Due to the static thrust capability of the hybrid rocket engine, the vehicle can take off under air-breathing mode, much like a conventional turbojet.[3] As the craft ascends and the outside air pressure drops, more and more air is passed into the compressor as the effectiveness of the ram compression drops. In this fashion the jets are able to operate to a much higher altitude than would normally be possible.
At Mach 5.5 the air-breathing system becomes inefficient and is powered down, replaced by the on-board stored oxygen which allows the engine to accelerate to orbital velocities (around Mach 25).
https://en.wikipedia.org/wiki/SABRE_(rocket_engine)#Engine
“which allows the engine to accelerate to orbital velocities (around Mach 25).”
The heat from friction at that speed will be a problem.
L
Thanks for the article. This is an excellent explanation of a combined cycle engine.