Skylon was a series of concept designs for a reusable single-stage-to-orbit spaceplane by the British company Reaction Engines Limited, 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 specially built reinforced runway, and accelerate to Mach 5.4 at 26 kilometres (85,000 ft) altitude (compared to typical airliner's 9–13 kilometres or 30,000–40,000 feet) using the atmosphere's oxygen before switching the engines to use the internal liquid oxygen (LOX) supply to accelerate to the Mach 25 necessary to reach a 400 km orbit. It would carry 17 tonnes (37,000 lb) of cargo to an equatorial low Earth orbit (LEO); up to 11 tonnes (24,000 lb) to the International Space Station, almost 45% more than the capacity of the European Space Agency's Automated Transfer Vehicle; or 7.3 tonnes (16,000 lb) to Geosynchronous Transfer Orbit (GTO). The relatively light vehicle would re-enter the atmosphere and land on a runway, being protected from the conditions of re-entry by a ceramic matrix composite skin. When on the ground, it would undergo inspection and necessary maintenance, with a turnaround time of approximately two days, and be able to complete at least 200 orbital flights per vehicle. In paper studies, the cost per kilogram (kg) of payload carried to LEO in this way is hoped to be reduced from the current £1,108/kg (as of December 2015), including research and development, to around £650/kg (718USD/kg), with costs expected to fall much more over time after initial expenditures have amortised. In 2004, the developer estimated the total lifetime cost of the Skylon C1 programme to be about $12 billion. As of 2017, only a small portion of the funding required to develop and build Skylon had been secured. For the first couple of decades the work was privately funded, with public funding beginning in 2009 through a European Space Agency (ESA) contract. The British government pledged £60 million to the project on 16 July 2013 to allow a prototype of the SABRE engine to be built; contracts for this funding were signed in 2015. Reaction Engines conducted tests of components of the SABRE engine in 2012 and 2024. Later in 2024, the company entered administration.
Research and development programme
Background and early work Skylon has its origins within a previous space development programme for an envisioned single-stage-to-orbit (SSTO) spaceplane, known as HOTOL. In 1982, when work commenced on the HOTOL by several British companies, there was significant international interest to develop and produce viable reusable launch systems, perhaps the most high-profile of these being the NASA-operated Space Shuttle. In conjunction with British Aerospace and Rolls-Royce, a promising design emerged to which the British government contributed £2 million towards its refinement; British engineer Alan Bond was amongst the engineers who worked on HOTOL. However, during 1988, the British government decided to withdraw further funding from the programme, resulting in development work being terminated. Aerospace publication Flight International observed that HOTOL and other competing spaceplane programmes were "over-ambitious" and that development on such launch systems would involve more research and slower progress than previously envisioned.
Following the setback of HOTOL's cancellation, in 1989 Alan Bond, along with John Scott-Scott and Richard Varvill decided to establish their own company, Reaction Engines Limited, to pursue the development of a viable spaceplane and associated technology using private funding. In 1993, Reaction publicly revealed its spaceplane proposal, which it named Skylon after the Skylon structure that had inspired Alan Bond at the Festival of Britain exhibition. Skylon was a clean sheet redesign based on lessons learned during development of HOTOL, the new concept again utilised dual-mode propulsion system, using engines that could combust hydrogen with the external air during atmospheric flight. Early on, Skylon was promoted by the company to the ESA for its Future European Space Transportation Investigations Programme (FESTIP) initiative, as well as seeking out both government or commercial investment in order to finance the vehicle's development. Reaction has also sought to form ties with other companies with the aim of producing an international consortium of interested firms to participate in the Skylon programme.
Project brief The Skylon design features several distinct differences when compared with the earlier HOTOL program. Whereas HOTOL would have launched from a rocket sled as a weight-saving measure, Skylon is to be equipped with conventional retractable undercarriage. The revised engine design, using the SABRE engine, is expected to offer higher performance than its predecessor. The rear mounted engine of HOTOL meant that the vehicle possessed intrinsically poor in-flight stability; early attempts to resolve this problem had ended up sacrificing much of HOTOL's payload potential, which in turn contributed to the failure of the overall project. Skylon's solution to the issue was to position its engines at the end of its wings, which located them further forward and much closer to the vehicle's longitudinal centre of mass, thereby resolving the instability problem.
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