A cryogenic rocket engine is a rocket engine that uses a cryogenic fuel and oxidizer; that is, both its fuel and oxidizer are gases which have been liquefied and are stored at very low temperatures.^[1] These highly efficient engines were first flown on the US Atlas-Centaur and were one of the main factors of NASA's success in reaching the Moon by the Saturn V rocket.^[1]

Rocket engines burning cryogenic propellants remain in use today on high performance upper stages and boosters. Upper stages are numerous. Boosters include ESA's Ariane 5, JAXA's H-II, ISRO's GSLV, LVM3, United States Delta IV and Space Launch System. The United States, Russia, Japan, India, France and China are the only countries that have operational cryogenic rocket engines.

Rocket engines need high mass flow rates of both oxidizer and fuel to generate useful thrust. Oxygen, the simplest and most common oxidizer, is in the gas phase at standard temperature and pressure, as is hydrogen, the simplest fuel. While it is possible to store propellants as pressurized gases, this would require large, heavy tanks that would make achieving orbital spaceflight difficult if not impossible. On the other hand, if the propellants are cooled sufficiently, they exist in the liquid phase at higher density and lower pressure, simplifying tankage. These cryogenic temperatures vary depending on the propellant, with liquid oxygen existing below −183 °C (−297.4 °F; 90.1 K) and liquid hydrogen below −253 °C (−423.4 °F; 20.1 K). Since one or more of the propellants is in the liquid phase, all cryogenic rocket engines are by definition liquid-propellant rocket engines.^[2]

Various cryogenic fuel-oxidizer combinations have been tried, but the combination of liquid hydrogen (LH2) fuel and the liquid oxygen (LOX) oxidizer is one of the most widely used.^[1][3] Both components are easily and cheaply available, and when burned have one of the highest enthalpy releases in combustion,^[4] producing a specific impulse of up to 450 s at an effective exhaust velocity of 4.4 kilometres per second (2.7 mi/s; Mach 13).

The major components of a cryogenic rocket engine are the combustion chamber, pyrotechnic initiator, fuel injector, fuel and oxidizer turbopumps, cryo valves, regulators, the fuel tanks, and rocket engine nozzle. In terms of feeding propellants to the combustion chamber, cryogenic rocket engines are almost exclusively pump-fed. Pump-fed engines work in a gas-generator cycle, a staged-combustion cycle, or an expander cycle. Gas-generator engines tend to be used on booster engines due to their lower efficiency, staged-combustion engines can fill both roles at the cost of greater complexity, and expander engines are exclusively used on upper stages due to their low thrust.^{[citation needed]}

Currently, six countries have successfully developed and deployed cryogenic rocket engines:

More information Country, Engine ...

Country	Engine	Cycle	Use	Status
United States	RL-10	Expander	Upper stage	Active
	J-2	Gas-generator	lower stage	Retired
	SSME (aka RS-25)	Staged combustion	Booster	Active
	RS-68	Gas-generator	Booster	Active
	BE-3	Combustion tap-off	New Shepard	Active
	BE-7	Expander	Blue Moon (spacecraft)	Active
	J-2X	Gas-generator	Upper stage	Developmental
Russia	RD-0120	Staged combustion	Booster	Retired
	KVD-1	Staged combustion	Upper stage	Retired
	RD-0146	Expander	Upper stage	Developmental
France	Vulcain	Gas-generator	Booster	Active
	HM7B	Gas-generator	Upper stage	Active
	Vinci	Expander	Upper stage	Developmental
India	CE-7.5	Staged combustion	Upper stage	Active
	CE-20	Gas-generator	Upper stage	Active
China	YF-73	Gas-generator	Upper stage	Retired
	YF-75	Gas-generator	Upper stage	Active
	YF-75D	Expander cycle	Upper stage	Active
	YF-77	Gas-generator	Booster	Active
Japan	LE-7 / 7A	Staged combustion	Booster	Active
	LE-5 / 5A / 5B	Gas-generator(LE-5) Expander(5A/5B)	Upper stage	Active

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