minor engineering quibble
from the SpaceDaily article quoted in the link:
he ship would have carried many small fission bombs, which would then have been ejected one by one behind the ship and detonated. A reaction mass would have been either incorporated into the bomblets or dropped separately. The explosion vapourising it into a plasma which would then bounce off the pusher plate.
.. and in case anyone is concerned about the robustness (especially over long term) of this -
Springs and cushioning would support the plate and absorb the impact of the explosion to drive the ship forward.
LOL .. nice try guys