It Turned Out the Backspin Was the Key to Busting Dams With Big Depth Charges.
On May 16th 1943, Royal Air Force (RAF) Avro Lancaster B Mark III Special bombers of 617 Squadron began Operation Chastise as dambusters against the Germans. Over the course of the attacks the Mohne and Edersee dams were breached and the Sorpe dam was damaged. Two hydroelectric power stations were destroyed and several more were damaged. The resultant flooding heavily damaged or removed much of the residential and industrial infrastructure located in the Ruhr and Eder valleys.
Targeted Even Before the War
The RAF had determined that the Ruhr valley, or more specifically the steel and mining industries located therein, was a strategic target even before World War II commenced in Europe. In addition the fresh water provided by the dams was used for drinking water and for the canal system used to transport German war materials. An effective method for attacking the dams had to be engineered by the RAF. Enter one Barnes Wallis.
It’s All In the Backspin
Wallis was the Assistant Chief Designer at Vickers Aircraft. In theory a relatively small explosive charge would cause a breach in a dam if it exploded underwater against the wall of the dam. The Germans had erected torpedo nets to protect the dams from conventional torpedoes and standard bombs could not be dropped with the necessary precision. Wallis and his team decided a bomb shaped like a depth-charge could be dropped from low altitude some distance from the dam. The bomber would induce backspin on the bomb, causing it to skip across the surface of the water- right over the German torpedo nets.
Low and Slow on the Bomb Run
When the bomb (code named Upkeep) reached the dam, it was theorized that because the device would likely still be spinning it would sink along the surface of the dam until its hydrostatic fuse detonated the device 30 under water. The bomber dropping the device would be forced to fly a low-altitude (60 feet), low-speed (240 mile per hour) approach to the drop point, making it vulnerable to all manner of German defenses. After tests during 1942 and early 1943, the concept was proven but the heavier bombs and the modified Lancaster bombers to deliver them were not yet ready.
Commonwealth Melting Pot
A new squadron was formed as part of Number 5 Group RAF. Initially the squadron was designated simply as X Squadron. Wing Commander Guy Gibson was chosen to command the RAF Scampton-based squadron. The men making up the 21 bomber crews in the squadron, eventually designated 617 Squadron, came from all over the RAF as well as from the Royal Australian Air Force (RAAF), the Royal New Zealand Air Force (RNZAF), and the Royal Canadian Air Force (RCAF). Operation Chastise was to be a Commonwealth show.
Reworked for the Dambusters
Avro Lancaster B Mark III bombers were modified and designated as B Mark III Specials. Most of the Lancaster’s protective internal armor and the upper machine gun turret were removed to save weight. Because of the bomb’s five foot length and four foot diameter and 9,250 pound weight the bomb bay doors were also removed. In flight the bomb would be carried under the fuselage. The motor used to induce backspin on the bomb before release was also mounted below the fuselage of the aircraft.