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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!

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    $\begingroup$ @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. $\endgroup$
    – uhoh
    Commented Jun 15, 2021 at 9:04
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    $\begingroup$ This seems to be a special case of a redox flow battery, see 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. $\endgroup$
    – Uwe
    Commented Jun 15, 2021 at 9:23
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    $\begingroup$ @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. $\endgroup$
    – uhoh
    Commented Jun 15, 2021 at 9:25
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    $\begingroup$ 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. $\endgroup$
    – Christopher James Huff
    Commented Jun 16, 2021 at 0:54
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    $\begingroup$ 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. $\endgroup$
    – Christopher James Huff
    Commented Jun 16, 2021 at 3:04

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