HOTOL, for Horizontal Take-Off and Landing, was a 1980s British design for a single-stage-to-orbit (SSTO) reusable spaceplane that was to be powered by an airbreathing jet engine. Development was being conducted by a consortium led by Rolls-Royce and British Aerospace (BAe).
HOTOL was to be fitted with a unique air-breathing engine, the RB545 or Swallow, that was under development by British engine manufacturer Rolls-Royce. The propellant for the engine technically consisted of a combination of liquid hydrogen/liquid oxygen; however, it was to employ a new means of dramatically reducing the amount of oxidizer needed to be carried on board by utilising atmospheric oxygen as the spacecraft climbed through the lower atmosphere. Since the oxidizer typically represents the majority of the takeoff weight of a rocket, HOTOL was to be considerably smaller than normal pure-rocket designs, roughly the size of a medium-haul airliner such as the McDonnell Douglas DC-9/MD-80.
While HOTOL's proof-of-concept design study was being carried out, attempts were made by both industry and the British government to establish international cooperation to develop, produce, and deploy the spacecraft. Despite American interest in the programme, there was little appetite amongst the members of the European Space Agency (ESA), and the British government was not prepared to depart from ESA cooperation. Additionally, technical issues were encountered, and there were allegations that comparisons with alternative launch systems such as conventional rocket vehicle using similar construction techniques failed to show much advantage to HOTOL.
In 1989, funding for the project ended. The termination of development work on HOTOL led to the formation of Reaction Engines Limited (REL) to develop and produce Skylon, a proposed spacecraft based on HOTOL technologies, including its air-breathing engine.
Development
Origins
The ideas behind HOTOL originated from work done by British engineer Alan Bond in the field of pre-cooled jet engines. Bond had specifically performed this research with the intention of producing a viable engine for powering a space launch system. In 1982, British Aerospace (BAe), which was Europe's principal satellite-builder, began studying a prospective new launch system with the aim of providing launch costs that were 20 per cent of the American Space Shuttle operated by NASA. BAe became aware of work by British engine manufacturer Rolls-Royce on a suitable engine, and soon conceived of an uncrewed, fully reusable single-stage-to-orbit (SSTO) winged spaceplane as a launch vehicle.
Thus, the project had soon become a joint venture between BAe and Rolls-Royce, led by John Scott-Scott and Bob Parkinson. Early on, there was an ambition to 'Europeanise' the project and to involve other nations in its development and manufacture as it was recognised that an estimated £4 billion would be needed to fund full-scale development. In August 1984, BAe unveiled a public display of the HOTOL satellite launcher project and released details on its proposed operations. In particular, some of the analysis seemed to indicate that similar technology applied to a pure rocket approach would give approximately the same performance at less cost.
Shutdown
By 1989, the outlook for HOTOL had become bleak. From the onset of the project, support between the British government and industrial partners had been uneven, while the United States had emerged as the only foreign nation that showed willingness to contribute to the programme,
In 1989, HOTOL co-creator Alan Bond and engineers John Scott-Scott and Richard Varvill formed Reaction Engines Limited (REL) which worked on a new air-breathing engine, SABRE, which used alternative designs to work around (and improve upon) the Rolls-Royce patents, and the Skylon vehicle intended to solve the problems of HOTOL. They first published these engine and spacecraft concepts in 1993, and continued developing the core technologies, particularly the engine and its frost-controlled pre-cooler; initially supported by private funding, but latterly with support from the European Space Agency, the British National Space Centre, the United Kingdom Space Agency, BAe, and the Air Force Research Laboratory. REL planned to demonstrate a flight-ready pre-cooler operating under simulated flight conditions in 2018, and statically test a demonstration engine core in 2020. REL fell into administration in 2024, ceasing all operations. Neither SABRE (a full-scale working engine; partial prototypes and components for testing not counted) nor Skylon were ever built.
Design
Overview
HOTOL was envisaged as an uncrewed, fully reusable single-stage-to-orbit (SSTO) winged spaceplane. The uncrewed craft was intended to put a payload of around 7 to 8 tonnes in orbit, at 300 km altitude. It was intended to take off from a runway, mounted on the back of a large rocket-boosted trolley that would help get the craft up to "working speed". The engine was intended to switch from jet propulsion to pure rocket propulsion at 26–32 km high, by which time the craft would be travelling at Mach 5 to 7. After reaching low Earth orbit (LEO), HOTOL was intended to re-enter the atmosphere and glide down to land on a conventional runway (approx 1,500 metres minimum). Only a single payload would have been carried at a time as BAe had judged this to be more economic as it removed any need for satellite interfacing and allowed for missions to be tailored to individual requirements.
During its high-altitude phase, its flight control system would have been linked to ground stations and to space-based global navigation system navigation, while radar would have been used during the take-off and landing phases. In addition to the placing of satellites into geosynchronous orbit or LEO, HOTOL was also projected as being able to also perform the retrieval of satellites and hardware from LEO. BAe promotional material depicts HOTOL docking with the International Space Station (ISS), a feat that the company claimed would have required crewed operation as automated systems were not capable of performing such docking manoeuvres at that time. HOTOL was designed to conduct fully automated uncrewed flights; however, it had been intended at a later stage to potentially re-introduce a pilot. Crewed operations would have required the installation of a dedicated pressurised module within the payload bay.
As designed, HOTOL would have been 62 metres long, 12.8 metres high, a fuselage diameter of 5.7 metres and a wingspan of 19.7 metres.
The final vehicle design (HOTOL-K) had a take-off mass of 275 tonnes. Approximately 82% of that mass was propellant with the vehicle structure being a further 16%. This left just 2% (approximately 5 tonnes) for the payload, leaving little margin for design changes to the basic vehicle structure.
It featured a wing design that had been derived from that of Concorde; its large area resulted in relatively low wing loading, which would have resulted in lower reentry temperatures (never rising above 1,400 °C). Built out of carbon composite materials, there would have been no need for the use of insulating tiles akin to those that comprised the Space Shuttle thermal protection system. The internally stowed landing gear would have been too small to carry the weight of the fully fuelled rocket, so emergency landings would have required the fuel to be dumped.
Almost the entire forward fuselage, ahead of the payload bay, comprised a single hydrogen tank.
HOTOL was designed to have a vertical fin, just aft of the nose, for lateral stability. However, later discussions with NPO Molniya about Interim Hotol made it clear that the fin would have been subject to excessive heating on re-entry (due to interactions between two shock-waves). Consequently, Interim Hotol switched back to a more conventional (but larger) tail-fin.