A scenario recently sketched out by Brian Wang at "Next Big Feature" is to launch a vehicle to escape velocity via using a nuclear pulse, but enclosed so the fall-out can be contained. I suggested it needed a working fluid, specifically hydrogen, to provide a mechanical coupling between vehicle and nuclear detonation. The original "Orion" nuclear pulse design used multiple small explosions over a long acceleration track up through the atmosphere - thus fallout issues. Brian supposed a large nuke might instead allow a very high acceleration of a payload in a short distance. Obviously it wouldn't be a manned vehicle, since the acceleration would be 10,000-100,000 gees. One problem I saw was the issue of energy transfer, since the original "Orion" used small charges that focussed the plasma from the blast, but interacted very briefly with the ultra-hot plasma, else the pusher plate would be in serious trouble. So what if a nuke was used to heat a volume of hydrogen gas, which was then allowed to expand and push the payload along a tube, say 2 km long. Basically an updated Verne Moon-Gun. I assumed the vehicle massed 10,000 tons, had a circular base (10 or 20 metres wide), was totally filled by either payload or a light, strong volume filling material. Boosted along a 2 km track, in a vacuum, until the end when a high-speed shutter would open. Final velocity would be roughly 13,500 m/s, allowing for some frictional loss. I could only make rough guesses as to what gas temperature would be required and I've no idea how efficiently a nuclear detonation's x-ray and neutron flash would transfer heat to the hydrogen, but I assumed a spherical gas reservoir either 100 to 200 metres across, probably under some positive pressure, buried in a mountain. For diatomic hydrogen initially at 300 K I ended up with a temperature of about 4600 K, in order for the expanding gas to do sufficient work to push the payload to the right speed. However I have no idea just how fast the gas would really expand - supersonic? sonic speeds? And how much would dissociate and ionise? How do you work that out? I know the specific energies for each, but not the actual proportions that would obtain. Any pointers, physics heads?