Dambusters Raid

Operation Chastise or commonly known as the Dambusters Raid was an attack on German dams carried out on the night of 16/17 May 1943 by 617 Squadron RAF Bomber Command, later called the Dam Busters, using special "bouncing bombs" developed by Barnes Wallis. The Möhne and Edersee dams were breached, causing catastrophic flooding of the Ruhr valley and of villages in the Eder valley; the Sorpe Dam sustained only minor damage. Two hydroelectric power stations were destroyed and several more damaged. Factories and mines were also damaged and destroyed. An estimated 1,600 civilians – about 600 Germans and 1,000 forced labourers, mainly Soviet – were killed by the flooding. Despite rapid repairs by the Germans, production did not return to normal until September. The RAF lost 53 aircrew killed and 3 captured, with 8 aircraft destroyed.

Background

Before the Second World War, the British Air Ministry had identified the industrialised Ruhr Valley, especially its dams, as important strategic targets. The dams provided hydroelectric power and pure water for steel-making, drinking water and water for the canal transport system. Calculations indicated that attacks with large bombs could be effective but required a degree of accuracy which RAF Bomber Command had been unable to attain when attacking a well-defended target. A one-off surprise attack might succeed but the RAF lacked a weapon suitable for the task.

Concept

The mission grew out of a concept for a bomb designed by Barnes Wallis, assistant chief designer at Vickers. Wallis had worked on the Vickers Wellesley and Vickers Wellington bombers and while working on the Vickers Windsor, he had also begun work, with Admiralty support, on an anti-shipping bomb, although dam destruction was soon considered. At first, Wallis wanted to drop a bomb from an altitude of about , part of the earthquake bomb concept. No bomber aircraft was capable of flying at such an altitude or of carrying such a heavy bomb and Wallis proposed the six-engined Victory Bomber for this purpose but this was rejected.Sweetman 1999, p. 40. Wallis realized that a much smaller explosive charge would suffice if it exploded against the dam wall under the water but German reservoir dams were protected by heavy torpedo nets to prevent an explosive device from travelling through the water.Sweetman 1999, pp. 44-45.

Wallis devised a bomb (more accurately, a mine) in the shape of a cylinder, equivalent to a very large depth charge armed with a hydrostatic fuse, designed to be given a backspin of 500 rpm. Dropped at and from the release point, the mine would skip across the surface of the water before hitting the dam wall as its forward speed ceased. Initially the backspin was intended to increase the range of the mine but it was later realized that it would cause the mine, after submerging, to run down the side of the dam towards its base, thus maximising the explosive effect against the dam.Sweetman 1999, pp. 58-61. This weapon was code-named ''Upkeep''.Sweetman 1999, p. 66.

Testing of the concept included blowing up a scale 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. A subsequent test suggested that a charge of exploded under water would breach a full-size dam; crucially this weight would be within the carrying capacity of an Avro Lancaster.Sweetman 1999, pp. 60-62. The first air drop trials were at Chesil Beach in December 1942; these used a spinning 4 ft 6 in sphere dropped from a modified Vickers Wellington, serial BJ895/G; the same aircraft was used until April 1943 when the first modified Lancasters became available. The tests continued at Chesil Beach and Reculver, often unsuccessfully, using revised designs of the mine and variations of speed and height.

Avro Chief Designer Roy Chadwick adapted the Lancaster to carry the mine. To reduce weight, much of the internal armour was removed, as was the mid-upper (dorsal) gun turret. The dimensions of the mine and its unusual shape meant that the bomb-bay doors had to be removed and the mine hung partly below the fuselage. It was mounted on two crutches and before dropping it was spun by an auxiliary motor. Chadwick also worked out the design and installation of controls and gear for the carriage and release of the mine in conjunction with Barnes Wallis. The Avro Lancaster B Mk IIIs so modified were known as ''Lancaster B Mark III Special (Type 464 Provisioning)''.Sweetman 1999, pp. 82-83.

In February 1943, Air Vice-Marshal Francis Linnell at the Ministry of Aircraft Production thought the work was diverting Wallis from the development of the Vickers Windsor bomber (which did not become operational). Pressure from Linnell via the chairman of Vickers, Sir Charles Worthington Craven, caused Wallis to offer to resign.Sweetman 1999, p. 78. Sir Arthur Harris, head of Bomber Command, after a briefing by Linnell also opposed the allocation of his bombers. Wallis had written to an influential intelligence officer, Group Captain Frederick Winterbotham, who 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. On 26 February 1943, Portal over-ruled Harris and ordered that thirty 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.Sweetman 1999, p. 79. With eight weeks to go, the larger Upkeep mine that was needed for the mission and the modifications to the Lancasters had yet to be designed.

Assignment

The operation was given to No. 5 Group RAF, which formed a new squadron to undertake the dams mission. It was initially called Squadron X, as the speed of its formation outstripped the RAF process for naming squadrons. Led by 24-year-old Wing Commander Guy Gibson, a veteran of more than 170 bombing and night-fighter missions, twenty-one bomber crews were selected from 5 Group squadrons. The crews included RAF personnel of several nationalities, members of the Royal Australian Air Force (RAAF), Royal Canadian Air Force (RCAF) and Royal New Zealand Air Force (RNZAF). The squadron was based at RAF Scampton, about north of Lincoln.

The targets selected were the Möhne Dam and the Sorpe Dam, upstream from the Ruhr industrial area, with the Eder Dam on the Eder River, which feeds into the Weser, as a secondary target. The loss of hydroelectric power was important but the loss of water to industry, cities and canals would have greater effect and there was potential for devastating flooding if the dams broke.

Preparations

Bombing from an altitude of , at an air speed of and at set distance from the target called for expert crews. Intensive night-time and low-altitude training began. There were also technical problems to solve, the first one being to determine when the aircraft was at optimum distance from its target. The Möhne and Eder Dams had towers at each end. A special targeting device with two prongs, making the same angle as the two towers at the correct distance from the dam, showed when to release the bomb. (The BBC
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