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@TildalWave's answer to Methane internal combustion engines for rovers on Mars and Moon. Feasibility? raises the possibility of using Titan's atmospheric methane for an energy source as long as you BYOO (bring your own oxygen).

With the advent of the MOXIE demonstration on the Perseverance rover, it's shown that CO2 can be converted to carbon monoxide and oxygen, which could recover half of the oxygen after burning CH4 + 2O2 to CO2 + 2H2O.

Suppose a Titan rover or helicopter used it's RTG for both electricity and as a heat source (as do Mars rovers Curiosity and Perseverance) and suppose to stay warm it used more heat than those on mars which thermodynamically must cut in to the amount of electricity available.

The idea there is that for the helicopter at least the RTG would charge the batteries at night (when there's no light for driving or flying and no headlights) and use high current from the battery for a short flight during the day. Basically the flights are limited by the mass and energy storage density of the space battery chosen, and if it ever got too cold for some reason, or just plain wore out, flight would no longer be possible.

Instead, consider the highly competitive energy density of liquid methane (LNG) and LOX per unit mass compared to space-qualified batteries. Titan is darn cold, so LOX might be easy to maintain and methane could be captured and stored as ice.

All you need to do (understatement) is capture the combustion products, separate out the CO2 and H2O and recover the oxygen via MOXIE-like processing of one and electrolysis conversion of the other.

Question: Could a closed oxygen cycle methane-burning internal combustion engine for Titan rover/helicopter works like a battery together with RTG and have a much higher energy density for longer and/or more frequent power-delivery sessions than a traditional space battery? If so, what are the most challenging challenges to making this work? Are there additional benefits?

note: Having a fuel and oxidizer can not only run an internal combustion engine for propulsion, it can power an electrical generator if you need higher instantaneous current than the RTG can generate, and the two could be fed to a fuel cell for direct conversion to electricity at any time!

Fred
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uhoh
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  • It'd be an extremely inefficient way of transferring power, with both the moxie and combustion engine/fuel cell having enormous losses. The point about the cold isn't really too important either because even if the 'batteries' don't need insulation, I'm pretty sure there are plenty of other electronics that will need to stay warm. In saying this, maybe the weight reduction would still make this favourable, however good luck scaling it down for something like ingenuity. – R. Hall Jun 15 '21 at 06:43
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    @ReubenFarley-Hall if you can do some fair comparisons in this context and put some actual numerical values to your "extremely inefficient" and "enormous losses" that could be developed into an answer. Until then I have no reason to believe you. One person's extreme is another persons's good enough; one person's enormous is another person's acceptable. – uhoh Jun 15 '21 at 09:04
  • @ReubenFarley-Hall for example the MMRTG converts 2000 watts of thermal power to 125 watts electrical power; only 6% efficient. Is it "extremely inefficient"? Compared to a Stirling engine, perhaps. But compared to anything else with no moving parts and a solid proven deep-space track record, it's excellent! These kinds of exclamations are not at all helpful unless they are quantitative and make direct comparisons to alternatives – uhoh Jun 15 '21 at 09:12
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    This seems to be a special case of a redox flow battery, see https://en.wikipedia.org/wiki/Flow_battery. But the use of gaseous methane and oxygen is a problem. Only liquids within the cycle would be easier. – Uwe Jun 15 '21 at 09:23
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    @Uwe check out the ambient temperature of Titan; it may not be a problem keeping them as liquids, but at low temperature some reaction rates might be a lot slower depending on the details. – uhoh Jun 15 '21 at 09:25
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    A simpler and likely much more efficient option would be a methane phase change engine...a methane-boiling steam engine. It wouldn't be as useful for providing bursts of power beyond what the RTG can provide, but you could get a lot more power density out of a radiothermal heat engine by boiling methane than, say, by running a Stirling engine. – Christopher James Huff Jun 16 '21 at 00:54
  • @ChristopherJamesHuff let's see if I understand; using an RTG as a heat source for basically a steam engine (where the phase change material is methane instead of water) will provide a larger burst of power for a given total mass added to a spacecraft than a Stirling engine? While perhaps lower in overall thermodynamic efficiency, if you need to "gun it" and power a two minute flight, then "burst of power" does sound attractive! Maybe a more general question about proposed forms of generating mechanical power from an RTG on a cyroworld is a good separate question. – uhoh Jun 16 '21 at 01:47
  • @ChristopherJamesHuff as would be one about proposed forms of storing power for mechanical use from an RTG; "Alternatives to traditional space batteries". But I don't think a steam engine can actually be more thermodynamically efficient that a Stirling engine, can it? (I really don't know, I break out in a cold sweat when I think about thermodynamics) – uhoh Jun 16 '21 at 01:49
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    A steam engine won't be more efficient, but it can be a lot smaller than a Stirling engine of the same power output. You would be sacrificing some of the power for a significant reduction in power system mass...a trade of interest when you're not just powering instruments as you coast through space, but actually powering your motion through a planet-like environment. – Christopher James Huff Jun 16 '21 at 03:04

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