Reaction Engines' SABRE precooler was a real propulsion breakthrough

Cooling Mach-5 intake air fast enough to protect a lightweight engine remains one of the hardest heat-transfer problems in aerospace

it's easy to dunk on SABRE. it's harder to admit they cracked one of the nastiest heat-transfer problems in propulsion: rapidly cooling air hot enough to melt the engine going at Mach 5. especially in that small and light of a form factor d