British Aerospace HOTOL: Difference between revisions

{{For|the generic concept of horizontal takeoff and landing|HTOL (disambiguation){{!}}HTOLCTOL}}

Designed as a [[single-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 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 propellantthe 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]]/[[McDonnell Douglas MD-80|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. In spite of 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|Reaction Engines Limited]] (REL) to develop and produce [[Skylon (spacecraft)|Skylon]], a proposed spacecraft based on HOTOL technologies, including its air-breathing engine.

The ideas behind HOTOL originated from work done by British Engineerengineer [[Alan Bond (engineer)|Alan Bond]] in the field of precooledpre-cooled jet engines. Bond had specifically performed this research with the intention of producing a viable engine for powering a space [[launch system]].<ref name="3rocket">{{cite avAV media |title=BBC Four: The Three Rocketeers |url=http://www.bbc.co.uk/programmes/b01mqv45 |title=BBC Four: The Three Rocketeers |publisher = [[BBC]] |accessdateaccess-date=14 September 2012}}</ref> In 1982, [[British Aerospace]] (BAe), whowhich 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]].<ref name {{sfn|Moxon|1986|p= "moxon 38"/>}} BAe became aware of work by British engine manufacturer [[Rolls-Royce plcHoldings|Rolls-Royce]] on a suitable engine, and soon conceived of an unmanned, fully reuseablereusable [[single-stage-to-orbit]] (SSTO) winged spaceplane as a launch vehicle.<ref name {{sfn|Moxon|1986|p= "moxon 38"/>}}

Thus, the project had soon becamebecome a joint venture between BAe and Rolls-Royce, led by [[John Scott-Scott (engineer)|John Scott-Scott]] and [[Bob Parkinson (aerospace engineer)|Dr Bob Parkinson]].<ref name = "moxon 38">{{sfn|Moxon |1986, |p. =38.</ref>}} 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.<ref name {{sfn|Moxon|1986|p= "moxon 38"/>}} In August 1984, BAe unveiled a public display of the HOTOL satellite launcher project and released details on its proposed operations.<ref name = "secretCoppinger" 2009"/>

In December 1984, a [[Department of Trade and Industry (United Kingdom)|Department of Trade and Industry]] (DTI) memorandum noted that [[West Germany]] was interested in the programprogramme, 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 the [[Department of Trade and Industry (United Kingdom)|Minister of Trade and Industry]] [[Geoffrey Pattie]], French diplomatic pressure to gather support for its own proposed [[Hermes (spacecraft)|Hermes]] space vehicle had inadvertently generated support and interest amongst [[European Space Agency]] (ESA) members in the HOTOL project.<ref name = "secretCoppinger" 2009"/> 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.<ref name = "secretCoppinger" 2009"/>

In March 1985, there were claims that Rolls-Royce was in the process of conducting [[Licensed production|licensing]] talks for HOTOL engine technology with American propulsion company [[Rocketdyne]].<ref name = "secretCoppinger" 2009"/> In April 1985, Pattie wrote to [[Secretary of State for Defence]] [[Michael Heseltine]] to propose a two-year £3 million [[proof of concept]] study be performed under a [[public-private partnership]] arrangement, 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.<ref name = "secret 2009Coppinger" /> According to aerospace publication ''[[Flight International]]'', the support of the [[Ministry of Defence (United Kingdom)|Ministry of Defense]] (MoD) was critical as the design of HOTOL's engine had been classified.<ref name = "secretCoppinger" 2009"/><ref name {{sfn|Moxon|1986|p= "moxon 38"/>}}

In July 1985, Rolls-Royce's technical director [[Gordon Lewis (engineer)|Gordon Lewis]] stated that the firm sought the involvement of the [[Royal Aircraft Establishment]]'s (RAE) propulsion group, and that Rolls-Royce was not prepared to invest its own funds into engine development for HOTOL.<ref name = "secretCoppinger" 2009"/> By the second half of 1985, work had commenced on the two-year concept-of-proof study.<ref name = "secretCoppinger" 2009"/> Early on, there was considerable pressure to demonstrate the project's feasibility and credibility in advance of final decisions being taken by the ESA on the Hermes and what would become the [[Ariane 5]] launch system, thus the work concentrated on the validation of critical technologies involved.<ref name {{sfn|Moxon|1986|p= "moxon 38"/>}}

By November 1985, DTI and RAE discussions noted that Rolls-Royce were seeking American data on [[ramjet]] technology to support their work on the engine, which it referred to by the name ''Swallow''.<ref name = "secretCoppinger" 2009"/> Reportedly, the [[United States Air Force]] were interested in the technology used in the Swallow engine for its own purposes.<ref name {{sfn|Moxon|1986|p= "moxon 38"/>}} In November 1985, discussions between [[Prime Minister of the United Kingdom|Prime Minister]] [[Margaret Thatcher]], [[Minister without portfolio (United Kingdom)|Minister without portfolio]] [[David Young, Baron Young of Graffham|David Young]] and [[President of the United States|US President]] [[Ronald Reagan]]'s scientific advisor [[George A. Keyworth, II|George Keyworth]] noted American interest in collaboration on developing [[hypersonic]] vehicles such as HOTOL, and that a prototype could be flying as early as 1990.<ref name = "secretCoppinger" 2009"/>

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.<ref name = "secretCoppinger" 2009"/> 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.<ref name {{sfn|Moxon|1986|p= "moxon 38"/>}} However, Rolls-Royce viewed transatlantic cooperation as necessary.<ref name = "secret 2009Coppinger" /> BAe's head of future business, Peter Conchie, stated that, if possible, HOTOL should become a part of the European space framework.<ref name {{sfn|Moxon|1986|p= "moxon 38"/>}} In early 1986, the British government formally approved the two-year study.<ref name = "secretCoppinger" 2009"/>

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 of [[Aerodynamic drag|drag]]. He also claimed that the vehicle offered no capability that was not already available; BAe responded that the criticisms made had been answered.<ref name = "secretCoppinger" 2009"/> In April 1985, the [[Ministry of Defence (United Kingdom)|Ministry 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".<ref name = "secretCoppinger" 2009"/>

In November 1985, the RAE issued an assessment of HOTOL's study proposal; the organisation believebelieved 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.<ref name = "secretCoppinger" 2009"/>

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.<ref>{{cite webinterview|last=Parkinson|first=Bob |author-link=Bob Parkinson (aerospace engineer)|interviewer=Thomas Lean|date=2010-11-08|title=Parkinson, Bob (Part 12 of 15)|url=httphttps://sounds.bl.uk/Oral-history/Science/021M-C1379X0005XX-0012V0|titlework=[[Oral historyHistory of British scienceScience]]|date=2010access-11-08|publisher=British Library|accessdatedate=19 July 2016}}</ref> 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.

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,<ref name = "secretCoppinger" 2009"/> in part because of the secrecy surrounding the programit. There was little prospect for European involvement, the ESA having elected to pursue development of what would become the [[Ariane 5]], a conventional space launch system.<ref name = "secretCoppinger" 2009"/> Rolls-Royce withdrew from the project, judging the eventual market for the engine was unlikely to be large enough to repay the development costs.<ref name="SpaceflightHempsell" 1993"/>. 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.

A cheaper redesign, '''Interim HOTOL''' or '''HOTOL 2''', which was to be launched from the back of a [[Antonov An-325|modified]] [[Antonov An-225 Mriya|Antonov An-225 ''Mriya'']] [[Cargo aircraft|transport 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.<ref>{{cite conference |last=Parkinson |first=R.C. |author-link=Bob Parkinson (aerospace engineer) |date=December 1991 |title=The An-225/Interim Hotol Launch Vehicle |conference=3rd International Aerospace Planes Conference |location=Orlando, Florida |doi=10.2514/6.1991-5006}}</ref>

In 1989, HOTOL co-creator [[Alan Bond (engineer)|Alan Bond]] and engineers John Scott-Scott and [[Richard Varvill]] formed [[Reaction Engines|Reaction Engines Limited]] (REL) which has since been working on a new air-breahingbreathing engine, [[SABRE (rocket engine)|SABRE]], which used alterativealternative designs to work around (and improve upon) the Rolls-Royce patents, and the [[Skylon (spacecraft)|Skylon]] vehicle intended to solve the problems of HOTOL. They first published these engine and spacecraft concepts in 1993,<ref name="Skylon 1993">{{cite magazine |first1last1=Varvill |first1=Richard |last1author-link1=Richard Varvill |first2last2=Bond |first2=Alan |last2author-link2=Alan Bond (engineer) |date=May 1993 |title= SKYLON: a Key Element of a Future Space Transportation System |date= May 1993 |pages= 162-166 |issue= 5 |volume= 35 |magazine= [[Spaceflight (magazine)|Spaceflight]] |publishervolume=35 [[British Interplanetary Society]]|issue=5 |issnpages= 0038-6340162–166 |placebibcode= London1993SpFl...35..162V}}</ref> and hashave since been developing the core technologies, particularly the engine and its frost-controlled precoolerpre-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,<ref>{{cite newsmagazine |last=Norris |first=Guy |date=1 November 2015 |title=BAE Takes Stake In Reaction Engines Hypersonic Development |url=http://aviationweek.com/technology/bae-takes-stake-reaction-engines-hypersonic-development |titleurl-access=BAE Takes Stake In Reaction Engines Hypersonic Developmentsubscription |last1=Norris |first1=Guy |date=1 November 2015 |website=aviationweek.com |publishermagazine=[[Aviation Week & Space Technology]] |accessdateaccess-date=1 November 2015}}</ref> and the [[Air Force Research Laboratory]]. {{As of | 2017}} REL plan to demonstrate a flight-ready precoolerpre-cooler operating under simulated flight conditions in 2018, and statically test a demonstration engine core in 2020.

HOTOL was envisioned as an unmanned, fully reuseablereusable [[single-stage-to-orbit]] (SSTO) winged spaceplane. The unmanned craft was intended to put a payload of around 7 to 8 [[tonne]]stonnes in orbit, at 300&nbsp;km altitude.<ref name {{sfn|Moxon|1986|p= "moxon 40"/>}} 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&nbsp;km high, by which time the craft would be travelling at [[Mach number|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.<ref name {{sfn|Moxon|1986|p= "moxon 40"/>}}

During its high-altitude phase, its [[flight control system]] would have been linked to ground stations and to space-based [[Global Positioning System|global navigation system]] navigation, while [[radar]] would have been used during the take-off and landing phases. In additionaladdition to the placing of satellites into [[geosynchronous orbit]] or LOE, HOTOL was also projected as being able to also perform the retrieval of satellites and hardware from LOE.<ref name {{sfn|Moxon|1986|p= "moxon 40"/>}} BAe promotional material depicts HOTOL docking with the [[International Space Station]] (ISS), a feat that the company claimed would have required manned operateoperation as automated systems were not capable of performing such docking manoeuvres at that time.<ref name {{sfn|Moxon|1986|p= "moxon 40"/>}} 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 dedicated [[Cabin pressurization|pressurised]] module within the payload bay.<ref name {{sfn|Moxon|1986|p= "moxon 40"/>}}

As designed, HOTOL would have been 62 metres long, 12.8 metres high, a fuselage diameter of 5.7 metersmetres and a wingspan of 19.7 metres.<ref name {{sfn|Moxon|1986|p= "moxon 40"/>}} 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&nbsp;°C).<ref name {{sfn|Moxon|1986|p= "moxon 40"/>}} 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.<ref name = "moxon 40">{{sfn|Moxon |1986, |p. =40.</ref>}}

|name country_of_origin = Rolls-Royce RB545United "Swallow"Kingdom

|country_of_origindesigner = [[UnitedRolls-Royce KingdomHoldings|Rolls-Royce]]

|fueloxidiser = [[Air]] =and [[liquid hydrogenoxygen]]<ref name="Spaceflight 1993"/>

|oxidiserfuel = [[Air]] and = [[Liquidliquid oxygen|LO₂hydrogen]]

|dry_weight nozzle_ratio = {{convert|2500|kg|lb}} (excluding intake and spill)100:1

|nozzle_ratiothrust(Vac) = 100:1= ≈&nbsp;{{cvt|735|kN}}

|thrust(SL) = Approx. ≈&nbsp;{{convertcvt|340|kN|abbr=on}}

|thrust(Vac)thrust_to_weight = Up to 14 = Approx. {{convert|735|kN|abbr=on}}(atmospheric)

|thrust_to_weightdry_weight = Up to 14 = {{convert|2500|kg|lb}} (atmosphericexcluding intake and spill)

|references = <ref name="Spaceflight 1993Hempsell" />{{rp|page=172}}

The exact details of this engine were covered by the [[Official Secrets Act]] of 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.<ref>[{{cite interview|last=Parkinson|first=Bob |author-link=Bob Parkinson (aerospace engineer)|interviewer=Thomas Lean|date=2010-11-08|title=Parkinson, Bob (Part 10 of 15)|url=http://sounds.bl.uk/Oral-history/Eminent-scientists/021M-C1379X0005XX-0010V0 Dr Bob Parkinson discusses HOTOL in an oral history interview recorded for the [[National Life Stories]] project |work=[[Oral History of British Science]]|access-date=3 atNovember the [[British Library]]]2023}}</ref><ref name="Spaceflight 1993Hempsell">{{cite magazine |first1last=Hempsell |first=Mark |last1author-link=Mark Hempsell |titledate=May 1993 |title=HOTOL's Secret Engines Revealed |date= May 1993 |pages= 168-172 |issue= 5 |volume= 35 |magazine= [[Spaceflight (magazine)|Spaceflight]] |publishervolume= [[British Interplanetary Society]]35 |issnissue= 0038-63405 |placepages=168–172 London|bibcode=1993SpFl...35..168H}}</ref>

Within the atmosphere, air is taken in through two vertically mounted [[Intakeintake ramp]]s, then the flow would be split, passing the correct amount to the precoolerspre-coolers, and the excess to spill ducts. Hydrogen from the fuel tanks would be passed through two [[Precooled jet engine|heat exchangers to precoolpre-cool the air]] prior to entering a high overall pressure-ratio [[turbojet]]-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.".{{efn|dueDue 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 couldn'tcould not provide thrust, because that was the responsibility of the engine, so instead it was a mechanism for reducing drag.<ref name="SpaceflightHempsell" 1993"/>}} These ramjets were typically depicted as two glowing red circles below the rocket engines in pictures of HOTOL.

To prevent the precoolerspre-coolers from icing up, the first precoolerpre-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 {{cvt|-50C, (-58F)50|C|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 precoolerpre-cooler elements. A water trap could have been added after the first precoolerpre-cooler if operating conditions resulted in an excess of moisture.<ref name="frost1">{{cite patent |country= GB |number= 2241537 |status= patent |title= Prevention of icing in the intakes of aerospace propulsors |inventinventor= [[John Scott-Scott]] |invent2 = Bryan Belcher |invent3= [[Alan Bond (engineer)|Alan Bond]] |assign1= Rolls -Royce Plc |fdate= 23 March 1988 |pubdate=4 September 1991-09-04 |gdate= 1992-08-12 |class= F02C7/047; F02C7/143; F28F19/006; Y02T50/675August 1992}}</ref>

When it was no longer possible to use the atmosphere for combustion, the RB545 would switch to using on-board liquid oxygen (LOX) to burn with the hydrogen as a high-efficiency hydrogen/oxygen rocket.<ref name = "secret 2009Coppinger">{{cite webmagazine |last=Coppinger |first=Rob |date=23 February 2009 |title=Secret files reveal US interest in UK HOTOL spaceplane |url=https://www.flightglobal.com/news/articles/secret-files-reveal-us-interest-in-uk-hotol-spacepla-322955/ |titlemagazine= Secret files reveal US interest in UK HOTOL spaceplane |publisher = [[Flight International]] |access-date =3 23November February 20092023 |first via= Rob |last = Coppinger[[FlightGlobal]]}}</ref>

* [[Rockwell X-30|NASP]] – a scramjet vehicle with which HOTOL would have competed

* [[Reaction Engines SkylonLAPCAT A2]] – a follow-on design thatfor attempts toa avoidhypersonic HOTOL'santipodal shortcomingsairliner

*{{cite Julianmagazine |last=Moxon, [|first=Julian |date=1 March 1986 |title=Hotol: where next? |url=http://www.flightglobal.com/pdfarchive/view/1986/1986%20-%200486.html Hotol: where next?], ''|magazine=[[Flight International]]'' |volume=129 |issue=4000 |pages=38, 140 March|via=[[FlightGlobal]] 1986|bibcode=1986FlInt.129...38M pp|archive-url=https://web.&nbsp;38–40archive.org/web/20121022194036/https://www.flightglobal.com/pdfarchive/view/1986/1986%20-%200486.html |archive-date=22 October 2012}}

* Alan{{cite magazine |last=Postlethwaite, [|first=Alan |date=25 March 1989 |title=Hotol fights for life |url=http://www.flightglobal.com/pdfarchive/view/1989/1989%20-%200764.html Hotol|magazine=[[Flight fights for lifeInternational]], ''Flight|volume=135 International'',|issue=4157 25|pages=34–38 March|via=[[FlightGlobal]] |bibcode=1989FlInt.135...34P |archive-url=https://web.archive.org/web/20131203013144/https://www.flightglobal.com/pdfarchive/view/1989/1989%20-%200764.html (With|archive-date=3 detailedDecember cutaway)2013}}

* {{webarchive |date=30 January 2013 |title=Cutaway drawing of the HOTOL |url=https://web.archive.org/web/20130130095436/http://www.flightglobal.com/airspace/media/missilespacesystemscutaways/images/10296/bae-hotol-cutaway.jpg|date=30 January 2013|origurl=http://www.flightglobal.com/airspace/photos/missilespacesystemscutaways/bae-hotol-cutaway-10296.aspx|title=Cutaway drawing of the HOTOL}}

* [httphttps://wwwpatents.google.com/patents?vid=USPAT5150858patent/US5150858 HOTOL-related patent on jetisonable control surfaces]