Would aliens on a entirely different planet to earth still use the same rocket fuel as us? If not what are the alternatives?

The planet has an escape velocity of 17 km/s, an atmosphere roughly composed of 25% sulfur dioxide, 50% nitrogen, 6% methane and 6% hydrogen sulfide, and is 55 °C. It also has a pressure of 11.98 atm and an air density of 5.5 kg/m3.

What fuel could be used for rocket engines and would they use the same engines and fuel as us?

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    $\begingroup$ John Drury Clark, Ignition! or, An Informal History of Liquid Rocket Propellants, 1972. Never dull, fun and very informative. Highly recommended before asking such questions. The point being that this site expects questions to be asked only after performing routine research; and if rocket fuels are of interest for the story you are developing there is no way that you didn't find out about Ignition. Yes, the book is really that famous among people with even a small interest in rocketry. $\endgroup$
    – AlexP
    Nov 10, 2021 at 12:58
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    $\begingroup$ Yep, I read that book twice during high school, once more in college, and eventually purchased a copy (it was back in print as recently as the early 2000s). $\endgroup$
    – Zeiss Ikon
    Nov 10, 2021 at 13:41
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    $\begingroup$ I also suggest Ignition. It's a great source of information on all kinds of wild projects from the late 20th century. And you really appreciate just how dangerous that kind of research was. "It is, of course, extremely toxic, but that’s the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively." $\endgroup$ Nov 10, 2021 at 14:06
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    $\begingroup$ The Falcon 9 with Dragon alone uses three different fuels: Kerosene/Oxygen for the Merlin engines of the Falcon 9 first and second stage, Monomethylhydrazine/Dinitrogen Tetroxide for the Draco and SuperDraco engines of the Dragon, and cold Nitrogen for the reaction control thrusters of the first and second stage. SpaceX Starship and Super Heavy use Methane/Oxygen. The Space Shuttle Main Engines / Space Launch System RS-25s use Hydrogen/Oxygen, its Solid Rocket Boosters use Polybutadiene Acrylonitrile/Ammonium Perchlorate. … $\endgroup$ Nov 10, 2021 at 20:16
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    $\begingroup$ @JörgWMittag, because each of those fuels was selected for a reason, and most of those reasons also apply to aliens. Of your fuels, I'd expect to see, at a minimum, hydrolox and cold nitrogen. $\endgroup$
    – Mark
    Nov 11, 2021 at 1:20

6 Answers 6


The chemical propellants in common use today (kerosene/oxygen, hydrogen/oxygen, methane/oxygen rising fast, plus variations on hydrazine and nitrogen oxides for storable hypergolics) are dictated by chemistry and physics. Energy density, latent heat of formation, reaction products, all contribute to exhaust velocity, which is the prime measure of efficiency for rockets.

Hydrocarbons and oxygen are rather common in the universe (oxygen is the second most common element in Earth's crust and atmosphere, for instance) and have relatively good working properties for this -- that's why, after roundly a century of development, almost all rocket boosters use some variation of this (except those that for one reason or another need to be stored ready to launch on short notice, which mostly use either solid propellants or hypergolics).

With your escape velocity being nearly 50% above Earth's, however, your aliens may not reach orbit (or be able to orbit substantial payloads, like capsules with people inside) barely forty years after the first liquid fueled rockets, as humans did. Adding 50% to orbital velocity (which is directly proportional to escape velocity) means that with the same exhaust velocity, the mass ratio of the rocket has to more than double -- that would be like needing a Titan II (the Gemini booster) to orbit a Sputnik, or needing a Saturn I to get Mercury capsules into orbit. That, in turn, will set back everything else in their space program (although they'll know where they're headed, it'll just take longer to make the bigger, more powerful rockets reliable enough to get the job done).

IMO, the real question here is whether they'll ever invent rocketry as we know it at all.

Rocketry started (on Earth) with the simplest solid propellant: gunpowder. Early gunpowder rockets ("fire arrows" as the Chinese called them a thousand years or so ago). With 12 atmospheres ambient pressure, fire arrows probably wouldn't produce enough thrust to fly, never mind be effective terror weapons by issuing hordes of smoking, roaring arrows falling almost at random. Without fire arrows and the firework rockets that were the other side of that coin, Congreve rockets, Hale rockets, and other early artillery rockets would never have developed, and without artillery rockets, it's likely Tsiolkovsky would have reported rockets as a dead end instead of writing about them in a way that inspired Goddard -- whose work with liquid fuel was the foundation of all modern orbital rocketry. BTW, guns would have been much less effective, too -- higher air drag means shorter range, higher ambient pressure slightly reduces muzzle velocity. Given the history of early cannon, that might have been enough to make them an "interesting historical side track" in the annals of war, instead of contributing to the development of rockets.

  • $\begingroup$ You are right of course. Deleting the useless comment. $\endgroup$
    – AlexP
    Nov 10, 2021 at 15:43
  • $\begingroup$ I don't understand how increased pressure causes decreased thrust from a rocket. Please sir, could I have some more? $\endgroup$
    – Willk
    Nov 10, 2021 at 23:09
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    $\begingroup$ @Willk, simplified version: the high atmospheric pressure pushes the exhaust back into the nozzle. A rocket operating in the vacuum of space is more efficient than one on the surface of the Earth, and a rocket for taking off from the surface of Venus would be quite the challenge. $\endgroup$
    – Mark
    Nov 11, 2021 at 1:25
  • $\begingroup$ Another way to think of it is that thrust, to a first approximation, is the difference between chamber pressure and ambient, times the area of the nozzle throat. $\endgroup$
    – Zeiss Ikon
    Nov 11, 2021 at 12:09
  • $\begingroup$ Rockets are driven by converting liquids or solids to gas and letting the gas expand through a nozzle, accelerating it in one direction and the rocket in the other. The higher the ambient pressure, the sooner the gas reaches equilibrium and stops expanding. Too high, and you can't even achieve supersonic flow, and you've got a big flamethrower instead of a rocket producing useful thrust. $\endgroup$ Nov 13, 2021 at 13:13

Chemistry and physics are the same all across the universe, therefore chemically propelled rockets will experience the same limitations as we see on Earth. Even the chemistry behind the propellants would be the same, so I don't expect exotic fuels.

There is a study which explored rocketry and other planets, from which it comes out that we might be living in a "chemical-propulsion rocket Goldilocks zone".

For the sake of his study, Loeb considered how we humans are fortunate enough to live on a planet that is well-suited for space launches. Essentially, if a rocket is to escape from the Earth’s surface and reach space, it needs to achieve an escape velocity of 11.186 km/s (40,270 km/h; 25,020 mph). Similarly, the escape velocity needed to get away from the location of the Earth around the Sun is about 42 km/s (151,200 km/h; 93,951 mph).

As Loeb indicates in his essay, the escape speed scales as the square root of the stellar mass over the distance from the star, which implies that the escape speed from the habitable zone scales inversely with stellar mass to the power of one quarter. For planets like Earth, orbiting within the habitable zone of a G-type (yellow dwarf) star like our Sun, this works out quite well.

Unfortunately, this does not work well for terrestrial planets that orbit lower-mass M-type (red dwarf) stars. These stars are the most common type in the Universe, accounting for 75% of stars in the Milky Way Galaxy alone. In addition, recent exoplanet surveys have discovered a plethora of rocky planets orbiting red dwarf stars systems, with some scientists venturing that they are the most likely place to find potentially-habitable rocky planets.

“The nearest star to the Sun, Proxima Centauri, is an example for a faint star with only 12% of the mass of the Sun,” he said. “A couple of years ago, it was discovered that this star has an Earth-size planet, Proxima b, in its habitable zone, which is 20 times closer than the separation of the Earth from the Sun. At that location, the escape speed is 50% larger than from the orbit of the Earth around the Sun. A civilization on Proxima b will find it difficult to escape from their location to interstellar space with chemical rockets.”

“Rockets suffer from the Tsiolkovsky (1903) equation : if a rocket carries its own fuel, the ratio of total rocket mass versus final velocity is an exponential function, making high speeds (or heavy payloads) increasingly expensive.”

For comparison, Hippke uses Kepler-20 b, a Super-Earth located 950 light years away that is 1.6 times Earth’s radius and 9.7 times it mass. Whereas escape velocity from Earth is roughly 11 km/s, a rocket attempting to leave a Super-Earth similar to Kepler-20 b would need to achieve an escape velocity of ~27.1 km/s. As a result, a single-stage rocket on Kepler-20 b would have to burn 104 times as much fuel as a rocket on Earth to get into orbit.

To put it into perspective, Hippke considers specific payloads being launched from Earth. “To lift a more useful payload of 6.2 t as required for the James Webb Space Telescope on Kepler-20 b, the fuel mass would increase to 55,000 t, about the mass of the largest ocean battleships,” he writes. “For a classical Apollo moon mission (45 t), the rocket would need to be considerably larger, ~400,000 t.”

While Hippke’s analysis concludes that chemical rockets would still allow for escape velocities on Super-Earths up to 10 Earth masses, the amount of propellant needed makes this method impractical. As Hippke pointed out, this could have a serious effect on an alien civilization’s development.

A possible alternative would be nuclear propulsion, but that falls out of the chemical fuels, and it is also questionable if a civilization would jump onto it without or with a less effective stage in the chemical propulsion realm.

  • $\begingroup$ Great answer! I wasn't aware of that study. $\endgroup$ Nov 10, 2021 at 14:15
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    $\begingroup$ Re "A civilization on Proxima b will find it difficult to escape from their location to interstellar space with chemical rockets." AFAIK, none of the probes that have so far escaped to interstellar space (or are en route) have done so entirely with chemical rockets. The Pioneers, Voyagers, and New Horizons all made considerable use of gravity assist. $\endgroup$
    – jamesqf
    Nov 10, 2021 at 19:14
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    $\begingroup$ @jamesqf Gravity assist required a partial escape; reaching the other planets required delta-v that was proportional to the potential between Earth's orbit and theirs. And if the start planet was deep within the star's gravity well, getting to the outer planets to pull off a slingshot would remain much harder. $\endgroup$
    – Yakk
    Nov 11, 2021 at 15:21
  • $\begingroup$ A launch into a near Earth orbit (very little deltaV besides escaping the Earth) can be altered by repeated Earth flybys to change the orbit. The Lucy probe will use this to explore the Trojans. Shorter years would speed this up in a red dwarf system. $\endgroup$ Nov 11, 2021 at 18:19
  • $\begingroup$ @Yakk: You can use gravity assist from inner planets (assuming there are any), or from the home planet itself. The Galileo, Cassini, and Juno missions all did this. $\endgroup$
    – jamesqf
    Nov 12, 2021 at 3:02

Seconding the comment to read Ignition. I recommend getting a hardcopy - thank goodness it's back in print now! - but it's available online: https://library.sciencemadness.org/library/books/ignition.pdf; scroll down to page 191 for a good summary of propellant types. As long as your aliens are in our universe, with the same subatomic particles and the same elements, they're going to come up with the same rocket fuels discussed in Ignition. Also, it's hilarious and a great read. One of my favorite quotes is:

[The chemist] then retreats hurriedly to his lair, pursued by the imprecations of the engineers, who complain that the density is too low ... to which he replies that he'd like a higher density himself, but that he's a chemist and not a theologian and that to change the properties of a compound you have to consult God about it...

In a fictional world, you can play God a bit, though "The aliens used an Unobtainum -hydroxide fuel, which was a non-toxic storable hypergolic monopropellant with a specific impulse of 1500s" is going to get the same kind of eye rolls from chemists as humans taking their helmets off and breathing the air on an unknown alien planet gets from biologists.

The same things will be hypergolic, peroxides will still be persnickety, halogens are still heavy, alcohols won't have enough energy, 12 bar of pressure is no help in keeping oxygen a liquid, and even less so at keeping hydrogen a liquid, and flourine will still consume everything it touches. It may be that some compounds will not be as toxic to your aliens as they are to humans, which will be a boon, but on the other side of that hopefully water vapor or carbon dioxide are not toxic to the aliens because most chemistries will produce a lot of them.

You're going to have to deal with fuels that might be sensitive to SO2/H2S contamination where our rocket engineers have to deal with fuels suffering from oxygen or water contamination. Sulfur in particular is a nasty contaminant, RP1 rocket fuels is basically just a grade of kerosene with very low sulfur and atomic masses kept near C12; we can find natural petroleum deposits with low sulfur but I suspect your aliens may be unable to do so.

Also, you may have a phase change for a few fuels, something that's true at 1 bar and 20C may not be true at 12 bar and 55C. For one example, hydrazine becomes a solid at 2C, so we don't use it as a fuel on an ICBM carried by an arctic submarine because turbopumps don't work well with solids: your aliens probably wouldn't be bothered by that. For another example, propane is a liquid at 10 bar; we don't use it because it's inferior to methane (if you're using a cryogenic fuel) or RP1 kerosene (if you're using a fuel that should be liquid at room temperature). But RP1 would (I think?) be a gas at 55C and therefore unsuitable for its current uses.

Do be aware that Ignition was written in the 70s. The original engines on which the Space Shuttle was based had been around for a while, but it was still a decade before the Shuttle would fly and four decades before SpaceX and other modern spaceflight organizations would fly. Chemistry hasn't changed in the intervening years, there have been no paradigm-shifting discoveries, but chemical engineers and especially controls engineers have gotten better. We can now generate fuels with higher purity, throttle engines much more deeply, and pilot rockets much more precisely, to the point of being able to land a booster propulsively. The 2018 reprint doesn't update any of the chemistry.


As L.Dutch notes in their answer, chemistry is the same everywhere. The rocket fuel / oxidizer combinations that work for us will work just as well on other planets too.

(For things like jet engines, atmospheric composition matters, since they use atmospheric oxygen to burn the fuel with. But rockets carry their own oxidizer, so that doesn't matter either.)

That said, practical rocket design isn't affected just by chemistry. Other things matter too, such as:

  • Cost and availability: Kerosene (a mixture of hydrocarbon compounds distilled from crude oil) is a popular fuel on Earth because it's cheap and easily available. It's so easily available because the Earth's crust happens to contain deposits of fossil hydrocarbons formed from biological material buried millions of years ago. Your world might have similar fossil fuel deposits, at least if it has some kind of carbon-based life on it, but the exact content of those deposits will likely be affect by differences in atmospheric and mineral chemistry, as well as the biochemistry used by life on your planet.

    If you don't have fossil fuel deposits, you can of course still produce kerosene (or some similar hydrocarbon mix) out of carbon-based biomass more or less directly, like biodiesel is currently produced on Earth. Depending on your world's biochemistry (which you haven't specified) this might require more or less effort. Even at best, though, it's probably still more work than just pumping the stuff out of the ground and distilling it. And once you're doing some non-trivial chemistry to produce your fuel anyway, you might as well start optimizing its chemical composition in other ways too, rather than just taking that cheap distilled fossil oil and calling it good enough.

    For that matter, cost and availability is also one major reason why liquid oxygen is so popular as an oxidizer — on Earth you can literally distill it out of thin air! Sure, it's also a pretty good oxidizer in general, and might be a decent choice even if it wasn't so easily obtainable, but the low cost certainly doesn't hurt.

  • Temperature: Another reason for the popularity of kerosene as a rocket fuel on Earth is that it's liquid at typical Earth surface temperatures and pressures, and thus doesn't need to be cooled down or compressed to liquify it. That makes storing and handling it a lot easier.

    Using a fuel or an oxidizer that isn't liquid at typical ambient temperatures on your world is certainly possible; after all, plenty of rockets used on Earth today use cryogenic fuels and/or oxidizers, such as liquid hydrogen and oxygen. But it's still an extra technical hurdle.

    (Of course, solid fuel rockets exist too, and have their own advantages and disadvantages. In terms of chemistry and engineering, they're almost completely different from liquid-fuel rockets, but they certainly are a viable alternative option for spaceflight. And let's not even get into hybrid rocket designs…)

  • Safety: Rockets are inherently dangerous things: a chemical rocket launch is basically a slow, controlled explosion, and it only takes a relatively minor failure to turn it into an uncontrolled explosion. And rocket fuels and oxidizers need to be reactive enough to ignite easily and to produce a lot of energy (and high-pressure gas!) when they burn, which tends to make them risky to store and handle. Basically, rocket fuels like to go "KABOOM!" (or at least "WHOOMPH!"), and a lot of rocket engineering goes into ensuring that they don't do that before they should.

    To some extent, these safety issues are universal, but they also vary somewhat depending on ambient chemistry. For example, oxygen is a pretty nasty and reactive chemical, but because our atmosphere is full of it, we tend to not use construction materials that aren't at least somewhat resistant to it and safe to handle in its presence. That means that we haven't needed to develop particularly specialized materials or procedures for handling it — ordinary steel or aluminum will do just fine, as long as you're careful.

    On the other hand, fluorine or some of its compounds (such as chlorine trifluoride) could potentially be even better oxidizers than oxygen. But despite some early experiments, neither fluorine nor its compounds have ever been used for spaceflight, mostly because they're just too difficult to handle safely using know materials and technology, and the potential performance gain over oxygen just isn't worth the cost and risk, not to mention the nasty toxic exhaust such a rocket would produce. But on a hypothetical planet that already had free fluorine in its atmosphere, the situation would presumably be different, and fluorine might well see use in rockets.

    Conversely, on a planet where free oxygen was uncommon, you might find that developing safe methods of storing and handling it could take a lot more trial and error (and fire and explosions…) than it did on Earth. Maybe even enough that other, safer oxidizers would be preferred — although there's really no such thing as a truly safe rocket oxidizer.

  • Environmental concerns: These kind of go together with the safety issues described above, but it's important to remember that any combustion products your rocket produces during launch will end up in your planet's atmosphere (and a lot of them will end up close to and downwind from the launch site).

    Liquid fuel rockets used on Earth tend to produce mostly water and carbon dioxide, which are fairly harmless to the environment here, concerns about the greenhouse effect notwithstanding. Some solid fuel rockets do produce nastier chemicals; for example, the NASA Space Shuttle solid rocket boosters used APCP fuel that contained about 20% chlorine by mass and produced about 200 tons of hydrogen chloride on each launch, which you certainly wouldn't want to breathe in. But even hydrogen chloride is only a local environmental hazard: it reacts with water to form hydrochloric acid, but once that has been sufficiently diluted and neutralized in the ocean, all you're left with is salty water.

    However, again, what's harmless and what's not depends on local chemistry. On Earth, for example, water is considered inert and harmless since it's everywhere, but on a planet with oceans of ammonia it would behave as an acid, just as hydrogen chloride does on Earth.

In the end, though, none of that might matter much, because — as already noted in other answers — the higher gravity and significantly denser atmosphere of your planet would make any kind of rocket launch from surface to orbit a lot harder than on Earth. (And launching an orbital rocket from Earth is already hard enough!) If the inhabitants of your world managed to develop orbital rocketry at all, it would likely involve huge multi-stage rockets with the most energetic fuels they can produce, cost and safety be damned.

Or else they might end up developing some kind of orbital air launch technology. That won't help with the higher escape velocity, but it does let you lift your rocket above the thickest part of the atmosphere before launching it. The down side, of course, being that you'll have to lift your rocket high into the atmosphere before launching it — just imagine the engineering effort needed to do that with something like the Saturn V.

(Also, a big and often neglected problem with air launch is that it's a lot easier to make a rocket lightweight if it only needs to be able to support its own weight when standing upright. Once you try to hang it sideways from a plane, you suddenly need to deal with lateral bending loads that require a lot more reinforcement, which eats into your already thin mass budget. Suspending the rocket from multiple points in some kind of a massive launch gantry to distribute the load could perhaps be a solution, but then you'd have to haul that gantry into the sky as well. Or I guess you could always decide to go with a giant balloon instead, which would at least let you lift the rocket into the air while keeping it upright. A dense atmosphere with a high average molecular mass would actually help with that, by making lifting gases more buoyant.)

  • $\begingroup$ They could use technology from super advanced humans (around 5 million years more advanced then us) in the same era the first rocket like things where made on by us earth. If I need to create some form of perfect rocket fuel, I can have future-us do that. $\endgroup$
    – Katze
    Nov 13, 2021 at 3:09

Given high escape velocity and high atmospheric pressure, space flight would be very challenging in this world.

Among the existing rocket engines, maybe only liquid hydrogen/oxygen can provide sufficient specific impulse to launch Sputnik from this planet. All other chemical propellants would be worse.

Inhabitants of this world may devise different methods of conquering space, for example Air-launch-to-orbit when the rocked is first lifted into the air by a plane which does not require as much fuel as would a rocket. In this case space flight can be achieved with greater variety of fuels.

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    $\begingroup$ Chamber pressures aren't dependent on the fuel -- Merlin engines in Falcon 9 have several times the chamber pressure of the Saturn V's F-1 engines, on the same fuel, so would suffer less from high surface pressure. Air launch is a real problem for a payload of any significant size, however, especially with 50% higher orbital velocity than Earth. Takes a BIG airplane to do that job. $\endgroup$
    – Zeiss Ikon
    Nov 10, 2021 at 18:56
  • $\begingroup$ @Zeiss Ikon the main concern here is air resistance rather than chamber pressure. BIG airplanes are sure a challenge, but imho smaller challenge than rockets the size of Burj Khalifa. $\endgroup$
    – Alexander
    Nov 10, 2021 at 19:10
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    $\begingroup$ Haven't played ,much Kerbal Space Program with Realism Overhaul, eh? By the time you get to orbital rockets, if they're built like a rocket, air resistance means almost nothing -- the rocket spends so little time in the thick air compared to what it takes to accelerate horizontally to orbital velocity that drag losses are fourth -- behind gravity, steering, and cosine losses. Which is to say, the only reason fairings are streamlined is to guide shockwaves near max Q, when speed is building but the air hasn't thinned enough yet to ignore. Less than one minute, usually, out of 8+ to orbit. $\endgroup$
    – Zeiss Ikon
    Nov 10, 2021 at 19:23
  • $\begingroup$ @Zeiss Ikon from my understanding, if we have air pressure 12 times the Earth level, max Q is going to be the hell of an issue. $\endgroup$
    – Alexander
    Nov 10, 2021 at 19:33
  • $\begingroup$ Woops, didn't catch the 12 atm -- then it's pretty simple, they'll probably never actually invent rocketry, because primitive gunpowder rockets won't even propel, they'll just be drab fountains. $\endgroup$
    – Zeiss Ikon
    Nov 10, 2021 at 19:38


Humans on Earth don't even agree on a single rocket fuel. As others have pointed out, there are only so many elements on the periodic table, and they only have so many promising combinations. Even so, assuming we've found the best rocket fuel defies history.

Humanity, in our first century of space flight, has already found and then shied away from "better" fuels, for good reason. Another species that evolved under different conditions might well have different risk tolerances and use these alternate fuels.

For a planet as difficult to get off of as the one you describe, nuclear propulsion similar to Project Orion or NERVA might be the way to go.

References: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/NERVA

These projects were abandoned on Earth before either was completed, for many excellent reasons. Orion drives only work on an unmanned craft, or an exceptionally large craft: The thrust from a nuclear pulse drive is so powerful that the g-forces would splatter a human crew if the ship wasn't big enough to spread the forces out. NERVA had heat-control issues: The drive has to be run hot to work. It's essentially a nuclear reactor on the edge of melting down. That's not only tricky to manage. It's also hard to find materials to build the hot parts out of that hold their shape at those temperatures. But probably neither of these problems were unsolvable. And those weren't the only problems left to solve.

As we get closer to solving fusion power, nuclear powered craft may come back into vogue. There's no beating the power-to-weight ratio of nuclear fuels. Our chemical rockets spend most of their weight on fuel. You're carrying fuel just to propel the fuel. Nuclear power is so much stronger that the fuel weight is comparatively zero.

Fission drives, ultimately, are dead because we don't want radioactive isotopes in the atmosphere. Again, that's something that fusion is likely to ultimately solve. From what we know (and don't know) right now, it's entirely imaginable that we won't be using chemical fuels a century from now. We might be using laser-ignited fusion pulse propulsion, or something else we can't yet imagine.

If humans still have fuels left to explore, there's no reason to expect aliens would settle on the same fuels we did.

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    $\begingroup$ Welcome beaudot. Please take our tour and refer to the help center as and when for guidance as to our ways. Enjoy worldbuilding. $\endgroup$ Nov 11, 2021 at 5:07

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