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Getting anywhere in the world within four hours seems impossible, but a UK company, Reaction Engines Ltd., successfully completed testing of a new propulsion system that could one day make that possible.
The idea behind the REL system is similar to what we now see with current jetliners, but would function as a rocket.
Called SABRE, it burns hydrogen and oxygen but gets the oxygen from the atmosphere, much like a jet engine breathes fuel. The system would be affixed to a 275-foot Skylon spaceplane. Taking off and landing like a regular airbus, the vehicle would then burn its onboard fuel supply once it reaches speeds of about Mach 5.
Because the engine gathers its oxygen supply from the air for take-off, a higher thrust-to-weight ratio is achieved in the absence of on-board fuel.