Summarizer

> It kinda does make sense if you consider that solar panels in space have been used for a very long time (to power satellites). It stops making sense the second you ask how you’d dissipate the heat any GPU would create. Sure, you could have vapour chambers. To where? Would this need square kilometers of radiators on top of square kilometers of solar panels? All this just to have Grok in space?

You have a dark radiating side on the back of the solar panels. You can spread the GPUs around the solar panels. All the energy in comes from the sun so the temperature should be much the same as any dark panel like object floating in sunlight in space.

>Getting rid of the copious amounts of heat that data centers generate might also be a challenge at 70 Celsius - normal for GPU - 1.5m2 radiates something like 1KWt (which requires 4m2 of panels to collect), so doesn't look to a be an issue. (some look to ISS which is a bad example - the ISS needs 20 Celsius, and black body radiation is T^4)

So for the ISS at 20c you'd get 481 W/m^2 so you'd only need 2.3m2. So comparing the ISS at 20c to space datacenters at 70c you get an improvement of 63%. Nice, but doesn't feel game-changing. The power radiated is T^4, but 70c is only about 17.1% warmer than 20c because you need to compare in kelvin.

You could immersion cool them and get radiation resistance as a bonus.

A large piece of aluminum with ammonia pumped through it?

Right up to the radiation limit and then you'll either have to throttle your precious GPUs or you'll be melting your satellite or at least the guts of it. You're looking at an absolutely massive radiator here, many times larger than the solar panels that collect the energy to begin with.

not really, for A_radiator / A_PV = ~3; you can keep the satellite cool to about 27 deg C (300K) check my example calculation (Ctrl-F: pyramid)

> > absolutely massive radiator here, many times larger than the solar panels > A_radiator / A_PV = ~3; Seems like you're in agreement. There's a couple more issues here-- 1. Solar panels are typically big compared to the rest of the satellite bus. How much radiator area do you need per 700W GPU at some reasonable solar panel efficiency? 2. Getting the satellite overall to an average 27C temperature doesn't necessarily keep the GPU cool; the satellite is not isothermal.

Where does the heat collected by amminia get evacuated?

Datacenter capacity (and thus heat) grows by the cube law, but the ability to radiate heat grows by the square law, so it seems like it would be advantageous to have a bunch of smaller satellites, if you were concerned about cooling them.

We have radiators on the ISS. Even if you kept the terrible performance of those ancient radiator designs (regularly exposed to sunlight, simplistic ammonia coolant, low temperature) you could just make them bigger and radiate the needed energy. Yes it would require a bit of engineering but to call it an "unsolved problem" is just exaggerating.

It's a solved problem. The physics is simply such that it's really inefficient. > ... we'd need a system 12.5 times bigger, i.e., roughly 531 square metres, or about 2.6 times the size of the relevant solar array. This is now going to be a very large satellite, dwarfing the ISS in area, all for the equivalent of three standard server racks on Earth. https://taranis.ie/datacenters-in-space-are-a-terrible-horri... The gist of it is that about 99% of cooling on earth works by cold air molecules (or water) bumping into hot ones, and transferring heat. There's no air in space, so you need a radiator 99x larger than you would down here. That adds up real fast.

I think you may be thinking of cooling to habitable temperatures (20c). You can run GPUs at 70c , so radiative cooling density goes up exponentially. You should need about 1/3 of the array in radiators.

That's not a new problem that no one has dealt with before. The ISS for instance has its External Active Thermal Control System (EACTS). It's not so much a matter of whether it's an unsolvable problem but more like, how expensive is it to solve this problem, what are its limitations, and does the project still makes economic sense once you factor all that in?

It's worth noting that the EACTS can at maximum dissipate 70kW of waste heat. And EEACTS (the original heat exchange system) can only dissipate another 14kW. That is together less than a single AI inference rack. And to achieve that the EACTS needs 6 radiator ORUs each spanning 23 meters by 11 meters and with a mass of 1100 kg. So that's 1500 square meters and 6 and a half metric tons before you factor in any of the actual refrigerant, pumps, support beams, valve assemblies, rotary joints, or cold side heat exchangers all of which will probably together double the mass you need to put in orbit. There is no situation where that makes sense. ----------- Manufacturing in space makes sense (all kinds of techniques are theoretically easier in zero G and hard vacuum). Mining asteroids, etc makes sense. Datacenters in space for people on earth? That's just stupid.

Your calculations are based on cooling to 20c, which is exponentially harder than cooling to 70c where GPUs are happy. Radiators would be roughly 1/3 the size of the panels for 70c.

I'm a total noob on this. I get that vacuum is a really good insulator, which is why we use it to insulate our drinks bottles. So disposing of the heat is a problem. Can't we use it, though? Like, I dunno, to take a really stupid example: boil water and run a turbine with the waste heat? Convert some of it back to electricity?

What do you do with the steam afterwards? If you eject it, you have to bring lots of it with your spacecraft, and that costs serious money. If you let it condensate to get water again, all you did is moving some heat inside the spacecraft, almost certainly creating even more heat when doing that.

It's a good question, but in a closed system (like you have in space) the heat from the turbine loop has to go somewhere in order to make it useful. Let's say you have a coolant loop for the gpus (maybe glycol). You take the hot glycol, run it through your heat exchanger and heat up your cool, pressurized ammonia. The ammonia gets hot (and now the glycol is cool, send it back). You then take the ammonia and send it through the turbine and it evaporates as it expands and loses pressure to spin the turbine. But now what? You have warm, vaporized, low pressure ammonia, and now you need to cool it down to start over. Once it's cool you can pressurize it again so you can heat it up to use again, but you have to cool it, and that's the crux of the issue. The problem is essentially that everything you do releases waste heat, so you either reject it, or everything continues to heat up until something breaks. Developing useful work from that heat only helps if it helps reject it, but it's more efficient to reject it immediately. A better, more direct way to think about this might be to look at the Seebeck effect. If you have a giant radiator, you could put a Peltier module between it and you GPU cooling loop and generate a little electricity, but that would necessarily also create some waste heat, so you're better off cooling the GPU directly.

You can't easily use low grade heat. However there are workarounds. People are talking like the only radiator design is the one on the ISS. There are other ways to build radiators. It's all about surface area. One way is to heat up a liquid and then spray it openly into space on a level trajectory towards a collecting dish. Because the liquid is now lots of tiny droplets the surface area is huge, so they can radiate a lot of heat. You don't need a large amount of material as long as you can scoop up the droplets the other end of the "pipe" and avoid wasting too much. Maybe small amounts of loss are OK if you have an automated space robot that goes around docking with them and topping them up again.

The ISS consumes roughly 90kW. That’s about *one* modern AI/ML server rack. To do that they need 1000 m^2 of radiator panels (EACTS). So that’s the math: every rack needs another square kilometer of stuff put into orbit. Doesn’t make sense to me.

1000m2 is not a square kilometer (1 square kilometer is 1mil m2)

1000 square meters really isn't that big in space.

Heat exchanger melts salts, salts boil off? Some kind of potential in there to use evaporants for attitude/altitude correction. Spitballing. Once your use case also has a business case, scope to innovate grows.

It makes sense to target a higher operating temperature, like 375K. At some point, the energy budget would reach an equilibrium. The Earth constantly absorbs solar energy and also dissipates the heat only by radiative cooling. But the equilibrium temperature of the Earth is still kind of cool. I guess the trick lies in the operating temperature and the geometry of the satellites.

Asking for a friend (who sucks at thermodynamics:) could you use a heat pump to cool down the cold end more and heat up the hot end much higher? Heat radiation works better the higher the temperature?

Just have to size radiators correctly. Not a physics problem. Just an economic one. Main physics problem is actually that the math works better at higher GPU temps for efficiency reasons and that might have reliability trade off.

if the thermal radiation panels have ~3 x the area of the solar panels, the temperature of the satellite can be contained to about 300 K (27 deg C). Ctrl+F:pyramid to find my calculations.

Does that include all the required radiators to vent heat?

and of course, the continuous opposite boost needed to prevent the heat vent from knocking them out of orbit.

I think this is all ridiculous, to be clear, but re: this problem couldn't the radiators in theory be oriented so that they vent in opposite directions and cancel out any thrust that would be generated?

The intractable problem is heat dissipation. There is to little matter in space to absorb excess heat. You'd need thermal fins bigger than the solar cells. The satellite's mass would be dominated by the solar panels and heat fins such that maybe 1% of the mass would be usable compute. It would be 1000x easier to leave them on the moon and dissipate into the ground and 100000x easier to just keep making them on earth.

> The intractable problem is heat dissipation. 3 times the area of the heat dissipating surface compared to solar panel surface brings the satellite temp down to 27 deg C (300 K): https://news.ycombinator.com/item?id=46862869 > There is to little matter in space to absorb excess heat. If that were true the Earth would have overheated, molten and turned to plasma long ago. Earth cools by.... radiative cooling. Dark space is 4 K, thats -267.15 deg C or -452.47 deg Fahrenheit. Stefan-Boltzmann law can cool your satellite just fine. > You'd need thermal fins bigger than the solar cells. Correct, my pessimistic calculation results in a factor of 3,... but also Incorrect, there wouldn't be "fins" thats only useful for heat conduction and convection.

Take the area of solar panels, multiply by 3, thats the area of black body thermal radiation surface. The sattelite will chillax to 27 deg C (300 K): https://news.ycombinator.com/item?id=46862869

> Figuring out how to radiate a lot of waste heat into a vacuum is fighting physics. Radiators should work pretty well, and large solar panels can do double duty as radiators. Also, curiously, newer GPUs are developed to require significantly less cooling than previous generations. Perhaps not so coincidentally?