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▲ pantalaimon 8 hours ago

Existing satellites manage to keep their equipment that already can consume several kW cool just fine.

You might need space for radiators, but there is plenty space in space.

▲nerdsniper 8 hours ago | parent | next [-]

5,000 Starship launches to match the solar/heat budget of the 10GW "Stargate" OpenAI datacenter. The Falcon 9 family has achieved over 600 launches.

The ISS power/heat budget is like 240,000 BTU/hr. That’s equivalent to half of an Nvidia GB200 NVL72 rack. So two international space stations per rack. Or about 160,000 international space stations to cool the 10GW “Stargate” datacenter that OpenAI’s building in Abilene. There are 10,000 starlink satellites.

Starship could probably carry 250-300 of the new V2 Mini satellites which are supposed to have a power/heat budget of like 8kW. That's how I got 5,000 Starship launches to match OpenAI’s datacenter.

Weight seems less of an issue than size. 83,000 NVL72’s would weigh 270 million lbs or 20% of the lift capacity of 5000 starship launches. Leaving 80% for the rest of the satellite mass, which seems perhaps reasonable.

Elon's napkin math is definitely off though, by over an order of magnitude. "a million tons per year of satellites generating 100 kW of compute power per ton" The NVL72's use 74kW per ton. But that's just the compute, without including the rest of the fucking satellite (solar panels and radiators). So that estimate is complete garbage.

One note: If you could afford to send up one of your own personal satellites, it would be extremely difficult for the FBI to raid.

▲alangibson 8 hours ago | parent | prev | next [-]

Several kW is nothing for a bank of GPUs.

Radiators in space are extremely inefficient because there's no conduction.

Also you have huge heat inputs from the sun. So you need substantial cooling before you get around to actually cooling the GPUs.

▲DoctorOetker 8 hours ago | parent [-]

you put the radiators and the rest of the satellite within the shade of the solar panels, you can still make the area arbitrarily large

EDIT: people continue downvoting and replying with irrelevant retorts, so I'll add in some calculations

Let's assume

1. cheap 18% efficient solar panels (though much better can be achieved with multijunction and quantum-cutting phosphors)

2. simplistic 1360 W/m^2 sunlight orthogonal to the sun

3. an abstract input Area Ain of solar panels (pretend its a square area: Ain = L ^ 2)

4. The amount of heat generated on the solar panels (100%-18%) * Ain * 1360 W / m ^ 2, the electrical energy being 18% * Ain * 1360 W / m ^ 2. The electrical energy will ultimately be converted to computational results and heat by the satellite compute. So the radiative cooling (only option in space) must dissipate 100% of the incoming solar energy: the 1360 W / m^2 * Ain.

5. Lets make a pyramid with the square solar panel as a base, with the apex pointing away from the sun, we make sure the surface has high emissivity (roughly 1) in thermal infrared. Observe that such a pyramid has all sides in the shade of the sun. But it is low earth orbit so lets assume warm earth is occupying one hemisphere and we have to put thermal IR reflectors on the 2 pyramid sides facing earth, so the other 2 pyramid sides face actual cold space.

6. The area for a square based symmetric pyramid: we have

6.a. The area of the base Ain = L * L.

6.b. The area of the 4 sides 2 * L * sqrt( L ^ 2 / 4 + h ^ 2 )

6.c. The area of just 2 sides having output Area Aout = L * sqrt( L ^ 2 / 4 + h ^ 2 )

7. The 2 radiative sides not seeing the sun and not seeing the earth together have the area in 6.c and must dissipate L ^ 2 * 1360 W / m ^ 2 .

8. Hello Stefan-Boltzmann Law: for emissivity 1 we have the radiant exitance M = sigma * T ^ 4 (units W / m ^ 2 )

9. The total power exited through the 2 thermal radiating sides of the pyramid is then Aout * M

10. Select a desired temperature and solve for h / L (to stay dimensionless and get the ratio of the pyramid height to its base side length), lets run the satellite at 300 K = ~26 deg C just as an example.

11. If you solve this for h / L we get: h / L = sqrt( ( 1360 W / m ^ 2 / (sigma * T ^ 4 ) ) ^ 2 - 1/4 )

12. Numerically for 300K target temperature we get: h/L = sqrt((1360 / (5.67 * 10^-8 * 300 ^ 4)) ^ 2 - 1/4) = 2.91870351609271066729

13. So the pyramid height of "horribly poor cooling capability in space" would be a shocking 3 times the side length of the square solar panel array.

As a child I was obsessed with computer technology, and this will resonate with many of you: computer science is the poor man's science, as soon as a computer becomes available in the household, some children autodidactically educate themselves in programming etc. This is HN, a lot of programmers who followed the poor man's science path out of necessity. I had the opportunity to choose something else, I chose physics. No amount of programming and acquiring titles of software "engineer" will be a good substitute for physicists and engineers that actually had courses on the physical sciences, and the mathematics to follow the important historical deductions... It's very hard to explain this to the people who followed the path I had almost taken. And they downvote me because they didn't have the opportunity, courage or stamina to take the path I took, and so they blindly copy paste each others doomscrolled arguments.

Look I'm not an elon fanboy... but when I read people arguing that cooling considerations excludes this future, while I know you can set the temperature arbitrarily low but not below background temperature of the universe 4 K, then I simply explain that obviously the area can be made arbitrarily large, so the temperature can be chosen by the system designer. But hey the HN crowd prefers the layers of libraries and abstractions and made themselves an emulation of an emulation of an emulation of a pre-agreed reality as documented in datasheets and manuals, and is ultimately so removed from reality based communities like physics and physics engineering, that the "democracy" programmers opinions dominate...

So go ahead and give me some more downvotes ;)

If you like mnemonics for important constants: here's one for the Stefan Boltzman constant: 5.67 * 10^-8 W / m^2 / K ^ 4

thats 4 consecutive digits 5,6,7,8 ; comma or point after the first significant digit and the exponent 8 has a minus sign.

▲alangibson 12 minutes ago | parent | next [-]

It seems straightforward to you because you're ignoring everything that makes this not work.

Here's a big one: you can't put radiators in shadow because the coolant would freeze. ISS has system dedicated to making sure the radiators get just enough sunlight at any given time.

▲perryprog 7 hours ago | parent | prev | next [-]

It's really not that simple. See this for a good explanation of why: https://taranis.ie/datacenters-in-space-are-a-terrible-horri...

▲tyg13 7 hours ago | parent | next [-]

It all basically boils down to: in order to dissipate heat, you need something to dissipate heat into, e.g. air, liquid, etc. Even if you liquid cool the GPUs, where is the heat going to go?

On Earth, you can vent the heat into the atmosphere no problem, but in space, there's no atmosphere to vent to, so dissipating heat becomes a very, very difficult problem to solve. You can use radiators to an extent, but again, because no atmosphere, they're orders of magnitude less effective in space. So any kind of cooling array would have to be huge, and you'd also have to find some way to shade them, because you still have to deal with heat and other kinds of radiation coming from the Sun.

It's easier to just keep them on Earth.

▲eldenring 2 hours ago | parent | next [-]

What you're describing is one of two mechanisms of shedding heat which is convection, heating up the environment. What the long comment above is describing is a _completely_ different mechanism, radiation, which is __more__ efficient in a vacuum. They are different things that you are mixing up.

▲DoctorOetker 6 hours ago | parent | prev [-]

for a square solar array of side length L, a pyramid height of 3*L would bring the temperature to below 300K, check my calculation above.

people heavily underestimate radiative cooling, probably because precisely our atmosphere hinders its effective utilization!

lesson: its not because radiative cooling is hard to exploit on earth at sea level, that its similarily ineffective in space!

▲DoctorOetker 5 hours ago | parent | prev [-]

that page has not a single calculation of radiative heat dissipation, seems like he pessimistically designed the satellite avoiding use of radiative cooling which forces him to employ a low operational duty cycle. Kind of a shame to be honest, given the high costs of launching satellites, his sat could have been on for a larger fraction of time...

▲tempestn 7 hours ago | parent | prev | next [-]

That helps with the heat from the sun problem, but not the radiation of heat from the GPUs. Those radiators would need to be unshaded by the solar panels, and would need to be enormous. Cooling stuff in atmosphere is far easier than in vacuum.

▲bdamm 7 hours ago | parent | next [-]

Not so. Look at the construction of JWST. One side is "hot", the other side is very, very cold.

I am highly skeptical about data centers in space, but radiators don't need to be unshaded. In fact, they benefit from the shade. This is also being done on the ISS.

▲tempestn 6 hours ago | parent | next [-]

That's fair. I meant they would need a clear path to open space not blocked by solar panels, but yes, a hot and cold side makes sense.

The whole concept is still insane though, fwiw.

▲DoctorOetker 6 hours ago | parent [-]

"I meant they would need a clear path to open space not blocked by solar panels, but yes, a hot and cold side makes sense."

This is precisely why my didactic example above uses a convex shape, a pyramid. This guarantees each surface absorbs or radiates energy without having to take into account self-obscuring by satellite shape.

▲RIMR 7 hours ago | parent | prev | next [-]

Look at how many layers of insulation are needed for the JWST to have a hot and cold side! Again, this is not particularly simple stuff.

The JWST operates at 2kw max. That's not enough for a single H200.

AI datacenters in space are a non-starter. Anyone arguing otherwise doesn't understand basic thermodynamics.

▲DoctorOetker 4 hours ago | parent [-]

The goal of JWST is not to consume as much power as possible, and perform useful computations with it. A system not optimized for metric B but for metric A scores bad for metric B... great observation.

▲lm28469 6 hours ago | parent | prev [-]

> Look at the construction of JWST.

A very high end desktop pulls more electricity than the whole JWST... Which is about the same as a hair dryer.

Now you need about 50x more for a rack and hundreds/thousands racks for a meaningful cluster. Shaded or not it's a shit load of radiators

https://azure.microsoft.com/en-us/blog/microsoft-azure-deliv...

▲DoctorOetker 6 hours ago | parent | prev [-]

this makes no sense, the radiation of heat from the GPU's came from electrical energy, the electrical energy came from the efficient fraction of solar panel energy, the inefficient fraction being heating of the solar panel, the total amount of heat that needs to be dissipated is simply the total amount of energy incident on the solar panels.

▲tempestn 4 hours ago | parent [-]

True, the solar panels would need to be enormous too.

▲alangibson 7 hours ago | parent | prev | next [-]

> arbitrarily large

Space is not empty. Satellites have to be boosted all the time because of drag. Massive panels would only worsen that. Once you boosters are empty the satellite is toast.

▲DoctorOetker 5 hours ago | parent | next [-]

the point wasn't that a 1 m^2 solar panel could theoretically be kept reasonably cool at the cost of a miles long radiator... nono, the point was that you could attain any desirable temperature this way, arbitrarily close to 4K.

for a reasonable temperature (check my comment for updated calculations) the height of a square based pyramidal satellite would be about 3 times the side length of its base, quite reasonable indeed. Thats with the square base of the pyramid as solar panel facing the sun, and the top of the pyramid facing away, so all sides are in the shade of the base. I even halved my theoretical cooling power to keep calculations simple: to avoid a long confusing calculation of the heat emitted by earth, I handicapped my design so 2 of the pyramidal side surfaces are reflective (facing earth) and the remaining 2 side triangles of the pyramid are the only used thermal radiative cooling surfaces. Less pessimistic approaches are possible, but would make the calculation less didactic for the HN crowd.

▲alangibson 12 minutes ago | parent [-]

It seems straightforward to you because you're ignoring everything that makes this not work.

Here's a big one: you can't put radiators in shadow because the coolant would freeze. ISS has system dedicated to making sure the radiators get just enough sunlight at any given time.

▲inglor_cz 7 hours ago | parent | prev [-]

"Satellites have to be boosted all the time because of drag."

On Low Earth Orbits (LEOs), sure, but the traces of atmosphere that cause the drag disappear quite fast with increasing altitude. At 1000 km, you will stay up for decades.

▲stingrae 7 hours ago | parent | prev | next [-]

arbitrarily large means like measured in square km. Starcloud is talking about 4km x 4km area of solar panels and radiative cooling. (https://blogs.nvidia.com/blog/starcloud/)

Building this is definitely not trivial and not easy to make arbitrarily large.

▲DoctorOetker 5 hours ago | parent | next [-]

When a physicist says arbitrarily large it could even be in a dimensionless sense. It doesn't matter how small or large the solar panel is:

for a 4 m x 4 m solar panel, the height of the pyramid would have to be 12 m to attain ~ 300 K on the radiator panels. Thats also the cold side for your compute.

for a 4 km x 4 km solar panel the height of the pyramid would be 12 km.

▲Daishiman an hour ago | parent | prev [-]

A size like that is going to be completely, absolutely obliterated by micrometeor collisions.

These people are all smoking crack.

▲adastra22 7 hours ago | parent | prev | next [-]

I’ve got a perpetual motion machine to sell you.

▲DoctorOetker 6 hours ago | parent [-]

this isn't even an argument?

▲adastra22 5 hours ago | parent [-]

> you put the radiators and the rest of the satellite within the shade of the solar panels, you can still make the area arbitrarily large

The larger you make the area, the more solar energy you are collecting. More shade = more heat to radiate. You are not actually making the problem easier.

▲DoctorOetker 4 hours ago | parent [-]

no the radiator planes are in the shade, so you can increase the height of a pyramidal shaped satellite for a constant solar panel base, and thus enjoy arbitrarily low rest temperatures, check my calculation which I added.

for a target temperature of 300K that would mean the pyramid height would be a bit less than 3 times higher than the square base side length h=3L.

I even handicapped my example by only counting heat radiation from 2 of the 4 panels, assuming the 2 others are simply reflective (to make the calculation of a nearby warm Earth irrelevant).

▲MuskIsAntidemo 7 hours ago | parent | prev [-]

[dead]

▲TheGRS 8 hours ago | parent | prev | next [-]

I'm not big on this subject, but I understand that heat transfer is difficult in space, because there's little to transfer to. If the solution is just making large radiators, then that means you're sending some big payloads full of radiators. Not to mention all the solar panels needed. I wanna live in sci-fi land too, but I don't see how it makes any sense compared to a terrestrial data center.

▲eldenring 8 hours ago | parent [-]

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

▲TheGRS 7 hours ago | parent [-]

If someone has a design out there where this works and you can launch it economically on a rocket today, I wanna see that. And then I wanna compare it to the cost of setting up some data centers on earth (which BTW, you can service in real time, it sounds like these will be one-and-done launches).

▲Aurornis 8 hours ago | parent | prev | next [-]

> keep their equipment that already can consume several kW cool just fine

That's equivalent to a couple datacenter GPUs.

> You might need space for radiators, but there is plenty space in space.

Finding space in space is the least difficult problem. Getting it up there is not easy.

▲eldenring 7 hours ago | parent | prev | next [-]

You can line the solar panels and radiators facing away from each other, and the radiators would take up less surface area. I think maybe the tricky part would be the weight of water + pipes to move heat from the compute to the radiators.

▲bdamm 7 hours ago | parent [-]

Water is not needed to move heat. Heat pipes do it just fine. There's one in your laptop and one in your phone too. It does scale up.

▲eldenring 7 hours ago | parent [-]

Interesting, That could surely simplify things.

▲wat10000 8 hours ago | parent | prev [-]

There's plenty of space in space, but there isn't plenty of space in rocket fairings, nor is there plenty of lift capacity for an unlimited amount of radiators.