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. , 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.
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. Skylon has been designed with the target of achieving no less than 200 flights per vehicle. According to the company, its business plan is to sell vehicles for $1 billion each, for which it has forecast a market for at least 30 Skylons, while recurring costs of just $10 million per flight are predicted to be incurred by operators. While the Reaction intends to manufacture some components directly, such as the engine precooler, other components have been designed by partner companies and a consortium of various aerospace firms is expected to handle full production of Skylon.
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. Amongst other prospective operations, Skylon would be capable of transporting payloads of up to 10-tonnes to the International Space Station. 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.
Funding Speaking in June 2011, Reaction estimated that it would require ultimately $12 billion to achieve an operational configuration, which was then estimated to be achieved around 2020, dependent on funding. The obtaining of additional financing for the Skylon programme from the British government has often been difficult. During 2000, Reaction issued an ultimately unsuccessful request for funding from the British government; according to the government, Reaction's proposal had involved an offer of a potentially large return on its investment. 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."
During February 2009, following on from a series of extended discussions with the British National Space Centre (which later became the UK Space Agency), it was announced that a major funding agreement had been established between the British National Space Centre, ESA and Reaction, committing €1 million ($1.28 million) for the purpose of producing a demonstration engine for the Skylon programme by 2011. 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. As a consequence of the enactment of the Technology Demonstration Programme, Reaction was able to transition from a Technology Readiness Level (TRL) of 2/3 up to 4/5 within a matter of months.
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. On 13 April 2011, Reaction announced that the design of the Skylon had passed several rigorous independent reviews. On 24 May 2011, ESA publicly declared the design to be feasible, having found "no impediments or critical items" to be present in the proposal.Emspak, Jesse. "REL’s Skylon spaceplane aims to take on SpaceX with a reusable rocket design." theverge.com, 8 March 2016. Speaking on the topic of Skylon in 2011, David Willetts, the UK Minister of State for Universities and Science, stated:
In June 2013, George Osborne, then Chancellor of the Exchequer, stated that the British government would be giving £60 million towards the further development of the SABRE engine."@George_Osborne". Twitter, 27 June 2013. The grant was contingent upon Reaction having an industrial partner. The first grant of £50 million was approved by the European Commission in August 2015.
In October 2015, British defence conglomerate BAE Systems entered into an agreement with Reaction Engines, under which it would invest £20.6 million in Reaction to acquire 20% of its share capital, as well as to provide assistance in the development of the SABRE engine.
In July 2016, the second grant of £10 million was approved by ESA.
In September 2017, it was announced that the U.S. Defense Advanced Research Projects Agency (DARPA) had awarded a contract to Reaction Engines for an undisclosed amount to conduct high-temperature airflow testing at a Colorado, United States site of a Reaction Engines precooler called HTX. Testing work was scheduled to start in 2018, and in March 2019 the performance of the precooler was validated at conditions simulating Mach 5.
In April 2018, Reaction Engines announced Boeing and Rolls-Royce would be joining BAE Systems as investors in the development of the SABRE engine. A total of $37.5 million of new funding is to be provided including contributions from Baillie Gifford Asset Management and Woodford Investment Management.
Development
In 2000, the firm completed work with University of Bristol testing the precooler.Wang, Brian. "Progress to the Skylon Single Stage to Orbit Spaceplane for 2019." Next Big Future, 31 January 2009.
From 2007 to 2009 Reaction worked with University of Bristol and Airborne Engineering on Project STERN (Static Test Expansion/Deflection Rocket Nozzle), which tested Reaction's engine ignition system, a Reaction designed air breathing hydrogen rocket engine, and investigated the flow stability and behaviour of Dr Neil Taylor's expansion deflection nozzle design via multiple test-firings by Airborne Engineering. An expansion deflection nozzle is capable of compensating for the changing ambient pressure encountered while gaining altitude during atmospheric flight, thus generating greater thrust and thereby efficiency.