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Comment by mecko23 | Hacker News Reader

Respectfully, I believe you are inaccurate on both accounts, sun synchronous orbits (SSO) only have no eclipse if their orbital plane is within a very degrees of the terminator, this of course widens with altitude but there exists far more possible SSOs with eclipses than without. Secondly with GEOs as with any very high altitude orbit the eclipse time trends towards zero so effectively at a GEO the eclipse time is minimal compared to the illuminated time. Of note the length for the eclipse time of GEO varies throughout the year.

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Respectfully, you are not correct that eclipse in Geo is insignificant. It drives many engineering constraints for spacecraft systems. Look I get why people are excited about space space solar power, but I've been in this industry for 15 years and when you dig into the numbers and understand the realities of spacecraft engineering, SBSP just seems like a fool's errand. It may not always be that way, but the technical challenges are extreme and costs are still nowhere near what would be required to make it work.

You can chalk my comments up to a grumpy engineer tired of the cyclical SBSP pushes that never go anywhere.

Hmm, well I totally understand how it drives spacecraft design and I don’t know off the top of my head the length of greatest eclipse but it cannot be greater than a small fraction of the period (upwards of 1/12). This is significant for a spacecraft itself but as far as a regionally integrated power grid is concerned it is a clear improvement over current solar PV systems. I get that there maybe some fatigue here with the idea and blue sky optimism that comes with SBSP, I think there are valid criticisms to level at it orbit selection and corresponding ground tracks and eclipses are not one.

> It drives many engineering constraints for spacecraft systems.

Here the only thing it's going to drive are thermal requirements-- which are admittedly significant problems. There's no need to continue to use large amounts of power for comms, etc, like on most GEO birds.

And, of course, the grid needs to deal with the power disappearing for an hour in the middle of the night for short periods of the year.

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

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.

Why in this case would it cause significant problems? Are you referring to the heating (that would make sense with such a large surface area of panels)? Concerning cooling- with such short times in eclipse I can’t imagine that it would have enough time to have cooling issues beyond flexing of the superstructure (if made from metals). Be interested to hear though if I’m missing something.

As to power disappearing with an adequately geographically integrated grid I don’t forsee that as really too much of a problem. Currently the grid deals with short term outs fairly well especially if they are planned for months in advance.

> Concerning cooling- with such short times in eclipse I can’t imagine that it would have enough time to have cooling issues

Lots of thin structure with 70 minutes to radiate, with the only thing shining on it the earth's albedo subtending a tiny angle. I'd imagine it creates rather significant demands on structure and electrical connections.

I've not run the numbers on a GEO solar spacecraft, but the smallsat group that I'm mentoring that would be "thicker" than a lot of the GEO craft... gets down to -30C without heaters during its 40 minutes in eclipse while much closer to Earth.

> As to power disappearing with an adequately geographically integrated grid I don’t forsee that as really too much of a problem. Currently the grid deals with short term outs fairly well especially if they are planned for months in advance.

Yup, that's the point I'm making. A space based solar power craft has smaller problems from eclipse than a typical comsat. Batteries, etc, are not nearly as much of a concern. It's mostly the thermals that are left.

Make it flexible. Connect panels with steel cables. Or steel reinforced aluminum conductor. Organize like spidernet. Let it slowly spin.

Have pizza: https://www.youtube.com/watch?v=lGDUmGlMJzU

Solar Radiation Pressure is non-negligible for navigation and attitude dynamics as well. Eclipse season changes things for the NAV team.

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