Liquid hydrogen
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Liquid hydrogen is a common liquid rocket fuel for rocket applications. In the aerospace industry, its name is often abbreviated to LH2. Hydrogen is found naturally in the molecular H2 form, thus the H2 part of the name. Hydrogen at normal temperature and pressure is a gas and to exist as a liquid must be pressurised and cooled to a very low temperature, 20.268 K (−423.188 °F). Once liquified it can be maintained as a liquid in pressurised and thermally insulated 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 to cool the engine nozzle and other parts before being mixed with the oxidizer and burned.fr:LH2 (astronautique) ja:液体水素 zh:液态氢