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I am working on a near-hard scifi story that mainly involves travel across a single widely populated solar system. I imagine that realistically there will probably be a wide variety of ship designs, but fuel types will probably have to become more standardized, much like different levels of octane grades in gasoline for cars, or diesel as an alternative all together. Assuming there's still a higher prevalence of more traditional rocket engines (rather than super booster engines fueled by handwavium) what are the advantages or disadvantages of using liquid hydrogen or liquid methane as the base rocket fuel?

I am looking to have at least a vague understanding of how they compare to each other as almost everything I find is comparing each of them to kerosene, which as far as we know can't be made anywhere that doesn't have liquid dinosaurs. I am also imagining a variety of craft that travel both exclusively in vacuum and trans-atmospheric. I'm trying to create a world(s) that feels real and lived in, so there should be all sorts of different kinds of craft. Rockets, space planes, bright red hotrod, or whatever the craft, I feel like part of what makes any of it realistic though is having a reason as to why it functions the way that it does and why one might be chosen over another.

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    $\begingroup$ You'll need different fuels for different things. LH2 is really too bulky for most air-breathing aircraft, but it's a waste to operate nuclear rockets on anything else, and spaceplanes almost require it for cooling and combustion speed. Methane is a much better launch vehicle fuel due to its density, which makes tanks far smaller and improves thrust density. And any decently developed industrial base will allow production of heavier hydrocarbons, for example by the Fischer-Tropsch process. And then there's various room-temperature chemical fuels and ion thruster propellants... $\endgroup$ Jun 15 at 2:35
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    $\begingroup$ Dinosaurs or organic material are not required to make kerosene. Methane and Kerosene are hydrocarbons, so made entirely of Carbon and Hydrogen atoms, kerosene is just made of more complex molecules than Methane. If you have hydrogen and carbon you can make hydrocarbons, so it's not 'realistic' to limit yourself to liquid hydrogen and methane. $\endgroup$
    – GdD
    Jun 15 at 7:42
  • $\begingroup$ If you have a power source you can synthesize the liquid fuels we call fossil fuels. And now the liquid fuel just becomes an energy carrier like a battery, not a primary energy source. The issue is, of course, you need sufficiently abundant energy to do this since some energy is always lost in the transfer which means to do this practically you need an abundant energy source (for example, fusion or lots and lots and lots of solar). If we had fusion power and it was abundant as everyone hopes it is, we would just be cooking up gasoline, diesel, methane, kerosene, etc. $\endgroup$
    – DKNguyen
    Jun 16 at 17:55

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There have been articles on why Space-X chose Methane. Here's a few points. Liquid Methane & liquid Oxygen have boiling points that are relatively close together. That way they can more easily have common bulkheads. Hydrogen's boiling point is so low, it would have to be insulated from LOX. Methane is much cleaner burning than Kerosene because it is a short molecule with only 1 carbon atom. Hydrogen is better in that regard, but not a lot. Hydrogen has a "Pain in the ass factor" because it is much harder to handle. It escapes through the tiniest crack, boils off and causes Hydrogen enbrittlement where metals absorb it and lose ductile qualities. It's so cold it's very dangerous. The biggest thing is pound for pound, hydrogen is the most bang for your buck, but that energy density is offset by its bulk. A pound of hydrogen is much larger than a pound of methane so much bigger fuel tanks are needed for the same amount of energy.

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    $\begingroup$ Common bulkheads were built more than 50 years ago for the Centaur and Saturn V, see history.nasa.gov/SP-4206/ch6.htm So hydrogen and oxygen may have a common bulkhead with thermal insulation. $\endgroup$
    – Uwe
    Jun 15 at 8:15
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    $\begingroup$ Another reason for SpaceX using methane is that it can be produced in-situ on Mars with relative ease using the Sabatier reaction. SpaceX's goal is to build a self-sustaining civilization on Mars, making humanity multi-planetary, which will require shuttling about one million tons of payload delivered to Mars. The only way to do that realistically is by reusing the transportation system, which means launching from Mars and returning to Earth. Which, in turn, is much easier if you don't have to bring your return fuel with you. $\endgroup$ Jun 16 at 8:08
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    $\begingroup$ @Uwe Just because it's possible to put enough thermal insulation between the LOX and LH2 tanks does not necessarily mean it's practical. $\endgroup$
    – Philipp
    Jun 16 at 9:05
  • $\begingroup$ "Hydrogen is better in that regard, but not a lot." ?? One of us is missing something there - one never gets soot from burning Hydrogen alone, and I believe that real-world Hydrogen is almost perfectly pure, whereas hydro-carbons may not be, and it is the impurities in the fuel that cause most of the trouble. $\endgroup$ Jun 16 at 10:57
  • $\begingroup$ Hydrogen embrittlement is a thing, but you can eliminate it by lining the tank with a thin polymer, so that's not a point anymore. The other issues stand, and are likely much more relevant than the weight of the liner. $\endgroup$
    – toolforger
    Jun 16 at 11:14
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It very much depends on the mission, and there are several factors.

If you are doing anything involving nuclear-thermal rockets you REALLY want to try to use hydrogen, the low exhaust mass means you get far greater Isp at a given chamber temperature and pressure. Conversely, hydrogen is a complete pain in numerous ways, including critical things like long term storage, maintaining a tank at say 20 kelvin is hard when your only way to dump heat is by radiation.

Now if you either want a long term storable or a lower stage propellant (Where Isp is less critical) then something like methane makes sense because you can store it at 100 kelvin (Much easier), and the improved density makes the tanks more reasonable.

Places where hydrogen makes sense : upper stages where you want a high impulse burn shortly after launch, nuclear thermal stages providing the duration of storage is not too long.

Places where a light hydrocarbon makes sense : Basically any time you don't NEED to use hydrogen and need too much thrust to make ion engines sane.

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