CRYOGENIC ENGINE

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1. INTRODUCTION

Cryogenic Engines are rocket motors designed for liquid fuels that have to be held at very low "cryogenic" temperatures to be liquid - they would otherwise be gas at normal temperatures. Typically Hydrogen and Oxygen are used which need to be held below 20°K (-423°F) and 90°K (-297°F) to remain liquid.
The engine components are also cooled so the fuel doesn't boil to a gas in the lines that feed the engine. The thrust comes from the rapid expansion from liquid to gas with the gas emerging from the motor at very high speed. The energy needed to heat the fuels comes from burning them, once they are gasses. Cryogenic engines are the highest performing rocket motors. The Space Shuttle's main engines used for liftoff are cryogenic engines. The Shuttle's smaller thrusters for orbital manuvering use non-cyogenic hypergolic fuels, which are compact and are stored at warm temperatures. Currently, only the United States, Russia, China, France, Japan and India have mastered cryogenic rocket technology



2. STEP TOWARDS CRYOGENIC ROCKET ENGINE

During World War II, when powerful rocket engines were first considered by the German, American and Soviet engineers independently, all discovered that rocket engines need high mass flow rate of both oxidizer and fuel to generate a sufficient thrust. At that time oxygen and low molecular weight hydrocarbons were used as oxidizer and fuel pair. At room temperature and pressure, both are in gaseous state. Hypothetically, if propellants had been stored as pressurized gases, the size and mass of fuel tanks themselves would severely decrease rocket efficiency. Therefore, to get the required mass flow rate, the only option was to cool the propellants down to cryogenic temperatures (below −150 °C, −238 °F), converting them to liquid form. Hence, all cryogenic rocket engines are also, by definition, either liquid-propellant rocket engines or hybrid rocket engines.



3. CRE IN DETAIL

Cryogenic Engines are rocket motors designed for liquid fuels that have to be held at very low "cryogenic" temperatures to be liquid - they would otherwise be gas at normal temperatures. Typically Hydrogen and Oxygen are used which need to be held below 20°K (-423°F) and 90°K (-297°F) to remain liquid.
The Space Shuttle's main engines used for lift off are cryogenic engines. The Shuttle's smaller thrusters for orbital manuvering use non-cyogenic hypergolic fuels, which are compact and are stored at warm temperatures. Currently, only the United States, Russia, China, France, Japan and India have mastered cryogenic rocket technology.



3.1 PRINCIPLE
In principle, cryogenic rocket engines generate thrust like all other rocket engines-by accelerating an impulse carrier to high speeds.
In conventional aircraft engines the surrounding air is the main impulse carrier and fuel is the energy carrier. This is why such an engine requires the atmosphere not only to burn the fuel but also to generate thrust.
But in cryogenic rocket engines the impulse and energy carriers are identical and are present as fuel in the launcher. The chemical energy stored in the fuel is converted into kinetic energy by burning it in the thrust chamber and subsequent expansion in the nozzle, in the process creating thrust.

3.2 SPECIFIC IMPULSE
Inorder to compare a variety of fuel combinations, a quantity known as specific impulse which determines the thrust per kilogram of emitted fuel per second, is used.
For example: Hydrazine has 230 seconds of specific impulse, for solid propellants it is around 290 seconds. The favourite fuel and oxidizer combination used during the boost phase are Liquid Hydrogen(LH2) and Liquid Oxygen(LOX) which provide a specific impulse of 445 seconds, almost double that of hydrazine.