Space Applications of High Power Microwaves

Summary form only given. A variety of schemes have been suggested for transferring energy from Earth-to-space, space-to-Earth, and space-to-space using high power microwave beams. All use power beaming, scaling of which will be summarized. This talk summarizes a recent review. Microwave beams have been studied for propelling spacecraft for launch to orbit, launch from orbit into interplanetary and interstellar space and deployment of large space structures. The microwave thermal rocket is a reusable single stage vehicle that uses a high power microwave beam to provide power to a heat-exchanger type propulsion system, called the microwave thermal thruster with double the specific impulse of conventional rockets. It could transform the economics of launch to space. The principle is analogous to nuclear thermal thrusters, which have experimentally demonstrated specific impulses exceeding 850 seconds. The cost of such large HPM systems is driven by two elements-the capital cost, including microwave source cost and radiating aperture cost, and the operating cost, the cost of the electricity to drive the system. Cost must also account for the learning curve, the decrease in unit cost of hardware with increasing production. For fixed effective isotropic radiated power, minimum capital cost is achieved when the cost is equally divided between antenna gain and radiated power. Microwave propelled sails are a new class of spacecraft that promises to revolutionize future space probes. Beam-driven sail flights have now demonstrated the basic features of the beam-driven propulsion. An early mission for microwave space propulsion is dramatically shortening the time needed for sails to escape earth orbit. Simulations of trajectories and escape time for sails driven by a microwave beam from the surface show that resonance methods can reduce escape times from earth orbit by over two orders of magnitude. A number of missions for beam-driven sails have been quantified: high velocity ma- pping of the outer solar system, Kuiper Belt, Plutinos, Pluto and the Heliopause, and the interstellar medium. The penultimate is the interstellar precursor mission. For this mission class, operating at high acceleration the sail size can be reduced to less than 100 m and accelerating power ~100 MW focused on the sail. At 1GW, sail size extends to 200 m and super-light probes reach velocities of 250 km/s for very fast missions. Will such sails riding beams be stable? Experiments have verified that beam-riding does occur. Beams can also carry angular momentum and communicate it to a sail to help control it in flight. Although technical feasibility is the focus, advocates must also deal with societal issues: interference with unintended targets in the sidelobes, spectrum allocation and potential weaponization.