Skylon is a design for a single-stage-to-orbit spaceplane by the British company Reaction Engines Limited (REL), using SABRE (Synergistic Air-Breathing Rocket Engine), a combined-cycle, air-breathing rocket propulsion system, potentially reusable for 200 flights. 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 using the atmosphere's oxygen before switching the engines to use the internal liquid oxygen (LOX) supply to take it into orbit. Once in orbit it would release its payload (of up to 15 tonnes). The vehicle will be unpiloted, but also be certified to carry passengers. All payloads could be carried in a standardised container compartment. The relatively light vehicle would then re-enter the atmosphere and land on a runway, being protected from the conditions of re-entry by a ceramic composite skin. When on the ground, it would undergo inspection and necessary maintenance. If the design goal is achieved, it should be ready to fly again within two days.