Dynamic America by General Dynamics and Doubleday published 1960
"Within this decade chemical propellants will reach their highest point of efficiency. But even this power increment will not be sufficient to develop spaceflight capability beyound a bare minimum. Nuclear powe seems most likely to provide the energy-weight ratio requisite to lift megatons into deep space. Theoretically feasible, such systems are under intense study at General Atomic Division (Project Orion) and elsewhere."