nasa facts rocket propulsion

8/7/2019 NASA Facts Rocket Propulsion

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N A S A ~ f f i ~ ~ AN EDUCATIONAL PUBLICATION OF TH E

NATIONAL AER ONAU TIC S AND SPACE ADMINISTRATION

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Transportation systems are dependent uponengines for locomotion. This is true whether thecargo is a pebble to be delivered by a man-poweredslingshot, or the Apollo spacecraft to be propelled to the moon by the giant Saturn V launchvehicle.

Somewhere along the engine power developmentcycle, oxygen is , or has been , required to give themotor 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 issupplied by a man, or boy, who breathes oxygenin the air. Here on the earth the supply of thatvital ingredient is virtually inexhaustible. The rocketcombustion engine differs from other transportationpower systems. It must carry its own oxygen toburn fuel because it is designed to operate mostlybeyond the ocean of air surrounding earth.Today, power for rocket engines is provided bychemicals which , when combined, generate gasesthat produce the force or thrust necessary fo r flight.The push which the rocket vehicle receives fromgases expanding out of the engine's nozzle is ex-plained in Newton's third law of motion: "For everyaction there is an equal and opposite reaction ."This can be illustrated by envisioning a skaterstanding upright with a bowling ball held near hisbody. He pushes the ball away. The ball goes flying one way and he will go flying the other way. Thefaster he pushes the ball away, the harder he willbe pushed in the opposite direction. All rocketswork on this principle.

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

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

A. Thrust is demonstrated by the balloon and rocket asgas gushes out of the nozzles. The action of the engineforces th e gases ou t of the nozzle.B. Reaction to the thrust makes the balloon or rocket move.


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phere. There are several other types of rocketpropulsion systems-theoretically, far more efficient-under development that will probably beused later for longdistance journeys. These include Nuclear, Solar, Arc , Ionic, and Fusion. Thesepower systems all require the use of nuclear poweror solar energy for the energy source. By far themost power, or exhaust velocity, could be obtainedfrom nuclear fusion. However, its use is still far inthe future .Meanwhile , although chemical propulsion is notvery efficient because of the tremendous amountof fuel requ ired to deliver the cargo , it is presentlythe best means available.

To move more weight into space and to move itfaster, we can either burn more fuel per second ,burn the fuel for a longer time , or increase thespeed at which the gases flow through the exhaustnozzle. The developers of our space vehicles areworking on all of these methods to increase theefficiency of chemically fueled rocket engines.

Most chem ical rockets used today burn hydro carbons, such as kerosene, which provide specificimpulses of about 300 seconds. Specific impulseis a measurement used to compare the energy available from various fuels. It represents thrust forcein pounds per second.

Mathematically, the actual force being generatedby a rocket engine, or its thrust , can be calculatedby Newton 's second law of motion: Force equalspropellant mass flow rate times exhaust velocity(ital icized words ours). Mass flow rate is primarilya matter of the size of the rocket engine. The exhaust velocity is limited by the chemical energy, orhea t of combustion of the propellants. Exhaustvelocity divided by "g" (force of gravity) is calledspecific impulse. The state it simply, specific impulse is the miles-per -ga llon figure for a rocketengine.

More advanced rockets, such as the Centaur

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upper stage and upper stages of th e 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 thebest chemical propellants have a fixed energy perpound , or heat of combustion . So far , our bestchemical rockets develop an exhaust veloc ity ofabout 9000 miles per hour (the speed of acceleration of the propellant, not the final speed the vehicle attains). To obtain higher exhaust velocit iesfor more ambitious space missions, scientists areworking to develop more advanced systems, such asnuclear and electric propulsion.

There are two types of chemical rocket enginesused widely today. One burns liquid propellantsand 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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REACTION-y regulating the flow of propellants to the combustionchamber, liquid engines can operate at different thrustlevels.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 ofunsymmetrical dimethyl hydrazine (UDMH) andhydrazine. Common oxidizers are liquid oxygen, inhibited red fuming nitric acid, and nitrogen tetroxide. In recent years, however, the development ofhigh-energy propellant technology has led to theuse of higher specific impulse fuels such as liquidhydrogen. And even higher energy is available whenliquid hydrogen and liquid fluorine are combined.The latter propellant combination is only in theexperimental stage.

Liquid rocket engines are more complex thansolid motors. A liquid propulsion system consistsof 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-controsystem, propellant injectors, and a cooled combustion chamber and rocket nozzle.

Th e simplest way to force propellants into thecombustion chamber is to pressurize the propellahttanks. Another way is to use turbinedriven pumpspowered by hot gases obtained in one of severaways: from a gas generator especially for thepurpose; by bleeding off propellant heated by theengine; or by tapping off combustion chambergases. By regulating the flow rate of propellants tothe combustion chamber, the liquid engine canoperate at different thrust levels.Solid rocket motors have been used primarily forsounding and military rockets so far. However,technology has been advancing rapidly and solidmotors as large as 260 inches in diameter havebeen fired and have produced more than threemillion pounds of thrust.

In a solid rocket, the propellant, consisting of asolid mass of mixed fuel and oxidizer , is bonded toor supported by, the inside of the motor case. Theslug of propellant , called a grain , usually has a holerunning th rough its center . Burning takes placealong the exposed surface of the grain , progressesradially toward the case , and produces combustionproducts which flow from the nozzle . The shapeof the hole determines the amount of burning surface of the grain at any time . The propellant itselfthus usually protects the motor case walls from thehot gases inside the motor. The larger the burningarea, the faster the propellant is burned , and thehigher the thrust .

Control of th e solidpropellant rocket is moredifficult than control of liquid propulsion systems;it is not as easily throttled and the usual way ofterminating its thrust is to open vents in the cham ber walls , which causes the pressure in the motorto drop, extinguishing the flame.

The usual vacuum specific impulse of solid pro -


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