Have you had a look at Project Daedalus ?

https://en.wikipedia.org/wiki/Project_Daedalus

Or Project Orion :

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

and the excellent book on Project Orion

https://www.amazon.com/Project-Orion-Story-Atomic-Spaceship/...

The book written by Freeman's son George Dyson is also very good:

https://www.amazon.com/Project-Orion-Story-Atomic-Spaceship/...

Alas there is no ebook and the print book is quite expensive.

There is a documentary on Youtube from the BBC about it all:

https://www.youtube.com/watch?v=uMcwVj0S6Ws