The precooled jet engine is a concept that enables jet engines with turbomachinery, as opposed to ramjets, to be used at high speeds. Precooling restores some or all of the performance degradation of the engine compressor (by preventing rotating stall/choking/reduced flow), as well as that of the complete gas generator (by maintaining a significant combustor temperature rise within a fixed turbine temperature limit), which would otherwise prevent flight with high ram temperatures. 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. For lower flight speeds precooling can be done with mass injection, known as WIPCC (water injection precompressor cooling) This method has been used for short duration (due to limited coolant capacity) increases to an aircraft's normal maximum speed. "Operation Skyburner", which gained a world speed record with a McDonnell Douglas F-4 Phantom II, and the Mikoyan Ye-266 (Mig 25). Both used a water/alcohol spray to cool the air ahead of the compressor. Precooling (as well as combustion chamber water injection) is used at the lowest flight speeds, i.e. during take off, to increase thrust at high ambient temperatures. Advantages and disadvantages of using precooler heat exchangers One main advantage of pre-cooling is (as predicted by the ideal gas law) for a given overall pressure ratio, there is a significant reduction in compressor delivery temperature (T3), which delays reaching the T3 limit to a higher Mach number. Consequently, sea-level conditions (corrected flow) can be maintained after the pre-cooler over a very wide range of flight speeds, thus maximizing net thrust even at high speeds. The compressor and ducting after the inlet is subject to much lower and more consistent temperatures, and hence may be made of light alloys. This reduces the weight of the engine, which further improves the thrust/weight ratio. Hydrogen is a suitable fuel because it is liquid at deeply cryogenic temperatures, and over its useful range has a