I am not sure where it is now, but I once found in my house a book on the rocketry of its time, which was the space race. Among its chapters was one discussing a proposed nuclear powered rocket, which used fissile material like uranium to provide thrust. Such a rocket was never built but the idea of a nuclear rocket has not disappeared as researchers at the University of Washington have designed and performed preliminary tests on what would be a nuclear fusion powered rocket engine.
If my memory serves me correctly, that fission powered rocket used the radiation of the fissile material to ionize a piece of metal, and that ionized metal would be projected out of the rocket, creating thrust. While the mechanism is completely different, the fusion rocket also uses the ejection of metal to provide thrust, but the similarity ends there. What the researchers have proposed is to create pellets of fuel-plasma that are shot into the rocket nozzle. There an intense magnetic field is used to crush a ring of metal down onto the plasma with enough force to trigger fusion. The fusion then ionizes the metal so quickly that it launches from the nozzle, and accelerates the rocket.
There is still a great deal of work to do before fusion rockets push people to Mars, but this is a very promising design. Nuclear fusion releases a great deal of energy, which will allow a fusion rocket to use less fuel, and that will greatly decrease its weight and cost.
Source: University of Washington