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Cryogenic Rocket Engine

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 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.

The use of liquid fuel rocket engines was first considered by the German, American and Soviet engineers independently, and all discovered that rocket engines needed high mass flow rate for both liquid oxidizer and fuel, for generating the necessary thrust. Higher thrust levels were achieved when liquid oxygen (LOX) and liquid hydrocarbon were used as fuel.

Image of Cryogenic rocket engine

At atmospheric conditions, liquid oxygen and low molecular weight hydrocarbons are in gaseous state and to get required mass flow rate, the only option is to feed them to the engine in liquid form. For that, these are stored in liquid form by cooling them down and hence the name cryogenic rocket engines. Various cryogenic fuels combination were tried and the liquid oxygen oxidizer and the liquid hydrogen (LH2) fuel combination caught special attention of engineers as: both components are easily and cheaply available, bio-friendly, non-corrosive and have the highest entropy release by combustion, among all non-toxic pairs. Liquefaction temperature of oxygen is 89 kelvins and at this temperature liquid oxygen achieves a density of 1,140 kg/m3 (1.14 g/cm3). And, for hydrogen it is 20 kelvins, a few kelvins above absolute zero, and gains a density of 70 kg/ m3 (70 mg/cm3). All cryogenic rocket engines work on expander cycle or gas-generator cycle or staged combustion cycle depending on thrust requirement, since the oxidizer and fuel are at sub-zero temperatures. LOX LH2 cryogenic rocket engines produce specific impulse up to 450 s (4.4 kN·s/kg).

Construction
The major components of a cryogenic rocket engine are:

  • The thrust chamber or combustion chamber,
  • Gas turbine,
  • Fuel injector,
  • Fuel turbopumps,
  • Pyrotechnic igniter,
  • Cryo valves,
  • Regulators,
  • The fuel tanks and
  • Rocket engine nozzle.

The fuel flow can be differentiated into a main flow or a bypass flow configuration. In the main flow design, the entire fuel is fed through the gas turbines, which intern drive the cryopump for fuel and oxidizer, and then injected to the combustion chamber. In the bypass configuration, the fuel flow is split, the main part is goes to the combustion chamber to generate thrust, while a small amount of the fuel goes to the turbine, to drive the cryopumps for fuel and oxidizer and is subsequently injected to combustion chamber.

Rocket engine can generates 5,000 degree steam and 13,800 pounds of thrust form icicles at the rim of its nozzle. It's cryogenic. The Common Extensible Cryogenic Engine (CECE) has completed its third round of intensive testing. This technology development engine is fueled by a mixture of -297°F liquid oxygen and -423°F liquid hydrogen.

The engine components are super-cooled to similar low temperatures. As CECE burns its frigid fuels, gas composed of hot steam is produced and propelled out the nozzle creating thrust. The steam is cooled by the cold engine nozzle, condensing and eventually freezing at the nozzle exit to form icicles. Using liquid hydrogen and oxygen in rockets will provide major advantages for landing astronauts on the moon. Hydrogen is very light but enables about 40% greater performance (force on the rocket per pound of propellant) than other rocket fuels. Therefore, NASA can use this weight savings to bring a bigger spacecraft with a greater payload to the moon than with the same amount of conventional propellants. CECE is a step forward in NASA's efforts to develop reliable, robust technologies to return to the moon, and a winter wonder.