3 ms·

Power consumption may not be the only concern but also dissipating the heat. You don't have air or water around you to dump the heat into so you're limited by w

by IanCal 2y ago

Power consumption may not be the only concern but also dissipating the heat. You don't have air or water around you to dump the heat into so you're limited by what you can radiate (is my understanding). Trying to radiate out 200W of heat vs 10-100kW is a dramatically different challenge.

vlovich123 2y ago
Fill a surrounding container with a volumetric shit ton of nitrogen gas (perhaps with stepped pressure gradients) and you might be able to radiate a significantly larger amount by increasing your surface area without drastically increasing your weight (or use nuclear power to mitigate the weight concerns). But yes, today’s design space generally prefers as low a power envelope as possible to not have to worry about dissipation.
- IanCal 2y ago
  Wouldn't the limiting factor be the outer layer? I'm not sure what layering would give you, or what adding a container would give you. For short bursts this may make a difference but the steady state means the system as a whole needs to radiate out that energy somewhere. More mass gives some kind of ballast I guess but kilowatts of energy is just a lot to get rid of.
  - vlovich123 2y ago
    It's not about ballast. Black body radiation, which I'm sure you know is the only way heat is dissipated in a vacuum (unless you're the Sun & even then it's still the primary way), is defined as: P = ε * σ * A * T^4 That's the constant, the emissivity of the material, the surface area A, and temperature. We want to increase the temperature differential above what the natural black body radiation can dissipate for a given device (let's take a space probe) and assume we can't change the material. The only thing we can play with is surface area. The point of the nitrogen gas is to leverage normal convection / conduction to dissipate into a larger gaseous volume which then naturally also has a larger surface area. This would then give you more surface area to dissipate across. The reason I'm thinking the layering with different pressure gradients might be useful is that it reduces the overall weight because you need less overall gas for a given dissipation profile (i.e. you need high density beside your heat source to transfer energy away quickly but less density further away because there's a larger surface area growing with the square of the distance away from the heat source). [1] https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law
    - mppm 2y ago
      Convection depends on buoyancy and hence gravity, and conduction in nitrogen gas is extremely low, so I don't think your idea would work. You need a conventional radiator with forced circulation.
- tuatoru 2y ago
  Fractal branching structures (trees), which maximise radiating surface area, don't make good pressurised vessels.
- mncharity 2y ago
  Inflatable envelopes in space are challenging (micrometeoroid damage). The envelope, and thus the gas, need to be hot, lest T^4 swamp R^2. It's feasible to cool a reactor to low-pressure gas (Mars atmosphere [1]) using forced convection, but that's cold gas and IIUC the heat exchanger is comparable to the size and mass budgets of a radiator. So attempting a steelman of a set of forced-convection radiators on poles, hosing hot gas against the envelope... hmm. Big picture, this attempts a low-mass heat transfer construct, using a large-scale low-pressure gas flow, where the cold-side surface is nearly as hot as the hot-side, and is extremely mass constrained (thus flat-ish). Seems hard? Fun thought experiment - thanks! [1] Colgan Mass Optimization [...] Forced-Convection Heat Exchanger for Mars Surface Waste Heat Rejection 2023 https://asset.library.wisc.edu/1711.dl/FYJCG5YME7O4X82/R/file-ce70a.pdf https://asset.library.wisc.edu/1711.dl/FYJCG5YME7O4X82/R/fil...

by IanCal 2y ago

vlovich123 2y ago

Fill a surrounding container with a volumetric shit ton of nitrogen gas (perhaps with stepped pressure gradients) and you might be able to radiate a significantly larger amount by increasing your surface area without drastically increasing your weight (or use nuclear power to mitigate the weight concerns). But yes, today’s design space generally prefers as low a power envelope as possible to not have to worry about dissipation.

IanCal 2y ago
Wouldn't the limiting factor be the outer layer? I'm not sure what layering would give you, or what adding a container would give you. For short bursts this may make a difference but the steady state means the system as a whole needs to radiate out that energy somewhere. More mass gives some kind of ballast I guess but kilowatts of energy is just a lot to get rid of.
- vlovich123 2y ago
  It's not about ballast. Black body radiation, which I'm sure you know is the only way heat is dissipated in a vacuum (unless you're the Sun & even then it's still the primary way), is defined as: P = ε * σ * A * T^4 That's the constant, the emissivity of the material, the surface area A, and temperature. We want to increase the temperature differential above what the natural black body radiation can dissipate for a given device (let's take a space probe) and assume we can't change the material. The only thing we can play with is surface area. The point of the nitrogen gas is to leverage normal convection / conduction to dissipate into a larger gaseous volume which then naturally also has a larger surface area. This would then give you more surface area to dissipate across. The reason I'm thinking the layering with different pressure gradients might be useful is that it reduces the overall weight because you need less overall gas for a given dissipation profile (i.e. you need high density beside your heat source to transfer energy away quickly but less density further away because there's a larger surface area growing with the square of the distance away from the heat source). [1] https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law
  - mppm 2y ago
    Convection depends on buoyancy and hence gravity, and conduction in nitrogen gas is extremely low, so I don't think your idea would work. You need a conventional radiator with forced circulation.
tuatoru 2y ago
Fractal branching structures (trees), which maximise radiating surface area, don't make good pressurised vessels.
mncharity 2y ago
Inflatable envelopes in space are challenging (micrometeoroid damage). The envelope, and thus the gas, need to be hot, lest T^4 swamp R^2. It's feasible to cool a reactor to low-pressure gas (Mars atmosphere [1]) using forced convection, but that's cold gas and IIUC the heat exchanger is comparable to the size and mass budgets of a radiator. So attempting a steelman of a set of forced-convection radiators on poles, hosing hot gas against the envelope... hmm. Big picture, this attempts a low-mass heat transfer construct, using a large-scale low-pressure gas flow, where the cold-side surface is nearly as hot as the hot-side, and is extremely mass constrained (thus flat-ish). Seems hard? Fun thought experiment - thanks! [1] Colgan Mass Optimization [...] Forced-Convection Heat Exchanger for Mars Surface Waste Heat Rejection 2023 https://asset.library.wisc.edu/1711.dl/FYJCG5YME7O4X82/R/file-ce70a.pdf https://asset.library.wisc.edu/1711.dl/FYJCG5YME7O4X82/R/fil...