Editing De Havilland Comet (section)

==Development==
===Origins===
[[File:Design Studies for the DH 106 Comet.jpg|thumb|upright=1.7|Design studies for the DH.106 Comet 1944–1947 (artist's impression)]]

On 11 March 1943, the [[Cabinet of the United Kingdom]] formed the [[Brabazon Committee]], which was tasked with determining the UK's airliner needs after the conclusion of the [[Second World War]].<ref name=TH88>Trischler and Helmuth 2003, p. 88.</ref> One of its recommendations was for the development and production of a pressurised, transatlantic mailplane that could carry {{cvt|1|LT|lb kg}} of payload at a cruising speed of {{cvt|400|mph}} non-stop.<ref name=Birtles124>Birtles 1970, p. 124.</ref> 

Aviation company [[de Havilland]] was interested in this requirement, but chose to challenge the then widely held view that [[jet engine]]s were too fuel-hungry and unreliable for such a role.{{refn|During the same era, both Lockheed with their [[Lockheed L-188 Electra]] and Vickers with the ground-breaking [[Vickers Viscount]] discounted the advantages of "pure" jet power to develop [[turboprop]]-powered airliners.<ref>Kodera et al. 2010, p. 16.</ref>|group=N}} As a result, committee member Sir [[Geoffrey de Havilland]], head of the de Havilland company, used his personal influence and his company's expertise to champion the development of a jet-propelled aircraft; proposing a specification for a pure [[turbojet]]-powered design.<ref name=TH88/>

The committee accepted the proposal, calling it the "Type IV" (of five designs),{{refn|The "Type IV" Specifications issued on 3 February 1943 provided for a "high-speed mail-carrying airliner, gas-turbine powered."<ref name=Jones60>Jones 2010, p. 60.</ref>|group=N}} and in 1945 awarded a development and production contract to de Havilland under the designation ''Type 106''. The type and design were to be so advanced that de Havilland had to undertake the design and development of ''both'' the airframe and the engines. This was because in 1945 no turbojet engine manufacturer in the world was drawing-up a design specification for an engine with the thrust and [[thrust specific fuel consumption|specific fuel consumption]] that could power an aircraft at the proposed cruising altitude ({{cvt|40000|ft}}), speed, and transatlantic range as was called for by the Type 106.<ref>Jackson 1988, p. 453.</ref> First-phase development of the DH.106 focused on short- and intermediate-range mailplanes with small passenger compartments and as few as six seats, before being redefined as a long-range airliner with a capacity of 24 seats.<ref name=Birtles124/> Out of all the Brabazon designs, the DH.106 was seen as the riskiest: both in terms of introducing untried design elements and for the financial commitment involved.<ref name=TH88/> Nevertheless, the [[British Overseas Airways Corporation]] (BOAC) found the Type IV's specifications attractive, and initially proposed a purchase of 25 aircraft; in December 1945, when a firm contract was created, the order total was revised to 10.<ref name=Jones62>Jones 2010, p. 62.</ref>

{{Quote box|align=left|width=21%|quote=During the next few years, the UK has an opportunity, which may not recur, of developing aircraft manufacture as one of our main export industries. On whether we grasp this opportunity and so establish firmly an industry of the utmost strategic and economic importance, our future as a great nation may depend.|source=<small>[[Duncan Sandys]], Minister of Supply, 1952.<ref name=TH90/></small>}}

A design team was formed in 1946 under the leadership of chief designer [[Ronald Eric Bishop|Ronald Bishop]], who had been responsible for the [[de Havilland Mosquito|Mosquito]] fighter-bomber.<ref name=Jones62/> Several unorthodox configurations were considered, ranging from [[Canard (aeronautics)|canard]] to [[Tailless aircraft|tailless]] designs;{{refn|From 1944 to 1946, the design group prepared submissions on a three-engined twin-boom design, a three-engined canard design with engines mounted in the rear, and a tailless design that featured a [[swept wing]] and four "[[podded engine|podded]]" engines.<ref name=Jones62/>|group=N}} All were rejected. The [[Ministry of Supply]] was interested in the most radical of the proposed designs, and ordered two experimental tailless [[de Havilland DH 108|DH 108]]s{{refn|The Ministry of Supply's order for DH 108s was listed as Operational Requirement OR207 to Specification E.18/45.<ref name=watkins39/>|group=N}} to serve as [[proof of concept]] aircraft for testing swept-wing configurations in both low-speed and high-speed flight.<ref name=Birtles124/><ref name=watkins39>Watkins 1996, p. 39.</ref> During flight tests, the DH 108 gained a reputation for being accident-prone and unstable, leading de Havilland and BOAC to gravitate to conventional configurations and, necessarily, designs with less technical risk.<ref>Darling 2001, p. 11.</ref> The DH 108s were later modified to test the DH.106's power controls.<ref name=Birtles125/>

In September 1946, before completion of the DH 108s, BOAC requests necessitated a redesign of the DH.106 from its previous 24-seat configuration to a larger 36-seat version.<ref name=Birtles124/>{{refn|BOAC's requested capacity increase was known as Specification 22/46.<ref name=Birtles124/>|group=N}} With no time to develop the technology necessary for a proposed tailless configuration, Bishop opted for a more conventional 20-degree swept-wing design{{refn|The wing was drastically redesigned from a 40˚ sweep.<ref name=Jones62–63/>|group=N}} with unswept tail surfaces, married to an enlarged fuselage accommodating 36 passengers in a four-abreast arrangement with a central aisle.<ref name=Winchester109/> Replacing previously specified [[de Havilland Goblin|Halford H.1 Goblin]] engines, four new, more-powerful [[Rolls-Royce Avon]]s were to be incorporated in pairs buried in the wing roots; [[de Havilland Ghost|Halford H.2 Ghost]] engines were eventually applied as an interim solution while the Avons cleared certification. The redesigned aircraft was named the DH.106 ''Comet'' in December 1947.{{refn|The name "Comet", previously used by the [[de Havilland DH.88]] racing aircraft, was revived.<ref>Jackson 1988, p. 356.</ref>|group=N}} Revised first orders from BOAC and [[British South American Airways]]{{refn|British South American Airways merged with BOAC in 1949.<ref name=Birtles124/>|group=N}} totalled 14 aircraft, with delivery projected for 1952.<ref name=Jones62–63>Jones 2010, pp. 62–63.</ref>

===Testing and prototypes===
[[File:Comet Prototype at Hatfield.jpg|thumb|upright=1.4|Comet 1 prototype (with square windows) at [[Hatfield Aerodrome]] in October 1949]]

As the Comet represented a new category of passenger aircraft, more rigorous testing was a development priority.<ref name=d17/> From 1947 to 1948, de Havilland conducted an extensive research and development phase, including the use of several stress test rigs at [[Hatfield Aerodrome]] for small components and large assemblies alike. Sections of pressurised fuselage were subjected to high-altitude flight conditions via a large [[pressure vessel|decompression chamber]] on-site{{Refn|The fuselage sections and nose simulated a flight up to {{cvt|70000|ft}} at a temperature of {{cvt|−70|°C|°F}}, with 2,000 applications of pressure at {{cvt|9|psi}}.<ref name=Birtles125>Birtles 1970, p. 125.</ref>|group=N}} and tested to failure.<ref name=d18/> Tracing fuselage failure points proved difficult with this method,<ref name=d18/> and de Havilland ultimately switched to conducting structural tests with a water tank that could be safely configured to increase pressures gradually.<ref name=Birtles125/><ref name=d18/><ref>{{Cite magazine |url=http://www.flightglobal.com/pdfarchive/view/1955/1955%20-%201835.html |title=Tank Test Mk 2. |magazine=Flight |publisher=Iliffe |date=30 December 1955 |pages=958–959 |access-date=26 April 2012 |archive-url=https://web.archive.org/web/20190131201738/https://www.flightglobal.com/pdfarchive/view/1955/1955%20-%201835.html |archive-date=31 January 2019}}</ref> The entire forward fuselage section was tested for metal fatigue by repeatedly pressurising to {{convert|2.75|psi|kPa}} overpressure and depressurising through more than 16,000 cycles, equivalent to about 40,000 hours of airline service.<ref name=DaviesandBirtles>Davies and Birtles 1999, p. 30.</ref> The windows were also tested under a pressure of {{cvt|12|psi|kPa}}, {{cvt|4.75|psi|kPa}} above expected pressures at the normal service ceiling of {{cvt|36000|ft}}.<ref name=DaviesandBirtles/> One window frame survived {{cvt|100|psi|kPa}},<ref>{{Cite magazine|url=https://www.flightglobal.com/pdfarchive/view/1953/1953%20-%200556.html |title=Comet Engineering |publisher=Iliffe |via=[[FlightGlobal]] Archive |magazine=[[Flight International|Flight]] |date=1 May 1953 |access-date=23 March 2019 |page=552 |archive-url=https://web.archive.org/web/20170202030058/https://www.flightglobal.com/pdfarchive/view/1953/1953%20-%200556.html |archive-date=2 February 2017}}</ref> about 1,250 per cent over the maximum pressure it was expected to encounter in service.<ref name=DaviesandBirtles/>

The first prototype DH.106 Comet (carrying [[United Kingdom aircraft test serials|Class B markings]] G-5-1) was completed in 1949 and was initially used to conduct ground tests and brief early flights.<ref name=d18>Darling 2001, p. 18.</ref> The prototype's maiden flight, out of Hatfield Aerodrome, took place on 27 July 1949 and lasted 31 minutes.<ref>Dick and Patterson 2010, pp. 134–137.</ref><ref name=green174>Green and Swanborough April 1977, p. 174.</ref> At the controls was de Havilland chief test pilot [[John Cunningham (RAF officer)|John "Cats Eyes" Cunningham]], a famous night-fighter pilot of the Second World War, along with co-pilot Harold "Tubby" Waters, engineers John Wilson (electrics) and Frank Reynolds (hydraulics), and flight test observer [[Tony Fairbrother]].<ref>Prins 1998, p. 43.</ref>

The prototype was [[United Kingdom aircraft registration|registered]] G-ALVG just before it was publicly displayed at the 1949 [[Farnborough Airshow]] before the start of flight trials. A year later, the second prototype G-5-2 made its maiden flight. The second prototype was registered G-ALZK in July 1950 and it was used by the BOAC Comet Unit at [[Bournemouth Airport|Hurn]] from April 1951 to carry out 500 flying hours of crew training and route-proving.<ref>Swanborough 1962, p. 45.</ref> Australian airline [[Qantas]] also sent its own technical experts to observe the performance of the prototypes, seeking to quell internal uncertainty about its prospective Comet purchase.<ref>Gunn 1987, p. 268.</ref> Both prototypes could be externally distinguished from later Comets by the large single-wheeled main [[landing gear]], which was replaced on production models starting with G-ALYP by four-wheeled [[bogie#Bogie (aircraft)|bogies]].<ref name=Walker25/>