Profile – DH Comet Pt I

Revolutionised air travel over the past 40 years. Those jet airliners which have individually made their contribution in shrinking distance and providing comfortable and reliable air transportation all owe something to that far-sighted team which nurtured the comet and created a truly different mode of travel. That the Comet was faced with immense difficulties is well known. That de Havilland was able to identify and conquer their problems so effectively was testimony to the depth of talent resident in the British aircraft industry. Their identification of the causes made an important contribution to the store of aeronautical knowledge worldwide.

The story of the development of the de Havilland Comet is with?ut parallel in the history of commercial aviation. It is a story of triumph, tragedy and rebirth of quite Shakespearean proportions. It is a story of aeronautical innovation and ingenuity at the highest levels which were unfortunately confronted with a chain of completely unforeseen events. But above all, it is a story of perseverance which ultimately won through, but did not reap its just rewards.

Aviation, like so many other fields of technical endeavour, had been dramatically influenced by the events of World War II. The rapid sequence of improvements made to combat aircraft saw many new ideas filter through to the civil aviation arena. Many of the new crop of postwar airliners either reflected a military parentage or incorporated numerous improvements in fresh designs. The US manufacturers particularly, were quick to draw on their experience gained during wartime production. This saw the intercontinental Boeing 347 Stratocruiser, Douglas DC-6 and Lockheed L749 Constellation dominate the transocean routes while the shorter range Convair 240 and Martin 2-0-2 were examples of the new breed of smaller airliner. All these aircraft had improvements in numerous areas compared with their prewar counterparts. All metal construction, pressurisation, more powerful powerplants and greater comfort are some of the more readily identifiable positive attributes. Detracting features were periodic lack of reliability of some of the more powerful piston engines and the continued inability of these airliners to climb above the weather.

During the immediate postwar years, the lack of competitive aircraft from the British aircraft industry saw many customers such as BOAC and Qantas, previously prominent purchasers of British equipment, turn to the US for their new fleets. Offerings such as the Avro Tudor or Handley Page Hermes were not in the same league as the US airliners. To maintain long distance services, BOAC re-equipped with Lockheed Constellations and Boeing Stratocruisers. The Brabazon Committee convened in Britain during 1943/44 identified five categories of airliner which would be needed after the war. In response to some of the identified needs, various well known aircraft such as the Airspeed Ambassador, Bristol Britannia and Vickers Viscount were designed and marketed with varying degrees of success. One of the types outlined at the initial 1943 committee meeting, the Type 4, was a pure jet aircraft which was intended to solely carry mail. During 1944, a second committee chaired by Lord Brabazon dispensed with the mail carrier role and altered the requirement to that of an aircraft with a 700-800 mile range carrying 14 passengers together with a small amount of mail. Specification 20/44 was issued by the Ministry of Aircraft production towards the end of 1944 embodying the general requirements for the jet transport outlined at the meetings.

Following the initial 1943 committee findings, de Havilland began preliminary design studies late that year. A design group headed by R E Bishop investigated a series of proposals during 1944 in answer to the Brabazon Type 4 specification which called for a pure jet airliner capable of carrying a one ton payload at a 400mph plus cruising speed. The de Havilland involvement in the project was officially confirmed in February 1945 when it was agreed that the company should proceed to design an aircraft which would meet the specification.

The type number DH. 106 was then given to the design. Amongst the configurations discussed were a tailless swept wing version, a canard rear engined type powered by Ghosts and a three engined aircraft with Goblins and twin booms. Also considered was a jet engined version of the earlier DH. 95 Flamingo airliner, an all-metal high wing aircraft powered by two 900hp Bristol Perseus XVls which first flew in 1938. Of these proposals, the tailless design seemed the most promising. However by the end of 1945, following the voicing of reservations about this configuration by both de Havilland and BOAC, a layout which included a swept tail unit and a wing with a 40° sweep at the quarter chord line was favoured. A contract tor two prototypes was placed by the Ministry of Supply in May 1946

for this revised design which also had a 7,500lb payload (including accommodation for 24 passengers), 96ft 7in wing span, 93. 000lb gross weight and a 535mph cruising speed. Further significant changes were made prior to the confirmation of the initial BOAC order. These included increases in passenger load to 32 with an appropriate increase in fuselage length, gross weight to 100,000lb and wing span to 111ft. The wing sweep was reduced to 20°, tail surfaces were to be unswept and the cruising speed was reduced to 505mph.

During October 1945, design work commenced on the Goblin powered DH. 108 research aircraft which was used to obtain swept wing data for both the jet airliner project and the DH. 110 jet fighter. The DH. 108 consisted of a Vampire fuselage married to a 39th span 43° swept wing. Three aircraft were built with the first flight of DH. 108 number one on 15 May 1946 with Geoffrey de Havilland Jnr at the controls. Aircraft numbers two and three differed in some respects from the initial version. In particular, the second DH. 108, which was first flown by John Cunningham from Hatfield on 23 August 1946, had its wing sweep increased to 45° and was equipped with the same powered controls to be fitted to the DH. 106. The experience gained from the DH. 108 resulted in a more conventional configuration being adapted by the new airliner. Detailed design work on the airliner now known as the Comet, commenced during September 1946. The rekindling of the name Comet, first used by the highly successful DH. 88 MacRobertson Air Race entrant, seemed an appropriate name for the new jet airliner.

A positive sign for de Havilland’s management during 1946 was the placement by BOAC for a production batch of Comets off the drawing board at an approximate unit cost of £500,000. This vital order enabled the company to plan the establishment of a production line. Another early order for six Comets was lodged by British South American Airways which subsequently amalgamated with BOAC. Some time was to pass before other orders were received.

Two Comet prototypes were ordered by the Ministry of Supply built to their specification 22/46. While construction of the first aircraft was underway, cockpit visibility in all weathers was evaluated in an Airspeed Horsa glider with a much modified nose section towed behind a Handley Page Halifax. This unlikely Comet/Horsa combination proved a very useful tool to the design team. The rollout of the first Comet G-5-1/G-ALVG took place on 2 April 1949 with the first engine runs being made the same day. The Comet was by far the heaviest aircraft to have been built by de Havillands up to that time. The previous holder of this distinction was the beautiful DH. 91 Albatross, first flown in 1937, which weighed up to 32,500lb in the mailplane version. At a gross weight of 102,000lb G-ALVG, although the lightest of all Comets, was more than three times the Albatross weight. The initial Comet powerplant was the de Havilland Ghost 50 turbojet, the first turbine to be type-approved for civil transport operation. It initially delivered 4,450lb st (static thrust) in the Mk. 1 version in the Comet 1 while the Mk. 2 installed in the Comet 1A produced 5,050lb st at 10,250rpm with a sea level static consumption of 1. 14lb/hr/b. Maximum continuous was 4,320lb st at 9,750rpm with a sisc of 1. 071b/hr/lb, while cruising was 3,230-3,940lb st at 9,000-9,500rpm with sisc of 1. 04-1. 05lb/hr/lb. Weight dry, including sump and tailpipe was 2,209 lb +2. 5%. The depth and versatility of the de Havilland Company was quite remarkable when one considers that the one organisation built both the airframe and engines, a feat they had been achieving for many years. In an aircraft as radically different as the Comet this was even more noteworthy.

Not long after the first flight in July, the public had their first opportunity to see the Comet put through its paces when G-ALVG was demonstrated at Farnborough in September. On 25 October it made its first overseas flight to Castel Benito in Libya and averaged 448mph for the journey. This was the first of several flights made to establish fuel consumption details. The first flight with a cabin pressure of 8,000ft at a cruising altitude of 40,000ft took place on 21 February 1950. During the first 11 months of test flying, over 300 hours were flown and the Comet inevitably captured many flight time records between London and several European destinations. The Comet ventured further afield to conduct the first tropical trials at Eastleigh, Nairobi in Kenya while Khartoum in the Sudan was selected for hot and high airfield trials. The second prototype Comet G-5-2/G-ALZK was first flown from Hatfield by John Cunningham and Peter Bugge on 27 July 1950, the first anniversary of the initial flight of G-ALVG.

An important decision regarding the Comet powerplants was taken fairly early in testing and despite the good performance of the Ghost, it was only used in a comparatively small number of Comets. The emergence of the superior Rolls-Royce Avon led to the decision taken in late 1950 to power Comets with the newer engine which was now being offered as a civil powerplant. The Avon eventually became the powerplant used by the Comet 2 and subsequent marks.

Both Comet 1 prototypes carried out a further testing and on 2 April 1951 the second Comet joined the BOAC Comet Unit at Hurn to commence a programme of crew training and route proving. While this was under way, a production line was set up at Hatfield. An interesting series of flights began on 7 May in which G-ALVG made takeoffs assisted by two Sprite rocket motors in the wings. In the expectation that some airlines may need additional takeoff power, provision had been made in the wing design for the accommodation of the Sprites. An important milestone for de Havillands was the delivery of BOAC’s first Comet 1 G-ALYP on 8 April 1952. History was made some weeks later on 2 May when G-ALYP made the world’s first revenue passenger flight from London to Johannesburg via Rome, Beirut, Khartoum, Entebbe and Livingstone. Deliveries to BOAC continued with a further eight Comets handed over by the end of September.

The Comet 2 was a version intended for use across the Atlantic. Fuel capacity was increased to 7,000 gallons, the fuselage was stretched by 3ft and gross weight was 120,000lb. The sixth Comet airframe produced, G-ALYT originally intended as a Comet 1, was purchased by the Ministry of Supply, was redesignated a Comet 2X and became the Comet 2 prototype. It was initially powered by 6,500lb st Avon RA. 9 Mk. 501s and had a 108,000lb gross weight. The first flight was on 16 February 1952.

Apart from their greater weight, the production versions also differed with their Avon RA. 25 Mk. 503 or 504 engines of 7,300lb st. During 1953, BOAC ordered 12 Comet 2s for use on its South American routes and the first of this type, G-AMXA first flew on 27 August 1953. Trials extended over the next few months and a highlight of these was the non-stop 3,064 mile flight from Hatfield to Khartoum in 61⁄2 hours flown by John Cunningham on 22 January 1954.

What was the view of those early days of the Comet from the test pilot’s point of view? During a recent visit to England, I had the pleasure of meeting John Cunningham, a renowned aviation identity so closely connected with many de Havilland aircraft. He commenced flying in 1935 and achieved considerable success during World War II as an accomplished night fighter pilot in Bristol Beaufighters and de Havilland Mosquitoes. After a distinguished wartime career in which he accounted for 20 enemy aircraft (including 19 at night), John Cunningham then returned to civilian life to make an important contribution to several new aircraft programmes at Hatfield. He now lives in retirement not far from Hatfield and understandably still takes a keen interest in the aviation scene.

When I asked him what was the nicest aircraft he had flown, his response was, “Well it is very difficult to give a balanced answer, because if you are closely associated with the development of an aeroplane you get to appreciate the qualities of each succeeding aircraft. However I must say the outstanding one was the Comet, because it put together in a large passenger carrying aircraft all the advances and enjoyment of flying a jet engined, high speed specialised aircraft. It was the biggest step forward because it really doubled the speed of commercial transports in one step. ” “What are your own feelings about the Comet? It never really achieved the success it deserved?” “No it did not. It had its unhappy pressure cabin failure in 1954. The Comet having entered passenger carrying service in 1952 operated very successfully on the routes to South Africa, India and the Far East. It was severely set back and checked by the failures in 1954 and it wasn’t until the Comet 4 was produced in 1958 and flown on the North Atlantic that the Comet got back into its stride. It lost the many years advantage it had when the Boeing 707 entered service after the Comet 4. ”

“Do you think Boeing learned a lot from what happened?” “Oh yes, undoubtedly. Both Boeing and Douglas were straightforward and said that they had learned a very great deal from de Havilland’s misfortune. We had naturally made available all the information we had to other manufacturers. Fortunately the pressure cabin failures have not been repeated. ”

“What was the feeling at the time when it first happened? I suppose at that stage it was not really identified?” “It was something we just could not understand. We could not imagine or believe that it was the pressure cabin. Because there was a fuselage being tested for pressure cabin life in a water tank and was subjected to all the loads to which the aircraft could be expected to go through. That particular test section had not turned up any sign of cracks or failures. But subsequently the enquiry at Farnborough showed, after a detailed examination of some of the fuselages that were in build on the production line, that there had been quite considerable differences in the way those fuselages were made compared with the section of fuselage that was being tested. Many lessons were learned from that. Methods, tolerances, skin gauges, reinforcing around cutouts and all the variations that could take place on production compared with the original test section which was made by the experimental section at de Havillands as opposed to the production department. ”

“What were your feelings on the first flight of the Comet?” “Oh, just great pleasure and joy because we combined in the Comet 100% hydraulic powered controls with no manual reversion. That was a great step which took a long time to convince the body of airline pilots that we had provided sufficient alternatives in the event of hydraulic power failure. We at Hatfield had already flown those power controls on one or two other aircraft experimentally and were quite happy with our experience with them. It was a practical way of operating aircraft flying controls. We had that experience and I had the Ghost development experience and had all the military Vampire and Venom flying before the Comet flew. So I was happy to see all these features combined into the one aeroplane. ”

“On the first flight how close to expectations was the performance?”

“Very close indeed. Hatfield had been fairly conservative. They had really always been pretty conservative with their performance estimates and the Comet came up extremely well. The very large Comet wing enabled us to go to a considerably higher operating altitude than we had planned. The Comet 1 would happily go well above 40,000ft and on many occasions we did do that. That was partly because of the very low wing loading. Bear in mind that people did not know much about flying through claims and thunderstorms. The met people generally assumed that they didn’t go above 35,000ft. On the very first flight back from East Africa where we had been on some development work in 1950, there was a continuous line from left to right of large thunderstorms and I climbed up and up and got to 45,000ft and still found that the stuff was above me. I went through this somewhere between 43,000ft and 45,000ft, not very happy. We did not have radar in those days and I was going through what looked like the heart of this enormous tropical front. When I landed at Khartoum the met office people just wouldn’t believe me, but I had a number of DOT people with me and others were suitably frightened going through this great front at that height. ” “Was that as high as you flew in the Comet?” “Oh no, I went to over 60,000ft in either a Comet 1 or 2. It was quite remarkable how, at a light weight, they would go very high indeed. ”

Considering the major step forward which the Comet represented, the time frame of less than three years from the initial design decisions to the first flight was quite an achievement. From the outset, de Havillands were of the opinion that the Comet should succeed or fail on commercial merits alone and not be primarily promoted for its speed benefits. They felt that speed without improvements in operating expenses would not make it an attractive proposition to prospective customers. To enhance compatibility with existing airline fleets and airport facilities, speed was not to be gained at the expense of slow flying ability. A moderate wing loading, in some cases below that of some existing piston engined aircraft, was selected to ensure that let-down, approach and landing speeds together with takeoff characteristics were similar to the airlines’ current equipment. The exceptionally clean engine installation was achieved by almost completely burying the powerplants in the wing root between the main spars wing. Entry of air directly to the engine compressors was through the front spar. Other benefits arising from this installation were that it was simple and light, had good fire prevention and anti-icing of the enginebay. Easeof maintenance was simplified with the majority of areas being accessible at shoulder height without the need for steps or platforms. In addition, it permitted the use of a short undercarriage with a commensurate saving in weight. The main undercarriage with a large single wheel which appeared on the prototypes was replaced with a bogie fitted with four smaller diameter wheels on production Comets. This reduced the wheel loading to make the Comet less dependent on major airport runways. In the light of future sales this proved to be a good move. The wheel loading on main wheels was takeoff 11,375Ib and landing 8,580Ib at 115psi and the nosewheels takeoff 4,500lb and landing 3,335Ib at 65psi.

On the structural side, the Redux metal to metal bonding process used for some time with the de Havilland Dove was also used with the Comet. All fuel was carried in the wings with bag tanks in the centre section and outboard integral tanks. Over wing pressure refuelling at a rate of 150 gallons and normal over wing refuelling was provided for. The cabin pressurisation system was designed to deliver a maximum cabin altitude of 8,000ft at 40,000ft at a maximum working differential pressure of 8. 25psi. Cabin fresh air was tapped from the compressors of all engines, passed through coolers and refrigerated if necessary before entry to the fuselage. The flight deck was designed for a crew of four with full dual controls for the pilots. Flying controls were power operated by completely duplicated hydraulic boosters with a duplicated source of power through two entirely separate systems. Flaps were split in the inboard section of the wings with plain flaps outboard and automatic aileron droop was applied when the flaps were lowered.

The Comet 1 production version had a 115ft wing span, 93ft length and 28ft 4in height. The gross weight was 105,000lb, later increased to 107,000lb, fuel capacity was 6,050imp gallons, passenger capacity 36-44 and the maximum range was 1,750 miles at maximum weight. A typical 36 passenger configuration saw eight passengers in the forward cabin arranged around fixed tables in facing pairs of fixed seats. The main cabin for the remaining 28 passengers was no less luxurious with the seven rows of four seats at a 45 inch pitch, two abreast on either side of the 17 inch gangway which had a minimum 6ft 5½in headroom. A total of 585cu ft was available in three compartments for freight, mail and luggage with access during flight to all compartments. Including the two prototypes, a total of 11 Comets were produced.

The next development was the Comet 1A powered by the 5,000Ib st Ghost 50 Mk. 2 with water-methanol injection. The main change in this 44 passenger seat version, was an increase in fuel capacity to 6,906 gallons and gross weight increase to an initial 110,0001b, later increased to 115,000lb. Ten examples were delivered, comprising Canadian Pacific Airlines 2, Royal Canadian Air Force 2, Air France 3 and Union Aeromaritime de Transport 3. The first Comet 1A service was operated by UAT between Casablanca and Dakar on 19 February 1953 and Air France placed the type into operation on 26 August on the Paris/ Rome/Beirut route.

The first Comet 1 A was CF-CUM ‘Empress of Vancouver’ which was a static exhibit at Farnborough in 1952. Canadian Pacific had been attracted to the Comet 1 A because of suitability for transpacific routes to Sydney. However because of later events which were to engulf the Comet, neither of the Canadian Pacific Comets entered service with that airline. The first RCAF Comet1 A 5301 made history on its delivery flight on 29 May1953 to Ottawa via Keflavik and Gander when it became the first jet transport to make the Atlantic crossing.

Next issue: The Comet saga continues as the problems of the first of the breed begin to take hold. The tracking down of the lethal fatigue problem and its implications for future pressurised aircraft design are fully detailed in addition to the later, and operationally successful, variants of this most famous airliner. Don’t miss it.