Space Forum / Space Flight / September 2005

Unfortunately, conventional explosives are *not* good rocket propellants,
and an external-combustion design like that (essentially a non-nuclear
Orion) is not generally an efficient rocket engine.  Unless there is some
subtle design trick that greatly boosts performance -- what it would be is
not obvious -- such a design needs an enormous mass ratio, much higher
than that of an ordinary chemical rocket.

The Orion guys did build and fly a small chemical-explosive demonstrator,
which is now in the Air & Space Museum, but it was built to show basic
feasibility and flight stability, not to have high performance.