Anatomy of a cryogenic rocket engine

Tuesday, 30 August 2011 - 4:15pm IST | Place: Ahmedabad | Agency: DNA
Ever wondered how a Cryogenic Rocket engine works? Retired ISRO engineer, tells us...

Evidently it should become a powerful phenomenon to generate power. Just as steam in the steam engine drives a locomotive or a steam turbine, combustion gases of hydrogen-oxygen mixture also have the capability to perform the same function.

The combustion product of fuel hydrogen mixed with oxygen is steam, generating a clean and environment friendly waste.
However, harnessing and controlling the burning of hydrogen fuel is highly complex and a great technological challenge. Its application as a power generator is thus constrained due to many practical and economical considerations.

If one has to use the concept of propelling a rocket into space using hydrogen as fuel, the technology becomes one order higher complex because of the severe limitations of size and weight.
Nevertheless, the merit of using hydrogen as fuel drives to develop the complex and safety intensive technology for certain specific applications. Rocket propulsion is one of them.

Now the curiosity arises as to what is cryogenic. Cryogenic is the science of using liquid gases below the temperature of minus 150 degrees centigrade (standard temperature of atmosphere is about 27 degrees centigrade).

The liquid temperatures of hydrogen and oxygen are below minus 253 and 183 degree centigrade respectively. The next question comes to mind is why to use it in rocket propulsion. To understand this one has to get a glimpse of the basic science behind rocket propulsion. We have all delightedly seen, during diwali festival, several cracker rockets going up with a colorful flame emanating downwards.

The keen observation here is that the cracker rocket moves up because the flame flows downward meaning thereby that any object bleeding out high pressure gases moves in the opposite direction of the flow of the flame. We call the flame a jet. This sets the principle of rocket propulsion. Other examples of common use of jet propulsion are garden water sprinklers, jet airplanes etc. All these examples follow the same principle of jet propulsion.
The jet of a rocket is nothing but a combustion product of a fuel burnt in a chamber and expelled through a narrow opening called nozzle.
Higher the pressure of combustion in the chamber, higher is the force with which the gases come out of the nozzle resulting in higher propulsive force applied to the chamber. We call it a part of the rocket body. Applying this theory of propulsion one can develop rockets of various capabilities, that can push its own weight upward. Although the rocket propulsion appears to be so simple, in reality it is not. The basic rockets have to be provided with numerous attachments with high sophistication and perfection.

These are needed to control its movement within a fraction of a second. The power generated by it is of the order of several lakhs horsepower and the weight of the rocket varies approximately between 15 to 5000 tones.

Now where the does power of such powerful propulsion come from? The rocket propulsion needs fuel and oxidizer to be stored on board to burn the fuel. This is unlike an aircraft which uses the oxygen of the atmospheric air as oxidizer and that is why an aircraft cannot go beyond the earth's atmosphere.

There are a number of fuel options available for rocket propulsion. Categorizing broadly, we can say solid and liquid fuels, which would be called solid or liquid propellants in rockets terminology.
In solid propellant propulsion system, the propellant is in the form of a cake cast into a combustion chamber.

It contains the fuel (normally an aluminium powder) and an oxidizer, which is in the form of an oxidizer chemical. Solid fuel burns in the storage chamber itself and the burnt gases expel through a nozzle leading to the jet propulsion effect. Whereas in liquid propulsion system, the liquid fuel and oxidizer are stored on board in separate chambers and flowed down into the combustion chamber for burning their mixture and expelling out as jet.

Now the fuels in liquid propulsion system itself can be of three types - earth-storable, semi cryogenic and fully cryogenic. Earth-storable fuels are liquid under earth atmospheric conditions and hence named as earth-storable liquid fuels.    

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