Read Larry Niven's "Footfall."
We (the humans) build an Orion-powered ship, underwater, in the Puget Sound near Bremerton in order to hide it from the elephant aliens that have conquered us... then we launch it and wipe out the entire west coast of Washington and engage in a space dogfight with the huffalump mothership that spans distances of several AU around the solar system.
I accept that it might be suitable for accelerating a ship once you have sufficient distance between the blast and the absorbing shield that catches the energy, but I don't accept that we humans have the engineering proficiency to LAUNCH something using nuclear fission explosions as the sole source of propulsion and have the ship survive. Even if cost and materials provisioning was no object.
'Splode a nuke under a huge metal sheet attached to a ship, and I suspect the huge metal sheet will rumple and fail if it's too close. Add in the resistance from the water on top of the spaceship during launch, and that energy seems more and more likely to drive straight through the metal and tear apart the blast shield.
Actually, it was under a big tent or thin dome. We let the Fithp think it was a food storage/distribution depot by painting the "surrendered Fithp" symbol on it. (They communicated any mis-use would cause all so marked facilities to be lasered/KEW'd.) so we only misused it that one time.
To confuse the Fithp we engaged in a coordinated launch (with the Soviets) of all our remaining nuclear missiles, allowing the launch to happen amidst all the visual distractions and EMP.
Once in space it used Iowa class 16" naval guns and nuclear shells, and kamikaze space shuttles that used their heat shields to keep the lasers off of them, and small kamikaze fighter craft built around 16" naval guns. (there being no way to retrieve the smaller craft, or return the Space Shuttles, so far out of their usual orbital maneuvering envelope.)
And they also used bomb pumped X-ray lasers that would fire when the ship made maneuver pulses.
http://www.up-ship.com/apr/michael.htmAnd as to the pusher plate crumpling, the specific impulse of a nuke is HUGE. The shield can be really really thick. The (real) engineers studying Orion thought of that. The main technical challenge was the shock absorber system.
If an Orion craft were assembled in orbit, by unmanned Orion cargo launches, then a more "delicate" Orion system that was to be manned would be more feasible. The biggest stresses on the system would be the sea or land launch when the first bombs would superheat the atmosphere underneath the craft. Once in space with only the bomb-mass and radiation to work with, it would have been much gentler.
The thrust/weight ratio was so huge for an Orion system, that the engineers were working on the FARM that would have gone inside the ship to feed the crew, and provide grazing space for the chickens and goats.
