Solar-powered space flight

7a. Achieving the desired level of optical precision: Operating in non-zero g conditions

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7.1          Achieving the desired level of optical precision s perhaps the biggest technical challenge facing the proposed vehicle design. The very low mass solar sails now being developed are in effect merely very thin floppy sheets strung out in front of sunlight. In our concept vehicle, the larger mirror (which forms the vast bulk of the mirror surface area) needs to be made from a very thin and therefore almost certainly non-rigid sheet, to minimise mirror mass. Given the degree of concentration involved, the angular accuracy of the main mirror positioning probably needs to be, say, 1 in 3000 or better. The small second mirror is much less of a problem, as it should contribute relatively little to the overall mirror mass.

7.2          Most previous investigators of solar-powered concentrators for astronautical purposes appear to have decided that it is impractical to achieve this level of positioning accuracy with a mirror akin to a solar sail. Instead they have typically proposed either refractive/diffractive arrangements or mirrored inflated concentrators. The former cannot typically be made very thin. The latter also typically have much higher masses per unit collector area. Using an inflatable collector at least doubles the mass per unit collector area (to surround the inflated region), and for a large collector the gas used within the inflated region can contribute significantly to the overall mass of the concentrator. The tension in the collector will also typically be inversely proportional to the radius of curvature of the inflated surface, which is likely to make it more difficult to achieve the desired accuracy with a thin film, unless its thickness varies across the collector surface (or more practically the sheet is likely to be thicker than it needs to be in some places to cater for more modest curvature in other places). The author has seen unpublished estimates which suggest that inflated concentrators might in practice have masses per unit area perpendicular to the sun’s rays circa 50 – 100 times more than those that appear to be achievable with a solar sail.

7.3          However, a presupposition such investigators appear to be making is that any solar-powered engine would necessarily be operating in an essentially zero g environment. This is potentially flawed logic, as it should be easier to achieve the desired optical positioning when a thin-film is experiencing effective accelerations closer to 1g. A simple way to justify this perhaps surprising claim is to apply an approximately uniform tension in all (tangential) directions to a sheet of some commercially available ultra-thin transparent sheet (e.g. kitchen cling-film). It adopts a well-defined (flat) shape that is sufficiently smooth to form a surprisingly good reflective image. So thin-film sheets such as those proposed for solar sails are likely to be able to achieve the desired level of optical precision as long as:

(a)    The material out of which the sheet is made is reasonably inextensible;

(b)   The sheet (or to be precise its mirrored surface) is sufficiently smooth;

(c)    The sheet’s natural shape matches that desired, i.e. if it were hypothetically put over a mould of exactly the desired shape then it would fit perfectly, with no kinks or slack; and

(d)   The sheet is sufficiently tensioned throughout flight (probably with transverse and radial tension being approximately equal for any given point on the mirror surface throughout flight).

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