Ulam considered the problem of nuclear propulsion of rockets, which was pursued by Project Rover, and proposed, as an alternative to Rover's nuclear thermal rocket, to harness small nuclear explosions for propulsion, which became Project Orion.
Ulam considerou o problema da propulsión nuclear dos foguetes, investigada no Proxecto Rover, e propuxo como alternativa ó foguete termonuclear o aproveitamento de pequenas explosións nucleares para a propulsión, o que se convertería no Proxecto Orion.