So you just keep firing off nukes? They would have to be extremely small nukes. Too big and you waste the energy by absorbing it so you don't kill your crew. The ideal method would be to make them small enough that you don't need shock absorbers of any kind. However, I think critical mass is required to make it explode (that may just be the amount required for a continuous reaction in a nuclear reactor). If that's the case, you can't make it small enough to not kill humans. Maybe you could have the humans breathe a liquid in a flooded, cushioned room while strapped in place, but we still don't have that tech. If we did have it, it could negate the need to absorb shock for the whole ship, drastically increasing energy efficiency. On top of that, they should be detonated in a tube, like a shaped charge. That's how modern engines work. Explosions expand outward in all directions. Only the energy directed toward the back of the space craft helps. Modern engines have a bell that the exhaust travels through. Exhaust gases can escape sideways, so pressure builds in the bell. It has no where to go but out the open end of the bell, away from the craft. Detonating a nuke in open space behind the craft will give you a kick, but lots of the energy will go sideways or be absorbed by the pistons. Ideally there would be a chamber attached to the ship that you would detonate the bombs in. The problem is that such a chamber would be very heavy if it could even be built, and I have my doubts that even modern tech could build it. Uranium and plutonium are also VERY heavy. The rocket equation dictates that you have to use fuel to carry fuel, so that's another problem. Maybe with nukes the power to mass ratio makes the weight negligible. Using a proper chamber would probably increase efficiency WAY more than not needing shock absorbers.

I don't think Orion was ever practical. In addition to all those problems, there are still tons of technologies required that we still don't have. Maybe a nuclear reactor system would work. You could allow the nuclear fuel to run extremely hot and boil water or something else. Channel the steam into a very strong and very small chamber with a very small opening in the back. The pressure would be incredible. The water might even turn to oxygen and hydrogen plasma. It would fly out the hole at high speed. But even that has some engineering mountains to conquer that are quite daunting.

I like the idea of a nuclear ship. I wish it worked and that we had it now. I still think we may figure it out and build one one day. There is exciting new research into fission going on right now that may help. Some craft already get electric power from small nuke generators for their electronics and ion propulsion systems.