Question

Why is liquid hydrogen used in spacecraft?

Answer 1

Rocket propellant is a material used either directly by a rocket as the reaction mass (propulsive mass) that is ejected, typically with very high speed, from a rocket engine to produce thrust, and thus provide spacecraft propulsion, or indirectly to produce the reaction mass in a chemical reaction. Each rocket type requires a different kind of propellant: chemical rockets require propellants capable of undergoing exothermic chemical reactions, which provide the energy to accelerate the resulting gases through the nozzle. Thermal rockets instead use inert propellants of low molecular weight that are chemically compatible with the heating mechanism at high temperatures, while cold gas thrusters use pressurized, easily stored inert gases. Electric propulsion requires propellants that are easily ionized or made into plasma, and in the extreme case of nuclear pulse propulsion the propellant consists of many small, non-weapon nuclear explosives of which the resulting shock wave propels the spacecraft away from the explosive, thereby creating propulsion. One such spacecraft was designed (but never built), being dubbed "Project Orion" (not to be confused with the NASA Orion spacecraft).

Answer 2

Liquid hydrogen is used as a fuel in spacecraft. Hydrogen is a light and extremely powerful rocket propellant has the lowest molecular weight of any known substance and burns with extreme intensity (5,500°F). In combination with an oxidizer such as liquid oxygen, liquid hydrogen yields the highest specific impulse, or efficiency in relation to the amount of propellant consumed, of any known rocket propellant.