Originally Posted by
GadgetFreak
I think that they wanted to try two different types of detonators. Also, I seem to recall that the Hiroshima bomb was uranium and the Nagasaki bomb was plutonium. And I think that there was difficulty in making enough enriched material for either. That might have had something to do with it.
At the time it was much harder to do isotopic separation (U235) than chemical separation (Pu238 made in reactors). At the same time, plutionium requires implosion detonation which is rather tricky while uranium can work in a simple gun-type bomb. Therefore it was felt that there was no need to test a uranium gun-type bomb, but there was a desire to test a plutonium gun-type bomb rather than have it go fizzle in a bombing meant to have frightful impact on an enemy...