At the risk of hijacking this thread and getting banned (
) - the problem was that one of the German physicists (either Teller or Schoedinger ?) was convinced that you needed about 1000 times more U235 to create a chain reaction - so they were of the opinion that the nuclear explosive was virtually impossible. I'm not sure where they went wrong, but there are a lot of calculations where a factor of 10 here and there can get inadvertently dropped.
The one who worked for NASA - are you thinking of Werner von Braun? Tom Lehrer wrote a song about him ....
