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I’m interested in realistic and practical fuel or power sources for vehicles (particularly military ones), where there are no fossil fuels available. The setting is 400 years in the future. The only hand-wavey bit is FTL travel to nearby star systems, to explore and colonise. Most of the planets discovered are lifeless rocks without liquid water, so they have no alien coalfields or alien oilfields to exploit.

My question is similar to this one but I’m interested in more than main battle tanks.

The types of places in the setting are:

  • Mars. Colonised 350 years ago. Terraforming has begun, but you can’t get far in 350 years, so most of the settlements are domed cities, para-terraformed sections of canyon or burrowed into rock.
  • Exoplanet X. Venus sized but without the runaway greenhouse effect. No liquid water because it is outside the Goldilocks zone – an orbit a bit further out than Mars’. No terraforming even attempted.
  • Earth. All the fossil fuel sources worth exploiting have been found, extracted and used up. However, compared to other less hospitable planets, Earth folks can plant huge tracts of land with palm oil plantations for biodiesel, sugar cane for ethanol, etc. And they still have plenty fissionable & fusionable (is that a word?) material.

Any planet can have vats of microbes churning out interesting hydrocarbons - if that is genetically and biochemically feasible?

Ignore electricity generation UNLESS it relates to transport (e.g. charging batteries or electric trains). Just assume for everyday electricity needs there are both fission and fusion reactors all over the place, and green energy sources on planets which have the appropriate ingredients (such as oceans and moon for tidal power on Earth). But in a war situation powerlines may be down… the enemy may hold the power stations and be denying your bit of the national grid any power… or you might have to cross an empty area of the planet which wasn’t on the grid to start with…

The types of vehicles to consider power needs for are:

  • Any and all planet-based military vehicles from infantry fighting vehicles to artillery to aircraft.
  • And not exactly vehicles, but also consider self-propelled weapons which thus require their own power, e.g. missiles.
  • Civilian city to city transport – by road, rail, air or sea.
  • Surface to orbit transport (military and civilian)

What fuels or power sources are feasible and practical? Especially for the military.

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    $\begingroup$ "All the fossil fuel sources worth exploiting have been found, extracted and used up." At this point I'd suggest fleeing Earth if there's a reasonable alternative. This would lead to absolutely devastating climate change. It's also rather unlikely to happen, unless Trump can asexually reproduce to other country's presidencies. $\endgroup$ – Feyre Nov 13 '16 at 12:58
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    $\begingroup$ @Feyre They've had 4 centuries to use it up and 4 centuries to sort out efficient carbon capture technologies to cope with the emissions. Also Earth has been so warm it hasn't had ice caps for lots of geological history. Polar bears will become extinct and coastal cities will be drowned, but it is not a "flee the planet" catastrophe. If they can terraform Mars, they can terraform Earth back to "factory settings". $\endgroup$ – DrBob Nov 13 '16 at 18:01
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    $\begingroup$ If there is a sun in range, then solar. If fission/fussion is so common there is no reason not to have each smaller region have its own. Certainly, civilian city to city is electric cars and above. Many, but not all could be converted to run off electricity, $\endgroup$ – cybernard Nov 13 '16 at 18:54
  • $\begingroup$ Carbon capture gives you the "fossil" fuels back (unless you're talking about the most primitive "put the carbon oxides/carbonates back deep underground). In fact, there's no reason why we couldn't capture all the carbon dioxide produced and turn it back into useful fuel with a bit of solar power - really, wood and other biofuels are just a very roundabout way of doing exactly that. It's not like the raw materials disappear - the only thing lost is energy, and we're getting tons of that from the Sun. $\endgroup$ – Luaan Nov 13 '16 at 19:36
  • $\begingroup$ @Luaan "A bit of solar power"?!? Fossil energy has been collected from the sun over a half a billion years, and sufficed for only say 200-300 years, so... good luck with that. Forget about cheap Joules giving you cheap tractors for the biofules or cheap factories of photovoltaics or cheap wind turbines. The Joules will get insanely expensive and so everything else will get insanely expensive. $\endgroup$ – kubanczyk Nov 13 '16 at 22:46
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Different power sources for different purposes.

  • Biofuel, including alcohol and oils.
  • Hydrogen, cracked from water using some source of electrical power.
  • Batteries, again recharged from electrical power source.
  • Electrical contact power, like trains using overhead power, and also induction power for maglevs.

Combat vehicles might run on biofuel, it is relatively easy to handle and store. Imagine big tanks of the stuff, possibly underground.

Civilian vehicles might go for hydrogen. Again there will be tank farms to even out seasonal changes in demand, but those could be vulnerable.

For missiles and artillery propellant, it would be highly refined propellants, possibly based on biofuel. Low amounts in absolute numbers, so it can be stored.

Surface to orbit will be difficult, and the quality of your engines and fuels will greatly affect the setting. If it is hydrogen/oxygen or refined biofuel/oxygen, you will have giant spaceports to put a few tons into orbit.

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    $\begingroup$ Biofuel is extremely inefficient. Plants convert around 1% of the light energy to biomass. $\endgroup$ – Karl Nov 13 '16 at 13:47
  • $\begingroup$ @Karl, biofuel could be easier to handle than hydrogen. And the military doesn't need much, compared to the civilian economy. $\endgroup$ – o.m. Nov 13 '16 at 14:35
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    $\begingroup$ @Zxyrra, one could recover the water in a closed cycle fuel cell, and run it through electrolysis again. $\endgroup$ – o.m. Nov 14 '16 at 6:01
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    $\begingroup$ @John Five percent is a blatant lie, and very few plants can reach 2%. Or do you have any reliable resource that says otherwise? en.wikipedia.org/wiki/Photosynthetic_efficiency (I do not know what they've dreamed up with the sugarcane in that article. The references given for that number are of very bad quality. The mere idea that it's four times better than other plants is ridiculous.) $\endgroup$ – Karl Nov 14 '16 at 7:26
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    $\begingroup$ @Zxyrra Of course you would. You only lose the binding energy - that's why the process needs an energy input. The use of hydrogen here is for energy storage, not as an energy source. It's just that even with modern technologies, it's a pretty decent energy storage system. The point is that the fuel cell is basically a battery, and closed-cycle hydrogen cells like that already exist. They're not magic. Your "cracking hydrogen from water" has exactly the same problem - even with 100% efficiency, you gain exactly as much energy from the oxidation as you lose to lysis. $\endgroup$ – Luaan Nov 14 '16 at 11:26
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Oxygen Fuels

Require oxygen (in the atmosphere or stored) to burn.

Hydrogen

Hydrogen is easy to produce - just crack water, but it has low density and needs heavy and expensive storage tanks. High efficiency when used as rocket fuel.

Methane

The Sabatier reaction (CO2 + 4 H2 → CH4 + 2 H2O + energy) can be used to generate methane from hydrogen and carbon dioxide (from the atmosphere), producing methane and water (which can be cracked into hydrogen and reused). Can be used as fuel on mars (95.9% of mars's atmosphere is carbon dioxide). Still needs pressurized/cryogenic tanks, although less so than hydrogen.

Liquid Hydrocarbons

The Reverse-water-gas-shift reaction (CO2 + H2 → CO + H2O) followed by the Fischer–Tropsch process ((2n + 1) H2 + n CO → C(n) H(2n+2) + n H2O) can be used to synthesize liquid hydrocarbons. Water can be recycled back into hydrogen.

Batteries

Lithium-Air

Lithium-air batteries have the highest possible specific energy of up to 12 kwh per kilogram (close to gasoline at 13 kwh/kg), and are rechargeable. They do require air to operate though, although it can be stored.

Beam-Powered Propulsion

Beaming power (either using lasers or microwaves) could be used for surface to orbit craft. The advantage is that the craft does not need to carry it's own power source. However, it is limited by the need for line-of-sight with the transmitter. Probably anything large enough to carry a beam receiver can carry a fission or fusion reactor.

Lightcraft

A laser beam reflects off a parabolic reflector on the spacecraft, focusing it onto a small area, causing the air to suddenly expand with each pulse, propelling the craft. Could be used for surface to orbit launch or large civilian aircraft in range of a laser installation.

Laser Thermal Rocket

A laser beam is focused onto propellant, heating it, the propellant would probably be hydrogen. Would probably be combined with Lightcraft propulsion.

Microwave Electrical Transmission

Transmitting electricity using microwaves, could be used for powering large ships from an orbital transmitter, or for wireless electrical power transmission of small craft using a rectenna.

In Summary

For small craft (drones, etc.), Batteries or Microwave Electrical Transmission would be used. For large craft, either beamed power or internal fission/fusion reactors. Bridging the gap would be liquid hydrocarbons (or maybe methane), with stored oxidizers (possibly using a fuel cell). And batteries with high energy densities or hydrogen for civilian crafts (as it would probably be more efficient to charge a battery or crack hydrogen than to synthesize hydrocarbons).

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  • $\begingroup$ Good call on the oxygen fuels needing oxygen. I had totally forgotten to take that into account. D'oh! $\endgroup$ – DrBob Nov 14 '16 at 13:19
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In a world with FTL ... Main battle tanks use atomic (fusion/fission) power of course, or superbatteries for green-minded states. Best rechargeable batteries today have an energy density of about 0.8 MJ/kg, compared with 48 MJ/kg (60× higher) for diesel fuel or 24 MJ/kg (30× higher) for ordinary coal; by the time we invent an FTL engine, batteries may well reach energy densities comparable with fossil fuels. Thirty years ago best rechargeable batteries had energy densities around 0.2 MJ/kg.

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Probably you will have lots of CO2's in your atmosphere but it lacks materials to be turned into hydrogen. If this is the case, using electricity, it is possible to separate CO2 into C and O. Pure carbon can be used as fuel in the presence of oxygen. This might be inefficient but can be done without the need of organisms. Probably in time, the necessity will interest scientists and engineers to build converters with better conversion efficiency. It will still not burn as good as petroleum products.

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for small vehicles and rockets, biofuels including methane are very feasible, there is a fungi that manufacture diesel a engineered form of it could do it easily. A bacterial vat used to turn waste into fuel is possible. better yet you can tailor your production instead of using fractionation. on Mars the Sabatier reaction is what you want.

for trains, transport infrastructure nuclear and solar are solid options.

as for orbital transport space elevators or launching railgun will drastically reduce the need for fuel since they can use the electrical grid.

few vehicles will work well on more than one planet, just due to the atmospheric and temperature differences.

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Frozen Methane

You can mine it from Earth's oceans (in the form of methane clathrate) and transport them to Mars etc. Or you can simply collect frozen/molten methane from the outer reaches of our solar system (or other solar systems, beyond the freeze line).

It is also highly likely that 500 years in the future, we would have devised a method to produce methane from an endothermic reaction between carbon and hydrogen. The reaction can be carried out quite slowly with current technology. With passing time, it would get easier and easier to speed the process up.

In this case, you would have huge factories running on nuclear power, producing and freezing methane, which is then sold on their chilled outlets, where chunks would be directly inserted into the vehicles' fuel tank, where parts of them would be gradually sublimated into gas form and burned as fuel.

Frozen Acetylene

Acetylene ($C_2H_2$) has an even higher energy yield than methane. Freezing acetylene and then using it, instead of methane, would be even better and energy-efficient choice. However, production of large quantities of acetylene is difficult unless exhaustible supplies (like Calcium Carbide) are used.

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  • $\begingroup$ To be fair, methane clathrates are a fossil fuel. It's just one that we aren't using yet, since oil and coal is much cheaper. Presumably, the OP's "fossil-fuel-less world" would have no significant reserves of methane clathrates. But yes, using stuff like this as energy storage, rather than an anergy source would make plenty of sense. $\endgroup$ – Luaan Nov 14 '16 at 11:31
  • $\begingroup$ Methane clathrate is not fossil fuel since it is not made from the fossils of any animals. Instead it is released by benthic marine creatures. $\endgroup$ – Youstay Igo Nov 14 '16 at 12:25
  • $\begingroup$ Well, that's not a definition of fossil fuels I've ever heard before (and I assume you include fossils of plants and microorganisms) :) Wikipedia even lists methane clathrates explicitly as fossil fuels, as do plenty of other studies and papers. Both are just solar energy stored by plants and animals over geologically significant times. Some even include nuclear fuel in fossil fuels (since it's not renewable) - it's not like definitions like this aren't stretched all the time. But I'm not here to argue about definitions - the OP must say whether methane clathrates are available or not. $\endgroup$ – Luaan Nov 14 '16 at 12:53
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If you've got the energy budget to do FTL, then you've got the energy budget to crack and reform atoms (from rock, for example) into any other useful atoms, like carbon, hydrogen and oxygen. Fuse the C & H and presto: hydrocarbons!

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  • $\begingroup$ We can already do this today. The problem is where you get the energy to do that - it's not like you can turn carbon dioxide and water into hydrocarbons and back into carbon dioxide and water while also extracting work from the process. Conservation of energy and all that. $\endgroup$ – Luaan Nov 14 '16 at 11:32
  • $\begingroup$ @RonJohn Thanks. I'm not sure how you do an energy budget for FTL, since it doesn't exist. Making the leap to hyperspace or opening a wormhole or whatever, may turn out to be cheap. (Building the spaceship will still be expensive!) So I think your solution is a bit high tech for what I'm after. I have however always liked the idea of smashing 3 iron atoms and 1 hydrogen together to make a gold atom! :-) $\endgroup$ – DrBob Nov 14 '16 at 13:29
  • $\begingroup$ @DrBob nonono, let it be cheap FTL, but building ship will also be cheap. Because I just say so and see how to implement. One of them is enslave other galaxy for them to produce those FTL ships. Do not get me wrong is is not only way I see, and least interesting one as for my taste. $\endgroup$ – MolbOrg Nov 14 '16 at 17:45

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