Microwave Starwisp Sails

Robert Forward was a physicist, an aerospace engineer, an inventor, a science writer and a science fiction author. In 1985, he published an idea to create ultra-light interstellar probes that used microwave beamed power sails for propulsion. Freeman Dyson had pointed out to him that microwaves could use a sail with a much larger grid size because of the longer wavelength. Forward used that concept to develop his idea and called it “starwisp”. Now, James Benford has published a paper on starship sails propelled by directed energy beams.

Remembering Starwisp – [centauri-dreams.org]

Mention beamed propulsion and people invariably think you’re talking about lasers. The idea seems obvious once you’ve gotten used to solar sail principles — if photons from the Sun can impart momentum to push a sail, then why not use a laser beam to push a sail much farther, into the outer Solar System and beyond? These are regions where sunlight is no longer effective, but a laser infrastructure of the kind envisioned by Robert Forward could produce a tightly collimated beam that could drive the sail to an appreciable fraction of the speed of light.

But are lasers the best way to proceed? Although he would sketch out a range of missions with targets like Alpha Centauri and, the most audacious of all, Epsilon Eridani (this for a manned crew, with return capability!), Forward himself quickly turned away from lasers and began exploring microwave propulsion. I’m fairly certain the turn to microwaves came at Freeman Dyson’s suggestion, and when I asked Dyson about it in an interview some years back, his response all but confirmed the fact. “It doesn’t matter who came up with it,” Dyson said, “the question is whether it would work. It’s problematic but a good system to look at.”

Problematic indeed. But also a system with serious advantages over lasers. Forward wanted to reduce the weight of his unmanned probe as much as possible, so he conceived of making it out of nothing more than a wire mesh a solid kilometer in diameter, one that weighed a mere sixteen grams and included microchips at each intersection in the mesh. The name Starwisp seemed a natural for this spider’s web of a starprobe, a mission so lightweight that it would actually be invisible to the eye.

Starship Sails Propelled by Cost-Optimized Directed Energy – James Benford – [arxiv.org]

Microwave propelled sails are a new class of spacecraft using photon acceleration. It is the only method of interstellar flight that has no physics issues. Laboratory demonstrations of basic features of beam-driven propulsion, flight, stability (‘beam-riding’), and induced spin, have been completed in the last decade, primarily in the microwave. It offers much lower cost probes after a substantial investment in the launcher. Engineering issues are being addressed by other applications: fusion (microwave, millimeter and laser sources) and astronomy (large aperture antennas). There are many candidate sail materials: carbon nanotubes and microtrusses, graphene, beryllium, etc. For acceleration of a sail, what is the cost-optimum high power system? Here the cost is used to constrain design parameters to estimate system power, aperture and elements of capital and operating cost. From general relations for cost-optimal transmitter aperture and power, system cost scales with kinetic energy and inversely with sail diameter and frequency. So optimal sails will be larger, lower in mass and driven by higher frequency beams. Estimated costs include economies of scale. We present several starship point concepts. Systems based on microwave, millimeter wave and laser technologies are of equal cost at today’s costs. The frequency advantage of lasers is cancelled by the high cost of both the laser and the radiating optic.

IKAROS Solar Sail
Solar Sails

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