Liquid hydrogen

Liquid hydrogen is hydrogen cooled below its boiling point, about 20.3 kelvins at atmospheric pressure. It is a cryogenic liquid used chiefly as a chemical feedstock and rocket propellant.

Hydrogen liquefaction: The process of cooling and compressing hydrogen until it becomes liquid. Liquefaction requires staged cooling because hydrogen does not cool readily through ordinary expansion.
Hydrogen fuel: Hydrogen used as an energy carrier in combustion or electrochemical systems. Liquid hydrogen is one storage form for hydrogen fuel, alongside compressed gas and chemical carriers.
Compressed hydrogen: Hydrogen stored as a gas under high pressure in a containment vessel. It avoids refrigeration but generally stores less hydrogen per tank volume than the liquid.
Cryogenic engineering: The engineering of materials, equipment, and processes operating at very low temperatures. Liquid hydrogen systems require cryogenic vessels, transfer lines, seals, and thermal management.
Joule–Thomson effect: The temperature change of a real gas during expansion at constant enthalpy. Hydrogen must be precooled before throttling, since expansion at room temperature warms it.
Space Shuttle External Tank: The Space Shuttle component that held liquid hydrogen and liquid oxygen for its main engines. Its enormous insulated tank illustrates the volume required to carry low-density liquid hydrogen.
Metal hydride: A compound formed when hydrogen atoms occupy sites within a metal or alloy structure. Metal hydrides store hydrogen without cryogenic temperatures but add substantial material mass.
Hydrogen safety: Practices and engineering controls for preventing and managing hazards involving hydrogen. Leaks can ignite easily, while cold liquid and rapidly expanding vapor create additional hazards.
Ortho-para hydrogen conversion: The conversion between hydrogen molecules with parallel and antiparallel proton nuclear spins. Conversion releases heat inside stored liquid unless it is promoted during liquefaction.
Saturn V: The American launch vehicle that sent Apollo astronauts toward the Moon. Its upper stages burned liquid hydrogen with liquid oxygen for high-efficiency propulsion.

Linked from 9 pages