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. To maintain lubrication for the hinge joints within the stalk, the entire stalk should be encased in a gas tight accordion fold sheath. A pressure of 0.1 psi should be sufficient to prevent 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 the day of equinox would the photovoltaic section be unable to shade the microwave tranmitter, 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.