Those guys at least weren't up on the state of the art. Early rough calculations indicated a huge mass might be required, which is why the Einstein–Szilárd let

Those guys at least weren't up on the state of the art. Early rough calculations indicated a huge mass might be required, which is why the Einstein–Szilárd letter to FDR said "A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air." On the other hand, with as far as I know no more info than the Germans had, Otto Frisch and Rudolf Peierls estimated in early 1940 that a kilogram of U-235 might do it, see https://en.wikipedia.org/wiki/Frisch%E2%80%93Peierls_memorandum https://en.wikipedia.org/wiki/Frisch%E2%80%93Peierls_memoran... They pretty much laid out the correct physics mechanism (fast neutrons) and were only off by a factor of 15 on the amount required; pity for Adolph that they were Austrian and German Jews self-exiled in England by then.