Liquid Hydrogen is a common fuel for rocket applications. In the aerospace industry, its name is often abbreviated to LH2. Hydrogen at normal temperature and pressure is a gas and to exist as a liquid must be compressed and cooled to a low temperature, 20 K (−423°F). Once liquified it can only be maintained as a liquid in thermally insulated (cryogenic) containers.
Liquid hydrogen has a very low density of 70.8 kg/m³ (at 20 K), so storage tanks for it have to be quite large.
In rocket engines, liquid hydrogen is frequently used as a coolant for the engine nozzle and combustion chamber before being mixed with the oxidizer and burned. This is referred to as regenerative cooling.