The Medusa design is a type of nuclear pulse propulsion which has more in common with solar sails than with conventional rockets. It was proposed in the 1990s in another BIS project when it became clear that ICF did not appear to be able to run both the engine and the ship, as previously believed.
A Medusa spacecraft would deploy a large sail ahead of it, attached by cables, and then launch nuclear explosives forward to detonate between itself and its sail. The sail would be accelerated by the impulse, and the spacecraft would follow.
Medusa performs better than the classical Orion design because its pusher plate intercepts more of the bomb's blast, its shock-absorber stroke is much longer, and all its major structures are in tension and hence can be quite lightweight. It also scales down better. Medusa-type ships would be capable of a specific impulse between 50,000 and 100,000 seconds (500 to 1000 kN·s/kg).
The January 1993 and June 1994 issues of the Journal of the British Interplanetary Society (JBIS) have articles on Medusa.
Read more about this topic: Nuclear Pulse Propulsion