Operation Chastise

February 1943. Barnes Wallis now had the go ahead to produce full size bombs. Suddenly, however, the enormity of what he was attempting dawned and he began to have doubts that he could do it in the time alloted. The target date for the operation was set at the middle of May at which time the dams would be at peak capacity. This gave him about 12 weeks to finish the job.

"I had won my battle and I had the terrible responsibility of making good all my claims and you can't imagine what a horrible feeling that is. When somebody has actually called your bluff you are depending upon your self confidence and past experience to guarantee that you can do something entirely new, which nobody has ever done before"

But in true Wallis fashion he mainly kept this to himself and plunged into the work required to get the job done. First was the bomb itself. While he'd not yet produced a full sized prototype, his research told him the weapon's dimensions and weight. It required 6500lbs (2800kgs) of explosive. The rest of the weight would be casing - one strong enough to survive the initial impact of dropping from 120 feet at 220mph then bouncing along the surface and the final impact against the dam wall.

His tests had also shown that the bomb needed to have spin. This acheived two aims - the first was stabilization. It was essential for the bomb to present a flat surface to the water to ensure it bounced ina straight line. If on side of the cylinder was lower than the other then it could veer off course, or even tumble end over end. Spinning was vital, the gyroscopic forces made it stable and predictable.

Spin was also essential as once the bomb struck the dam wall, its spin help it "adhere" or cling close to the wall as it sank through the water. ( An effect caused by Magnus lift. Magnus Lift is similar to Bernoulli's Principle. Bernoulli's Principle states that fluid pressure decreases at points where the speed of the fluid increases. A spinning cylinder moved sideways through a fluid will have a high pressure area on the advancing surface)

The Wellington tests had shown there was no problem creating a mechanism to spin up the bomb before launch, but Wallis was concerned that a full sized bomb weighing over 10,000 lbs spinnng at 500 rpm would behave like a giant gyroscope. (the bomb was now code named UPKEEP) Would this affect the aircraft's performance? The only way to find out was to try.

Because he knew the spin and release mechanism would virtually remain the same irrespective of the internal structure of the bomb, his team rushed through a design of these components plus the modifications that would be essential for all aircraft involved. Modifications to the Lancasters included removing the bomb bay doors, creating a forward fairing for the bomb bay area, removing the mid upper gunners turret and reducing weight wherever possible. as training progressed other modifications evolved.

While Wallis and his team worked on the bomb, the RAF set about organising the aircraft and crews needed for the raid. It was decided to form a special squadron, 617 based at Scampton, rather than take an existing squadron off the line for training. In March 1943, Wing Commander Guy Gibson DFC was chosen as th squadron's commander. In some ways Gibson was an enigma - his colleagues either hated or loved him, however all agreed he was a good leader and a man who could get the job done.

Guy Gibson joined the RAF and by the outbreak of the Second World War was a bomber pilot with 83 Squadron. In July 1940 on Bomber Commands first raid of the war he won the Distinguished Flying Cross. After completing his first tour of duty, by volunteering for Fighter Command, he then avoided the mandatory six-month rest from operations by serving at a flying training establishment.

As a night fighter pilot he gained four kills and won a bar to his DFC. Promotion to wing commander followed and at the age of 23 he was posted back to Bomber Command in 1942. During the next 11 months he led 106 Squadron on 172 sorties.

His experience enabled him to assess what was required for the dam raids - although at that stage he didn't know what the actual targets were. He did know however, that the mission required pilots who were skilled at low flying and who could fly accurately. Gibson and his staff hand picked all the crews involved - a total of 21 crews plus his own.

On the 21 March, 1943, 617 Squadron was formerly raised at Scampton. Initially the squadron was supplied with Mk111 Lancasters from the RAF supply pool, these aircraft were to be used for training while their new modified Mk111s were being manufactured. Pilots were briefed and flying training commenced - again no one knew what the targets were, all they knew was that it required exceptional low flying skills with an ability to release a special bomb at a certain height and distance from the target while flying over water.

Practices took place in the English Lake District and in the county of Kent, in daylight at first because low level flying at night was considered too dangerous. Night flying was simulated by covering the windshield with tinted plastic and having the crew wear amber goggles.

The movie, the Dam Busters, gives a reasonable summary of the training process and how most of the problems associated with releasing the bomb were solved, much of which is now folk legend. However innacuracies have crept into some accounts of how these problems were solved.

The height problem

Wallis's original specifications called for the bomb to be released at a height of 120ft at a speed of 220 knots at a precise distance from the dam wall. Flying at night over water, it was soon found that the altimeter was useless for determining height. A better method had to be found. The movie attributes the idea of using triangulated spotlamps to Gibson who, after being inspired by a visit to the theatre where he sees spotlights being aimed from either side of the stage on the performer below, figures that a similar system would work from an aircraft. This is a piece of folk legend that's not true.

In fact it was Benjamin Lockspeiser (Chief Scientist at the Ministry of Supply), who came up with the idea. In the film, the spotlamps are shown under the nose and tail of the Lancasters, when in reality the second spotlamp was in the rear of the bomb bay. In many art prints of the dams raid, the spotlamp beams are incorrectly shown shining down directly beneath the aircraft (where they could not have been seen from the cockpit). Instead they were offset to the starboard side to allow the observer to see them from the cockpit side blister.

Distance from the Target

Another vital parameter was the release point for the bomb. Accounts vary but the most likely distance was 300 yards. In 1943, there were no precision electronic instruments capable of doing this, so once again ingenuity intervened. The "backroom boys" at Farnborough came up with a simple aiming device that solved the problem.

Bombsight 3D Model courtesy Sardon Technologies

Referred to as the "Three-penny bombsight", by holding the sight in one hand and looking through the peephole, the bomb aimer could then line up the towers on each dam. When the two towers coincided with the forward sight "blades" the aircraft was at the correct distance to drop the bomb. Bombsights were made to suit the distances for tower spacings for each dam.

In practice, some of the bomb aimers found it very difficult to keep the sight steady with one hand while the aircraft was making its bomb run. Some devised their own based on the same concept, such as marking the clear vision panel in the nose and a piece of string to line up with the towers of the dam.

At the same time crews of No 617 Squadron trained intensively for nearly six weeks, practicing low level flying and navigation both at night and during daylight hours.  Practice attacks against Uppingham and Derwent reservoirs went a long way toward improving confidence and bombing technique.

During the raid communications between aircraft would be critical. The success of this mission would depend upon not only the teamwork within each aircraft but also within the entire squadron. In a normal night bomber mission, aircraft attacked in a continuous stream dropping their bombs almost simultaneously on a broad target. The attack on the dams however, relied upon individual precision attacks. Gibson would have to act as the master bomber calling each aircraft in one at a time, essentially co-ordinating the attack and making adjustments as required. Good air to air communications were essential, but the standard HF R/T radio sets performed badly at low level. To solve this they fitted VHF sets normally used by fighter aircraft

Tests with the full size Upkeep bomb continued, with Wallis experiencing a string of embarrassing failures when the bomb casing shattered on impact. Finally he concluded that to lessen the stress on the casing, the bomb would have to dropped from 60 ft instead of 120 ft. Gibson agreed to try and recalibrated their light "altimeters" to the new height. In further practices they found they could fly at this altitude with equal accuracy. Wallis succeeded with new models of the bomb dropped from 60 ft and the design was put into production.

The aniticipated problems with handling the aircraft when the bomb was spun up to its full 500rpm proved not to be an issue. They found that the weapon did exert a gyroscopic effect, however as this was at a point when the aircraft was flying straight and level, the effectwas minimised, in fact it was thought that it probably contributed to the stabilty of the aircraft during the critical bomb run.

In the movie the Dam Busters, the drop tests show a De Havilland Mosquito making the drop tests at Reculver. In fact these movie clips are of later trials of an anti shipping version of the bomb known as highball. The actual tests used Lancasters and the following film clip shows one problem encountered.

The above clip shows a Lancaster dropping a full size (dummy) Upkeep. The splash from the bomb impacts the underbelly of the aircraft and removes a section of fairing (seen falling away and splashing down) From this segment it looks as if the aircraft also took on a load of water - it can be seen trailing astern as the Lancaster climbs away. This proved to be a problem when pilots flew too low and in his diaries Guy Gibson describes how many aircraft were damaged during training with full size dummy bombs. He was quite pragmatic about it, stating that if the aircraft were damaged on the actual raids it wouldn't really matter as the job would have then been done and the damaged aircraft would have to find their way home as best they could.

Finally all problems are overcome and by the beginning of May, 617 Squadron is declared ready...

Next. The Raids on the Dams