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Liquid Fueled Rocket Engine

Liquid Fueled Rocket Engine Basics

Bi-propellant rocket motor Credit NASA

As an aerospace technician you may find yourself around liquid fueled rocket engines daily or as in some cases of SpaceTec alumni, actually working on them.  Either way, it is a good idea to know some basics about these engines, especially the more common bi-propellant engines.


Liquid fueled rocket engines were first proposed by Konstantin Tsiolkovsky in his book The Exploration of Cosmic Space by Means of Reaction Devices published in 1903.  It remained a theory until the first successful flight by a rocket named “Nell”, lasting only 2 and ½ seconds and traveling 41 feet, done by Robert Goddard on March 16th, 1926.

Goddard and “Nell” Credit NASA

What is a bi-propellant liquid fueled rocket engine?  Well, as the name describes, it is a rocket engine that is fueled by a liquid fuel and a liquid oxidizer.   The entire set up is deceptively simple and consists of two tanks to hold the fuel and oxidizer, two turbo pumps, a combustion chamber, and a nozzle.

The two tanks can be small such as what was on Robert Goddard’s rocket or can be as large as the External fuel tank on the Space Shuttle system (which holds the tanks for both the fuel and oxidizer in one large shell).

The pumps are critical in pushing the fuel and oxidizer to their explosive meeting in the combustion chamber while maintaining adequate pressure to ensure proper flow rate and to prevent the combustion chamber from collapsing under its own weight due to atmospheric pressure.  As Dr. Jerry Grey wrote in his book Enterprise, “Much of the difficulty with liquid-propellant rockets arises in maintaining constant flow and rapid, efficient mixing of the propellants…the need for tanks heavy enough to withstand the pressure dictated the use of pumps instead…pumps the size of washing machines which had to have the power of a diesel locomotive…” (Grey, Ph.D., 1979)  One of the factoids listed at the Kennedy Space Center Visitor Center Launch Gantry says that one turbo pump from one Space Shuttle Engine (SSME) can actually drain an Olympic size swimming pool in just 25 seconds.  That’s a lot of fuel being pushed along the pipes leading into the combustion chamber, yet it takes all three Space Shuttle Main Engines nearly 8 minutes to drain the External Tank!

The combustion chamber is where all the action happens.  This is where the fuel and oxidizer meet and combust forming the explosive energy required to lift the rocket.  “Chamber walls less than a sixteenth of an inch thick that withstood pressures of one hundred atmospheres and temperatures of 6,000 F.” (Grey, Ph.D., 1979)  Basically an explosion occurs that is continuous as long as there is fuel or until the engine is shut down.  This is why it is said sometimes that a rocket launch is nothing but a vehicle or payload riding a controlled explosion.

After the fuel combusts the resulting energy has to go somewhere or the chamber will explode.  The resulting energy or flame exits out the familiar nozzle we see on all rockets.  Nozzles experience great heat from the flames and actually have to be made of a material that can withstand it and/or have a way of being cooled so they can maintain their shape and form during the launch.  The nozzles for the SSME’s actually have some of the liquid hydrogen routed through tiny radiator tubes lining the nozzle using the super cold liquid hydrogen to keep the nozzle cool and to warm the hydrogen before it reaches the combustion chamber making it easier to ignite.

As I said, deceptively simple but as many technicians and engineers that have worked on these engines for nearly 100 years now can attest, this “simple” system has resulted in many exploded rockets and engines (and some loss of life) during the trial and testing that still goes on today.

For further learning about this subject:

Grey, Ph.D., J. (1979). Enterprise. New York: William Morrow and Company, Inc.


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Tuesday, December 14th, 2010 History, Introduction to Aerospace Comments Off on Liquid Fueled Rocket Engine Basics