[DoctorOetker]

what makes you believe this?

radiators can be made as long as desirable within the shade of the solar panels, hence the designer can pracitically set arbitrarily low temperatures above the background temperature of the universe.

[c1ccccc1]

Radiators can shadow each other, so that puts some kind of limit on the size of the individual satellite (which limits the size of training run it can be used for, but I guess the goal for these is mostly inference anyway). More seriously, heat conduction is an issue: If the radiator is too long, heat won't get from its base to its tip fast enough. Using fluid is possible, but adds another system that can fail. If nothing else, increasing the size of the radiator means more mass that needs to be launched into space.

[DoctorOetker]

please check my didactic example here: https://news.ycombinator.com/item?id=46862869

"Radiators can shadow each other," this is precisely why I chose a convex shape, that was not an accident, I chose a pyramid just because its obvious that the 4 triangular sides can be kept in the shade with respect to the sun, and their area can be made arbitrarily large by increasing the height of the pyramid for a constant base. A convex shape guarantees that no part of the surface can appear in the hemispherical view of any other part of the surface.

The only size limit is technological / economical.

In practice h = 3xL where L was the square base side length, suffices to keep the temperature below 300K.

If heat conduction can't be managed with thermosiphons / heat pipes / cooling loops on the satellite, why would it be possible on earth? Think of a small scale satellite with pyramidal sats roughly h = 3L, but L could be much smaller, do you actually see any issue with heat conduction? scaling up just means placing more of the small pyramidal sats.

[c1ccccc1]

Kudos for giving a concrete example, but the square-cube law means that scaling area A results in A^(3/2) scaling for the mass of material used and also launch costs. If you make the pyramid hollow to avoid this, you're back to having to worry about heat conduction. You assumed an infinite thermal conductivity for your pyramid material, a good approximation if it's solid aluminum, but that's going to be very expensive (mainly in launch costs).

In reality, probably radiator designs would rely on fluid cooling to move heat all the way along the radiator, rather than thermal conduction. This prevents the above problem. The issue there is that we now need to design this system with its pipes and pumps in such a way that it can run reliably for years with zero maintenance. Doable? Yes. Easy or cheap? No. The reason cooling on Earth is easier is that we can transfer heat to air / water instead of having to radiate it away ourselves. Doing this basically allows us to use the entire surface of the planet as our radiator. But this is not an option in space, where we need to supply the radiator ourselves.

In terms of scaling by instead making many very small sats, I agree that this will scale well from a cooling perspective as long as you keep them far enough apart from each other. This is not as great from the perspective of many things we actually want to use a compute cluster for, which require high-bandwidth communication between GPUs.

In any case, another very big problem is the fact that space has a lot of ionizing radiation in it, which means we also have to add a lot of radiation shielding too.

Keep in mind that the on-the-ground alternative that all this extra fooling around has to compete with is just using more solar panels and making some batteries.

[DoctorOetker]

At no point did I propose a massive block of solid aluminum. I describe the heated surface and I describe a radiating surface, so programmers understand the concept of the balance of energy flow and how to calculate rest temperature with Stefan Boltzmann law, if they want to explore the details they now have enough information to generalize, they can use RMAD and run actual calculations to optimize for different scenarios.

Radiation hardening:

While there is some state information on GPU, for ML applications the occasional bit flip isn't that critical, so Most of the GPU area can be used as efficiently as before and only the critical state information on GPU die or host CPU needs radiation hardening.

Scaling: the didactic unoptimized 30m x 30m x 90m pyramid would train a 405B model 17 days, it would have 23 TB RAM (so it can continue training larger and larger state of the art models at comparatively slower rates). Not sure what's ridiculous about it? At some point people piss on didactic examples because they want somebody to hold their hand and calculate everything for them?

[alangibson]

Shading does work; JWST does this. However I don't see how you can make it work for satellite data centers. You would constantly be engaging attitude control as you realigned the panels to keep the radiators in shade. You'd run out of thruster fuel so fast you'd get like a month out of each satellite

[DoctorOetker]

attitude control doesn't need to consume propellant, there's reaction wheels.

but you'd rarely ever need it though: it just needs to rotate at a low angular velocity of 1 rotation per year to keep facing the sun.

[T-A]

https://en.wikipedia.org/wiki/Spacecraft_attitude_determinat...

[eldenring]

these same comments pop up every time someone brings up satellite data-centers where people just assume the only way of dissipating heat is through convection with the environment.

[wat10000]

No, we just "assume" (i.e. know) that radiation in a vacuum is a really bad way of dissipating heat, to the point that we use vacuum as a very effective insulator on earth.

Yes, you can overcome this with enough radiator area. Which costs money, and adds weight and space, which costs more money.

Nobody is saying the idea of data centers in space is impossible. It's obviously very possible. But it doesn't make even the slightest bit of economic sense. Everything gets way, way harder and there's no upside.

[DoctorOetker]

> No, we just "assume" (i.e. know) that radiation in a vacuum is a really bad way of dissipating heat, to the point that we use vacuum as a very effective insulator on earth.

In space or vacuum radiation is the best way to dissipate heat, since it's the only way.

I believe the reason the common person assumes thermal radiation is a very poor way of shedding heat is because of 2 factoids commonly known:

1. People think they know how a vacuum flask / dewar works.

2. People understand that in earthly conditions (inside a building, or under our atmosphere) thermal radiation is insignificant compared to conduction and convection.

But they don't take into account that:

1) Vacuum flasks / dewars use a vacuum for thermal insulation. Yes and they mirror the glass (emissivity nearer to ~0) precisely because thermal radiation would occur otherwise. They try their best to eliminate thermal radiation, a system optimized to eliminate thermal radiation is not a great example of how to effectively use thermal radiation to conduct heat. The thermal radiation panels would be optimized for emissivity 1, the opposite of whats inside the vacuum flask.

2) In a building or under an atmosphere a room temperature object is in fact shedding heat very quickly by thermal radiation, but so are the walls and other room temperature objects around you, they are reheating you with their thermal radiation. The net effect is small, in these earthly conditions, but in a satellite the temperature of the environment faced by the radiating surfaces is 4K, not a temperature similar to the object you are trying to keep cool.

People take the small net effect of thermal radiation in rooms etc, and the slow heat conduction through a vacuum flasks walls as representative for thermal radiation panels facing cold empty space, which is the mistake.

[wat10000]

Well no, it’s because conduction/convection into a fluid is so much more effective.

Just look at a car. Maybe half a square meter of “radiator” is enough to dissipate hundreds of kW of heat, because it can dump it into a convenient mass of fluid. That’s way more heat than the ISS’s radiators handle, and three orders of magnitude less area.

Or do a simple experiment at home. Light a match. Hold your finger near it. Then put your finger in the flame. How much faster did the heat transfer when you made contact? Enough to go from feeling mildly warm to causing injury.

[DoctorOetker]

Yes, it's so much more effective, ... at sea level Earthly conditions.

[wat10000]

What’s more effective: conduction/convection on the ground, or radiation in space?

[verzali]

Additional radiator area means bigger spacecraft, implies more challenge with attitude control. Lower down you get more drag so you use propellant to keep yourself up, higher up you have more debris and the large area means you need to frequently manoeuvre to avoid collisions. Making things bigger in space is not trivial! You can't just deploy arbitrarily large panels and expect everything to be fine.

[DoctorOetker]

space is vast

they could go near a Lagrange point

there are so many options

heavier boats are also slower to accelerate or decelerate compared to smaller boats, does this mean we should ban container ships? having special orbits for megastructure lanes would seem a reasonable approach.

[eldenring]

The radiators would be lighter compared to the solar panels, and slightly smaller surface area so you can line them back to back

I don't think dissipating heat would be an issue at all. The cost of launch I think is the main bottleneck, but cooling would just be a small overhead on the cost of energy. Not a fundamental problem.

[lm28469]

If you solved this problem apply at nasa because they still haven't figured it out.

Either that or your talking out of your ass.

FYI a single modern rack consumes twice the energy of the entire ISS, in a much much much much smaller package and you'll need thousands of them. You'd need 500-1000 sqm of radiator per rack and that alone would weight several tonnes...

You'll also have to actively cool down your gigantic solar panel array

[DoctorOetker]

eldenring is slightly wrong: for reasonable temperatures the area of the radiating panels would have to be a bit more than 3 times the area of the solar panel, otherwise theres nothing wrong.

No need to apply at NASA, to the contrary, if you don't believe in Stefan Boltzmann law, feel free to apply for a Nobel prize with your favorite crank theory in physics.

[eldenring]

Whats your definition for reasonable temp? my envelope math tells me at 82 celsius (right before h100s start to throttle) you'd need about 1.5x the surface area for radiators. Not exactly back to back, but even 3x surface area is reasonable.

Also this assumes a flat surface on both sides. Another commenter in this thread brought up a pyramid shape which could work.

Finally, these gpus are design for earth data centers where power is limited and heat sinks are abundant. In the case of space data centers you can imagine we get better radiators or silicon that runs hotter. Crypto miners often run asics very hot.

I just don't understand why every time this topic is brought up, everyone on HN wants to die on the hill that cooling is not possible. It is?? the primary issue if you do the math is clearly the cost of launch.

[wat10000]

The pertinent thing is that it’s not an advantage. It may be doable but it’s not easier than cooling a computer in a building.

[DoctorOetker]

The distinction is that you don't need to compete for land area, that you don't cause local environmental damage by heating say a river or a lake, that you don't compete with meatbags for energy and heat dissipation rights.

Without eventually moving compute to space we are going to have compute infringe on the space, energy, heat dissipation rights of meatbags. Why welcome that?!?

[defrost]

How efficient is thermal radiation through a vacuum again?

Sure, it occurs, but what does the Stefan–Boltzmann law tell us about GPU clusters in space?

[wat10000]

The land area and heating is completely insignificant on a terrestrial scale.

[DontBreakAlex]

Radiators can only be made as long as desirable because there's gravity for the fluid inside to go back down once it condenses. Even seen those copper heat pipes in your PC radiator?

[ginko]

Fluid in heat pipes moves through capillary action.

[ares623]

what? the heat is coming from inside the house

[DoctorOetker]

which house?