Cryogenic rocket engine

[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.

Boosters include ESA's Ariane 5, JAXA's H-II, ISRO's GSLV, LVM3, United States Delta IV and Space Launch System.

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.

[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).