annodomini2 wrote:
Air with a turbine is only practical up to about Mach 2.5 at which point the air temperature entering the engine is so high that it literally melts the engine.
You can use a pre-cooler such as the SABRE design from Reaction Engines in the UK http://www.reactionengines.co.uk/
They are aiming to be able to have one engine that can seamlessly accelerate from 0 through mach 5, then transition to using a small onboard oxygen tank to morph into a rocket for the last bit to orbit, then all the way back.
(incidentally they are based on the same site at Culham where the JET and MAST tokamaks are)
