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Since radioisotope thermoelectric generators (RTGs) operate by turning the flow of heat into electricity, and Titan's atmosphere can absorb heat faster through conduction than a vacuum through radiation, would an RTG (or a set of them) produce enough power on the surface of titan, say, for a manned base?

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    $\begingroup$ If you have a set of RTGs, at some mass of them the heat output increases significantly. We call this a "critical mass".... $\endgroup$
    – Aron
    Jul 26, 2022 at 5:43
  • $\begingroup$ Global warming on Titan could be quite a thing. $\endgroup$
    – fraxinus
    Jul 26, 2022 at 6:35
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    $\begingroup$ What is an RTG? As a casual observer I'd love to know even if I can't provide any help on the topic! $\endgroup$
    – Ed HP
    Jul 26, 2022 at 9:24

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RTGs would be far more effective for the reason you describe: the job of a large radiator could be done with a small heat exchanger, and a larger heat exchanger could get the cold side temperature much lower. Taking the MHW-RTG used on the Voyager missions as an example, the cold junction temperature is ~600 K, an RTG designed for Titan could get it much closer to 100 K.

However, the MHW-RTG already has a Carnot efficiency of (1273-573)/1273 = 0.55. If you redesigned the thermocouples to keep the same hot-side temperature while the cold-side was dropped, you might get that up to ~0.9, but you aren't going to even double the electrical power output, all else being equal. Even if you improved the conversion efficiency independently of the Carnot efficiency, there's only ~500 W/kg of thermal power to work with. You can get a healthy boost to the power budget of a rover or something (provided they don't need even more for heating), but RTGs are still not a good option for heavy power demands like keeping a base running.

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    $\begingroup$ Nice answer. I think it could be improved by defining acronyms (RTG, MHW) the first time you use them. $\endgroup$
    – Dast
    Jul 26, 2022 at 10:12
  • $\begingroup$ I added a mention that the MHW-RTG was the RTG design used on the Voyager probes and linked to the Wikipedia article in case anyone wants to learn more. RTG is now defined in the question and MHW-RTG is just the name of the design, the exact meaning of MHW isn't relevant here. (Or anywhere, really, they just threw some technically-accurate but not terribly descriptive words together to make an acronym.) $\endgroup$ Jul 30, 2022 at 2:34
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would an rtg (or a set of them) produce enough power on the surface of titan, say, for a manned base?

Maybe, but you've forgotten something fairly important.

That very cold, highly thermally conductive atmosphere that's boosting your Carnot efficiency is also going to be sucking a huge amount of heat out of your manned base and all of your surface equipment.

Moon rovers like Lunokhod 1 had to survive the chilly lunar nights, which get down to 140K for a couple of weeks. They used host radioisotopes not for power generation, but to stop them freezing to death, a risk even for electronic equipment at that temperature. The average surface temperature on Titan is under 100K... that's quite significantly colder.

Your base is going to need some pretty significant insulation and plenty of heating especially for anything that ventures out of the main insulated bubble. The savings you make in terms of heat engine power generation are likely to be offset by heat losses from everything you'd like to not die of cold.

It is also well worth remembering that the power density of RTGs isn't great, and their lifetime isn't amazing... their output drops continuously during their life, too. They're great for deep space probes that have to be launched from Earth, because engineering an economical compact lightweight nuclear reactor is a pretty tall order, even without the environmental concerns (real or otherwise) of an accident.

If you can drop an entire habitat on Titan, you won't be mucking about with short-lived RTGs coughing up a disappointing and shrinking amount of power, you're going to send them the real thing, because they'll need it.

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  • $\begingroup$ True that the power output of an RTG is relatively small, but the half life is around 75 years, probably longer than the mission duration. Also great point regarding the effect of cold on the life support needs. Andy Weir touches on this in "The Martian." $\endgroup$
    – Jason
    Jul 26, 2022 at 16:04
  • $\begingroup$ @Jason, the isotope half-life isn't the limiting factor on RTG lifespan. The limiting factor is radiation damage to the thermocouples. $\endgroup$
    – Mark
    Jul 27, 2022 at 0:38
  • $\begingroup$ @Mark part radiation and part thermal diffusion. SiGe thermocouples are made of a solid solution of germanium in silicon, and apparently at high temperatures the germanium slowly precipitates out. Dopant materials can similarly diffuse around and even sublimate out of the junction material. Long-lived junctions could be constructed of more stable materials, but there'd probably be a tradeoff in mass and/or efficiency. $\endgroup$ Jul 27, 2022 at 15:26

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