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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

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.

mecko23 3y ago
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.
mlyle 3y ago
> 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.
- spacemark 3y ago
  It drives electrical and battery system requirements more than anything else. Thermal effects impact pointing/ADCS, which impacts propulsion, etc.
  - mlyle 3y ago
    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.
    - giantrobot 3y ago
      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.
      - mlyle 3y ago
        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.
        dabluecaboose 3y ago
        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.
        mlyle 3y ago
        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.
        giantrobot 3y ago
- mecko23 3y ago
  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.
  - mlyle 3y ago
    > 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.
    - LargoLasskhyfv 3y ago
      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 https://www.youtube.com/watch?v=lGDUmGlMJzU
- dabluecaboose 3y ago
  Solar Radiation Pressure is non-negligible for navigation and attitude dynamics as well. Eclipse season changes things for the NAV team.

mecko23 3y ago

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.

mlyle 3y ago

> 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.

spacemark 3y ago
It drives electrical and battery system requirements more than anything else. Thermal effects impact pointing/ADCS, which impacts propulsion, etc.
- mlyle 3y ago
  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.
  - giantrobot 3y ago
    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.
    - mlyle 3y ago
      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.
      - dabluecaboose 3y ago
        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.
        mlyle 3y ago
        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.
      - giantrobot 3y ago
mecko23 3y ago
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.
- mlyle 3y ago
  > 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.
  - LargoLasskhyfv 3y ago
    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 https://www.youtube.com/watch?v=lGDUmGlMJzU
dabluecaboose 3y ago
Solar Radiation Pressure is non-negligible for navigation and attitude dynamics as well. Eclipse season changes things for the NAV team.