There are two options for the so-called 'gravity disc'. The first uses EM waves to generate a plasma around it and pushes it backward, and so as it is an air-breathing engine, it unsuited for anything except first-stage use, and as it needs a lot of power, a nuclear jet would probably be better (or even better an Nulclear liquid air cycle engine). The other so-called gravity disc utilises a superconducting annulus, spinning at high speed, to pruduce an apparent upward force on anything above it. However, the experiment has only rarely been repaeted (not enough to prove that the results of the original experiment was not due so some other factor. If this works however, this has only been seen above a horizontal spinning disc, and so is more use for simulating zero-G than as propulsion. In fact the only (theoretical) propulsive effect that this branch of science has produced a design for, is a gravitational/electromagnetic transducer, and this design doesn't even have a disc (it has a slab).
