Liquid Hydrogen (LH2) (H₂) is a cryogenic fuel propellant used as a fuel. Extremely difficult — deep cryogenic, rapid boil-off. Typical ISP: 420–465 s (with LOX) seconds.
Liquid hydrogen is the highest-performing chemical rocket fuel by specific impulse, offering unmatched exhaust velocity when burned with liquid oxygen. However, its extremely low density and cryogenic requirements (-253°C) impose significant engineering challenges. It is the fuel of choice for upper stages and missions where performance is paramount over vehicle size.
First used on the Centaur upper stage in 1963. Powered the Saturn V upper stages, Space Shuttle main engines, and SLS core stage.
Highest Isp of any chemical propellant, clean exhaust (water vapor), non-toxic
Very low density requires huge tanks, extreme cryogenic handling, hydrogen embrittlement risk
