An air-precooling system before compression is indispensable to extend the flight envelope and the improvement of the performance of turbo-based air breathing engines for the space plane. One of the critical problems on a shell-and-tube-type precooler is a deterioration of its heat exchange and pressure recovery performance due to the thick frost formation on its tube surface. An innovative method is proposed to mix a condensable additive like ethanol, methanol, etc. in the airflow as a defrosting system. The defrosting effectiveness and essential factors on the additive were investigated by using a small heat exchanger under two different cooling temperature conditions, that is, lower and higher cooling wall temperatures than the melting point of mixture of the water vapor and the additive. It was cleared in the test that most of the alcohols had good effectiveness with methanol the best. This methanol addition concept was applied in the precooler of the practical ATREX engine. A methanol injection system worked well and the thick frost layer formed on the tube surface at the entrance side of precooler could be eliminated. The required methanol mass along the ATREX engine flight path is estimated to be less than 3% of fuel hydrogen consumption.
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