Doing without SARJ Structures
Doing without SARJ type structures in Space Based Solar Power
The International Space Station has illuminated problems in using solar power by the difficulties it encountered. In particular, the Solar Alpha Rotary Joints (SARJ) have been a long time source of problems. The core of the problem is the rotation of the solar array with respect to the rest of the space station while electrical power is conveyed through slip rings. One way of handling such difficulties in Space Based Solar Power (SBSP) is by eliminating the requirement for rotation of the photovoltaic array section of the SBSP relative to the microwave transmitting section. This is possible because the photovoltaic array has no requirement to avoid roll as it is facing the sun. The microwave transmitting section must point with great precision to the receiving rectenna on Earth. The north edge of the electrically steerable dipole array should continue to be the north edge. The edge on the forward orbital side should stay forward. The photovoltaic array can be on a long stalk like the stalk of a sunflower. For a 10 Gigawatt satellite the microwave transmitting section can be a 4 kilometer diameter disc and the photovoltaic section can be an 8 kilometer diameter disk. The two can be connected by an 8 kilometer long stalk connecting the centers of the two discs. The stalk can flex by hinge joints within the stalk so that the photovoltaic section constantly points at the sun and shades the rest of the satellite. If the stalk connecting the centers of the microwave antenna and photovoltaic cells is a square truss-work tower, the successive levels are squares with structural elements for edges and at least one diagonal. The shape of levels would not change when the stalk bends, but structural elements connecting successive levels would change in length as hydraulic pistons and rotate where their ends are pinned as hinges. The elements that are hydraulic pistons would be enclosed in gas tight sheaths with accordion folds to be stretchable. Maintaining gas pressure would make lubrication practical for the hydraulic piston. A sheath would end in flanges that are pressed against matching flanges where the structural element is pinned. Matching flanges would press a gasket between them as they are pressed together by a threaded ring on the sheath flange mated to threads on the square level section where the hydraulic piston is pinned. The entire stalk should be encased in a multiple sheets of accordion folded micrometeoroid shield which do not hold pressure. So the whole tower could flex like a sun flower stalk and point at the sun at all times while electrical wires conducting power would be continuous and without slip-rings from the solar cell arrays to the microwave antenna. The wire would only be interrupted by switches that connect the wire to control and regulating equipment. Such switches would be soldered in and non-rotating. A pressure of 0.1 psi should be sufficient to prevent special low vapor pressure lubricant from evaporating. The gas tight sheath should in turn be within a few layers of micrometeoroid shield also with accordion folding. Three layers of aluminum sheet alternating with three eight centimeter layers of vacuum might be sufficient. For increasing protection against micrometeoroids, the aluminum sheets should not be made thicker, but rather be made more numerous with more alternating layers of vacuum. Only near midnight on days near the equinox would the photovoltaic section be unable to shade the microwave transmitter, because then it would get in the way of the microwave beam.
It is probably too late for design changes in the ISS, but avoiding rotary joints with electrically conducting slip rings is still possible for SBSP.