Liquid hydrogen is the element hydrogen, in the liquid state. It 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.

At normal temperature and pressure, Hydrogen 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/-252.882222°C). Liquid hydrogen is typically used as a practical form of storing hydrogen. As in any gas, storing it as liquid takes less space them storing it as a gas at normal temperature and pressure. Once liquified it can be maintained as a liquid in pressurised and thermally insulated containers. Liquified Hydrogen can be used as a fuel in an internal combustion engine not much different from the engines used with gasoline. Various concept Hydrogen vehicles were already built using this form of hydrogen mainly by BMW.

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. Even with thermally insulated containers it is impossible to avoid the heating of such a cold liquid, and the hydrogen will gradually evaporate, typically 1.7 percent per day.

In terms of energy density, liquid hydrogen takes a lot more space them other fuels to store the same amount of energy. You need 4 gallons of Liquid Hydrogen to match the same energy content of one gallon of gasoline. On the other hand it is also one of lightest fuels avaiable, 1 Liter of hydrogen weights only 0.07 kg. That's a density of 70.8 kg/m³ (at 20 K).

Hydrogen | Rocket fuels | Coolants | Cryogenics

LH2 (astronautique) | 液体水素 | 液态氢