Nah -- when we're talking about how much it takes to power 70kW of GPUs, we need to include some kind of power utilization efficiency number. If 70kW is really 100kW, then we need to make this ridiculously big design 40% larger.
> >You can pick a color that absorbs very little *visible light* but readily emits in *infrared*-
> how well it absorbs photons at that wavelength as well as how efficiently it sheds energy at that wavelength.
Yes. Planetshine is infrared, 290K-ish; sunshine is 5500K-ish and planetary albedo is close enough to this, with a very small portion of its light being infrared. You are being long winded and not even reading what you reply to.
So, for example, white silicate paint or aluminized FEP has a equilibrium temperature in full sun, with negligible heat conducted to or away from it, somewhere in the span of -70 to -40C depending upon your assumptions. It will happily net radiate away heat from above-room temperature components while facing the sun.
It will also happily net radiate away heat when facing the planet because the planet is under room temperature and the planet doesn't subtend a whole hemisphere even in LEO.
I don't really like argument from authority, but... I will point out that I am the PI for multiple satellite projects and have owned thermal design, and that the stuff I've flown in space has ended up at very close to predicted temperatures. I don't feel like this is an easy thermal problem.
I mean, it's easy in the sense of "it takes a radiator area about the same as the floor area of my house". It's not easy in the sense of "holy shit I need to launch a radiator that's bigger than my house and somehow conduct all that heat to it while keeping the source cool."
> of course I realize there will be a thermal gradient from base to apex of the pyramidal satellite
No, there will be a thermal gradient from the hot thing -- the GPU -- to the radiator surface. S-B analysis is OK for an exterior temperature, but it doesn't mean the stuff you want to keep cool will be that average temperature. This is why we end up with heat pipes, active cooling loops, etc, in spacecraft.
If this wasn't a concern, you could fly a big inflated-and-then-rigidized structure and getting lots of area wouldn't be scary. But since you need to think about circulating fluids and actively conducting heat this is much less pleasant.