07-04-2012, 01:05 PM
Cryogenic rocket engine and propellents
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Rocket engine
A rocket engine, or simply "rocket", is a jet engine[1] that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law. Since they need no external material to form their jet, rocket engines can be used for spacecraft propulsion as well as terrestrial uses, such as missiles. Most rocket engines are internal combustion engines, although non combusting forms also exist.
Types of rocket engines
Rocket motor (or solid-propellant rocket motor) is a synonymous term with rocket engine that usually refers to solid rocket engines.
Liquid rockets (or liquid-propellant rocket engine) use one or more liquid propellants that are held in tanks prior to burning.
Hybrid rockets have a solid propellant in the combustion chamber and a second liquid or gas propellant is added to permit it to burn.
Thermal rockets are rockets where the propellant is inert, but is heated by a power source such as solar or nuclear power or beamed energy.
Monopropellant rockets are rockets where the propellant is one chemical, typically hi-test (85%+) hydrogen peroxide, which is decomposed by a catalyst producing steam and oxygen. There is no flame.
Liquid Propellants
In a liquid propellant rocket, the fuel and oxidizer are stored in separate tanks, and are fed through a system of pipes, valves, and turbopumps to a combustion chamber where they are combined and burned to produce thrust. Liquid propellant engines are more complex than their solid propellant counterparts, however, they offer several advantages. By controlling the flow of propellant to the combustion chamber, the engine can be throttled, stopped, or restarted.
Cryogenic Propellants
In a cryogenic propellant the fuel and the oxidizer are in the form of very cold, liquefied gases. These liquefied gases are referred to as super cooled as they stay in liquid form even though they are at a temperature lower than the freezing point. Thus we can say that super cooled gases used as liquid fuels are called cryogenic fuels.
Drawbacks of Cryogenic Propellants
Boil off Rate
Since these propellants are in extremely low temperature conditions they are very hard to handle. They must be protected from heat so as to prevent boiling of gases. When liquid propellants are stored at temperatures above their boiling point they vaporize. If these vapors are contained in a tank, then the pressure increases with temperature.
Leakage
One of the most major concerns is leakage. At cryogenic temperatures, which are roughly below 150 degrees Kelvin or equivalently (-190oF), the seals of the container used for storing the propellants lose the ability to maintain a seal properly. Hydrogen, being the smallest element, has a tendency to leak past seals or materials.