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.
Nuclear explosions release mostly heat and radiation. Are you turning your pusher plate into high energy plasma with the explosion and using that to propel your spacecraft? My gut feeling is that your total ISP for this is disappointing compared to the amount of mass you add for the huge store of nuclear bombs.
> Are you turning your pusher plate into high energy plasma with the explosion and using that to propel your spacecraft?
I think the idea was for the bomb to vaporize a certain amount of propellant. According to wikipedia, the propellant was supposed to be tungsten, but I imagine that any substance would do. For example ice. The vaporized propellant hits the pusher plate and is reflected, resulting in an exhaust jet of very high velocity. The ISP was initially calculated to be between 4000 and 6000 seconds (so 10 times higher than the Space Shuttle), but later when they did the calculations with fusion bombs they concluded that an ISP of 75000 seconds is possible.
Not sure if this is the same idea as the one above, but the ones I’ve heard send the nukes up separately, so that the final launch craft is not impacted by the mass of the nukes. Instead as the craft passes by each nuke, they are detonated, and so it accelerates.
This plan is somehow even more insane than the original one. Now you have to launch bombs into space many years before launching your craft, then somehow fly in absolute precision with them to pass close enough to get an even push but not so close as to collide.
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...