Rocket fuel in liquid form. It is a gas, normally hydrogen, that has been cooled to a liquid by a cryogenic process. The liquid fuel is combined in the combustion chamber with a liquid oxidizer, usually liquid oxygen, to convert it back into a pressurized gas. This is called a bipropellant system.

Russian scientist Konstantin Tsiolkovsky designed a rocket in 1903 using liquid hydrogen and liquid oxygen and US rocket pioneer Robert Goddard introduced petrol as a fuel in 1930. Germany in the 1940s used alcohol for their wartime V-2 rockets; the USA in the 1950s chose kerosene for the first Atlas military rockets (see atlas rocket) and, in the next decade, liquid hydrogen for the upper-stage Centaur engine; and the Soviets in the 1960s tested nitric acid for their intercontinental ballistic missiles.