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The setting I'm trying to build is perhaps best described as "Mad Max in Space" - except civilization hasn't collapsed and tanks and airplanes are still a thing.

  • Planetary colony established for ca. 250 years
  • Mostly cut off from earth (interstellar flight = super rare)
  • Thus, colony mostly utilizes resources found on the planet
  • There's a big war going on: easily fixable low-tech machines rule

Now I'm wondering how those air and ground vehicles might be powered.

I could just explain that the colony planet once had native life and oceans that provided the environment needed to produce oil, identical to earth, but is that even plausible?

What is the next likely fuel source that could be used in fighting vehicles that are maintainable + fixable by average Joes?

  • Nuclear fission is out. Too dangerous when vehicles are damaged.
  • Electricity + simple lead acid batteries probably can't hold enough power
  • Could natural gas occur without historical extraterrestrial life?
  • Hydrogen fuel cells sound okay, pressure explosions could be acceptable

Additional info:

  • Whether the planet was terraformed for the colony is still open. It would help if it was /lightly/ terraformed (i.e. habitable temperature range, only some oxygen-producing bacteria were needed)
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  • $\begingroup$ Why are hydrogen explosions acceptable, but nuclear is too dangerous? $\endgroup$
    – Logan R. Kearsley
    Commented Jul 18, 2020 at 18:38
  • $\begingroup$ @LoganR.Kearsley I assume because of the longevity of radioactivity. There is a war going on after all and vehicles are going to be blasted. You don't want a bunch if potential dirty bombs running around being targeted by guns if your planet is terraformed or fighting in a domed colony. $\endgroup$
    – DKNguyen
    Commented Jul 18, 2020 at 18:53
  • $\begingroup$ Yes, that was my thinking. I'm assuming that hitting a reactor would either shatter the fuel and spread radioactive pieces around or cause a meltdown where the fuel melts into the ground and contaminates the groundwater. $\endgroup$
    – Cygon
    Commented Jul 18, 2020 at 20:17
  • $\begingroup$ @Cygon You cannot cause a meltdown by crashing into a reactor. If the fuel is not contained, the chain reaction will naturally stop and the fuel materials cool down. Meltdowns can only occur in outdated industrial power plant designs which could continue to sustain a reaction after control systems entered fail-deadly state. $\endgroup$
    – Logan R. Kearsley
    Commented Jul 18, 2020 at 20:46
  • $\begingroup$ @Cygon Shattering and scattering the fuel pellets is a more realistic concern, but it is not hard to design a vehicle-scale reactor casing that makes that risk negligible. Nuclear batteries designed for spacecraft, for example, can survive uncontrolled re-entry and meteoric impact with the ground with no risk of environmental contamination. $\endgroup$
    – Logan R. Kearsley
    Commented Jul 18, 2020 at 20:47

4 Answers 4

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Did you know there are terrestrial bodies in this solar system with oceans of methane? Namely, the moons of the gas giants. Just add oxygen which you need anyways.

Of course, as you already mentioned there is also hydrogen from the gas giants. You can use that in fuel cells or you can just burn it. You need oxygen either way.

You never did say if the colony planet was terraformed or not.

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  • $\begingroup$ I'm still open on whether the colony used terraforming. Getting hydrogen from extraplanety sources and using it to power things and release oxygen into the atmosphere sounds great, though! $\endgroup$
    – Cygon
    Commented Jul 18, 2020 at 18:14
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    $\begingroup$ @Cygon Burning hydrogen or methane for power will not release oxygen into the atmosphere. In that sort of environment, you would still need to acquire oxygen as part of your fuel. $\endgroup$
    – Logan R. Kearsley
    Commented Jul 18, 2020 at 18:40
  • $\begingroup$ @LoganR.Kearsley Oops, you're right. I had it backwards, hydrogen is combined with oxygen to release energy and produce water. Oxygen would be released when producing hydrogen from water, though (but I guess it'd require a /lot/ of hydrogen storage and a large ocean to enrich the atmosphere with 20% oxygen). $\endgroup$
    – Cygon
    Commented Jul 18, 2020 at 18:48
  • $\begingroup$ @Cygon I guess you would need to dump water in from elsewhere and spend energy to electrolyze it. Or dump CO2 in from elsewhere and use energy to crack that. If you're synthesizing fossil fuels then you could re-use that carbon and combine it with hydrogen. Either way, you're going to need a really abundant primary energy source since you're paying double or triple to produce and burn stuff. $\endgroup$
    – DKNguyen
    Commented Jul 18, 2020 at 18:49
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    $\begingroup$ @Logan R. Kearsley: In a methane atmosphere (Titan, for instance) oxygen would BE the fuel. $\endgroup$
    – jamesqf
    Commented Jul 19, 2020 at 17:44
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Good ol' alcohol (either methyl or ethyl) distilled from local organics on the colony.

  • Easy to make, easy to store, easy to burn, relatively easy to build engines for.
  • Nice and boom-y when the dune buggies go careening off the cliff.
  • If you make ethanol right, you can drink it. It's poisonous...but when has that ever stopped anybody?
  • Methanol has higher energy density, but is also more toxic.
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    $\begingroup$ Right on! Mr. Average Joe could certainly manage to ferment and distill whatever the colony has for grapes. And it's even lower tech than hydrocarbon or hydrogen production. Alcohol probably wouldn't have covered the colony's energy needs pre-war, but it makes sense that for specialty uses (dune buggies, drones?) alcohol-powered combustion engines should exist. $\endgroup$
    – Cygon
    Commented Jul 18, 2020 at 20:06
  • $\begingroup$ @Cygon: Alcohol (or biodiesel, or other biofuels) would have HAD to cover the colony's needs, if there were no native fossil fuels. It's certainly technically possible, as Brazil runs a large part of its vehicle fleet on biofuel: en.wikipedia.org/wiki/Ethanol_fuel_in_Brazil You could also have electric vehicles using PV, hydro, wind, geothermal, or nuclear electricity. $\endgroup$
    – jamesqf
    Commented Jul 19, 2020 at 5:44
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If their technical level is up to the challenge, solar, tidal and geothermal energy can be captured and transformed into electrical energy which can then be used to synthesize combustible hydrocarbons from base elements.

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  • $\begingroup$ You could use any energy source you have in sufficient abundance, including fission or other dangerous sources not suitable to be put into mobile platforms. The fuel essentially becomes more like a rechargeable battery which is just a container for energy rather than a primary energy source like how we use fossil fuels right now. $\endgroup$
    – DKNguyen
    Commented Jul 18, 2020 at 18:01
  • $\begingroup$ Producing hydrocarbon (or hydrogen) does fit my plot if the produced fuel is trivially and widely available (which is likely, assuming farming machines & transportation). However, focus of most battles would then logically be on control of those hydrocarbon/hydrogen factories (and their connected power plants). Hrm... $\endgroup$
    – Cygon
    Commented Jul 18, 2020 at 18:34
  • $\begingroup$ @Cygon, if you want to break the focus of battle away from the factories and power sources, just make the fuel creation process require a specific rare catalyst. Platinum might be a good choice. Now as the enemy troops are breaking through the outer defenses, the heroine can grab the one critical component, a shiny rod of pure platinum, and escape. In taking the critical component away from the initial battle location, she can lure the enemy away from civilians, saving the day while continuing the conflict in a location with less potential collateral damage. $\endgroup$
    – Henry Taylor
    Commented Jul 18, 2020 at 18:51
  • $\begingroup$ continued.from previous comment.. The machines to make fuel and even the electrical power to make it from might be commonplace due to the ubiquitous need for fuel when the colony was young. But the platinum catalyst rods might be scarce due to some mysterious earlier event, leaving the survivors to fight for the few rods which still exist. $\endgroup$
    – Henry Taylor
    Commented Jul 18, 2020 at 18:52
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Algal based bio-diesel and/or alcohol Since you hinted the colonist bought bacteria with them why not certain specific strains of algae?

Research is being conducted now into various strains of algae that can produce a range of fuels including butanol, diesel, ethanol and aircraft fuel.

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