James Benford’s JPL experiments pushing an ultralight carbon sail with a microwave beam were the first solid demonstration that the beamed sail concept would work. Both James and brother Gregory were deeply involved in the design of the Cosmos 1 solar sail mission, and understandably disappointed that its microwave experiment — aimed at demonstrating a microwave push on the orbiting craft from the Deep Space Network’s Goldstone antenna on Earth — was never completed.
But an interesting offshoot of the JPL study was that while photon pressure on the sail was clearly demonstrated, the power of the beam did not account for all the observed acceleration. Something else was clearly at work, evidently the evaporation of absorbed molecules from the hot side of the sail, a phenomenon known as desorption. In a March, 2005 paper for Acta Astronautica, the Benford brothers suggest using this effect to achieve additional thrust over conventional solar sail designs. In fact, a microwave sail designed around these principles could have advantages over solar and laser sails.
The reason: microwaves do not damage sail materials and can heat them less destructively than lasers. So why not paint a sail with a compound that can sublime away with heating, giving the sail a powerful boost early in the mission and allowing it, once the coating has been expended, to function as a standard solar sail? Such a combination would allow a beamed sail in low-Earth orbit to be boosted quickly into an interplanetary trajectory; it would then use solar photon pressure for the duration of the journey, having started out at much higher speeds than would have been possible without the coating.
“Solar sails are plagued in mission plans by low accelerations, which dictate long orbital times,” the authors say. “Laser sails have problems with atmospheric distortion if the laser beam is fired from the ground, which microwave beams do not. A natural collaboration emerges between subliming sails driven by beams in LEO, converting to greatly accelerated solar sails for the long mission.”
The coating itself needs to be easy to apply to a sail surface, and one that sublimes readily when heated by microwaves. Finding candidate compounds — and the Benfords discuss the possibilities in this paper — would allow remarkable advantages. “The upper temperature range of thermal desoption-driven sails,” they write, “promises higher specific impulse than liquid rockets…A major thrust of future work should be to study such embedding and the resultant thermal desorption rates of both painted materials and desorption of embedded atoms.”
The paper is Gregory Benford and James Benford, “Acceleration of Sails by Thermal Desorption of Coatings,” in Acta Astronautica Vol. 56, No. 6 (March, 2005), pp. 593-599. James Benford’s thoughts on the design for a future Cosmos-series sail can be found here. An earlier Centauri Dreams article discussing thermal desorption is also available.
