An idea originated by Robert P. Carmichael in 1955[REF] is that hydrogen-fueled engines could theoretically have much higher performance than hydrocarbon-fueled engines if a heat exchanger were used to cool the incoming air.
This idea leads to plausible designs like Reaction Engines SABRE, that might permit single-stage-to-orbit launch vehicles,[REF] and ATREX, which could permit jet engines to be used up to hypersonic speeds and high altitudes for boosters for launch vehicles.