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The mission developed out of a bomb designed by Barnes Wallis and developed into a working device by a team at Vickers. Wallis was an aircraft designer and had the successful Wellesley and Wellington to his credit. While working on the Warwick, he also began work on bomb design with dams specifically in mind.
His initial idea was for a 10-ton bomb to be dropped from 40,000 feet (12,200 m). However, research showed that a bomb sufficient to breach dam without a direct hit would be too heavy for any available bomber to carry. A much smaller charge would suffice if it could be exploded directly against the dam wall below the surface of the water. The major German dams were protected by heavy torpedo netting to prevent such an attack, and Wallis's breakthrough was to overcome this. A drum-shaped bomb, spinning rapidly and dropped from a sufficiently low altitude at high speed would skip for a short distance over the surface of the water in a series of bounces before halting and sinking. An accurate drop could bypass the dam protection and let the bomb be detonated against the dam with a hydrostatic fuse. After testing, and many meetings, the idea was adopted on February 26, 1943. The bomb was codenamed 'Upkeep'. The dams were to be bombed in May of that year, when water levels would be highest.
The operation was given to 5 Group which formed a new squadron to undertake the mission. Initially called Squadron 'X', it was led by wing Commander Guy Gibson, a veteran of over 170 missions. A further 21 crews were chosen from 5 Group to join the new squadron based at RAF Scampton.
The targets were the three key dams in the Ruhr area, the Moehne and the Sorpe on the RuhrEssen-Kettwig The Ruhr is a large river in western Germany starting near the town of Winterberg in Sauerland and ending in the Rhine in the city of Duisburg. Along the river, a number of large industrial cities are found, forming the Ruhr area. German riv River and the Eder DamThe Eder is a river in Germany (ca. 180 km long). It is a tributary of the Fulda River which at its confluence with the Werra river creates the Weser river which flows into the North Sea north of Bremen. A dam (47 m high, 400 m long) completed in 1914 nea on the EderThe Eder is a river in Germany (ca. 180 km long). It is a tributary of the Fulda River which at its confluence with the Werra river creates the Weser river which flows into the North Sea north of Bremen. A dam (47 m high, 400 m long) completed in 1914 nea River. The loss of hydroelectric power was important but the loss of water to industry, cities and canals would have greater effect.
The aircraft were adapted Avro LancasterThe Avro Lancaster was a four-engined World War II bomber aircraft made initially by Avro for the Royal Air Force. First used in 1942, together with the Handley-Page Halifax it was the main heavy bomber of both the RAF and the RCAF, and the most heavily u Mk IIIs, dubbed Special Bs. To reduce weight, much of the armour was removed, as was the mid-upper turret. The substantial bomb and its unusual shape meant that the bomb doors were removed and the bomb itself hung, in part, below the body of the aircraft. It was mounted in two crutches and before dropping, it was spun up to speed by an auxiliary motor.
Bombing from 60 feet (18 m) at 240 mph (390 km/h), at a very precise distance from the target, required expert crews, intensive night and low-altitude flying training, and the solutions to two technical problems. The first was to know when the airplane was the correct distance from the target. The two key dams at Moehne and Eder had a tower at each end. A special aiming device (a triangle similarSeveral equivalence relations in mathematics are called similarity . For similarity between people, see similarity (psychology). Geometry Two geometrical objects are called similar if both have the same shape. One can be obtained from the other by uniform to that created by the two towers and an airplane at the correct distance from the dam) showed when to release the bomb. The second problem was to measure the airplane's altitude (the usual barometric altimeters were insufficiently accurate). Two spotlights were mounted under the nose and under the fuselageIn an aircraft, the fuselage is the main body section that holds crew and passengers or cargo. In single engine aircraft it will usually contain an engine, athough in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselag such that their beams would intersect 60 feet (18 m) from the underside of the plane. At the correct height, the two spots of light would merge into one on the surface of the water.
The bombs were delivered to the squadron on May 13, after the final tests on April 29. With promising weather reports the pilots, navigators and bomb aimers were informed of the targets on May 15, the rest of the crews on the following day.