Project Orion (nuclear propulsion)

Project Orion was a study conducted in the 1950s and 1960s by the United States Air Force, DARPA, and NASA into the viability of a nuclear pulse spaceship that would be directly propelled by a series of atomic explosions behind the craft. Following preliminary ideas in the 1940s and a classified paper co-authored by physicist Stanisław Ulam in 1955, DARPA (then known as ARPA) agreed to sponsor and fund the program in July 1958.

Source: Wikipedia — Project Orion (nuclear propulsion) (CC BY-SA 4.0)