|In 1967, Richard MacNeal introduced a dramatically innovative design concept that ultimately received serious consideration for a viable, working solar sail.* Termed the heliogyro, MacNeal's design consists of long, thin blades connected to a central core. The blades are rigidized by centrifugal force and pitched to provide attitude control, much like a helicopter. The initial design concept had two blades 5700 meters long, 1.5 meters wide, and 6 microns thick, and weighed 250 kg, but MacNeal also conceived of extremely large, advanced configurations over 30 kilometers in radius and weighing 45000 kg.
At first glance, it might seem inconceivable that a very thin sheet of plastic with a large aspect ratio could be sufficiently controlled and pitched, but preliminary studies have indicated that it is possible. In an experiment carried out by Richard MacNeal**, a sample blade 80 microns thick, 2 cm wide, and 2 meters long was successfully put through pitch maneuvers in a rotating room. The results from the experiment indicate that the heliogyro concept is feasible.
In fact, in the 1970's a design team from the Jet Propulsion Laboratory (JPL) and NASA's Ames and Langley research centers conducted a heliogyro solar sail design study in 1977-78 for implementation in a 1981 Halley's comet rendezvous mission. The design they came up with was a two-tiered, twelve-blade heliogyro, with the blades 7.5 kilometers long. Although their solar sail design was narrowly beaten out for final consideration for the mission by a solar-powered ion thruster spacecraft, the feasibility of solar sails, particularly heliogyro solar sails, was conceptually proven in their design study.
* Friedman, L., et. al., "Solar Sailing-The Concept Made Realistic", AIAA paper 78-82, 16th Aerospace Sciences meeting, Jan. 16-18, 1978.
** MacNeal, R. H., "Structural Dynamics of the Heliogyro", NASA-CR-1745A, 1971.
Drawings from MacNeal, R. H., "The Heliogyro, An Interplanetary Flying Machine", NASA Contractor's Report CR 84460, June 1967.