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Comment by spacemark | Hacker News Reader

It drives electrical and battery system requirements more than anything else. Thermal effects impact pointing/ADCS, which impacts propulsion, etc.

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There's no real intrinsic electrical or battery system requirements for a giant solar power satellite. The energy use is comparably trivial: no big transponders to run in eclipse like a comsat.

But you do need everything to survive the cold and thermal cycling.

Not just thermal cycling but a giant solar collector is literally a giant solar sail. There would be significant force applied to a giant collector.

You'd need propulsion to maintain position and orientation. You'd also need a number of propulsion units to balance solar pressure gradients as the collector entered and existed the Earth's shadow as well as the thermal expansion/contraction of the structure.

It's likely not a lot of power but a non-trivial amount of fuel.

You probably maintain orientation with a control moment gyro and periodically desaturate it with thrust. You also use thrust to reboost and stay in orbital slot.

High-impulse ion engines, etc, are a good match for this task.

This is exactly how the NOAA GOES sats work. Every day at a proscribed time we used to desaturate the reaction wheels, so they could more or less keep running constantly and keep the satellite pointing where it should be.

Yup. This would be big enough that a control moment gyro would be "worth it," too-- and could store a whole lot of momentum and allow less frequent desaturation burns.

You'd need a rigid structure to keep the gyro from ripping itself loose from the structure. Even constructed as a giant space frame, that's a lot of mass to deal with the torsion of the structure rotating.

A structure 100m on a side would be just at the bounds of current technology (the ISS's control moment gyros). With 30% efficient panels that's only about 4MW before conversion and path losses.

The high impulse ion engines to desaturate the gyros would still need to be refueled regularly. I think you're hand waving a lot of complexity that even if completely solved still leaves a solution that's orders of magnitude costlier than solar panels on the ground.

I am not saying SBSP is easy. I am saying eclipse is a small amount of the problem compared to other geostationary craft.

Ideally sail effect would be used for position keeping, and to reduce loads needed to keep the assembly together. I suppose that would reduce amount of area available for soaking the sun, but might still be lighter than trying to build stiff enough large structures, plus you wouldn't constantly need to ferry more fuel.

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