According to Reaction, an aircraft using such engines could take off from a runway and accelerate to more than five times the speed of sound, before switching to a rocket mode which would propel the aircraft into orbit.
Reaction has designs for such a plane, which it calls Skylon.
One of the challenges for the engineers at Reaction is to manage the very hot air entering the engine at high speed.
These gases have to be cooled prior to being compressed and burnt with onboard hydrogen.
Reaction engineers have developed a module containing arrays of extremely fine piping that can extract the heat and plunge the inrushing air to about -140C in just 1/100th of a second.