Meet HOTOL: The British-designed space plane that never flew a mission

HOTOL (which stands for Horizontal Take-Off and Landing) was a British-designed space plane developed in the mid-1980s. The aircraft was designed to be a single-stage-to-orbit (SSTO) vehicle that would be powered by a revolutionary airbreathing jet engine, designed by Rolls-Royce. The project was led by British Aerospace (now part of Airbus) and included teams from several disciplines across the aviation firm.

What were the plans for HOTOL?

HOTOL was designed as a reusable winged launch vehicle. The aircraft would take off under its own power from a conventional runway and land back as an unpowered glider once its mission was completed. Designed to be an aerial launch platform for satellites, HOTOL was supposed to be cheaper to build and more efficient to operate (by around 20%) than its nearest competitor, the US-built Northrop Space Shuttle

The uncrewed craft was intended to have the ability to put a payload of around seven to eight tonnes (15,450lbs to 17,650lbs) into orbit at around 187 miles (300 km) altitude. HOTOL was designed to take off from a runway mounted on the back of a large rocket-boosted trolley that would help get the craft up to a speed where it could take off using conventional lift generated by the wings.   

The engine was intended to switch from jet propulsion to pure rocket propulsion at 16-20 miles (26–32 km) altitude, by which time the craft would be travelling at Mach five to seven. After reaching low-earth orbit, HOTOL was then intended to re-enter the atmosphere and glide down to land like a conventional airplane. Payload restrictions meant that only one payload would be carried at a time.

The proposed propellant for HOTOL’s engine technically consisted of a combination of liquid hydrogen mixed with liquid oxygen. The powerplant itself was designed to utilize a new method of dramatically reducing the amount of oxidizer needed to be carried on board by utilizing atmospheric oxygen as the spacecraft climbed through the lower atmosphere after take-off.

Since the oxidizer typically represents the majority of the take-off weight of a space vehicle rocket, HOTOL was designed to be considerably smaller than normal pure-rocket designs, roughly the size of a medium-haul airliner such as a Boeing 737.

Dimensions and capabilities

HOTOL was designed to have an overall length of 62 metres (203ft), a height of 12.8 metres (42ft), a fuselage width of 5.7 meters (18.8ft), and a wingspan of 19.7meters (64.6 ft). The final design had a take-off mass of 275 tonnes (606,000lbs). Approximately 82% of the total weight would be propellant, with the vehicle structure being a further 16%. This left just 2% for payload.

HOTOL featured a ‘delta’ wing design that British Aerospace had derived from Concorde. The unique wing design was adopted to provide relatively low wing loading, resulting in lower re-entry temperatures. Ahead of its time, and built entirely out of carbon composite materials, the design of HOTOL meant that there was no need for thermal insulation tiles such as those of the underside of the US Space Shuttle.  

AlchetronAlmost the entire forward fuselage, ahead of the payload bay, comprised a single hydrogen tank, with no need for a cockpit as the vehicle was to be unmanned. Additionally, unlike its US competitor, HOTOL would feature a vertical fin, just aft of the nose, for lateral stability. However, this was later supplemented by a larger vertical rear stabilizer for enhanced lateral control.  

While HOTOL was designed to conduct fully automated uncrewed flights, it had been intended at a later stage to potentially re-introduce a crew.  At an early stage of development, British Aerospace released futuristic images of HOTOL docking with the forthcoming International Space Station. This would have required a crewed operation, as automated systems at the time were not capable of performing such docking manoeuvres.

Early development and concept

The concept behind HOTOL originated from work done by Alan Bond, a British aeronautical engineer who specialized in the field of pre-cooled jet engines. Bond had specifically performed this research with the intention of producing a viable engine to power a reusable space vehicle. In 1982, Europe’s leading satellite manufacturer, British Aerospace, began its own studies to build a new launch system that it could use to launch its own satellite products, hence reducing the reliance on other space programs such as NASA’s Space Shuttle or Europe’s Ariane rocket to carry out the function.

While the two worked independently to begin with, they eventually joined with Rolls-Royce (which had been developing its own engine technology) to work on the development of an uncrewed, fully reusable SSTO winged spaceplane that could be used as a suitable satellite launch vehicle.

KVDP / Wikimedia CommonsGiven the extensive projected costs of such a program (estimated at the time as £4 billion / $5.4 billion),  British Aerospace began to market the concept to other European nations in the hope of further collaboration and cost sharing. By 1984, to promote the program, British Aerospace built and displayed a large-scale mock-up of HOTOL to market the project to other nations, which toured around Europe.

However, European interest was initially hard to find. It was reported by the UK Department of Trade and Industry (DTI) in late 1984 that West Germany might be interested in participating in HOTOL. However, another key ally that it was hoped would be involved, France, was reticent about the project and was not interested in participating. The French government saw HOTOL as a potential competitor to its own space program (known as ‘Hermes’) and did not have the finances available to be involved in both projects simultaneously.

Additionally, despite the tentative interest of West Germany, the general feeling was that without widespread European support and collaboration, there were too many hurdles to overcome, not least the enormous costs involved, for a British company to develop and deliver such a technically advanced and costly space vehicle on its own. These feelings set a tone for the whole HOTOL project; that British Aerospace, along with Rolls-Royce, were very much “on their own” in terms of developing the new space vehicle.

Engine and propellants

For its part in the HOTOL project, Rolls-Royce developed its RB545 engine, which was later given the same ‘Swallow ‘ by its manufacturer. The Swallow was to be an air-breathing rocket engine, with the capability to function as an integrated dual-role powerplant. This meant it would be capable of running on conventional air while operating within the atmosphere, while also operating as a rocket engine when HOTOL had reached low Earth orbit. The engine would have also been capable of powering the spacecraft to hypersonic speeds, reducing overall costs and making the overall project more attractive as a result.

The Swallow engine was designed so that, within the atmosphere, air would be taken in through two vertically mounted intakes. It would then split the airflow and pass the correct amount to the pre-coolers, and the excess to the spill ducts. Hydrogen from the fuel tanks would be passed through two heat exchangers to pre-cool the air before it was passed into a turbojet-type engine, with the heated hydrogen driving a compressor to compress and feed the cooled air into the rocket engine. One there, it would be combusted with hydrogen used to cool the air. The majority of the remaining hot hydrogen was released from the back of the engine.

Secret Projects.co.ukWhen it was no longer possible to use the atmosphere for combustion, the RB545 would switch to using on-board liquid oxygen to burn with the hydrogen as a high-efficiency hydrogen/oxygen rocket. Little else is known about the exact design or workings of the Swallow engine, as its entire development program was classified, with all aspects of the project being protected under the UK Official Secrets Act.

Other international involvement

In March 1985, it was reported that Rolls-Royce was in the process of conducting talks for HOTOL engine technology with American propulsion company Rocketdyne, the firm that had helped Charles ‘Chuck’ Yeager break the sound barrier in 1947.

However, according to British government files, neither British Aerospace nor the UK Ministry of Defence was enthusiastic about the prospects of American involvement in the HOTOL space program. Both parties stated at the time that they were expressing reluctance out of a belief that the outcome of such a move could result in the UK becoming a junior member in a project that it had once been the leading developer.

Kim Shiflett / NASA There was also a belief that if Britain chose to pair up with the United States, it would find itself frozen out of work on future European space vehicles, with its involvement with the European Space Agency (ESA) being either reduced or even frozen entirely. However, with development costs already spiralling, Rolls-Royce said that transatlantic cooperation was essential for the project to work. This put Rolls-Royce and British Aerospace at loggerheads with the UK government, and progress on the HOTOL project began to stall as a result.

Widespread Scepticism

In December 1984, a project management consultant, David Andrews, issued an eight-page critique of the HOTOL program. Andrews said that the design was “optimised for the ascent while exposing itself to extended thermal loads during descent due to a low level of drag”. He also claimed that the vehicle offered no capability that was not already available elsewhere on the open market.

Further adding to HOTOL’s problems, in April 1985, the Ministry of Defence’s research and development department stated that HOTOL lacked any real justification, as there was no defence requirement for such vehicles. He also noted that the “engineering problems are considerable” and that it was unlikely to enter service until the 2020s.

In November 1985, the UK’s Royal Aircraft Establishment (RAE) issued an assessment of HOTOL’s study proposal. The RAE had been brought in to assist Rolls-Royce earlier in 1985 with the costs and complexities of developing the Swallow powerplant. The organisation believed that HOTOL would take up to 20 years to develop, rather than the 12-year timetable that had been envisioned by British Aerospace. The RAE also projected that the project’s estimated costs would rise to around £5 billion ($6.75 billion), representing a 20% increase over initial cost estimates.  

Adding to HOTOL’s growing list of issues, 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.

The 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 project economics “ambiguous”. In particular, some of the analyses seemed to indicate that similar technology applied to a pure rocket approach would give approximately the same performance at less cost, thus putting the future of HOTOL in real jeopardy.

Despite repeated approaches to the European Space Agency by the British Government for possible collaboration and funding, there was little appetite for HOTOL among senior officials at the ESA. However, the UK government saw participation from ESA as an essential part of the HOTOL project. Without it, said the government at that time, there was little future for the HOTOL project.

Program shutdown

With no tangible involvement from the Americans other than a passing interest, and without ESA involvement at any meaningful level, UK government funding for the HOTOL was finally withdrawn in 1989.  The secrecy that surrounded the Swallow engine had not helped, as the UK government was simply unwilling to share this technology with any other international party, thereby giving a ‘protectionist’ feel towards Rolls-Royce, which other nations deemed to be unfair.

By this point, ESA began investing much more time and money into its own space project, which would later develop into the successful Ariane rocket program. Around the same time, Rolls-Royce decided to end its development of the Swallow engine, deciding that, because of the dwindling potential market for HOTOL, there was almost no chance of a return on the huge capital outlay required.

It was reported that even as the project was being wound down, it still faced massive development issues linked to aerodynamic problems and operational or economic disadvantages. Equally, the US Space Shuttle program was gathering strength in the satellite launching market, with ESA’s Ariane rocket development following closely behind, largely eradicating the need for HOTOL.

What came next for HOTOL?

British Aerospace, desperate not to lose all the money that they had spent to develop HOTOL, quickly produced a cheaper redesigned HOTOL2 concept, which, with a lighter overall weight, would be launched off the back of a modified Antonov An225 transport aircraft, in the same manner that the Russian ‘Buran’ space shuttle was to operate. However, this design was swiftly rejected by the authorities in the UK.

Matt Brown / FlickrAs the HOTOL project officially ended in 1989, Alan Bond, along with two colleagues, formed Reaction Engine Limited (REL) to revive plans for a new low-Earth-orbit spaceplane and develop a new air-breathing engine, known as ‘Sabre’. This new spaceplane design, to be called ‘Skylon’, would build on the technology that Rolls-Royce had already developed, and would refine the aeronautical shortcomings of HOTOL.

REL first published these engine and spacecraft concepts in 1993, while continuing to develop the engine technology. Funded in part with support from ESA, British Aerospace, the UK Space Agency, and others, REL aimed to demonstrate a flight-ready engine operating under simulated flight conditions by 2017 and run demonstration tests by 2020.

However, REL called in the administrators in 2024, ceasing all operations and bringing the hopes of a UK-designed spaceplane and the legacy of HOTOL to a conclusion. No working model of the Sabre engine was ever built.  

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