Design Rules and Scaling for Solar Sails

Abstract

Useful design rules and simple scaling models have been developed for solar sails. Chief among the conclusions are 1. Sail distortions contribute to the thrust and moments primarily though the mean squared value of their derivatives (slopes), and the sail behaves like a flat sheet if the value is small. The R M S slope is therefore an important figure of merit, and sail distortion effects on the spacecraft can generally be disregarded if the R M S slope is less than about 10% or so.2. The characteristic slope of the sail distortion varies inversely with the tension in the sail, and it is the tension that produces the principle loading on the support booms. The tension is not arbitrary, but rather is the value needed to maintain the allowable RMS slope. That corresponds to a halyard force about equal to three times the normal force on the supported sail area. 3. Both the AEC/SRS and L'Garde concepts appear to be structurally capable of supporting sail sizes up to a kilometer or more with 1AU solar flux, but select transverse dimensions must be changed to do so. Operational issues such as fabrication, handling, storage and deployment will be the limiting factors. L Sail PerformanceWrinkles and billow in the sail can reduce the thrust and even produce moments. It should be immediately obvious, however, that such sail distortion is a matter of degree and that a sail should ultimately behave as a flat sheet as the surface slopes become "small". In fact, analytical results shown in Fig. 1 were presented three years ago at the SSDM Conference in Denver (Ref. l), predicting that wrinkle aspect ratios of as much as several percent should produce minimal effects under most conditions of interest.The findings appear to have gone unappreciated, however, and considerable effort has been spent on modeling and testing of what could be regarded as a "non-problem" since tension can be used quite effectively to flatten the sail (Section 11. ) It will be shown here, using a perturbation approach that has proved quite useful with optics, that their results can be generalized in terms of the FWS slope as a single "figure of merit" for sail distortion characterization and control.First, considering for simplicity a sail with ideal specular reflectivity, McInnis (Ref. 2, Eqn. 2.20) has shown that the force per unit sail area is given by where (5 = 9.12 Nkm' = 2.05 lb/km2 @ 1AU is twice the solar pressure p, is its unit flux vector, and n' is the unit normal to the surface.

Solar Sail Scalability and a "Truly Scalable" Architecture: The Space Tow

“…Analytical skill and state-of-the-art computational tools cannot fill the void: various guiding principles and equations ("rules of thumb," thrust models, design methodology, or back-of-the-envelope calculations [30][31][32][33]) are bound to rely on unverified, sometimes admittedly inappropriate, assumptions. Among the questionable assumptions are that of elastic film behavior (true only for membranes stretched wrinkle-free which results in high structural loads), wrinkle models that ignore or Table 1 Dimensional challenges in solar sail engineering in general and relevance to the cord mat design and the space tow architecture (the --indicates irrelevance, nonapplicability)…”