Yes, it keeps the bell of the nozzle from melting off the back of the rocket. And that is a really cool trick, extremely tough to pull off in the original F1 en

Yes, it keeps the bell of the nozzle from melting off the back of the rocket. And that is a really cool trick, extremely tough to pull off in the original F1 engines of the Saturn V (lots of welds) but easier to do now that you can 3D print the engine. However, the SABRE has a different challenge. Rather than cool metal (Inconel) which is a very good conductor of heat, the SABRE heat exchanger is cooling the air which has gained temperature for becoming compressed by the moving aircraft. Air, for all of its wonderful properties, is not a particularly good heat conductor[1]. Further, since heat transfer by conduction is a function of time in which the two dissimilar temperature materials are in contact, and hyper sonic craft are going very fast, it has to achieve this heat transfer in a literal "blink of an eye." It has been compared to shooting an acetylene torch through it and having a cool breeze come out the other side. Really amazing stuff. [1] This is fortunate as it makes down coats warm rather than cold.