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 altitude (compared to typical airliner's ) 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 of cargo to an equatorial low Earth orbit (LEO); up to to the International Space Station, almost 45% more than the capacity of the European Space Agency's Automated Transfer Vehicle; or 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 (), 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. 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.
thumb|left|The Skylon was developed from the British [[HOTOL project.]]
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.
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.
thumb|left|A computer-generated image of the Skylon spaceplane climbing to orbit.
Reaction intends ultimately to operate as a for-profit commercial enterprise which, upon the completion of development, shall manufacture Skylon vehicles for multiple international customers who shall operate their fleets directly, while being provided with support from Reaction.
In service, Skylon could potentially lower the cost of launching satellites which, according to evidence submitted to the UK parliament by Reaction, is forecast to be around £650/kg; as of 2011, the average launch cost using conventional methods was estimated to be roughly £15,000/kg. Reaction has also completed internal studies into the use of Skylon as a launch platform for a network of space-based solar power satellites, which have been historically unfeasible due to high launch costs. However, several officials have emerged as proponents and advocated for the official backing of the Skylon programme. Speaking in 2009, the former UK Minister for Science and Innovation, Lord Drayson, stated of Reaction: "This is an example of a British company developing world-beating technology with exciting consequences for the future of space." The initiative, known as the Technology Demonstration Programme, was scheduled to last approximately 2.5 years, during which further financing in the form of €1 million was made available by ESA. The 2009 agreement allowed Reaction to involve several external companies, including EADS-owned Astrium, the University of Bristol and the German Aerospace Center (DLR), in further development work.
By 2012, according to the UK Space Agency, the funding required to develop and construct the entire craft has not yet been secured; as such, research and development work was at that point mainly focused on the engines alone, which was supported by an ESA grant of €1 million. In January 2011, Reaction submitted a proposal to the British Government requesting additional funding for the Skylon project. Speaking on the topic of Skylon in 2011, David Willetts, the UK Minister of State for Universities and Science, stated: