Officials from the ESA, the UK Space Agency, and their industrial partner Reaction Engines have been developing a SABRE engine prototype for nearly ten years, and for good reason. If successful, a SABRE engine could dramatically reduce the overall engine weight of a rocket and provide a reusable thruster for orbital flight. These two attributes could make space flight less expensive and more frequent.
To achieve these aims, a SABRE engine acts like a traditional jet engine, breathing in air as it accelerates toward the Mach 5 threshold. Once it passes Mach 5, the SABRE transitions to a rocket engine and rapidly accelerates through the Mach-teens, up towards a theoretical top end of Mach 25. That’s somewhere in the range of 17,500 mph (28,200 km/h).
While that simplistic overview of the SABRE makes travelling at hypersonic speed seem trivial, there are several critical physical issues with flying at such a velocity.
Chief among these issues is controlling the heat generated while the SABRE accelerates to Mach 5. During this phase of flight, the air flowing through the engine is whipping through at such a rate that the friction being exerted on the engine’s surfaces would heat and melt any material. To solve this issue, engineers have been developing a precooler that recirculates compressed helium to cool the incoming air. During this recirculation process, ambient air traveling through the fore of the engine is cooled from nearly 1000°C to -150°C in a fraction of a second. Using this sub-arctic breath of air, the SABRE can accelerate to speeds that aren’t possible with normal jet engines. Because the SABRE is already travelling at hypersonic speed when it fires up its rocket engine phase, vast amounts of heavy rocket fuel can be eliminated from the engine payload, making the overall weight of a space-faring vehicle lighter, and capable of carrying more cargo.
Within the next 18 months, Reaction Engines, the ESA and the UK Space Agency expect to begin testing their first SABRE engine. If those tests prove successful, a new era of frequent and less expensive space flight might emerge.