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Liquid hydrogen is the liquid state of the element hydrogen. It is a common liquid rocket fuel for rocket applications. In the aerospace industry, its name is often abbreviated to LH2 or LH₂. Hydrogen is found naturally in the molecular H₂ form, hence the H2 part of the name.

To exist as a liquid, H₂ must be pressurized and cooled to a very low temperature, 20.27 K (−423.17 °F/−252.87°C).^[1] 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 pressurized and thermally insulated containers.

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. The resulting exhaust of such LH2 - LOX engines is very clean water with traces of ozone and hydrogen peroxide.

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 (see BMW H2R).

Hydrogen has one the highest gravimetric energy densities of all available fuels, which means it has very high energy content per pound (143 MJ/kg, 40 percent more than other rocket fuels).[1].

As one of the lightest fuels available, one liter of hydrogen weighs only 0.07 kg. That is a density of 70.8 kg/m³ (at 20 K).

Producing "zero emissions", the byproducts of its combustion are mainly water vapor.

In terms of volumetric energy density, liquid hydrogen requires much more volume than other fuels to store the same amount of energy. Four liters of liquid hydrogen are needed to match the same energy content of one liter of gasoline.

Requires complex storage technology such as the special thermally insulated containers.

Even with thermally insulated containers it's difficult to keep such a low temperature, and the hydrogen will gradually evaporate. Typically it will evaporate at a rate of 1% per day.[2]

Requires special handling common to all cryogenic substances. Same as Liquid oxygen.

The last 2 reasons account for the switch to solid propellants in ballistic missiles. The solid fuel remains stable for years inside a missile in contrast with LH2, which delayed the launch by requiring a last refuel.

1. ^ Chemical elements data references