Reaction Engines' Skylon reusable spaceplane project has been given a boost, with analysis by the United States Air Force Research Laboratory (AFRL) confirming the feasibility of the SABRE engine cycle concept that lies at its heart.

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The feasibility study conducted as part of a Cooperative Research and Development Agreement (CRADA) with the AFRL's Aerospace Systems Directorate (AFRL/RQ) looked at the thermodynamic cycle of the SABRE concept. That is, whether the engine is able to do what Reaction Engine claims it can do. According to AFRL, there's no theoretical problem with the concept if the engine is properly built and integrated.

The SABRE (Synergetic Air-Breathing Rocket Engine) is a scramjet. That is, it reduces the propellant load because it acts as a jet while in the atmosphere and a rocket in space, so it doesn't have to carry as much oxygen to burn the liquid hydrogen fuel. It does so at velocities above Mach 5 (4,500 mph, 7,200 km/h) before flying into space, when it switches to rocket mode to achieve the even faster speeds needed to reach orbit.