Hawking predicted a plasma engine that doesn’t need fuel.

The engine that doesn't need fuel is not quite true. A plasma engine that collects plasma from space and heats it with microwaves can push the craft forward without tank-stored fuel. But thrust cannot form from emptiness. The magnet collects plasma from the craft's environment. And then microwaves turn that plasma superheat. Then the plasma travels to the exhaust tube forming thrust. 

Hawking predicted an ion- or plasma engine that doesn't need fuel or electricity. But how can this be possible? How can an engine operate without fuel? Before we answer that question, we must realize how the combustion engine works. In combustion engines. The system ignites the fuel-air mixture. And that expands the fuel-air mixture. 

An expanding mixture pushes pistons down or sends exhaust gas, which pushes jet aircraft forward. It is also possible to use nano-carbon or some other organic nano-powder. Those organic materials can combine with oxygen explosively. 

The combustion engine can operate in a vacuum if the system injects oxygen with the fuel into the cylinder. Then we can think about things like electric jet engines. The electric jet engines don't necessarily require an outside energy source. The idea of those systems is that the electric arc or some other thing heats the air in the chamber. There is the possibility that the system pumps air from the sides of the chamber using a compressor. And then the EM-system just 

That expands the air, and the heated air expands and erupts backward. And that creates thrust for that engine. It is also possible to use mechanical heating that can be the rotating plate. That causes friction that heats the air. Or the acoustic systems can create pressure waves which heat the air. That kind of system requires an electric source like a nuclear reactor that gives electricity to the system. 

The nuclear-powered submarines can also make this kind of propulsion system. The system can use lasers or microwaves to expand the water in the chamber. When that radiation heats water that system can push the submarine forward because of the expanding water. 

But things like ion engines don't need electric arcs to make thrust. The system can pull ions into it using magnetic fields. Those ions can travel through particle accelerators and then hit iron or gold sails. That thing can push the craft forward. The other version is that the system pulls ions into the rocket chamber where lasers or microwaves will warm that ion plasma. The system can pull ions through the craft and then increase their energy level by using electromagnetic stress. 

Powerful magnets, similar that CERN uses, can collect plasma from the solar wind to the point where the system focuses microwaves. Those microwaves turn that plasma superheat. 

When we think about cases in which the plasma engine creates super-heated plasma using only microwaves and magnetic fields, we must realize that the system uses gas that normally surrounds everything. So if that Hawking plasma engine were in the atmosphere that system simply heats air around the focus of the microwave systems.  Same way, in space those systems use the gas that they can collect around the microwave focus using a powerful magnet. The magnet can pull lots of plasma that comes from the sun around it. And then the microwaves can turn it into superheat. 

The system can also shoot positrons (anti-electrons) into the plasma. This kind of ion-plasma engine is the new way to think about ion systems. This kind of system makes it possible to create the ultimate Medusa drive. 

The system pulls ions into the system and injects them between the craft and a massive solar sail that is connected to the craft. Then the system injects antimatter into that space. The craft itself must be very aerodynamic because that denies the braking effect. And the annihilation between the sail and craft pushes that system forward. 

https://glassalmanac.com/stephen-hawking-predicted-it-a-new-plasma-engine-needs-no-fuel-or-electricity/

https://www.worldanvil.com/w/project-kentaurus-nearlyoctagonal/a/medusa-drive-technology