For higher flight speeds precooling may feature a cryogenic fuel-cooled heat exchanger before the air enters the compressor. After gaining heat and vapourising in the heat exchanger, the fuel (e.g. H2) burns in the combustor. Precooling using a heat exchanger has not been used in flight, but is predicted to have significantly high thrust and efficiency at speeds up to Mach 5.5. Precooled jet engine cycles were analyzed by Robert P. Carmichael in 1955. Pre-cooled engines avoid the need for an air condenser because, unlike liquid air cycle engines (LACE), pre-cooled engines cool the air without liquefying it.