The “bicycle wheel” concept could use centrifugal force to place a thin circular membrane of integrated transmitters in tension. Since the size is large and the tension required small, a very low rotation rate, «1 RPM, is sufficient to provide tension. Bracing cables from a central hub provide the requisite out-of-plane stiffness. If necessary, a counter-rotating flywheel can compensate for angular momentum. Alternately, the rotation is not required if beams are used instead of wires or an inflated torus (tire) is used to provide the tension. In the sphere configuration, the solar cell/microwave transmitter elements cover the surface of an inflated sphere as in the Echo satellite or are on a film stretched across the sphere. For a sphere radius of many hundreds of meters the surface/ volume ratio is extremely low, and the gas pressure required to hold the form, and the associated leak rate, can be made small. If a planar solar array is not normal to the sun, there will a decrease in power proportional to the cosine of the solar angle. Likewise, if the transmitted beam at the receiver is larger than the receiver rectenna, there will be a cosine-dependant decrease in the fraction of power received when the transmitter array is not pointed directly at the receive. A first look at a satellite in an inclined continuous-sun orbit around the moon at beaming to a base on the lunar surface [7] indicates that the combined “cosine loss” from a non-tracking array is approximately 30%. In addition to this loss would be other off-axis effects due to inefficient operation of the antenna elements. Thin-film PV elements are light enough that high power to weight ratios may be maintained even if they do not track the sun and must be oversized to compensate for these losses. An alternative would be to use a lightweight turning mirror [S] to redirect the light from the sun to an integrated array pointing toward the receiver(s). These options need to be evaluated in a more extensive system study.
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