Particularly for reusable liquid engines :)
Edit: can't reply
> But.. why?
Missile Defense and C4ISR [2]. Seconds matter, so most of the newer generation of missile defense systems are experimenting with how to offload compute at the edge to reduce C2 latency.
Most orbital sat startups in the US, China, and India are partially backed by military and intel oriented funds (eg. Starcloud and IQT/In-Q-Tel).
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[0] - https://www.nro.gov/news-media-featured-stories/news-media-p...
[1] - https://www.livemint.com/companies/news/agnikul-cosmos-iceye...
[2] - https://idsa.in/wp-content/uploads/2026/01/book-MISSION-SUDA...
1. How do you cool your chips? Vacuum is a thermal insulator, so radiators are required to remove heat. nVidia (or even ASICs) require much and specialised cooling.
2. How does one radiation harden a H100?
3. I'm also seeing where TCO for these are 78x their terrestrial equivalents [0]. Is that financial sustainable?
And there still remain issues with power supply, regulations, and bandwidth. This feels more like a thought experiment rather than an actual serious engineering or business case.
1 and 2 are still open questions, but these are not aimed to be commercial grade DCs - this is basically edge compute (think a handful of racks). 3 is not a problem for defense usecases. (EDIT: Discussion here seems to point out that data OP is using might be flawed [1]).
Ignoring the fact that just about every orbital data center startup in the US is funded by IQT and China's CMF has been doing something similar is bad from a defense perspective.
Based on dealflow, these aren't being targeted for consumer usecases in the short-term and whoever has been saying that is misleading.
> especially with the possibility of Kessler Syndrome...
India, Russia, China, and the US don't care about Kessler Syndrome - they have already launched and deployed ASATs. This also comes after India and China had a near collision in 2024 that was treated as an offensive action [0].
[0] - https://www.bloomberg.com/news/articles/2025-09-22/india-pla...
Worse, the technology for firing any meaningful payloads from an electric gun (8" Small Diameter Bomb equivalents, guided, airburst, incendiary) simply does not exist.
Same as with Musk's California Vacuum Tunnel (which diverted attention from passenger rail). And his Neuralink. In the 1980s, it was nuclear pumped space-based lasers and Soviet particle beam weapons.
All of the above can be debunked with 2 years undergrad physics and a Casio calculator. Yet they were still taken seriously by high-level politicians and business, some of whom were deeply connected with the military-industrial complex.
People have shrugging of questions like 1 and 2, only for 3 to hit them very hard. But we will see if they can launch 600 of these satellites as they claim.
My response:
Carnot's theorem and refridgeration cycles rear their head. Your heat pump still needs power to pump heat uphill., negating any savings from a smaller radiator. And what happens when you shrink a radiator? It becomes a high temperature radiator, meaning the GPU must operate outside its operational tmperature of about 70 degrees C. So small radiator = big pump + extra solar panels and batteries + dead GPU
Also, how do you deal with the added cost and complexity for the solar tracking mechanisms for the "edge-on-to-the-Sun" radiator?
I am regularly assured by HN denizens that orbiting data centers will never work.
For C4ISR usecases they solve the latency problem and are worth the spend needed.
Everyone on HN is talking about a commercial usecase that isn't under serious consideration.
But.. why?
Musk bandwagon-hopping? Investor bamboozlery? (Same diff?)
Is my borderline childish understanding of basic physics THAT off because I cannot see the value or utility (beyond a small smear of niche edge cases) of space-based compute?
- ed, disclosure: You seem to have edited your response whilst I was typing mine, adding in valuable links. Thanks!
In the real world, "physics" is not necessarily the gating factor. There is a major concern about the environmental footprint of terrestrial data centers, to the point where major U.S. states are enacting moratoriums: https://www.governor.ny.gov/news/first-statewide-moratorium-.... These legal and social roadblocks must be accounted for in analyzing the viability of orbital data centers.
As to power, Caltech is already at 2-3 pounds per square meter: https://magazine.caltech.edu/post/sspp-space-solar-power-pro...
> Another way to think about it: An SSPP spacecraft with a 60-meter-by-60-meter surface area made using today’s space PV-cell technology would cost $36 million and weigh nearly 9,000 pounds, or almost as much as a Ford F-450 truck. With the ultra-lightweight PV-cell technology Atwater envisions, it would cost just $450,000 and weigh about 300 pounds, or about as much as an IKEA three-seat sofa
That’s megawatt-level solar power under 5 tons using today’s leading edge technology. Starship super heavy can launch 100 tons into LEO.
As to bandwidth, Starlink V3 backhaul capacity is 1 terabit. Microwave radio frequencies have an insane amount of bandwidth.
SSPP has already sent prototypes to space: https://www.caltech.edu/about/news/space-solar-power-project....
There's a ton of work being done on lightweight solar panels for space. E.g. https://ascentsolar.com/asti-technology-and-unique-advantage...
The cost of replacement is exorbitant for commercial usecases, but is acceptable for defense usecases.
The issue is too many people are looking at the commercial usecase while ignoring the defense usecase that is what is actually driving the conversation and dealflow in this segment.
[0] - https://www.mikhailklassen.com/posts/orbital-data-centers/or...
First, it assumes the radiator is at the same temperature as the GPU. But radiators become dramatically more effective as temperature increases, with radiated power increasing as the fourth power of absolute temperature. So a heat pump that drives the radiator at higher temperature could make the radiator far smaller. More power would be required (and the radiator would have to radiate this energy too) but the radiator could become much smaller.
The other problem is assuming the radiator is intercepting sunlight. But it can be shaded by reflective films or kept edge-on to the Sun.
> First, it assumes the radiator is at the same temperature as the GPU. But radiators become dramatically more effective as temperature increases, with radiated power increasing as the fourth power of absolute temperature. So a heat pump that drives the radiator at higher temperature could make the radiator far smaller.
Carnot's theorem and refridgeration cycles rear their head. Your heat pump still needs power to pump heat uphill., negating any savings from a smaller radiator. And what happens when you shrink a radiator? It becomes a high temperature radiator, meaning the GPU must operate outside its operational tmperature of about 70 degrees C. So small radiator = big pump + extra solar panels and batteries + dead GPU
Also, how do you deal with the added cost and complexity for the solar tracking mechanisms for the "edge-on-to-the-Sun" radiator?
Sorry, that's all nonsense. Yes, the extra power needs to be radiated. But the advantage of operating at high temperature is so extreme that the more effective radiation will overwhelm that unless the heat pump is extraordinarily inefficient. If the heat pump would be perfect, operating at the Carnot limit, then if it doubled the absolute radiator temperature it would double the amount of energy to be radiated, but the area of the radiator would decrease by a factor of (2^4)/(2) = 8.
As for the second point, no, this does not require the GPU to operate at higher temperature. What made you conclude it would?
> Also, how do you deal with the added cost and complexity for the solar tracking mechanisms for the "edge-on-to-the-Sun" radiator?
I don't need to; I'm just debunking a bad argument. What you are doing there is called "moving the goalposts". But satellites normally have means of orienting PV toward the Sun. So, maybe have the radiator perpendicular to those? Those claiming the idea violates the laws of physics and using solar absoption on the radiator as part of the argument need to show no such scheme can work, even in principle.
Smaller radiator = bigger heat pump + more batteries and solar panels
Satellites have a mass and power budgets. Your scheme only looks at temperature. If you want to build infinitely large AI Data satellites, go ahead.
Don't worry, most of the people online didn't get the memo on this.
One of the major selling points of orbital compute is power supply - more specifically, it turns out that, compared to beaming power from space, it's projected to be cheaper to move compute upwell instead - atmosphere losses for beamed power are just too big. Of course this doesn't matter if you can get cheap, clean power from elsewhere (e.g. nuclear).
This is in general data center case. Here, GP says the motivation is reducing C2 RTT, which makes sense for military applications.
The other comparison point is of course plain old terrestrial power sources, including energy storage in the mix if it relies heavily on solar power without a sufficiently global grid. Not having to launch into space buys a lot of power.
C2 RTT times and edge computation of large datasets collected in space make much more sense since unit economics aren't the driving factor, but are unlikely to need datacentres on the same scale as inference compute for the general population (another reason why this use case makes more sense)
[1]but I think the underrated problem with beamed power isn't just the low maturity of the technology, but that as soon as you start talking about sending power to earth via RF or laser you're going to encounter political opposition that makes objections to terrestrial datacentres seem tame.... [2]chips can in theory be replaced on orbit, but the "million satellite" filings are disposable. Replenishing propellant for a few large power stations, potentially on longer cycles, is a simpler task.
We could. The usual stumbling block is how to ship fissile material upwell without the risk of a launch failure spreading highly radioactive material over several countries.
> What's the power loss from beaming down power?
I think it's about 10% from atmosphere alone, but you have to add losses from other components in the system, including light -> current -> RF and RF -> current legs, and I've seen estimates ranging from 15% to 40% efficiency end to end; this random article includes breakdown with estimates, that multiplies down to 37.5% efficiency end-to-end.
https://www.sciencedirect.com/topics/earth-and-planetary-sci...
EDIT:
This system design gives 7-14% end-to-end efficiency: https://arxiv.org/pdf/2206.08373
EDIT2:
Also to spell out another non-obvious aspect of beamed power, it turns out that it's not the efficiency that's the limiting factor per se, but land - you can improve efficiency by building larger rectennas, but it gets very expensive very quickly once you consider paying for land under them.
SNAP-10A
BES-5 (oops, sorry 'bout that Canada!)
TOPAZ-I
Kiwi, Phoebus, and NRX (Mars here we come!)
RD-0410 (Dossvidanya Solar System!)
SP-100
TOPAZ-II
Kilopower
The fact that the US, China, Russia, and India have already deployed ASATs means a Kessler effect if a question of when and not if.