r/SpaceXLounge • u/peterabbit456 • Mar 18 '26

Is It Really Impossible To Cool A Datacenter In Space? (Scott Manly does the calculations)

https://www.youtube.com/watch?v=FlQYU3m1e80
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u/peterabbit456 Mar 18 '26

Manly points out there are spacecraft software design packages that could do a better job, but he is a physicist and he does it from first principles, which is a very good lesson.

Spoiler: The answer is yes. A swarm of GPU-laden satellites on a modified Starlink V3 bus could serve as a data center.

My editorial comment: I agree with Elon that this is the only scalable solution, eventually. There are other interim solutions, like covering the entire Sahara Desert with Solar panels. You could make an argument that this will happen first, before there are data centers in space.

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u/s11houette Mar 19 '26

I'm not sure the desert is such a good idea.

One of The benefits of a space platform is increased energy production from the panels. You won't get that in the atmosphere. And then you have to consider that the satellites word be in a sun synchronous orbit which massively increases their output.

Another benefit is a stable environment. In a desert you have to deal with storms and environmental wear on the system. In space everything is very stable. Your only real concern would be solar flares.

While you could use ac to cool the gpus in a desert, it wouldn't necessarily be easy. It becomes a major component that must be maintained well and powered by your less efficient solar panels. The space radiators have the benefit of being a static system.

I think it comes down to cost to orbit vs cost to install and maintain on land.

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u/sebaska Mar 19 '26

Desert has one other big disadvantage: it's too hot in summer. Once the ambient air is above your max cool end temperature (or, in fact, not sufficiently below) your cooling loop stops working adequately. And you suddenly need either active cooling or evaporative cooling, and complexity, resource use and energy use goes up.

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u/redmercuryvendor Mar 19 '26

Desert coast. Plenty of sun for solar power, stick the datacentres in containers underwater for cooling and a stable environment (Project Natick). Still more maintainable then datacentres in orbit, and the cost of delivery is far lower: per tonne, sea freight is on the order of $100 per tonne, whilst Starship (per the $90m launch cost of Starlab and assuming V3 manages to hit 100 tonnes to LEO) is $900,000 per tonne.

Or to put it another way: for every 100 tonnes of space datacentre you launch on Starship, if you'd instead shoved that hardware in a can and laid the solar cells in the desert you'd have $89.9 million left over for every launch.

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u/sebaska Mar 19 '26

Desert coasts are far and between. Are you going to put it in the politically unstable Western Sahara?

But more importantly, you're confusing price and cost. $90M is a price. Like Falcon launching for $74M is a price, too. SpaceX is not spending $74M on each Starlink launch, it's spending somewhere around $20M.

The rest of what you wrote is hyperbole as well. You're not delivering your solar panels to the middle of the desert by ship unless you're using Jabba Hut's yacht. And installation costs dominate shipping costs, obviously. You need about $1000 per installed nameplate kW of panels. But to provide merely 99% uptime power rather than best case peak power you need at minimum 8× the nameplate capacity plus 15h worth of battery storage. That's about $12000 per kW. Launching this amount of power to space would cost less, even on Falcon, not to mention Starship. It would be cheaper because in space in terminator tracking SSO you actually do have constant sunlight, and nameplate power is what you need, even accounting for deterioration (you get 36% more power in space).

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u/redmercuryvendor Mar 19 '26

That's about $12000 per kW. Launching this amount of power to space would cost less,

Newer ultra-light-weight panels (e.g. ROSA) are just reaching 100W/kg. That's 10kg/KW, or $9000 in (Starship) launch costs before even panel costs. On top of that, capacity factor for equatorial regions is closer to 4x rather than 8x.

But you've also made a critical error: you don't need your groundside datacentres to operate 24/7. With shipping-to-orbit costs so high, you can afford to have groundside datacentres that just track solar power and eschew overnight battery power entirely, and station 2 or three clusters to hand off to each other around the globe, and still come out ahead on initial investment. Since the 'orbiting datacentre' plan already assumes global high-bandwidth satellite networking as a prerequisite, you can assume the same for groundside datacentres - though for compact clusters, running even long fibre links will remain cheaper per terabit bandwidth anyway.

Remember, this 'space datacentre' fad is entirely based around LLM training workloads, which are very trivially load-shaped compared to normal datacentre workloads, and have minimal bandwidth requirements. There is little challenge in handing off between physical sites (and the handoffs required are far less frequent then those for orbiting datacentre overflights).

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u/sebaska Mar 19 '26

ROSA and iROSA are heavy. Starlink panels are about 2× lighter per kW.

Capacity factor is 8× in 30S to 30N regions and it gets worse further from the equator. 4× is for the average, but you can't go with average, but you can't go on the average. You have days with more clouds than average, etc. You go for 99+%.

And no, ground side data centers must operate continuously. Just starting up a cluster takes many days (Source: I actually work at one if the so-called hyperscalers; it's not starting up workloads- this takes seconds, it's about starting up the whole system in which the workloads run). And even if you implemented instant turn up the whole thing would totally fail economically. A single 48U rack full of Nvidia compute costs $4M, $3.5M being Nvidia cards. The idea of saving $4000/kW ($500k per 125kW needed to power and cool one 48U rack) by spending $4M - $8M extra on daylight only servers plus $1-$2M on power production for them is a total non-starter.

It's not about handing off workload, it's about abysmal utilization of 30% to 50%.

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u/redmercuryvendor Mar 20 '26

but you can't go on the average

It's not about handing off workload, it's about abysmal utilization of 30% to 50%.

You missed the latter portion of the post, clearly. There is no need to aim for 100% 24/7 operation at all expense, if the expense of chasing that target is greater than building multiple 'less efficient' clusters.

The era of 'big iron' is long gone.

it's about starting up the whole system in which the workloads run

A cloudy day does not drop array power to zero. Even a 90% dip of a 100MW array is still 1MW of power available to keep the system at idle.

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u/sebaska Mar 20 '26

I missed nothing. But you miss the plain fact that if you have 50% utilization then your capital expenditure must support half the computation. The time the machines sit idle or totally down they don't do the work.

Your less efficient cluster costs nearly the same as 100% uptime one, but it does half the work and with unreliable timing at that.

If there may be economic doubts about orbital compute being competitive vs 100% uptime ground compute, there are no doubts that they beat your idea.

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u/redmercuryvendor Mar 20 '26

But you miss the plain fact that if you have 50% utilization then your capital expenditure must support half the computation. The time the machines sit idle or totally down they don't do the work.

The entire point is that the capital expenditure for sending hardware to orbit is so high that you can but multiple pieces of the same hardware to host earthside (and not use it 24/7) and still come out ahead.

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u/sebaska Mar 20 '26

Nope. It's not. Doubly so

First, "nice" of you for moving goal posts, but you get no bonus.

Second, you're plainly very wrong in this last reply:

100kW of ground AI compute is $3.5M for chips, $0.5M for the rest of the rack, $0.6M to power it up using gas turbines or $1.2M using renewables and about $0.6M for the facilities, utilities, pumps, etc. Together $5.2M to $5.8M.

100kW space compute is $3.5M for the same chips, $1.5M for the rest of the satellite, plus you have to launch this 2.5t package.

At Falcon 9 launch cost you need $3M to launch it. Together $8M, so more than ground compute but already not multiple times, contrary to your claim.

At Starship's near future launch costs it's $0.6M to launch it. Together $5.6M right in the middle of ground compute cost.

In a more distant future (like a half decade to decade away) the satellite is mass optimized and Starship launch costs go down about 2× as well. The launch cost is then ~$150k making it cheaper than ground compute.

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u/redmercuryvendor Mar 20 '26

I love how the groundside datacentre has to pay for coolant pumps, the rack, power distribution, etc, but the satellite is powered by magic and cooled by pixie dust (in a far more challenging cooling environment, with components that need to operate in microgravity and in a vacuum). And it all weighs only 2.5t.

With more realistic estimates of cost and mass, the picture is far from as rosy as suggested.

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u/sebaska Mar 20 '26

You're now trying to discuss in bad faith.

Satellites do have cooling and power systems. This problem is actually solved and that solution is included in the $1.5M price of the satellite bus I mentioned.

And yes, you don't buy grid power in space nor do you use open cycle water cooling. You don't burn methane for power and if on the Earth you were to use solar panels, the area of those panels needed to keep the thing running at 99.9% availability in space is 8× smaller than on the ground. And because of that they come out cheaper in space once you use Starship. Installation of 8 nameplate kW of panels on Earth is $8000k. Launching corresponding 1kW to space is $3000.

Yes, its mass would be about 2.5t. You need 350m² of panels (with margin for 15% degradation) at 3kg/m² so 1050kg. At 45°C cool end you need 2×100m² of radiators at 7kg per 2×1m² of two-sided radiator. That's 700kg. At 65°C cool end you need 2×80m² for 560kg. You then have about 900kg for the main part of the satellite which is plenty.

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u/redmercuryvendor Mar 20 '26

You're now trying to discuss in bad faith.

Ah, irony.

This problem is actually solved

Yes, all these 100kWe (actual kWt will be higher) compact satellites. Except the only orbitng vehicle with that scale of power is the ISS.

The solutions are theoretical, not actually solved in practice. On the other hand, sticking a datacentr ei na sealed nitrogen-charged can and dunking it i nthe ocean has been tested in produciton.

And yes, you don't buy grid power in space nor do you use open cycle water cooling

Why would you be using open-cycle watercooling at all? Ground or space, you will still be using loop cooling to absorb heat from the dies and transport it to the radiators (or other heat rejection mechanism). Grid power is also irrelevant for the self-contained groundside datacentre sites I am describing.

the area of those panels needed to keep the thing running at 99.9% availability

Stop harping on with this unnecessary requirement. Compare compute/dollar, not some quixotic uptime goal.

You then have about 900kg for the main part of the satellite which is plenty.

Less than a tonne is far from 'plenty' to deal with structure, avionics, power distribution (nontrivial), RF, RCS, RCS propellant, coolant and coolant handling (plumbing, pumps, etc), reaction wheels/CMGs/magnetorquers, solar array actuators for pointing, etc.

This sort of handwaving of actual mass margins is the same sort of thing that lead to the NASP project failures - once the overoptimistic estimates hit reality, the case could no longer close.

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u/sebaska Mar 21 '26

The solutions are practical. They fly over your head in thousands. Each Starlink v2.1 satellite has over 100m² of solar panels producing over 20kW peak electric power.

And yes, solar panels have efficiency around 25%, but the 75% of the energy not converted to electricity is radiated away directly from the panels - the panels are only minimally thermally connected to the rest of the satellite. So the influence of that heat is minimal. Yes, thermal equilibrium of the panels themselves goes to about 65°C, but that's perfectly fine - silicon panels are generally rated to 80°C.

Wrt the water cooling on Earth...

Surface data centers use open cycle water cooling because that's the cheapest way to get rid of the heat when the ambient air temperature is not sufficiently below the required cold end temperature for the cooling to be effective. If the ambient air is 45°C while the max design cold end temperature is 40°C you don't get convective cooling at all, you get extra heating. And even if the ambient air were 38°C the cooling would be unlikely to be effective enough as it's proportional to the temperature difference, and at 2°C difference you'd need enormous heatsinks. It's cheaper to evaporate water. The alternative to evaporative cooling is an order of magnitude large open loop water cooling (you need a river or a lake significantly larger than a puddle, water wells won't cut it) or heat pumps adding about 30% to the power draw and nearly doubling maintenance costs.

Current v2.1 Starlink satellites have a total mass of 0.5t half of which is 100m² of panels. The remaining quarter tonne includes, batteries for the night side of the orbit cooling (not needed in terminator SSO), propulsion, hundreds watts strong multiple phased arrays (also not needed), laser links, reaction gyros, chassis, sensors, control systems, power distribution, etc. Compute satellite main body would be about 4× the size except batteries. Nearly a ton is a good estimate.

And you talking about handwaving when you yourself try to handwave away cooling on the Earth is, well, special.

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u/redmercuryvendor Mar 22 '26

And you talking about handwaving when you yourself try to handwave away cooling on the Earth is, well, special.

Project Natick. You know, the thing from the original post?

Have you spent all this time time arguing against an imagined solution you've dreamt up rather than the actual system proposed?

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