Darpa Project Orion (1958) - Theoretical 8 million ton spacecraft the size of a small city.

Darpa Project Orion (1950-60s)

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.

Non-nuclear tests were conducted with models, but the project was eventually abandoned for several reasons, including the 1963 Partial Test Ban Treaty, which banned nuclear explosions in space, and concerns over nuclear fallout.

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)

While Project Orion never progressed beyond the conceptual and early design phases, it remains a fascinating chapter in the history of space exploration. Its audacious approach to propulsion demonstrated the creativity and ambition of scientists and engineers during the early days of the Space Age. Although the project was never realized, it contributed valuable lessons and ideas to the field of astronautics and propulsion technology.

https://www.photonicsonline.com/doc/nuclear-dreams-the-race-to-build-project-orion-0001