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British Aerospace HOTOL

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HOTOL
An artist's depiction of HOTOL
General information
TypeSpaceplane
National originUnited Kingdom
Designer
StatusCancelled
History
Developed intoSkylon

HOTOL,forHorizontal Take-Off and Landing,was a 1980s British design for asingle-stage-to-orbit(SSTO)spaceplanethat was to be powered by anairbreathing jet engine.Development was being conducted by a consortium led byRolls-RoyceandBritish Aerospace(BAe).

Designed as asingle-stage-to-orbit(SSTO) reusable winged launch vehicle, 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 liquidhydrogen/liquidoxygen;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 theMcDonnell Douglas DC-9/MD-80.

While HOTOL'sproof-of-conceptdesign 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. In spite of American interest in the programme, there was little appetite amongst the members of theEuropean 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 ofReaction Engines Limited(REL) to develop and produceSkylon,a proposed spacecraft based on HOTOL technologies, including its air-breathing engine.

Development

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Origins

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The ideas behind HOTOL originated from work done by British engineerAlan Bondin the field of pre-cooled jet engines. Bond had specifically performed this research with the intention of producing a viable engine for powering a spacelaunch system.[1]In 1982,British Aerospace(BAe), which was Europe's principalsatellite-builder, began studying a prospective new launch system with the aim of providing launch costs that were 20 per cent of the AmericanSpace Shuttleoperated byNASA.[2]BAe became aware of work by British engine manufacturerRolls-Royceon a suitable engine, and soon conceived of an unmanned, fully reusablesingle-stage-to-orbit(SSTO) winged spaceplane as a launch vehicle.[2]

Thus, the project had soon become a joint venture between BAe and Rolls-Royce, led byJohn Scott-ScottandBob Parkinson.[2]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.[2]In August 1984, BAe unveiled a public display of the HOTOL satellite launcher project and released details on its proposed operations.[3]

In December 1984, aDepartment of Trade and Industry(DTI) memorandum noted thatWest Germanywas interested in the programme, while France had adopted a critical attitude towards HOTOL, which the ministry viewed as potentially due to it being seen as a competitor to French-led projects. According to Minister of Trade and IndustryGeoffrey Pattie,French diplomatic pressure to gather support for its own proposedHermesspace vehicle had inadvertently generated support and interest amongstEuropean Space Agency(ESA) members in the HOTOL project.[3]Despite this climate of tentative interest and possible European support, there was a general attitude of reluctance within the British government to take the lead on a new space launcher.[3]

American interest and design study

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In March 1985, there were claims that Rolls-Royce was in the process of conductinglicensingtalks for HOTOL engine technology with American propulsion companyRocketdyne.[3]In April 1985, Pattie wrote toSecretary of State for DefenceMichael Heseltineto propose a two-year £3 millionproof of conceptstudy be performed under apublic-private partnershiparrangement, consisting of £1 million provided by the UK government and the remainder being financed by Rolls-Royce and BAe themselves. Pattie reasoned that the project would serve Britain's "strategic capability", and that tests of key technologies could foster international collaboration.[3]According to aerospace publicationFlight International,the support of theMinistry of Defense(MoD) was critical as the design of HOTOL's engine had been classified.[3][2]

In July 1985, Rolls-Royce's technical directorGordon Lewisstated that the firm sought the involvement of theRoyal Aircraft Establishment's (RAE) propulsion group, and that Rolls-Royce was not prepared to invest its own funds into engine development for HOTOL.[3]By the second half of 1985, work had commenced on the two-year concept-of-proof study.[3]Early on, there was considerable pressure to demonstrate the project's feasibility and credibility in advance of final decisions being taken by ESA on the Hermes and what would become theAriane 5launch system, thus the work concentrated on the validation of critical technologies involved.[2]

By November 1985, DTI and RAE discussions noted that Rolls-Royce were seeking American data onramjettechnology to support their work on the engine, which it referred to by the nameSwallow.[3]Reportedly, theUnited States Air Forcewere interested in the technology used in the Swallow engine for its own purposes.[2]In November 1985, discussions betweenPrime MinisterMargaret Thatcher,Minister without portfolioDavid YoungandUS PresidentRonald Reagan's scientific advisorGeorge Keyworthnoted American interest in collaboration on developinghypersonicvehicles such as HOTOL, and that a prototype could be flying as early as 1990.[3]

According to British government files, neither BAe nor the MoD were enthusiastic for the prospects of American involvement in the programme, 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 once led.[3]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 launchers.[2]However, Rolls-Royce viewed transatlantic cooperation as necessary.[3]BAe's head of future business, Peter Conchie, stated that, if possible, HOTOL should become a part of the European space framework.[2]In early 1986, the British government formally approved the two-year study.[3]

Problems and criticism

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In December 1984, project management consultant David Andrews issued an eight-page critique of the programme, noting that the design was optimised for the ascent while exposing itself to extended thermal loads during descent due to a low level ofdrag.He also claimed that the vehicle offered no capability that was not already available; BAe responded that the criticisms made had been answered.[3]In April 1985, theMinistry of Defence's research and development department deputy controller James Barnes claimed that HOTOL lacked a justification, and that 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; Barnes also observed the HOTOL engine to be "ingenious".[3]

In November 1985, the RAE issued an assessment of HOTOL's study proposal; the organisation believed that HOTOL would take up to 20 years to develop, rather than the 12-year timetable that had been envisioned by industry. The RAE also projected that the project would have an estimated total cost of £5 billion (as of its value in 1985), £750 million of which would be required in a six-year definition phase and an estimated £25 million in a pre-definition feasibility study.[3]

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.[4]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

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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,[3]in part because of the secrecy surrounding it. There was little prospect for European involvement, ESA having elected to pursue development of what would become theAriane 5,a conventional space launch system.[3]Rolls-Royce withdrew from the project, judging the eventual market for the engine was unlikely to be large enough to repay the development costs.[5]The British government declined to offer further funding for HOTOL. The project was almost at the end of its design phase while much of the plans remained in a speculative state; the craft was reportedly still dogged with aerodynamic problems and operational disadvantages at this point.

Successors

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A cheaper redesign,Interim HOTOLorHOTOL 2,which was to be launched from the back of amodifiedAntonov An-225transport aircraft,specifically was promoted by BAe in 1991; however, this proposal was rejected as well. The design for Interim HOTOL was to have dispensed with an air-breathing engine cycle and was designed to use a more conventional mix of LOX and liquid hydrogen as fuel instead.[6]

In 1989, HOTOL co-creator Alan Bond and engineers John Scott-Scott andRichard VarvillformedReaction Engines Limited(REL) which has since been working on a new air-breathing engine,SABRE,which used alternative designs to work around (and improve upon) the Rolls-Royce patents, and theSkylonvehicle intended to solve the problems of HOTOL. They first published these engine and spacecraft concepts in 1993,[7]and have since been developing the core technologies, particularly the engine and its frost-controlled pre-cooler; initially supported by private funding, but latterly with support from theEuropean Space Agency,theBritish National Space Centre,theUnited Kingdom Space Agency,BAe,[8]and theAir Force Research Laboratory.As of 2017REL plan to demonstrate a flight-ready pre-cooler operating under simulated flight conditions in 2018, and statically test a demonstration engine core in 2020.

Design

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Overview

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HOTOL was envisioned as an unmanned, fully reusablesingle-stage-to-orbit(SSTO) winged spaceplane. The unmanned craft was intended to put a payload of around 7 to 8 tonnes in orbit, at 300 km altitude.[9]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 atMach5 to 7. After reachinglow 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.[9]

During its high-altitude phase, itsflight control systemwould have been linked to ground stations and to space-basedglobal navigation systemnavigation, whileradarwould have been used during the take-off and landing phases. In addition to the placing of satellites intogeosynchronous orbitor LOE, HOTOL was also projected as being able to also perform the retrieval of satellites and hardware from LOE.[9]BAe promotional material depicts HOTOL docking with theInternational Space Station(ISS), a feat that the company claimed would have required manned operation as automated systems were not capable of performing such docking manoeuvres at that time.[9]HOTOL was designed to conduct fully automated unmanned flights; however, it had been intended at a later stage to potentially re-introduce a pilot. Manned operations would have required the installation of a dedicatedpressurisedmodule within the payload bay.[9]

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.[9]It featured a wing design that had been derived from that ofConcorde;its large area resulted in relatively low wing loading, which would have resulted in lower reentry temperatures (never rising above 1,400 °C).[9]Built out ofcarbon compositematerials, there would have been no need for the use of insulating tiles akin to those that comprised theSpace 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.[9]

Engine

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Rolls-Royce RB545 "Swallow"
Country of originUnited Kingdom
DesignerRolls-Royce
ApplicationSingle-stage-to-orbit
AssociatedLVHOTOL
PredecessorSATAN
StatusCancelled
Configuration
Nozzle ratio100:1
Performance
Thrust, vacuum≈ 735 kN (165,000 lbf)
Thrust, sea-level≈ 340 kN (76,000 lbf)
Thrust-to-weight ratioUp to 14 (atmospheric)
Specific impulse,vacuum4,500 N‑s/kg (460 s)
Specific impulse,sea-level14,780 N‑s/kg (1,507 s)
Dimensions
2,500 kilograms (5,500 lb) (excluding intake and spill)
References
References[5]: 172 

The RB545, which was given the name "Swallow" by its manufacturer, British engine maker Rolls-Royce, was an air-breathing rocket engine.[3]It would have functioned as an integrated dual-role powerplant, having been capable of air-breathing while operating within theatmosphereand operating in a similar manner to that of arocketwhen having attained close to and within LEO.[10]This engine would have also been capable of powering the spacecraft tohypersonicspeeds. It was a crucial element of the programme, having been publicly attributed as "the heart of Hotol's very low launch costs".[9]

The exact details of this engine were covered by theOfficial Secrets Actof the United Kingdom; consequently, there is relatively little public information about its development and on its operation. However, material was later declassified when government policy changed to prevent the keeping of secret patents without an attributed justification.[11][5]

Within the atmosphere, air is taken in through two vertically mountedintake ramps,then the flow would be split, passing the correct amount to the pre-coolers, and the excess to spill ducts. Hydrogen from the fuel tanks would be passed through twoheat exchangers to pre-cool the airprior to entering a high overall pressure-ratioturbojet-like engine cycle — the heated hydrogen driving a turbine to compress and feed the cooled air into the rocket engine, where it was combusted with some of the hydrogen used to cool the air. The majority of the remaining hot hydrogen was released from the back of the engine, with a small amount drawn off to reheat the air in the spill ducts in a ramjet arrangement to produce "negative intake momentum drag".[a]

To prevent the pre-coolers from icing up, the first pre-cooler cooled the air to around 10 degrees above freezing point, to liquefy the water vapour in the air. Then liquid oxygen (LOX) would have been injected into the airflow to drop the temperature to −50 °C (−58 °F) flash freezing the water into microscopic ice crystals, sufficiently cold that they wouldn't melt due to kinetic heating if they struck the second pre-cooler elements. A water trap could have been added after the first pre-cooler if operating conditions resulted in an excess of moisture.[12]

When it was no longer possible to use the atmosphere for combustion, the RB545 would switch to using on-board LOX to burn with the hydrogen as a high-efficiency hydrogen/oxygen rocket.[3]

See also

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Notes

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  1. ^Due to the split in responsibilities of the HOTOL project, Rolls-Royce were responsible for the engine, and BAe were responsible for the aerodynamics. When BAe wanted to add the ramjet it could not provide thrust, because that was the responsibility of the engine, so instead it was a mechanism for reducing drag.[5]

References

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Citations

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  1. ^BBC Four: The Three Rocketeers.BBC.Retrieved14 September2012.
  2. ^abcdefghiMoxon 1986,p. 38.
  3. ^abcdefghijklmnopqrstCoppinger, Rob (23 February 2009)."Secret files reveal US interest in UK HOTOL spaceplane".Flight International.Retrieved3 November2023– viaFlightGlobal.
  4. ^Parkinson, Bob(8 November 2010)."Parkinson, Bob (Part 12 of 15)".Oral History of British Science(Interview). Interviewed by Thomas Lean.Retrieved19 July2016.
  5. ^abcdHempsell, Mark(May 1993). "HOTOL's Secret Engines Revealed".Spaceflight.Vol. 35, no. 5. pp. 168–172.Bibcode:1993SpFl...35..168H.
  6. ^Parkinson, R.C.(December 1991).The An-225/Interim Hotol Launch Vehicle.3rd International Aerospace Planes Conference. Orlando, Florida.doi:10.2514/6.1991-5006.
  7. ^Varvill, Richard;Bond, Alan(May 1993). "SKYLON: a Key Element of a Future Space Transportation System".Spaceflight.Vol. 35, no. 5. pp. 162–166.Bibcode:1993SpFl...35..162V.
  8. ^Norris, Guy (1 November 2015)."BAE Takes Stake In Reaction Engines Hypersonic Development".Aviation Week & Space Technology.Retrieved1 November2015.
  9. ^abcdefghiMoxon 1986,p. 40.
  10. ^Moxon 1986,pp. 38, 40.
  11. ^Parkinson, Bob(8 November 2010)."Parkinson, Bob (Part 10 of 15)".Oral History of British Science(Interview). Interviewed by Thomas Lean.Retrieved3 November2023.
  12. ^GB patent 2241537,John Scott-Scott;Bryan Belcher &Alan Bond,"Prevention of icing in the intakes of aerospace propulsors", published 4 September 1991, issued 12 August 1992, assigned to Rolls-Royce Plc

Bibliography

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