[causal]

What doesn't make sense to me here is that even on Earth, where we have an atmosphere to disperse heat into, we find that closed-loop cooling is too expensive and so use evaporative cooling.

If the economics make it too expensive not to use freshwater on Earth, I don't see how closed-loop cooling suddenly becomes affordable in space where dispersing heat is already more difficult.

[db48x]

Here in datacenters we use cooling systems to move heat away from the computers and out of the building. If each rack in the data center were outdoors and 100 yards away from every other then almost no cooling would be required. Just some fans to suck in air at the bottom and eject it out the top. Even less would be required if the individual GPUs were somehow separated from each other. Then we would truly be dispersing heat into the atmosphere with no cooling system at all.

Similarly, a satellite only needs a cooling system so that it can move the heat from the internal components outward towards the hull. A satellite containing a rack’s worth of GPUs might literally have heat spreaders that touch the chips on one side and the outer hull of the satellite on the other. Combine that with some heat pipes or something to spread the heat out efficiently and you hardly need anything else. A satellite the size and shape of a Starlink v3 already has enough surface area to dissipate something like 28kW at 80°C, and more if you run it hotter. If you want more than ~30–40 GPUs per satellite then you might need a small radiator to increase the surface area, or you might just make the thing thinner and wider instead. You’ll need more solar panel area anyway, so making the bus wider to match the wider solar panels is fine. The “closed–loop cooling system” you say is so unaffordable might be no more than a bunch of heat pipes. Or it might be an aquarium motor that pumps a few kilos of ammonia through some pipes or channels in the hull of the satellite.

[NetMageSCW]

Those economics only apply when there is a cheaper alternative available. Closed loop becomes economic when alternatives aren’t available.