The rocket propellant oxidizer, normally combined with liquid hydrogen. Spacecraft must carry an oxidizer as well as fuel in order to be able to burn the fuel, there being no oxygen in space. The oxygen and hydrogen (LOX–LH) are in a cryogenic state whose super-coldness turns them into liquids; LOX is chilled to −148 °C. It then expands by a factor of 900 when turned into gas on combustion.

US rocket pioneer Robert Goddard used liquid oxygen mixed with petrol for his first liquid-fuelled rocket engine, and Germany's V2 World War II rocket was propelled by liquid oxygen and alcohol.