The 1945 Trinity test was run to prove the complicated “Fat Man” design would even work. But why were scientists so confident of the “Little Boy” design that the didn’t feel the need to do a full test of the design? The design was simpler, but still, no one had ever done it before. Was it purely for time? Lack of materials? Or was the design and principles so sound, it couldn’t fail?
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There were early plans to build a plutonium cannon bomb called Thin Man, but they came to nothing. The bomb would use plutonium 239, but the metal coming out of the Hanford reactors contained a sizeable amount of the Pu240 isotope. Plutonium 240 has a much higher spontaneous fission rate than Pu239 which means there are more available neutrons.
In the time a uranium cannon took to assemble a critical mass, it was found highly likely that contaminated Pu240 would have triggered a premature chain reaction; the bomb would ‘fizzle’ – it’d be perhaps a few tonnes equivalent.
The way it could have worked would have been to have a much higher muzzle velocity for the cannon which required a longer barrel. Whilst the required velocity was *just* about possible, the weapon would be too long for any existing or planned bomber and Thin Man was abandoned in July 1944.
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