"SmartIR’s graphene-based radiator launches on SpaceX Falcon 9" [1]. This could be the magic behind this bet on heat radiation through exotic material. Lot of blog posts say impossible, expensive, stock pump, etc. Could this be the underlying technology breakthrough? Along with avoiding complex self-assembly in space through decentralization (1 million AI constellation, laser-grid comms).
[1] https://www.graphene-info.com/smartir-s-graphene-based-radia...
Quote: "emissivity higher than 0.99 over a wide range of wavelengths". Article title "Perfect blackbody radiation from a graphene nanostructure" [1]. So several rolls of 10 x 50 meters graphene-coated aluminium foil could have significant cooling capability. No science-fiction needed anymore (see the 4km x 4km NVIDIA fantasy)
[1] https://opg.optica.org/oe/fulltext.cfm?uri=oe-21-25-30964
The limiting factor isn't the emissivity, it's that you're having to rely on radiation as your only cooling mechanism. It's super slow and inefficient and it limits how much heat you can dissipate.
Like the other person said, you can't do any better than blackbody radiation (emissivity=1).
They are usually white, because things in a spacecraft are not hot enough to glow in visible light and you'd rather they not get super hot if the sun shines on them.
The practical emittance of both black paint and white paint are very close to the same at any reasonable temperature-- and both are quite good, >90% of this magical material that you cite ;)
Better materials -- with less visible absorption and more infrared emittance -- can make a difference, but you still need to convect or conduct the heat to them, and heat doesn't move very well in thin materials as my sibling comment says.
The graphene radiator you cite is more about active thermal control than being super black. Cheap ways to change how much heat you are dumping are very useful for space missions that use variable amounts of power or have very long eclipse periods, or what move from geospace to deep space, etc. Usually you solve it on bigger satellites with louvers that change what color they're exposing to the outside, but those are mechanical parts and annoying.
Thus the extremities of the foil, which are far from the satellite body, will be much cooler than the body, so they will have negligible contribution to the radiated power.
The ideal heatsink has fins that are thick close to the body and they become thinner towards extremities, but a heatsink made for radiation instead of convection needs a different shape, to avoid a part of it shadowing other parts.
I do not believe that you can make an efficient radiation heatsink with metallic foil. You can increase the radiating surface by not having a flat surface, but one covered with long fins or cones or pyramids, but the more the surface is increased, the greater the thermal resistance between base and tip becomes, and also the tips limit the solid angle through which the bases radiate, so there must be some optimum shape that has only a limited surface increasing factor over the radiation of a flat body.
30% power added efficiency is the state of the art up in Ku band if you need a low compression budget. And it's important to note that this doesn't include the substantial power spent in modulation of complex signals or the power conversion, etc, before the transmitter. Or, the power lost in the connection to the antenna and its matching-- can easily exceed 2dB.
