Designed as a single-stage-to-orbit (SSTO) reusable winged launch vehicle, it was to be fitted with a unique air-breathing engine, the RB545 or Swallow, to be developed by the Rolls-Royce company. The engine was technically a liquid hydrogen/liquid oxygen design, but dramatically reduced the amount of oxidizer needed to be carried on board by utilising atmospheric oxygen as the spacecraft climbed through the lower atmosphere.
Since propellant 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. Ultimately, comparison with a rocket vehicle using similar construction techniques failed to show much advantage, and funding for the vehicle ceased.
HOTOL would have been 63 metres long, 12.8 metres high, 7 metres in diameter and with a wingspan of 28.3 metres. The unmanned 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 orbit, HOTOL was intended to re-enter the atmosphere and glide down to land on a conventional runway (approx 1,500 metres minimum). HOTOL was designed for automatic, unmanned flights, although later stages would reintroduce a pilot. The internal 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.
The RB545 was an air-breathing rocket engine. The exact details of this engine were covered by the UK's Official Secrets Act, and there is consequently little public information about its operation, although it has apparently been declassified.
Within the atmosphere, hydrogen fuel would be used in a heat exchanger to precool air entering a high overall pressure ratio turbojet-like engine cycle, before being burnt with the air in a rocket motor, to produce a very high velocity propulsive jet.
Once out of the atmosphere, the RB545 would be capable of burning the hydrogen with on-board liquid oxygen as a high-efficiency hydrogen/oxygen rocket. The engine was given the Rolls Royce name "Swallow".
Formal development began with government funding in 1982. The design team was a joint effort between Rolls-Royce and British Aerospace led by John Scott-Scott and Dr Bob Parkinson. About the same time, the Rockwell X-30 scramjet programme was announced in America.
During development, it was found that the comparatively heavy rear-mounted engine moved the centre of mass of the vehicle rearwards. This meant that the vehicle had to be designed to push the centre of drag as far rearward as possible to ensure stability during the entire flight regime. Redesign of the vehicle to do this required a large mass of hydraulic systems, which cost a significant proportion of the payload, and made the economics unclear. 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.
In 1988 the government withdrew further funding. The project was almost at the end of its design phase but the plans were still speculative and dogged with aerodynamic problems and operational disadvantages.
A cheaper redesign, Interim HOTOL or HOTOL 2, to be launched from the back of a modified Antonov An-225 Mriya transport aircraft, was offered by BAe in 1991 but that too was rejected. Interim HOTOL was to have dispensed with an air-breathing engine cycle and was designed to use more conventional LOX and liquid hydrogen.
In 1989, HOTOL co-creator Alan Bond formed Reaction Engines Limited (REL) which has since been working on the Skylon vehicle intended to solve the problems of HOTOL. In November 2012, REL conducted tests on an engine observed by the European Space Agency and declared the tests a success and that a major technical obstacle had been removed. In July 2013 the UK government announced a £60m investment in REL.
- NASP – a scramjet vehicle with which HOTOL would have competed
- Reaction Engines Skylon – a follow on design that attempts to avoid HOTOL's shortcomings
- Reaction Engines A2 – a design for a hypersonic antipodal airliner
- Liquid air cycle engine – a related engine cycle that liquifies the air
- Flight international 1 March 1986
- Dr Bob Parkinson discusses HOTOL in an oral history interview recorded for the National Life Stories project Oral History of British Science at the British Library
- Coppinger, Rob (23 February 2009). "Flight Global: secret files reveal US interest in UK HOTOL spaceplane".
- BBC Four: The Three Rocketeers. Retrieved 14 September 2012.
- "Oral history of British science". British Library. 2010-11-08. Retrieved 19 July 2016.
- Amos, Jonathan (28 November 2012). "Skylon spaceplane engine concept achieves key milestone". BBC News. Retrieved 19 July 2016.
- Amos, Jonathan (16 July 2013). "UK government excited by 'disruptive' Sabre engine". BBC News. Retrieved 19 July 2016.
- Julian Moxon, Hotol: where next?, Flight International, 1 March 1986. (Two page discussion with technical description).
- Alan Postlethwaite, Hotol fights for life, Flight International, 25 March 1989. (With detailed cutaway)
- Rob Coppinger, Secret files reveal US interest in UK HOTOL spaceplane, Flight International, 23 February 2009. (Government thinking in 1984-5).