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 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, but inserting a helium cooling loop between the air and the cold fuel avoids problems with hydrogen embrittlement in the air precooler.
However, the dramatic cooling of the air raised a potential problem: it is necessary to prevent blocking the precooler from frozen water vapour and other fractions. As of November 2012, the cooling solution itself has been demonstrated, comprising of a fine pipework heat exchanger, able to cool the hot in-rushing atmospheric air down to the required -150 °C in 0.01s
Read more about this topic: SABRE (rocket Engine), Design