This answer to Mass ratio of solar-electric versus radioisotope thermo-electric power for propulsion; beyond how many AU do RTGs win? estimates a crossover at about 4.3 AU, so a trip to the outer planets will likely need a self-contained power source like an RTG or similar.

Suppose a future (as yet unspecified) mission wanted to slow down somewhat before a flyby of an outer planet. I'll ask a separate question about propulsion, but this means that it would need much more of its mass for propulsion for a given trip time.

Wikipedia lists the mass of the MMRTG at about 45 kg.

Could a similar design be scaled down in such a way that the mass-specific power output was similar? If a 45 kg RTG can produce 125 Watts at the beginning, could a similarly designed 4.5 kg RTG generate close to 12.5 Watts? Could a 1 kG RTG produce 2.7 Watts?

Answers to Requirements to orbit Pluto describe fairly large spacecraft, I'm asking here about going in the other direction.

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    $\begingroup$ No doubt about thermal power being easy to scale down, that's simple physics, but that may not be true for how much can be recovered as electricity. $\endgroup$ – SE - stop firing the good guys Feb 14 at 7:41
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    $\begingroup$ RTG powered pace makers existed, but would not be producing the power levels useful for a space probe. Probably a key thing here would be safety. Making a large re-entry survivable canister is easier than a small one. If you are assembling in orbit and take off the can to fly just fuel, radiators and the generator plates it is both lighter and probably more efficient electrically. Wonder if the protective shielding mass of the New Horizons RTG is pubic. $\endgroup$ – GremlinWranger Feb 15 at 1:48
  • $\begingroup$ @GremlinWranger I thought about asking "what's the smallest RTG ever deployed in space" as a separate question, but it seems too related to this to ask separately. I'll wait first to see if that question is answered here as part of an example. $\endgroup$ – uhoh Feb 15 at 1:51
  • $\begingroup$ @SE-stopfiringthegoodguys But squared cube law might work against RTGs at smaller sizes. $\endgroup$ – DKNguyen 2 days ago

The plutonium used in RTGs produces a continuous amount of heat through decay per KG of material regardless of size so can be widely scaled. What does matter is that efficient harvesting of heat energy depends on maximising the difference between the hot and cold sides of the system, so a massive single block would melt everything it touched when built, and a very small element would produce too little heat to be effective for power (but useful for pure heat)

So there will be a sweet spot where a given volume of material produces useful levels of heat but does not melt itself during assembly or use. The currently used RTGs are made from sub elements that are listed as 1.44kg and producing 250 watts each with a 600 degree surface stacked to produce the required total power.

Which indicates that a 250W/1.5kg power element is available. The specs listed for this suggest half the overall weight is the elements with rest being radiators and power hardware making a 3kg, 250W heat/16W electric system possible.

Here suggests a figure of 540W/kg for the raw Pu material so that 1.5kg power element is only 1/3 Pu by weight (500g), potentially meaning by accepting more risk a 500g raw PU element with 500g of supporting radiators could produce a 1kg 16W electric power system that nobody would want to go anywhere near.

Going smaller from that size would loose thermal efficiency, going somewhat larger and working closer to the ~1000 melting point of the Pu would improve it, as would pushing conversion efficiency from 16% closer to 27% claimed for the Advanced Stirling generator.

  • $\begingroup$ I don't understand why efficiency can't be preserved for small devices 1 kg and below. Can't one always engineer the thermal coupling to keep the source at the desired equilibrium temperature? Is there some material property that limits this? $\endgroup$ – uhoh Feb 16 at 0:43
  • $\begingroup$ @Uhoh, the available power per junction is directly related to the temperature differential, you can add more junctions but that increases your series resistance into diminishing returns. You also want to avoid losing heat through conduction along the wires, so adding more junctions increases that as well. This does mean advances in materials science giving a highly electrically conductive but thermally insulation pair of materials that produce useful junctions would change the size assumptions. $\endgroup$ – GremlinWranger Feb 16 at 5:22
  • $\begingroup$ I'm probably missing something blindingly obvious, but why can't a sub-kilogram source run at the same temperature as a larger one? Why can't a device be shrunk dimensionally while keeping the same component temperatures? $\endgroup$ – uhoh Feb 16 at 5:25
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    $\begingroup$ @uhoh, you haven't missed anything blindingly obvious - l have - the above only applies if you are building a conventional RTG harvesting energy from all sides equally. If you make a sub 1 KG unit you just reduce the junction area and insulate the remainder to achieve the same temperature differential. Will need to do a major edit since there are still losses at that point but not as described. $\endgroup$ – GremlinWranger Feb 16 at 10:56

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