By far the most realistic engine for deep space travel is the Orion project [1]. You load a large rocket with lots of nukes, and detonate one at a time behind a pusher plate.
Of course, humanity being what it is, we'll never trust each other with the idea of building thousands of nuclear bombs with the "firm promise" that they'll only be used for space travel.
Of course, humanity being what it is, we'll never trust each other with the idea of building thousands of nuclear bombs with the "firm promise" that they'll only be used for space travel.
[1] https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...