Thread

1. zactato · 1790267254 · focus · HN ↗

   How are they solving the heat dissipation issues?

   1. lopsotronic · 1790268994 · focus · HN ↗

      For real. It's an enormous problem solved only with 1) sheer scale, and 2) Science Fiction.

      Both of those are expensive as hell, by the way.

      Cooling via radiation follows Stefan–Boltzmann: P = εσAT⁴. Let's assume a good surface (emissivity ~0.9) at 300 K (27 °C) at 400 W per square meter per side. A flat panel radiating from both faces into deep space gets 800 W/m, not including the losses from, say, the Sun, or from IR coming off the Earth. Now, input power. Sunlight in orbit ~1,360 W/m², assume ~22% cell efficiency, we got 300 W/m². So each 1 MW compute, 3,300 m² of solar panel and minimum 1,200–1,500 m² of radiator.

      In case ya didn't know - 1 MW is tiny from a present-day-datacenter perspective. It's like 8 racks. So we're talking orbital megastructures here, many many many square kilometers, and this is with all the best case assumptions, and magic radiator panels that never see the sun, or the earth, or the moon.

      This is just the basic numbers here, by the way. There's a garbage truck full of other unsolvable problems if you poke your head in there.

      Aside from the "Avoid Regulations" aspect, and the "Everything That Burns Deorbiting is Depreciation" aka "The Starlink Trick", I'm not sure what the hell the draw is.

      1. JoeAltmaier · 1790285575 · focus · HN ↗

         Surface area is a materials problem? Folded microstructure, atomic-scale textured surface or some other science-fiction solution could have square kilometers of surface area in a shoebox.

         1. sgsjchs · 1790289497 · focus · HN ↗

            It needs to be facing open space instead of other parts of itself, otherwise the radiation is just reabsorbed.

            1. JoeAltmaier · 1791126449 · focus · HN ↗

               Would prisms fitted between the faces, directing everything outward, change the result?
         2. AlotOfReading · 1790289652 · focus · HN ↗

            Imagine you have two blackbody radiators with the same bulk properties, except one has surface area shenanigans like aerogels. In the far field as a whole, it seems like both should radiate essentially the same regardless of the internal details. You can shape emissive direction, or improve efficiency of non-ideal materials, but even ideal materials don't fix the issues pointed out by the parent.

            1. peri-cl · 1790300631 · focus · HN ↗

               Right; it's only area exposed to the exterior that counts. A physical object can't thermally radiate more power than a perfect blackbody spanning its convex hull.

               (This follows because a physical object can't absorb more light than a perfect blackbody spanning its convex hull. A perfect blackbody by definition absorbs 100% of incident light, which is a hard upper bound. Any line incident on an object is also incident on its convex hull).

               (Consider an isothermal object that emits more power than a blackbody in the shape of its hull at the same temperature. If you were to place that object in a closed system at thermal equilibrium, the interior of an insulated emissive sphere—combining assumptions, it would emit more power than it absorbs, in violation of the 2nd law. Starting from an isothermal system, the object would grow colder, and the enclosing container hotter).

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