One Shot

Project Orion has problems.

You have to toss nuclear bombs under the pusher plate and explode them right on schedule. Plans on how to do this has only been generally sketched out. A trap door, exposed to regular fission blasts and that will work without fail? Catapult(s) on the rim of the plate, tossing them to go off at just the right time and place? There are no detailed plans on the delivery method.

The bombs would have to be rebuilt to fit whatever catapult is used. Machining radioactive metal is a delicate operation. Wouldn't it be nice to use existing, surplus weapons for propulsion?

This twist is promising. One blast, underground, pushes the craft up a collapsing tunnel past escape velocity. The explosion is largely contained and the plate only endures one blast. At the right moment small rockets can kick it into orbit.

The acceleration is too fierce for people, but most kinds of cargo won't mind. And we're talking about a lot of cargo; easily 100,000 tons. The greater the load the less the atmosphere effects the trajectory.

This also uses up existing bombs. Of course, one could surround the bomb pit with garden variety Uranium-238 and make Plutonium for further use...

Of all the plans that probably won't happen this is one of my favorites.

http://nextbigfuture.com/2009/03/underground-nuclear-tests-salt.html