LH2

Liquid hydrogen (LH2) is hydrogen gas cooled below its boiling point to a liquid at cryogenic temperatures. It has a boiling point of about 20.28 K (−252.87 °C) at 1 atmosphere, a very low density when liquid (around 70–71 kg/m3), and the lowest molecular mass of any element. Its energy content is high by mass (approximately 120–142 MJ/kg), but its volume energy density is low compared with hydrocarbon fuels, making storage and handling complex. Hydrogen is colorless and odorless; when burned in air it forms water, but its flammable range in air is wide, roughly 4–75% by volume, and ignition energy is very low.

Production and storage involve cryogenic technologies. LH2 is mainly produced from natural gas via steam methane reforming or by electrolysis of water. It is stored in specialized cryogenic dewars with vacuum insulation to minimize heat input and boil-off. Because of the extremely low temperature, materials must resist hydrogen embrittlement, and systems require careful venting and pressure control. Boil-off losses are a practical consideration in long-term storage and transportation.

Uses of LH2 are dominated by aerospace propulsion and energy research. In rocket engines, LH2 is used with liquid oxygen (LOX) to achieve high specific impulse, as in many launch vehicles and space programs. LH2 is also used in fuel cells and other experimental energy systems. Environmental impacts are favorable in combustion, as it yields only water vapor; however, production and handling energy and safety concerns remain significant considerations.