The SABRE engine

One of the problems with building a true spaceplane is that an engine needs to operate within the atmosphere of Earth and the vacuum of space. Reaction Engines is currently hard at work on its futuristic spaceplane known as Skylon, which will employ a revolutionary engine known as SABRE that enables the operation of the vehicle in both environments. While taking off from the ground the engine remains in air-breathing mode, drawing in oxygen like a jet engine. When above the atmosphere it enters rocket mode, using onboard liquid oxygen to reach orbital speeds. Components of the engine have already been tested, and it could be one of the most significant advancements in spaceplane technology yet.