Skylon is a design for a spaceplane by the British company Reaction Engines Limited (REL). It uses SABRE, a combined-cycle, air-breathing rocket engine, to reach orbit in a single stage. A fleet of vehicles is envisaged; the design is aiming for re-usability for more than 200 times. In paper studies, the costs per kilogram of payload are hoped to be lowered from the current £15,000/kg to £650/kg (as of 2011), including the costs of research and development (R&D), with costs expected to fall much more over time after the initial expenditures have amortised. In 2004, the developer estimated the total lifetime cost of the programme to be about $12 billion.

The vehicle design is for a hydrogen-powered aircraft that would take off from a conventional runway, and accelerate to Mach 5.4 at 26 kilometres (16 mi) altitude using atmospheric air before switching the engines to use the internal liquid oxygen (LOX) supply to take it to orbit. It would then release its payload, which can weigh up to 15 tonnes, and re-enter the atmosphere. The vehicle is to be unpiloted, but certified for carrying people, and the payload would be carried in a standardised payload container or passenger compartment. During re-entry the relatively light vehicle would fly back through the atmosphere and land back at the runway, with its skin protected by a ceramic composite. It would then undergo inspection and any necessary maintenance and, if the design goal is achieved, be able to fly again within two days.

As of 2012, only a small portion of the funding required to develop and build Skylon had been secured. The research and development work on the SABRE engine design is proceeding under a small European Space Agency (ESA) grant. In January 2011, REL submitted a proposal to the British Government to request additional funding for the Skylon project and in April REL announced that they had secured $350 million of further funding contingent on a test of the engine's precooler technology being successful. Testing of the key technologies was successfully completed in November 2012, allowing Skylon's design to advance to its final phase.