Precooler
As the air enters the engine at supersonic/hypersonic speeds, it becomes very hot due to compression effects. The high temperatures are traditionally dealt with in jet engines by using heavy copper or nickel based materials, by reducing the engine's pressure ratio, and by throttling back the engine at the higher airspeeds to avoid melting. However, for an SSTO craft, such heavy materials are unusable, and maximum thrust is necessary for orbital insertion at the earliest time to minimise gravity losses. Instead, using a gaseous helium coolant loop, SABRE dramatically cools the air from 1000 °C down to −150 °C[18] in a heat exchanger while avoiding liquefaction of the air or blockage from freezing water vapour.
Previous versions of precoolers such as HOTOL put the hydrogen fuel directly through the precooler. SABRE inserts a helium cooling loop between the air and the cold fuel to avoid problems with hydrogen embrittlement in the precooler.
The dramatic cooling of the air created a potential problem: it is necessary to prevent blocking the precooler from frozen water vapour and other air fractions. As of October 2012, the cooling solution was demonstrated for 6 minutes using freezing air.[19] The cooler consists of a fine pipework heat exchanger and cools the hot in-rushing atmospheric air down to the required −150 °C in 0.01s
