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Indeed, this I believe is the key insight into the stunning success of the Manhattan project. The scientists worked pretty hard as soon as uranium fission and it's details were discovered, Frisch and Peierls critically got all the fast fission concepts right in 1940, see (https://en.wikipedia.org/wiki/Frisch%E2%80%93Peierls_memoran... And Frisch's story is particularly interesting, see Rhodes' book, doing Christmas vacation with his aunt, who just happened to be the first physicist her back in Germany colleague sent his results to right then.... Vs., for example, again from Rhodes' book, a clerical mixup ruining the saving throw for the German effort, the Nazi political types got invitations for the wrong seminar, one on very technical stuff instead of the pitch for atomic stuff (which, if they'd done everything right, they could have pulled off, I think).

But it took a long time to light a fire under the American authorities, and it wasn't until the absolutely critical replacement of his name is a footnote in history with Groves that things really got rolling, on the industrial scale needed, and the scientists and engineers sufficiently focused on the design and execution of the bombs themselves (which for various reasons didn't end up being the afterthought some expected). And he of course picked Oppenheimer to lead that effort, which was opposed by most, albeit he was one of the few uncommitted physicists capable at that level.

These two men organized more than 100,000 people for the industrial production of the required fissionables (90% of the work per Wikipedia), and Grove's drive got those ready in time to forestall Operation Downfall. Heck, they went from the first real test to putting metal on target in 21 and 24 days....

And the design and fabrication of "the bomb" turned out to be massively harder than they expected due to weapons grade plutonium not being suitable for a gun assembly bomb (which is also grossly wasteful of fissionable, if the Little Boy is any guide, as I recall it had 3x critical mass, and a fair amount if it wasn't as pure U-235 as they'd have wanted). Making the implosion concept work was hard, and they got it right the first time....

Read Rhode's book, especially the latter half after the nuclear physics discoveries take a back seat (https://www.amazon.com/Making-Atomic-Bomb-25th-Anniversary/d...) and Grove's autobiography (https://www.amazon.com/Now-Can-Be-Told-Manhattan/dp/03068018...) to learn the organization and management details, they're amazing.

And had much wider effects on the world at large, that we could indeed do such things led to the Apollo program, and of course to too much conceit that "If we can put a man on the moon, why can't we [do something very different and a lot more intractable, probably without even a clearly defined goal]?"

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