A cryogenic propellant is a form of liquid propellant for rocket engines that must be kept at very low temperatures to remain liquid. The commonest examples are liquid hydrogen and liquid oxygen. Cryogenic propellants require special insulated containers and vents to allow gas from the evaporating liquids to escape. The liquid fuel and oxidizer are pumped from the storage tanks to an expansion chamber and injected into the combustion chamber where they are mixed and ignited by a flame or spark.
Because of the low temperatures of cryogenic propellants, they are difficult to store over long periods of time. For this reason, they are less desirable for use in military rockets which must be kept launch ready for months at a time. Also, liquid hydrogen has a very low density (0.59 pounds per gallon) and, therefore, requires a storage volume many times greater than other fuels. Despite these drawbacks, the high efficiency of liquid hydrogen/liquid oxygen makes these problems worth coping with when reaction time and storability are not too critical. Liquid hydrogen delivers a specific impulse about 40% higher than other rocket fuels.
Liquid hydrogen and liquid oxygen are used as the propellant in the high efficiency main engines of the Space Shuttle. LH2/LO2 also powered the upper stages of the Saturn V and Saturn lB rockets as well as the second stage of the Atlas-Centaur launch vehicle – the United States' first LH2/LO2 rocket (1962).