High Quality Content by WIKIPEDIA articles! A precooled jet engine is a concept for high speed jet engines that features a cryogenic fuel-cooled heat exchanger immediately after the air intake, to precool the air entering the engine. After gaining heat and vapourising in the heat exchanger system, the fuel (e.g. H2) is burnt in the combustor. Precooled jet engines have never flown, but are predicted to have much higher thrust and efficiency at speeds up to Mach 5.5. Precooled jet engines were described by Robert P. Carmichael in 1955.