Why has liquid hydrogen been used as a rocket fuel?

Liquid hydrogen is utilized as a rocket fuel primarily because it offers the highest exhaust velocity of any fuel, which is a critical factor in achieving high-performance propulsion.12

In the context of liquid-propellant rocket engines, the efficiency of the engine is measured by its specific impulse, which is directly related to the exhaust velocity. Because hydrogen has a very low molecular weight, it produces reaction products with lower molecular masses, which, combined with high combustion temperatures, results in superior exhaust velocities compared to other fuels.13

Despite these performance advantages, liquid hydrogen presents significant engineering challenges. It is a cryogenic substance, meaning it must be stored at extremely low temperatures to remain in a liquid state, and it has a very low density, which requires larger, lightweight fuel tanks. Consequently, while it is often the preferred fuel for upper-stage applications where high performance is essential, engineers must balance its use against the practical requirements of storage and tank design.1