Developed from HOTOL (Horizontal Take-Off and Landing) project Skylon is a series of designs for a single-stage-to-orbit spaceplane by the British company Reaction Engines Limited (REL), using SABRE, a combined-cycle, air-breathing rocket propulsion system. The vehicle design is for a hydrogen-fuelled aircraft that would take off from a purpose-built runway, and accelerate to Mach 5.4 at 26 kilometres (16 mi) altitude (compared to typical airliners' 9–13 kilometres (6–8 mi)) using the atmosphere's oxygen before switching the engines to use the internal liquid oxygen (LOX) supply to take it into orbit.