Orion was meant to operate in the atmosphere as well as in space...
http://www.islandone.org/Propulsion/ProjectOrion.html
The propulsion was indeed supposed to be ablative; they demonstrated
this with a bunch of proof-of-concept tests (at ground level) where
nukes kicked nearby stuff around (graphite spheres, aluminum, etc.)
(The Orion project team did later come up with a chemically launched
version which would have gone nuclear only once in orbit, but that was
because atmospheric nuclear explosions had become politically
infeasible, not because of any concern about technical feasibility).
rst