How much of that power is radiated as the radio waves it sends?

Good point - the comms satellites are not even "keeping" some of the energy, while a DC would. I _am_ now curious about the connection between bandwidth and wattage, but I'm willing to bet that less than 1% of the total energy dissipation on one of these DC satellites would be in the form of satellite-to-earth broadcast (keeping in mind that s2s broadcast would presumably be something of a wash).

	▲	adrian_b 19 minutes ago \| parent [-]
		I am willing to bet that more than 10% of the electrical energy consumed by the satellite is converted into transmitted microwaves. There must be many power consumers in the satellite, e.g. radio receivers, lasers, computers and motors, where the consumed energy eventually is converted into heat, but the radio transmitter of a communication satellite must take a big fraction of the average consumed power. The radio transmitter itself has a great efficiency, much greater than 50%, possibly greater than 90%, so only a small fraction of the electrical power consumed by the transmitter is converted into heat and most is radiated in the microwave signal that goes to Earth's surface.

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mlyle 7 hours ago | parent | prev | next [-]

I doubt half the power is to the transmitter, and radio efficiency is poor -- 20% might be a good starting point.

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synctext 7 hours ago | parent | next [-]

Is the SpaceX thin-foil cooling based on graphene real? Can experts check this out?

"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...

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ajnin 4 hours ago | parent [-]

This coating looks like it can selectively make parts of the satellite radiators or insulators, as to regulate temperature. But I don't think it can change the fundamental physics of radiating unwanted heat and that you can't do better than black body radiation.

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synctext 3 hours ago | parent [-]

Indeed, graphene seems capable of .99 of black body radiation limit.

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

	▲	adrian_b 5 minutes ago \| parent \| next [-]
		Aluminum foil of great surface will not work very well, because the limited conductivity of a thin foil will create a great temperature gradient through it. 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.
	▲	habinero 2 hours ago \| parent \| prev [-]
		It's not as exciting as you think it is. "emissivity higher than 0.99 over a wide range of wavelengths" is basically code for "it's, like, super black" 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).

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PunchyHamster 6 hours ago | parent | prev [-]

Entirely depends on band, at 10GHz more like 40%, at lower frequencies more, for example FM band can even go to 70%

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adgjlsfhk1 9 hours ago | parent | prev | next [-]

the majority is likely in radio waves and the inter satellite laser communication

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hdgvhicv 7 hours ago | parent [-]

Inter sat comms cancels out - every kw sent by one sat is received by another.

	▲	mlyle 7 hours ago \| parent [-]
		It doesn't, because the beams are not so tight that they all fall on the target satellite, and not all of that is absorbed :P

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nosianu 3 hours ago | parent | prev [-]

The radio receiver and transmitter are additional hardware and energy consumption. They add to the heat, not subtract from it.

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jeltz 3 hours ago | parent [-]

I think you missed the point. If you have a 100 MW communicstion satellite and a 100 MW compute satellite those are very different beasts. The first might send 50% of the energy away as radio communication making it effectively a 50 MW satellitefor cooling purposes.

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habinero 2 hours ago | parent [-]

No, they didn't. You can't "send away" thermal energy via radio waves. At the temperatures we're talking about, thermal energy is in the infrared. That's blackbody radiation.

	▲	mortehu an hour ago \| parent [-]
		Your answer makes it seem like you too missed the point. If a Starlink sends a 1000W signal to Earth, that is 1000W of power that does not heat the satellite.