system
Given the following post, respond as if you're commenting on Reddit:
human
Little Boy v Fat Man: why were different cores used?

LB was a uranium bomb and FM was a plutonium bomb.  Why wasn't the same design used for both drops?
gpt
This touches on a core aspect of the Manhattan Project and how exceptional it was as an R&D project. Compare to a more normal R&D project like the Apollo Program where scientists and engineers work together to come up with the best design for a system, build it, and ultimately work towards making it operational. But where there's potential to end up in some blind developmental alley or to have the whole program stall out due to some core aspect of the program being delayed or just plain unsuccessful.

The Manhattan Project didn't work that way, they didn't figure out the best route to the bomb and then pursued that. Instead they pursued *every route to the bomb simultaneously*. They pursued 3 different fissile fuels, 2 of which worked. They pursued 5 different methods of production for weapons grade fissile materials, 4 of which worked, including 3 different techniques for isotopic enrichment, which they ultimately just used as different stages in a pipeline to increase production capacity on a short timescale. They pursued 2 different bomb designs for each fuel (so in total 4 different designs), but ended up finding each fuel had a design more suited to it, at least in the short term.

This is why the end result of the Manhattan Project wasn't just one bomb design with one type of assembly, one type of fuel, and one method of producing that fuel. It ended up being 2 completely different designs using completely different fuels one of which had one method of production and the other of which had 3 different methods of production. It easily could have ended up being more designs/fuels/production methods had things worked out a little differently. Or fewer, had engineering or manufacturing of various facilities lagged behind.

This shotgun approach was a core reason why the program was so successful so rapidly (just 8 years after the discovery of the phenomenon of nuclear fission, in fact). Had they instead used the traditional approach they almost certainly would have gone with Uranium-235 as the fuel using gun assembly and centrifugal isotopic separation for enrichment. These we're all the best and most promising bets beforehand. And centrifugal separation is indeed one of the better methods for enrichment, but it took literally decades to advance the state of the art to the point where it was practical. The Manhattan Project attempted centrifuges, but it ended up not working out, while other methods of isotopic enrichment were easier to achieve with 1940s technology. Pursuing that route would have resulted in the program hitting a roadblock on fissile material production. It also would have resulted in a very slow pace of bomb production due to the sheer amount of material needed in gun assembly devices (about 10x as much as with implosion), it would have taken years after 1945 for the first bombs to become available.

This also helps explain the enormous costs of the Manhattan Project, of course. Because it wasn't just "one" project but effectively several different projects all working in parallel, some of which succeeded some of which failed.