[db48x]

It’s very easy to overestimate the difficulty of cooling things in space, unless you actually run the numbers. So please follow along as Scott Manley runs the numbers: <https://www.youtube.com/watch?v=FlQYU3m1e80>.

Basically a Starlink v3 satellite has an estimated power budget of 20kW. Add in the heat absorbed from the environment (both directly from sunlight and reflected off of the Earth) and you’ll find that it must reject about 22kW of heat. That’s a fair amount, but at 65°C it can radiate it all away just using it’s own surface area! No radiator required at all!

Of course the power density of computer racks has been going up over the years. If you want to reach 100kW per satellite then they will need a modest radiator, but nothing extravagant. It would still be smaller than the solar panels, and far smaller than the ones on the ISS. And don’t forget that because radiated heat goes up as the fourth power of temperature, raising the temperature of the system by even a small amount raises the radiation emitted by a lot. If you design the system to run hotter you can minimize the size of the radiator. Most chips these days are designed to max out at 100°C to 110°C without lasting damage, although running them at that temperature 24/7 may reduce their lifespan. There will be some sweet spot in the middle.

And it turns out that a Starlink v3 already has a volume somewhat larger than a 48U rack. You talk about launching 250k satellites in order to have 250k GPUs in orbit, but that’s ridiculous. A real compute swarm will be hundreds or thousands of satellites each equivalent to a whole rack of GPUs.

But you’re not wrong to be skeptical. The economics might not work out even if the cooling is easy enough. It’s just that rejecting the idea takes a lot more than back–of–the–envelope calculations.

[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.

[AntiUSAbah]

I'm not rejecting the basic idea in itself. There is nothing in this idea which we as humans can't do today. No issues here. Its just so much more expensive than just doing it in a dessert and putting fibre and solar panels and batteries there.

The Starlink v3 doesn't exist yet in space, it also needs Starship apparently and Musk said it will have the size of a Boeing 737 fully deployed. So it will not be small and its not proofen yet.

A rack with 48u will either have 12 or 24 GPUs which equals to 9kW or 17kW. Than its not 250k satellites for a 'small' 300MW DC but only 25k. Still a very crazy number.

I would love to see all of this scifi stuff happening. Spaceship in space, travel gates, dyson sphere but there is just no current breakthrough in our society which would indicate that this makes sense.

In my opinion, we as a society will have to get rid of capitalism first before we will do the next step and just because Musk needs a story to sell to keep his construct alive, doesn't mean its the right time.

[db48x]

> The Starlink v3 doesn't exist yet in space, it also needs Starship apparently

True but not really relevant. All prior versions of Starlink worked well enough, so there’s no reason to suspect that v3 won’t.

> … it will have the size of a Boeing 737 fully deployed. So it will not be small…

Irrelevant. Size is a weird measure here. You should ignore it because it is a marketing thing. All that is meant by it is that once the solar panels are extended they span a distance larger than the wingspan of a 737, nothing more. A Starlink v3 satellite has far less mass, interior volume, or complexity than a 737. I could get a long 30m rope and tell you that it was “larger than a 737”, but you wouldn’t be very impressed.

> A rack with 48u will either have 12 or 24 GPUs which equals to 9kW or 17kW. Than its not 250k satellites for a 'small' 300MW DC but only 25k. Still a very crazy number.

Are you sure? That fits within the estimated power budget of a Starlink v3, but I was assuming that GPUs were denser than that these days. I’m not an expert though. I figured a rack would hold somewhere between 96 and 128 GPUs depending on whether they had to be in 3u or 4u servers. They would need between 60kw and 90kW of power, and would need a modest radiator. The solar panels would be far larger than the radiator.

> I would love to see all of this scifi stuff happening. Spaceship in space, travel gates, dyson sphere but there is just no current breakthrough in our society which would indicate that this makes sense.

Ok, see, your problem is that you haven’t properly distinguished between different types of fiction. You put those three things in one category as if they were all equally fictional, but that cannot be true.

The first one is ambiguous, since the Space Shuttle was definitely a “spaceship in space”, so perhaps you just mean FTL travel like in Star Trek or Star Wars. By “travel gates” I assume you mean something like the eponymous gates from Stargate SG–1. Those are both ruled out by the laws of physics, and we can only tell stories about them because people willingly suspend their disbelief. Campbell said that a true science fiction story can only include one thing that requires the reader to suspend their disbelief. If it includes more than that then it is a fantasy instead.

But a Dyson sphere, or more accurately a Dyson swarm, is not an impossibility at all. If I can put one solar–powered satellite in orbit around the sun then if I am really industrious I could put ten, or a hundred, or a trillion. As many as I wanted and could afford, right? The laws of physics don’t say that a sun can only have 8 satellites around it, or any other number. Dyson knew that enough such satellites would eventually blot out the sun. They would absorb all the sunlight it could emit, and then the satellites would all emit infrared waste heat. If all of those satellites were doing something useful then whoever put them there would have a lot of useful work being done at their command. Even if it’s just trillions of GPUs making AI–powered cat memes, or simulating the minds of a bunch of human uploads, or even if they’re all just mirrors to redirect that light somewhere else, that’s a lot of power at our command. It is fictional only because it is an idea that nobody has actually gotten around to implementing yet. Once we’ve done it then it won't be fictional anymore.

> Its just so much more expensive than just doing it in a dessert and putting fibre and solar panels and batteries there.

No one here is arguing that it isn’t. It might still be cheaper to build datacenters on Earth. But most of the people who say that it’s _definitely_ still cheaper to build them on Earth are overestimating the difficulties, and therefore the costs, of doing it in orbit instead. We’re getting to the point where launch costs are low enough that it is no longer a given that it is cheaper.

For one thing, actually building things in the desert is expensive. You have to build all the necessary infrastructure yourself. Roads, power, water, fuel, etc, etc. You might as well be launching everything into space! No, if you want it to be cheaper you need to build somewhere closer to home, like Ohio.