part of Japan's ISAS (now JAXA) study into an Air-Turbo Ramjet (ATR, later ATREX after the addition of an expander cycle) intended to power the first stage of a TSTO spaceplane.
A liquid nitrogen precooled hydrogen burning test engine was flown at Mach 2 at Taiki Aerospace Research Field in September 2010.
[2] For higher flight speeds, precooling may feature a cryogenic fuel-cooled heat exchanger before the air enters the compressor.
"Operation Skyburner", which gained a world speed record with a McDonnell Douglas F-4 Phantom II,[4] and the Mikoyan Ye-266 (Mig 25)[5] 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.
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.