I may be giving away an important plot element, but Project Orion appears as an important plot element in Niven and Pournelle’s Footfall.
http://www.amazon.com/Footfall-Larry-Niven/dp/0345323440
Good stuff, as are most if not all of their collaborations.
I first heard of Dyson spheres in a Larry Niven's "Ringworld."
That's a sun in the middle of a gigantic (artificial) Dyson sphere.