Liquid hydrogen

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.

To exist as a liquid, H₂ must be pressurised and cooled to a very low temperature, 20.27 K (−423.17 °F/−252.87°C) . One common method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a practical form of storing hydrogen. As in any gas, storing it as liquid takes less space than 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. Various concept hydrogen vehicles have been 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 (regenerative cooling) and other parts before being mixed with the oxidizer (often liquid oxygen (LOX)) and burned. In such a rocket, even with thermally insulated containers it is impossible to avoid the heating of such a cold liquid, and the hydrogen will gradually evaporate. The resulting exhaust of such LH2 - LOX engines is very clean water with traces of ozone and hydrogen peroxide.

In terms of energy density, liquid hydrogen requires much more volume than other fuels to store the same amount of energy. Four litres of liquid hydrogen are needed to match the same energy content of one litre of gasoline. On the other hand it is also one of the lightest fuels available; 1 litre of hydrogen weighs only 0.07 kg. That is a density of 70.8 kg/m³ (at 20 K).