Basically an atom bomb with wings??
Nope, A HUGE thick hemisphere of steel with a crew compartment on top and injection ports to squirt a hydrogen bomb under it to detonate a few microseconds later. See this site
http://www.islandone.org/Propulsion/ProjectOrion.html
We don't need no stinkin' wings!
You put a nuke under a big steel plate and I guarantee you that sucka will FLY! You just gotta make sure you set off the next one before you fall back down and crash. Two, maybe three "pulses" and you're well out of the atmosphere, assuming you start in the atmosphere.
I think the most rational use of Orion is exo-atmospheric, however. But then you're still left with the problem of getting out of this gravity well before you can really get started.
By the way, I think the best fictional description of Orion I've seen is in Niven and Pournelle's Footfall. That's where the "this sucker will move" line comes from. Pournelle is a long time proponent of the "big dumb booster," including "old bang bang."
We WILL go back to the stars, riding a fire in the sky!