The SABRE engine concept is a contraption that lies at the hart of the Skylon reusable space plane project being developed by Reactoin Engines.
The SABRE (Synergetic Air-Breathing Rocket Engine) acts as a jet in the atmosphere and a rocket in space. This allows it to reduce the propellant load and means it doesn't need to carry as much oxygen. Once in space, the SABRE engine concept switches to rocket mode to help the space plane achieve the great speeds required to reach orbit.
Reaction Engines are currently working on coming up with concept vehicles that would be able to use the SABRE engine. This project holds exciting applications for the space launch industry as well as the military.