fired and have produced more than three million pounds of thrust.

In a solid rocket, the propellant, consisting of a solid mass of mixed fuel and oxidizer, is bonded to , or supported by, the inside of the motor case. The slug of propellant , called a grain , usually has a hole running through its center. Burning takes place along the exposed surface of the grain , progresses radially toward the case , and produces combustion · products which flow from the nozzle. The shape of the hole determines the amount of burning sur­face of the grain at any time. The propellant itself thus usually protects the motor case walls from the hot gases inside the motor. The larger the burning area, the faster the propellant is burned , and the higher the thrust .

Control of the solid·propellant rocket is more difficult than control of liquid propulsion systems; it is not as easily throttled and the usual way of terminating its thrust is to open vents in the cham ­ber walls , which causes the pressure in the motor to drop, extinguishing the flame.

The usual vacuum specific impulse of solid pro-

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I 100 GAL IIIQNO.ll!.ETHYlHYORAIIHE (REACTIOH COtoITROL

610 GAL ~'TROCe,., TETROXIOE (RF.A.CTlOI'I CONTROL SYSTEM) I 2,SOO GAL NITROGEN TETROXIDE

2, 11)..; GAL H Y~R "ZI"'E UHSYMMETRICAL QUIETHYL HYDRAZIHE

LUNAR m"U" L<-""

1,000 GAL HITRoc.elll TETROXIDE

I PITCH MOTOR (SOLID) ] ,000 LBS THRUST

TOWER JETTISON MOTOR (SOLID) 40,000 LBS THRUST

r-- LAUNCH ESCAPE SYSTEM

I P 22K S (L10UID) 21 ,900 lBS THRUST

Ani TUDE CONTROL ENGINES (LIQUID) 100 POUHOS THRUST EACH

ASCENT EI'IGI/'IE ILIOUID) l ,SOO LBS THRUn 1 (LUNAR ~OouLE ASCENT DECE"'T STAGE)

1,200 GAL HYDRAIIHE (~:::::~~~I~~S~~:~T~~~E~~H;T ;";::"0;;'.;) · -fj~q.~L_ DEsceNT ENGINE (LIQUID) I ,Osa TO 10,500 LBS THRUST

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I.' 900 CAL LlOUID HYOI'OOI'H-~-U_

267.700 CAL LlOUID HYDROGEN ___ +-_

87.400 GAL UOUID OXYO.H - __ I¥--J:k

2",200 GAL RP· I (KEROSENE)

STAGE

ATTITUDE CONTROL eNClt-IES (L10U10) 147 LSS THRUST EACH

2 ULLAGE ENGINES (LIQUID) 72 lBS THRUST eACH

RETRO,lolOTORS (SOUD) ]S.700 LSS THRUST EACH

J·2 ENGINE (LIOUID) 200.000 LBS THRUST

STAGE

ULLAGE ""OTORS (SOLID) 22 700 LBS THRUST EACH

J· 2 ENGINES (LiOUID) 200.000 LBS THRUST EACH

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RETRO ""OTORS (SOLID) 87.900 LBS THRUST EACH

F 1 ENGII'IES (UOUID) 1.500.000 LBS THRUST EACH

THE APOLLO/SATURN V

pellant rockets is about 240 sec· onds; some propellants can per· form as high as 350 seconds.

In the drawing at left you will see the various liquid propellant and solid rockets used in the Apollo / Saturn V space vehicle which will send three astronauts to the moon, and return them to earth. You will see that enormous amounts of fuel are used to propel the vehicle. The chart explains the thrust and use of the engines. This one rocket has a total of 91 en­gines, both liquid and solid.

QUESTIONS:

1. Why is it necessary for space vehicles to use liquid oxygen in­stead of gaseous oxygen for space flight?

2. What would be the advantage of using nuclear or ionic propul­sion for space travel to distant planets?

REFERENCE:

Thrust into W. Hunter, II. ence, Series and Winston .

Space, by Maxwell Holt Library of Sci ­III. Holt, Rinehart

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