Testing of the Upkeeps was continuing while the crews were on training. Vickers' Mutt Summers and Avro's chief test pilot Sam Brown had been dropping the bombs from the height specified of 150 feet but they continued to break up when first hitting the water.
It is a long living myth that Upkeep was a spherical shape when in actual fact it was more like an oil drum (cylindrical). This is mainly due to the 1954 film 'The Dambusters', which portrays Upkeep as a spherical shape. This was not entirely inaccurate because Wallis had indeed intended his weapon to be spherical. The reason for the change is outlined below and is due to the bomb breaking apart upon impact with the water. The reason the film kept the bomb as a spherical shape was because at the time it was made, Upkeep was still on the secrets list and the film makers were unable to show its actual shape. Upkeep was not removed from the secrets list until 1963 despite the Germans knowing literally everything about it.
When Wallis was experimenting with half size prototypes he found that the casing of the bomb which gave it its spherical shape broke upon impact with the water. This was because the casing was made from wood due to the lack of steel available at the time. Despite efforts to increase the strength of the casing it continued to break. Wallis eventually decided to forget about the casing because the cylinder inside the container which contained the explosives often continued and did indeed bounce as he had expected.
VIDEO: Casing breaks appart on test drop
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However when Summers and Brown were testing the actual full size prototypes of the weapon, they totally broke apart upon impact with the water when dropped from 150 feet. The only way to stop them from breaking apart was to reduce the height from which they were dropped. Wallis recalculated and found that 60 feet was the maximum height from which the bombs could be dropped without being torn apart.
'Y' shaped bomb sight
With only three weeks to go to the planned date of the raid, Wallis asked Gibson if he and his boys could fly at the perilously low level of 60 feet. Gibson took a deep breath and promised that they would.
The final major problem to be solved was judging the distance at which the bomb had to be released to hit the dam exactly where it needed to be.
Once again the Royal Aircraft Establishment came up with the solution. They produced simple hand held Y-shaped wooden sights. The bomb aimer would simply look through an eyepiece at the base of the Y and when the two nails on the arms lined up with the towers on the dam, it was the correct range for releasing the weapon. This was about 400 - 450 yards (366-411 meters).
Establishing Range: How the bomb sight works
In practice, some of the bomb aimers found it very difficult to keep the aimer steady with one hand while the aircraft was bouncing around. Some aimers therefore devised their own means based on the same idea. These ideas generally involved marks on the clear vision panel in the nose and a piece of string to line up with the towers of the dam. It was a very simple idea involving basic mathematics, but it worked!
Communication was going to be critical on the raid. The success of this mission in particular would depend upon not only the teamwork within each aircraft but also the whole squadron.
In a normal bomber stream, aircraft attacked in a conveyor belt system dropping their bombs almost simultaneously on a broad target. This raid 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 communication was therefore essential, but the standard R/T radio sets performed badly at low level at night. To solve the problem they fitted VHF sets normally used by fighter aircraft.
On May 11th 1943, just 5 days before the night of the attack, the squadron began training with actual Upkeep bombs at Reculver (although they were not actually filled with explosives). They were amazed to see the drums bouncing over the water right up to the beach. Still they did not know their targets! After seeing the weapon in operation, it reignited talk that the target was the Tirpitz or even U-boats.
While on training with real Upkeeps at Reculver, both Shannon and Maudslay damaged their Lancasters by dropping their bombs too low and being caught in a huge column of water thrown up after it hit the water. By the time the attack came five days later, Maudslay's aircraft could not be repaired and the attacking force was down to 19 aircraft from the 20 originally intended by Gibson. Gibson had picked 21 crews for the squadron, the 20 to fly and one reserve. Coincidently, both Divall and Wilson had sickness and their crews would not fly. This therefore left 19 aircraft and 19 crews. The attacking force had been determined out of Gibson's hands.
VIDEO: Damage to Lancaster at Reculver test
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