Stanislaw Ulam appears to have been the originator of the concept of using nuclear explosions in sequence to drive a vehicle, and he did in fact work on the Manhattan Project. In the John McPhee bio of Ted Taylor, who started the actual Orion project idea in 1958 (he was too young to have worked on the original A-bombs), he says that the idea came to him from the workings of a soda vending machine. But Major Hoople's contribution has never, to my knowledge, been made a part of the background history of the pulsed-explosion propulsion concept. As far as I can tell, the true beginnings of that concept were independently developed by a Russian and a German way back in the late 19th Century (conventional explosives, of course.)