page_title: Cryogenic rocket engine
section_title: Cryogenic propellants
hierarchical_section_title: Cryogenic rocket engine / Cryogenic propellants
caption_attribution_description: English: RL-10 engine characteristics. The RL-10 engine was developed under the management of the Marshall Space Flight Center (MSFC) to power the Saturn I upper stage (S-IV stage). The six RL-10 engines, which used liquid hydrogen and liquid oxygen as propellants, were arranged in a circle on the aft end of the S-IV stage.
context_page_description: A cryogenic rocket engine is a rocket engine that uses a cryogenic fuel and oxidizer, that is, both its fuel and oxidizer are gases liquefied and stored at very low temperatures. 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.
  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, and the United States Delta IV and Space Launch System. United States, Russia, Japan, India, France and China are the only countries that have operational cryogenic rocket engines.
context_section_description: 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 the simplest fuel hydrogen. 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 [90 K] and liquid hydrogen below −253 °C [20 K]. Since one or more of the propellants is in the liquid phase, all cryogenic rocket engines are by definition either liquid-propellant rocket engines or hybrid rocket engines.
  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.  Both components are easily and cheaply available, and when burned have one of the highest enthalpy releases in combustion, producing a specific impulse of up to 450 s at an effective exhaust velocity of 4.4 km/s.
caption: RL-10 is an early example of cryogenic rocket engine.
exif: {}