The J-2: a high-energy engine

While the F-l used conventional kerosene-type fuel for brute force, the J-2 achieved almost double the efficiency through the use of relatively exotic liquid hydrogen. However, despite being more efficient, it could not match the raw power levels attained by the F-l, which made it more suitable for an upper stage. A single engine could balance over 100 tonnes and it could be restarted in space. It traced its origins to work done in the 1950s to create a hydrogen-burning rocket engine, but its development funding came solely from NASA who wanted the inherent benefits of hydrogen applied to its Saturn vehicles.