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It seems possible to produce methane and oxygen from the atmospheric CO2 and icy soil on Mars, and from icy regolith in the polar craters of the Moon. It has been suggested that CH4+O2 could be used not only as rocket fuel, but also for internal combustion engines (ICE) in rovers and other industrial machinery on the Moon and Mars. The advantage over solar- or RTG-powered electric engines would be that an ICE can deliver great effect relative to the mass needed for a system. Electric rovers are slow without pre-charged batteries or nuclear reactor respectively, and heavy with them onboard.

But wouldn't an ICE overheat in the vacuum of the Moon? Would the thin Martian atmosphere be enough to cool a powerful car-sized ICE? Or would a methane and oxygen combustion engine have a completely different design than a conventional gasoline ICE, maybe with a hot gas exhaust like a rocket?

Non heat related comments on the feasibility of locally supplied "methane rovers" on the Moon and Mars would be welcomed too.

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Convective heat transfer within tenuous atmospheres wouldn't work, lunar exosphere is near vacuum, and it would be fairly limited on Mars with its average of ~ 0.6% mean sea-level Earth's atmospheric pressure, so yes, ICE blocks would have to be redesigned to either facilitate fuel and oxidizer also as a closed-loop liquid coolants (and also preheat them in the process to improve combustion), or use separate coolants (e.g. there plenty of dry ice precipitate in and above Martian regolith, so that's a lot of solid form CO2). Whichever option would be used, these coolants would be transporting excess heat into likely large and heavy radiators, losing heat predominantly via thermal radiation alone. But since you need to pump oxidizer to the ICE on top of fuel, they would have to be redesigned anyway.

So designs might be substantially different to what we're used to here on Earth, possibly depending on how you store your oxidizer (cryogenic LOX?), how much heat you're producing, and where else in the system you might want to use it. Mars obviously has an advantage in cooling ICE blocks with still some non-negligible atmospheric pressure and an average temperature at ~ -55°C, but that might also mean that you'd have to first heat the system up before even starting it. Excess heat could as well be redirected to heat up the cabin space, or otherwise made useful. But designs would be completely different for each celestial, even just local environment on any of them. For example, on Titan, you'd really only require the oxidizer, since there's plenty of methane in its lower atmosphere.

Our ICE designs would adapt and evolve, like they have here on Earth for all kinds of environments, from dry and hot deserts, to arctic conditions. How? Well, I expect lots of new innovations, and some of them will be written in big letters in the history of any of the new worlds we'd colonize. And these innovation processes have already started, for example, Wickman Spacecraft & Propulsion Co. (WSPC) developed a way to directly burn Martian atmospheric CO2 with their Mars Jet Engine. Not an ICE, but there might be others with goals of developing an air-breather ICE suitable for Mars. The race to win the best design has barely started. Would they use CH4 + O2, Trisilane Si3H8 + CO2, something else entirely? Who knows ...

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  • $\begingroup$ Would cooling be much less of a problem for powerful electric engines? Would a Tesla car overheat on the Moon? $\endgroup$
    – LocalFluff
    Mar 16 '14 at 17:17
  • $\begingroup$ @LocalFluff Oh yes, most certainly. It's simply not designed to lose all that excess heat via radiation alone and doesn't have sufficient radiator surfaces. But perhaps more importantly, physics of the whole thing changes (gravity, drag, diurnal temperature cycles, terrain,...) and engines we use here are designed for a completely different environment. It would be like taking an engine that's designed to work in water and run it in the air. Well, even more different. $\endgroup$
    – TildalWave
    Mar 16 '14 at 17:26
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    $\begingroup$ Cooling is much less of a problem for powerful electric engines - roughly speaking for each watt of useful power, an efficient internal combustion engine produces 4 watts of wasted energy, while an efficient electric engine would make just 0.1-0.2 watts waste heat. $\endgroup$
    – Peteris
    Mar 16 '14 at 19:09
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Who cares about going fast? The primary concern is just going. Going fast? Those Mars rovers, for example, are anything but fast.

The primary advantage of a methane-powered rover over a solar-powered rover is that the methane-powered rover can run at night. This advantage pretty much disappears for a solar-powered rover operating in the nearly perpetually-lit polar regions of the Moon. Burning methane and oxygen might be a handy backup to solar power in case the rover accidentally drives itself into one of the nearly perpetually-shadowed areas in the same locale.

The primary disadvantage of a methane-powered rover is that this is "eating your seed corn." The methane and oxygen used to power that rover would be extremely valuable if only they weren't used to move the rovers around. Initially (and probably for a long time to come), it will make more sense to apply those precious resources to other uses than to use it to make a fast go-cart.

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  • $\begingroup$ If you land geologists on the Moon or on Mars, you want them to use their exploration time efficiently. Their manhours are too expensive to waste in an electric golf cart which moves slower than walking speed. And robotics might need combustion power to do heavy lifting or drilling. Until we can land lokomotive sized electric engines there. $\endgroup$
    – LocalFluff
    Mar 16 '14 at 17:08
  • $\begingroup$ @LocalFluff One other thing with going fast is also that it might not be too safe to do that. We're likely talking of untraversed landscapes here, with e.g. dry ice pockets bursting in geyser-like fans during Martian springs, non-settled land collapsing, off-road terrain with sharp non-weathered rocks, well hidden in regolith lava tubes and cave chimneys, and so on and so forth. If you want fast, then by air would be likely the best way to do that. Heavy industry machinery can as well go slow and rather deliver with more certainty. $\endgroup$
    – TildalWave
    Mar 16 '14 at 17:20
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    $\begingroup$ @LocalFluff - You are creating a false dilemma. Electric vehicles can go pretty dang fast these days. A rover that carries people by necessity won't go nearly as fast as a Prius. Safety concerns take precedence over productivity concerns. With regard to landing a locomotive-sized engine: That's a problem for future generations to solve. We can't do that, and we won't be able to do that for decades. Trying to second guess the future is a task for science fiction authors and futurologists (aka science fiction authors). $\endgroup$ Mar 16 '14 at 17:33
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We only need to generate electricity. You can calculate the amount of energy and change the amount of displacement per cylinder. But generating electricity would be more beneficial than direct drive systems. If solar and wind go down, use the "gas" generator as backup...

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  • $\begingroup$ To a certain extent this makes sense; a similar thing happens with very heavy duty surface mining trucks. All heavy duty mining trucks have a diesel engine; the biggest ones use a locomotive engine. Up to a certain size of truck, based on tonnage, the drive system is a conventional engine, gear box, drive shaft. Over that size, mechanical properties of metals aren't strong enough to allow this arrangement. Instead, the biggest trucks have the locomotive engine power an electric generator which then powers electric motors in the hubs of the rear axles. ... $\endgroup$
    – Fred
    yesterday
  • $\begingroup$ ... If weight constraints allowed, an internal combustion engine, or a small gas turbine, on a rover could be used to power a small electric generator that could then power electric motors on the rover. As the answer suggest, it could be used as a back up system or as a night time driving system. $\endgroup$
    – Fred
    yesterday
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SpaceX has already developed a CH4/O2 ICE: the Raptor. It was a very difficult job since stoichiometricaly efficient CH4/O2 combustion produces temperatures above the melting point of available materials. The solution was a very complex staged combustion design where mixture was initially very rich (or very lean) to keep combustion temperatures down. The partially combusted exhaust was then combined for final stoichiometricaly balanced combustion.

Air-breathing hydrocarbon-burning ICEs (like auto engines) don’t need to deal with this complexity since air is 80% N2. This lowers combustion temperatures. Air-breathing H2-burning ICEs do have combustion temperature challenges, but in terrestrial applications this manifests itself as NOX pollution.

A CH4/O2 ICE on Mars will need to deal with the combustion temperature issue. It could be dealt with by the same strategy as the Raptor: staged combustion. A three cylinder engine could have 3 different mixtures. One would be CH3-rich, the second O2 rich. Exhaust from each would feed into the third cylinder for completion of combustion.

Cylinder timing and displacement could be chosen for thermodynamic optimization.

Of course, there is still all that heat to dump. There is no atmospheric convection available. Heat pipe convection with finned aluminum heat pipes would likely be a light, cheap, reliable option.

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doable, but with what oxygen? Martian atmosphere isn't very thick or oxygen rich. An air breathing engine simply doesn't work. On the other hand if it carried it's own oxidizer like liquid oxygen, etc.

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