Maintaining ideal curvature (flatness) of a solar sail

Question

If a solar sail is to be of any use, it needs to be a rigid structure and maintain its ideal curvature and orientation, otherwise it would eventually fold-up over its center of mass like an umbrella does in a strong wind due to torque increasing radially with distance to its geometric center. Furthermore, for a solar sail to effectively translate weak momentum force of radiation pressure onto its rigid structure, it needs to be absolutely enormous, further increasing its radius, and ideally, the center of its mass would be along the axis of movement at its geometric center on a 2-dimensional plane perpendicular to the radiation source, otherwise its total torque translates into a spin.

So, this means that even though we're talking of relatively small force applied per its square area, the larger it is (to increase net force on it), the larger its radial distance towards its central axis, and the torque differential with it. So the larger it gets, the stronger its frame holding it together would have to be, which, with large area useful for any missions of reasonable length of time means, we're fast approaching limits of maximum stress loads on any materials known to man. Either that, or we're increasing its ability to cope with structural loads by adding more mass to it (reinforcing the structure), negating the point in having enormous solar sail in the first place. Thus my question.

How does one maintain flatness of a large solar sail, so it doesn't bend into a less-than-ideal concave / convex dish or fold onto itself? What kind of support structures are proposed to maintain this flatness, what are the limitations (maximum size) of such materials proposed, and could these limits be somewhat stretched by use of, say, Electroactive Polymers (EAP), e.g. Carbon Nanotubes can be used for ionic EAP that are also some of the strongest materials known to science, to maintain its flatness / ideal curvature by applying voltage to the frame?

Answer 1

Rotate the sail. Centrifugal force will help maintain flatness. Unless there's some engineering requirement that dictates otherwise, it would probably work better to spin both craft and sail. Getting the rotation on the craft up to speed before deploying the sail should do the trick. JAXA's IKAROS solar sail demonstrator used rotational deployment.

However, there is a major disconnect between the popular conception of solar sails and the designs of missions flown or contemplated. As C. Town Springer mentioned in his comment above, many people think of solar sails deployed by lines in front of the ship, as in Arthur C. Clarke's The Sunjammer.

Robert Forward's design of interstellar beam-driven solar sails also featured tow lines. In practice, however, solar sail projects have looked more like this:

IKAROS, NanoSail-D, and the Planetary Society's LightSail-1 all feature this design, a square-rigged sail deployed from the body of the craft by unfolding booms.

If one thinks about the way an atmospheric parachute deploys, the impracticality of mimicking this design for a proof-of-concept solar sail becomes obvious. Typically a drogue chute catches the air and proceeds to pull the main chute out, which (hopefully) expands out to its desired shape and pulls the lines taut. No such mechanism of an equivalent order of magnitude is available to a solar sailcraft.

The flat-surface-normal-to-axis design, in this light, is mechanically easier to deploy. More sophisticated sailcraft may very well use other forces to deploy their sales, such as electrostatic or centrifugal force, as in this answer, but expect designs that owe more to the umbrella than to the traditional parachute in the near term.