They tried four different types of separation: centrifugation, thermal diffusion, gaseous diffusion, and electromagnetic separation. Centrifugation proved unwor

They tried four different types of separation: centrifugation, thermal diffusion, gaseous diffusion, and electromagnetic separation. Centrifugation proved unworkable at the time and the remaining processes were used in concert (taking the slightly enriched products of one into the feed of another) to produce HEU. The Manhattan Project was so expensive largely because it was a shotgun approach to development. They tried 6 different ways to produce 3 different possible bomb fuels and two different bomb designs. Ultimately the most promising methods didn't work (centrifugation) but this didn't cause the program to halt or have to rethink things, it kept chugging on everything else running in parallel and ultimately succeeded with producing 3 different ways of making HEU and one for making Plutonium plus a bomb design that worked for each of the two fuels.