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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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You are confusing energy generation with energy storage. Solar, RTG, and nuclear are means of energy generation for a space vehicle. Batteries and CH4/O2 are means of short-term energy storage. So the comparison would be between using batteries vs. CH4/O2, including all the associated equipment (tanks, pressurization or cryogenic maintenance, etc.) and inefficiencies (conversion of energy from solar/nuclear to CH4/O2, conversion to mechanical with an ICE). –  Mark Adler Mar 16 at 23:57
    
You are also confusing the means of conversion of energy to mechanical motion with speed. Electric vehicles can be just as fast or faster, and can convert energy more efficiently. (Take a Tesla for a test drive and floor the accelerator to see what I mean. I have, and it's quite the experience.) In fact, it would likely be more efficient to use fuel cells to combust the CH4 and O2 to make electricity and use that to drive electric motors, than to use an ICE. The main reason you don't see a lot of cars with fuel cells is that they're expensive. (But I have seen one.) –  Mark Adler Mar 17 at 0:00
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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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Would cooling be much less of a problem for powerful electric engines? Would a Tesla car overheat on the Moon? –  LocalFluff Mar 16 at 17:17
    
@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. –  TildalWave Mar 16 at 17:26
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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. –  Peteris Mar 16 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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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. –  LocalFluff Mar 16 at 17:08
    
@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. –  TildalWave Mar 16 at 17:20
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@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). –  David Hammen Mar 16 at 17:33
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