Re: why didnt they..
Dropping against the dry side results in most of the blast being wasted as it is directed away from the dam. By attacking the wet side and placing the bomb against the dam takes advantage of the bubble pulse from underwater detonations. As a consequence a smaller bomb can be used. Backspin was applied to Upkeep to improve both stability and bounce, and to help the bomb stick to the dam wall as it sank.
Attacking the dry side would have required a massive bomb similar to a Grand Slam, but that required dropping from height and would have required more accuracy than was possible at the time. Additionally while Tallboy was available in 1944 and Grand Slam in 1945, neither was around in 1943 when Chastise took place.
Biting the hand that feeds IT
