4 ms·
How much of that power is radiated as the radio waves it sends?
by tensor 8mo ago
How much of that power is radiated as the radio waves it sends?
- adgjlsfhk1 8mo agothe majority is likely in radio waves and the inter satellite laser communication
- hirsin 8mo agoGood 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 8mo agoI 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.
- tullianus 8mo agoUnfortunately this is not the case. The amplifiers on the transmit-side phased arrays are about 10% efficient (perhaps 12% on a good day), but the amps represent only ~half the power consumption of the transmit phased arrays. The beamformers and processors are 0% efficient. The receive-side phased arrays are of course 0% efficient as well.
- klaff 8mo agoI'm curious. I think the whole thing (space-based compute) is infeasible and stupid for a bunch of reasons, but even a class-A amplifier has a theoretical limit of 50% efficiency, and I thought we used class-C amplifiers (with practical efficiencies above 50%) in FM/FSK/etc. applications in which amplitude distortion can be filtered away. What makes these systems be down at 10%?
- adrian_b 8mo agoYes, a 10% efficiency is very weird if true. Nowadays such microwave power amplifiers should be made with gallium nitride transistors, which should allow better efficiencies than the ancient amplifiers using LDMOS or travelling-wave tubes, and even those had efficiencies over 50%. For beamformers, there have been research papers in recent years claiming a great reduction in losses, but presumably the Starlink satellites are still using some mature technology, with greater losses.
- mlyle 8mo agoI doubt half the power is to the transmitter, and radio efficiency is poor -- 20% might be a good starting point.
- synctext 8mo agoIs 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-radiator-launches-spacex-falcon-9 https://www.graphene-info.com/smartir-s-graphene-based-radia...
- ajnin 8mo agoThis 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.
- synctext 8mo agoIndeed, 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 https://opg.optica.org/oe/fulltext.cfm?uri=oe-21-25-30964
- habinero 8mo agoIt'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).
- nosianu 8mo agoThe radio receiver and transmitter are additional hardware and energy consumption. They add to the heat, not subtract from it.
- jeltz 8mo agoI 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.
- habinero 8mo agoNo, 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 8mo agoYour 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.
- nosianu 8mo ago[flagged]
- adrian_b 8mo agoYou missed the point. Nobody describes a satellite by specifying the amount of heat that it produces, but by the amount of electrical energy that it consumes. In a communication satellite, a large fraction of the consumed electrical energy goes into the radio transmitter. Radio transmitters are very efficient and most of the consumed power is emitted as radio waves and only a very small part is converted into heat, which must be handled by the cooling system. So in any communication satellite, a significant fraction of the consumed energy does not become heat.