In 1968 Freeman Dyson calculated that there “Momentum Limited” spacecraft using atomic pulse propulsion could reach Alpha Centauri in 133 years.
“The biggest design above is the “super” Orion design; at 8 million tonnes, it could easily be a city.[12] In interviews, the designers contemplated the large ship as a possible interstellar ark. This extreme design could be built with materials and techniques that could be obtained in 1958 or were anticipated to be available shortly after.”
https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#Theoretical_applications
What's a 'there “Momentum Limited” spacecraft?'