The Microwave Thermal Thruster Concept
The microwave thermal thruster heats propellant via a heat-exchanger then expands it through a rocket nozzle to produce thrust. The heat-exchanger is simply a microwave-absorbent structure through which propellant flows in small channels. Nuclear thermal thrusters are based on an analogous principle, using neutrons rather than microwaves, and have experimentally demonstrated specific impulses exceeding 850 seconds. A microwave equivalent will likely have a similar specific impulse, since both nuclear and microwave thermal thrusters are ultimately constrained by material thermal limits, rather than the energy-density limits of chemical propellants. We present the microwave thermal thruster concept by characterizing a novel variation for beamed-energy launch. In reducing the thruster concept to practice, the enabling physical process is microwave absorption by refractory materials, and we examine semiconductor and susceptor-based approaches to achieving this absorption within the heat-exchanger structure.