Thursday, March 10, 2016

RAF 1936 Bomber Specifications

A feature of the 1936 bomber specifications that had unlooked for beneficial repercussions was the requirement that the medium bomber P. 13/36 should be capable of modification to carry torpedoes. When tenders to specification P. 13/36 were received, it was found that provision to carry two 18 in. torpedoes (which were 18 ft long) without altering the main structure of the aircraft, or losing performance, was causing design difficulties. This led the DDOR (Oxland) to review the discussion on this issue that had taken place at the two Operational Requirements Committee meetings on P.13/36. He told the DCAS (Peirse) that the Coastal Command representative had given his C-in-C’s view that the aircraft was too large and expensive for a torpedo bomber. Even so, the then DCAS (Courtney) had argued that whilst there was the possibility of a limitation on the numbers of first-line aircraft, it was desirable that every unit should be as effective in war as possible. As we have seen, a muted form of the torpedo-carrying requirement was therefore included in the requirements.

Oxland recommended that as a dedicated torpedo bomber was now under development (B.10/36 — Bristol Beaufort) the torpedo requirement should be deleted from P.13/36. Alternatively, he said, provision could be made for a limited number of the aircraft to have larger bomb doors and so on.
The Operations and Plans branches of the Air Staff did not agree with deletion of the torpedo requirement. They advised Peirse that the Admiralty had yet to be persuaded that, ‘the “B” bomb is in every way a more efficient weapon with which to attack ships’, and that a torpedo bomber version of P. 13/36 should be developed until the Admiralty was convinced otherwise. (The ‘B’ bomb was designed to be dropped in the path of a ship, sink, and then rise to strike the bottom of the ship as it passed over.)

This discussion was made redundant when the Operational Requirements branch announced that it had new information on the size of torpedoes. It now found that the Avro P. 13/36 could carry only one internally, and that in any case existing torpedoes could not be released at 150 mph from 200 ft. If this was relevant, the DCAS (Peirse) must have wondered how the idea of torpedo carrying for P.13/36 had arisen in the first place. It appears that the Operational Requirements branch had given no more thought to the operational problems of torpedo dropping than they were later found to have given to catapult launching. Peirse decided that torpedo carrying would no longer be asked of the P.13/36 bombers. Nevertheless, the long bomb bay that had been required was to prove valuable when bombs larger than the 2,000 lb were found to be needed.

A common misconception regarding RAF bomber specifications is that they always sought to combine bomber and troop transport requirements, and it is suggested that this applied to the 1936 bomber specifications. We have seen that the requirement that a heavy bomber should be designed so as to carry troops was indeed included in the first draft specification for the B.3/34 (Whitley). The Air Staff had been led to believe that this additional role could be obtained without a reduction in its performance as a bomber. It was dropped from the specification after discussions with industry, and after the DTD admitted it would result in a loss of 10 mph in speed.

The 1936 bomber specifications (B.12/36 and P.13/36) stated:
Consideration is to be given in design for fitting a light removable form of seating for the maximum number of personnel that can be accommodated within the fuselage when the aircraft is being used for reinforcing Overseas Commands.

This was certainly not demanding provision for troop carrying. Seating was to be fitted in the fuselage, not that the fuselage was to be designed to take seating. Moreover, it referred to the need to transport RAF ground crew to RAF Overseas Commands — a concomitant of the introduction of a reinforcement range into bomber requirements. Significantly, only after the 1936 bomber specifications had been issued did the Air Staff investigate using them as transports, and proposed a provisional allocation of funds for a new transport in case this was not possible. But when this proposition was discussed it was decided that one of the bombers ‘must’ be used as a transport. In a later lecture to the Higher Commanders’ Course the point was made that these bombers ‘will have all the necessary cabin space, lift capacity and range to fulfil the bomber transport primary role and its secondary functions as well’. Nevertheless, the lecturer noted that ‘by reason of the multiplicity of internal installations in the fuselage the troops may not enjoy the same degree of comfort available in present types’. Indeed, when Bomber Command officers inspected the mock-up of the Supermarine design to B.12/36, far from finding accommodation for fully armed troops, they were concerned as to whether there was adequate room for the crew. They reported that headroom throughout the fuselage was restricted, and that even the captain and navigator did not have room to stand. Clearly a troop carrying requirement did not dominate — or even influence — the design of RAF bombers.

Another aspect of the future development of the aircraft designed to the 1936 bomber specifications was of great significance. We have seen that when Sir Edward Ellington saw the Air Staff Requirement for the new heavy bomber B.12/36, he asked for 20mm cannon armament to be considered. The Air Staff advised that this was neither possible nor necessary. Their reasoning was unsound, and the policy was soon reversed, but it was then too late to modify any of the designs to the 1936 bomber specifications, although attempts were made.

When the cannon fighter (F.37/35) was devised, the replacement of eight 0.303 in. machine guns by half that number of 20mm cannon was regarded as a major increase in armament. Yet, in response to Ellington’s request for consideration of cannon armament for bombers, Oxland examined only the replacement of machine guns by the same number of 20mm cannon. From this premise he argued that for a four-gun tail turret, the extra weight of cannon so far aft of the centre of gravity was unacceptable — hardly an insurmountable obstacle for aircraft which were yet to be designed. He added that recoil loads would give grave problems except for firing almost directly astern. For a two-gun midships turret, Oxland claimed that whilst the weight would be half that of the tail turret, the weight of the ammunition needed for an aircraft which would spend long periods over hostile territory was unacceptable. We will see that this self-contradictory argument was replaced in 1938 by recognition that it was worth exchanging half the bomb load for ammunition if that made it more likely that the remainder would get through. For the nose turret, Oxland said that cannon would obstruct the bomb aimer, and would be too heavy if beam fire was wanted. He then claimed that these difficulties could be avoided because a bomber did not need the extra range of a big gun.

This argument had appeared in the Operation Requirements branch’s review of fighter and bomber armament that we noted in our discussion of fighter firepower. It reasoned that when attacked from astern the effective range of a bomber’s firing was considerably shortened as compared with that of the attacking fighter. If we think of a fighter flying directly astern of a bomber, and at the same speed, then from the moment a projectile leaves the bomber the fighter is flying towards it, thus closing the effective range. Conversely, the range of the fighter’s firing opens, because the bomber is moving away from it. This theory was irrelevant to defence against beam or frontal attacks, and therefore to midships and nose turrets. Nevertheless, Oxland claimed that it largely disposed of one of the two supposed advantages of 20mm guns. As regards the other advantage of cannon — an explosive shell — he said that the stage had not been reached ‘where this can be utilised effectively without severe disadvantages’. There had been no mention of such difficulties when he and Sorley had advocated cannon armament for RAF fighters in the previous year — they then claimed that one hit from a 20mm round could be decisive.

Little more than a year after Oxland had argued against 20mm cannon armament for the B.12/36 he was advising Plans branch of the Air Staff that bombers of the immediate future would need to be armed with 20mm guns, and later with 37—40 mm guns.96 This was confirmed in a review of bomber armament in June 1938.

Plans were made to fit 20mm cannon to Mark II versions of the Stirling, Halifax and Manchester, but by then the centre of gravity issue was decisive because it had not been designed for in 1936. Experiments with twin 20mm cannon upper and lower midships turret for the Stirling and Halifax found that it was difficult to balance the aircraft even with the tail turret omitted entirely. W.S. Farren (then DD/RDA) explained to the Air Fighting Committee in 1940 that nevertheless this was the only way of having 20mm guns on existing bombers. He said that to have cannon in a tail turret, ‘they would have to start again from the beginning’.

The outcome of the 1936 bomber specifications was remarkable. On the one hand, the prospect of catapult take-off led to a requirement for a relatively small heavy bomber to carry a very large bomb load or have a longer range than that sought in earlier specifications. On the other hand, the desire for a multi-role high-speed medium bomber with a maximum range of 3,000 miles led to a relatively large aircraft of this type. Misleading interpretations of the Air Staff’s intentions in 1936 most likely arise from a retrospective view of the development of the aircraft designed to meet these requirements. It transpired that the aircraft designed to the medium bomber specification (P.13/36) embodied the potential for development into more successful heavy bombers (Halifax and Manchester/Lancaster) than that designed to the heavy bomber specification, B.12/36 (Stirling). That this was possible can be traced to three technical features of the medium bomber specification — gross overloading with catapult take-off, fuel tankage for a range of 3,000 miles, and provision for the internal stowage of torpedoes.

Initial designs to specification P.13/36 needed to be stressed for catapulting at maximum overload, and to have internal stowage for the overload bomb and fuel load. In addition, provision had to be made for an unobstructed bomb bay if some of the aircraft were to be modified to serve as torpedo bombers. These additions to the normal requirements gave scope for the future development of the aircraft which followed from the specification after both torpedo carrying and catapult take-off had been abandoned. There was space for much larger bombs than were envisaged in 1936, and the potential for operation with large bomb loads using a longer conventional take-off run.

The first step towards the transformation of the intended medium to a heavy bomber came from Handley Page. Soon after commencing design to P. 13/36 the company concluded that the aircraft would be very similar to their on-going design to B.1/35 — the ‘Americanised’ B.3/34 heavy bomber specification. They asked the Air Ministry if they could stop work on their contract for B.1/35 and absorb it into their P.13/36 design. This was agreed.

Both the Handley Page and Avro P.13/36 bombers were initially designed to be powered by two Vulture engines as anticipated by Verney. But on Air Ministry instructions the Handley Page design was soon changed to four Merlins, and was thought to meet the P.13/36 maximum overload requirement without assisted take-off, albeit with a long conventional take-off. Avro continued with the Vulture engine, but this proved a failure, and the P.13/36 Manchester was modified to the Lancaster, also with four Merlins. Thus the Air Ministry’s misplaced faith in the catapult scheme finished back where Liptrot’s first estimates for a new heavy bomber had started — with bombers powered by four Merlins — albeit derived from requirements for a medium bomber.

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