Concept
The mission grew out of a concept for a bomb designed by Barnes Wallis and developed by his team at Vickers. Wallis was Assistant Chief Designer at Vickers. He had worked on both the Vickers Wellesley and Vickers Wellington bombers. While working on the Vickers Windsor he had also begun work, with support of the Admiralty, on a bomb designed initially for attacking ships, though dam-destruction was soon considered.
Wallis's initial idea was to drop a 10 t (9.8 long tons; 11 short tons) bomb from an altitude of about 40,000 ft (12,200 m). This idea was part of the earthquake bomb concept. However, at that time no bomber aircraft was capable of flying at that altitude with such a heavy payload. A much smaller explosive charge would suffice, if it could be exploded directly against the dam wall below the surface of the water, but the major German reservoir dams were protected by heavy torpedo nets to prevent such an attack.
Wallis's breakthrough overcame this. A drum-shaped bomb spinning backwards at over 500 rpm, dropped at a sufficiently low altitude at the correct speed, would skip for a significant distance over the surface of the water in a series of bounces before reaching the dam wall. Its residual spin would run the bomb down the side of the dam to its underwater base. Using a hydrostatic fuse, an accurate drop could bypass the dam's defences and enable the bomb to explode against the dam.
Initial testing of the concept included blowing up a plaster model dam at the Building Research Establishment, Watford in May 1942 and then the breaching of the disused Nant-y-Gro dam in Wales in July 1942. The first full-scale trials were at Chesil Beach in January 1943. This demonstrated that a bomb of sufficient size could be carried by an Avro Lancaster rather than waiting for a larger bomber such as the Windsor to be built. However Air Vice-Marshal Francis Linnell at the Ministry of Aircraft Production thought the work was diverting Wallis from the development of the Windsor. Pressure from Linnell via the chairman of Vickers, Sir Charles Worthington Craven, caused Wallis to resign. Sir Arthur Harris, head of Bomber Command from a briefing by Linnell also opposed the allocation of his bombers. However Wallis had written to an influential intelligence officer, Group Captain Frederick Winterbotham. Winterbotham ensured that the Chief of the Air Staff, Air Chief Marshal Charles Portal heard of the project. Portal saw the film of the Chesil Beach trials and was convinced. Over-riding Harris, Portal ordered on 26 February 1943 that 30 Lancasters were to be allocated to the mission and the target date was set for May, when water levels would be at their highest and breaches in the dams would cause the most damage. With eight weeks to go, the larger bomb, code-named 'Upkeep', that was needed for the mission, and the modifications to the Lancasters had yet to be designed.
Read more about this topic: Operation Chastise
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