# Maintaining ideal curvature (flatness) of a solar sail

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?

• Ideal curvature is zero, for all practical uses (otherwise, the law of $\cos (\alpha)$ rears its ugly head, resulting in thrust losses). Jan 13, 2014 at 12:13
• @DeerHunter Well yes, but that changes nothing for the question, which is how to maintain it as the sail increases in size. It only means that ideal curvature = 0. Not plus, nor minus. So how do you assure that? Jan 13, 2014 at 12:25
• Wires, mostly. Apart from deployment, the crucial problem is ensuring (and coping with) torque during periapses - to get the most $\Delta V$ from Oberth effect. We haven't been able to manage tethers, let alone flexible sails that large. Jan 13, 2014 at 12:48
• I think the key is to balance frame size to line connectors. If you have a frame & sail that will maintain it's shape at one unit of size and you need nine units of size to provide thrust, than you have 18 connecter lines to the 18 corners of a 9 unit grids (shared corners in square pattern). Each line would need to be connected to separate "winch" that would allow for adjustment. Jan 13, 2014 at 15:47
• @james jenkins You are describing a sci-fi short story from somewhere like Boys Life in the 1960's, controlling dozens of lines, and all. As for solution, a perimeter that is angled inward or rearward to provide a force that pulls out from the center. Or consider that as a sail moves from flat to umbrella shaped, the angles of reflection will produce outward forces. Has Tethers Unlimited worked on this? Jan 15, 2014 at 0:47