Back in the 1950s GE, P&W and others researched various nuclear powered gas turbines for aviation. The motive was long range flight. Instead of a combustor they heated the air with a liquid metal heat exchanger cooled reactor.
My guess is flying nukes for human travel will never become politically correct, even if they catch bin Laden. Nevertheless it gave me an idea:
Since thermal efficiency isn't such a big factor when getting rid of fuel is the only objective, maybe it would be possible to alter the reaction at the plant to yield high temperature waste.
Instead of a combustor the air would be heated by the nuclear waste.
The scram would be fired from a cannon and once it reaches Mach 5 the heat from the waste would power the scram up to escape velocity.
To be sure, however high the heat transfer situation the time for the heat to be transferred to the air would be so short a really long heat transfer chamber would be necessary.
This thing would have to be very simple to be reliable and to survive the zillion Gs of being fired out of a gun.
Maybe a combination of compressed air and then an explosion would work.
Bret Cahill