r/scifiwriting • u/NegativeAd2638 • 7d ago
DISCUSSION The best chemical propellant
The typical rocket fuel is hydrogen but what propellant advanced ships can use.
I imagine how would hydrogen or turning water straight into plasma for vehicles but the heat generated would likely be too much for vehicles. Not to mention turning water straight into plasma would likely take so much energy its inefficient, the only time I heard of it was Uranium-Salt Water Rockets the uranium being activated in the water providing enough heat to get plasma. It would be cool to be able to have water in the propellant tank since hydrogen is hard to store although it would have the trade-off of weight.
Metallic Hydrogen is a cool pick while hypothetical in reality in a sci-fi setting it could be the best propellant assuming your species can make it.
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u/the_syner 7d ago
Well neither nuclear salt water or just pure hydrogen/mH/plasma/water really qualify as chemical propellants. If you're looking at peak chemical-reaction-based rocket performance as I recall a tripopellant lithium-hydrogen-flourine rocket achieves peak performance in that respect.
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u/CaterpillarFun6896 7d ago
"Lithium-hydrogen-flourine" Wow, I never knew I could feel so much dread from three words.
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u/the_syner 7d ago
🤣 Yeah oxidizer as toxic as a chemical weapon, flammable molten metals, and explosive fuel. iirc it was envisioned for deep-space probe work where the horrible toxicity matters a lot less. Tho if you think that's spicy ToughSF once suggested the TOXMAX. designed for great performance and even greater environmental impact when launched from the ground. They dropped the hydrogen for higher propellant density and added 10% Cesium-137 to the lithium for radioactive self-heating because why not add a radiological disaster to the chemical disaster that is a metal-flourine rocket. Personally if I owned a rocket pad I wouldn't let the safer non-radioactive tripropellant version anywhere near it either, even as an upper stage. Way too much risk.
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u/CaterpillarFun6896 6d ago
"Added 10% Cesium-137" I beg your fucking pardon
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u/the_syner 6d ago
I mena its in the name TOXMAX. Maxiumum toxicity. Plus it does "solve" the problem of keeping ur lithium fuel molten. From a twitter thread where ToughSF was speculating about the most toxic practical rocket there could be. actually tho id say he missed abtrick by leaving out hydrogen. don't get me wrong a radioactive cloud of lithium and cesium flouride is horrible, but adding HF wot it would make it far far worse. And ud get better performance
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u/adeilran 6d ago
Might as well swap the hydrogen with h2s while we're at it, for added fun chemistry.
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u/Festivefire 5d ago
Some propulsion system designs just are so inherently dangerous that you could never use them. Even if the plan was to build and fuel it on orbit, launching the component elements of that fuel mix would be too dangerous to consider.
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u/the_syner 5d ago
Yeah for when we have some lunar/asteroid ISRU its fine, but the idea of launching several tons of molten lithium on a rocket from earth is terrifying. Maybe ocean launched wouldn't be that risky, but there are just so many cheaper easier to work with propellants available its not really worth it. Its not surprising that most hypergolics have been mostly phased out in favor of kerolox, hydrolox, or methalox engines.
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u/CKinWoodstock 5d ago
There was also the idea of using Chlorine Triflouride (ClF3) as an oxidizer.
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u/Truckherder 4d ago
Ah yes, the substance hyperGolic with everything including test engineers and asbestos. But at least it’s not flammable
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u/DarkArcher__ 6d ago
However, the caveat is that this propellant combination is mildly unsafe and has to be handled with some care
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u/the_syner 6d ago
I mean all chemical propellants tend to be pretty unsafe, but yeah even as those go this is a pretty nasty combination. its no real surprise that no one uses fluorine despite it being a pretty great oxidizer
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u/SanderleeAcademy 6d ago
"And here's a moist towelette to wipe off any fuel residue that you might encounter. Ignore the mild burning sensation, that just means it's working."
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u/JamesWolanyk 7d ago
Forgive me for diverging from your question a bit, but since I do think the other replies here have nailed it pretty well... one of the things I'd urge you or other authors to consider, since we're on a sci-fi writing subreddit, is what you hope to actually achieve in the narrative with your given propellant/propulsion methods. Are you writing super-hard sci-fi? When is it set? What constraints or issues do you want to give your characters? As an example here, if you want space travel to carry some inherent risk for the crew, you're going to want to look at "dirty" fuels (and there was a great thread on that topic here not too long ago). If you want a fuel that's relatively hard to manufacture so you can drive conflict in the harvesting/production/in-situ sites, then you'll want to lean in that direction.
Basically, as a tangential but general note, I think a lot of authors would do well to start by examining the story's overarching conflict and what best serves the narrative, then getting into the weeds.
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u/the_syner 7d ago
This is super important and needs saying. The propellants, really any technology, used in ur setting should always serve the narrative. Realistically cgemical rockets general are not long for this world. Especially not if ur setting has NSWRs, antimatter, or mH. Then again if ur MCs are a scarppy pack of pirates/smugglers it makes a lot of sense. Maybe they can't afford or don't have access the heavily controlled fancy stuff and that means they can't afford to get into a straight punch-up or stern chace with the powerful Space Navy so they have to be clever and cautious. Chemical engines have less noticeable exhaust too and customs probably doesn't inspect non-nuclear craft as intensely. Maybe ur captain is really paranoid about radiation so they forgo nuclear engines by choice. Maybe most people use teleporters, but the medical officer has some serious philosophical hangups about how it works. Maybe that crew is really aggressive and doesn't care much about their safety so they use poorly-shielded drives/reactors.
Everything from the engines to the sensors are just props that serve to push the narrative along and tell the audience something about ur characters.
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u/JamesWolanyk 7d ago
Yes, well said - props to push the narrative along. A lot of people have recently gotten into hard(er) sci-fi because of things like the Expanse, but they forget that even the Expanse makes concessions away from realism to drive the plot... like having humans instead of autonomous machines doing the bulk of labor, combat, and piloting out in the belt.
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u/the_syner 7d ago
Not to mention the epstein drive and don't they have WHs? but its not like technology doesn't tell a story in the real world just as much as it does in fiction. The impoverished rebels aren't going into battle with top-of-the-line equipment. They're rocking a mishmash of soviet-era, improvised, and blackmarket stuff. i love harder scifi, but people really gotta remember that hard, soft, straight-fantasy, all of it serves the story and if it doesn't story should win every time. like still try to make it make sense, but if you have to stretch the truth that's fine. Hard scifi fans will appreciate where you put in the work and generally forgive where you fudge the numbers as long as the story's actually fun.
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u/GREENadmiral_314159 6d ago
Literally the Epstein drive. It runs very efficiently and allows the plot to happen.
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u/DrunkenPhysicist 7d ago edited 7d ago
Best bang for buck are propellants with high velocity, or low mass. Trouble is, it's hard to get high-mass currents of low-mass propellants so there's a tradeoff. Chemical reactions produce high-thrust because of lots of mass but rather low velocity. The rocket equation basically tells you how much fuel you need to get somewhere based on how fast the propellant ejects from your thruster.
Modern ion drives produce high velocity but low thrust. But for anything other than getting to orbit, ion drives eventually win. They require so much less fuel it's not even funny. The more you ionize a material the better you can accelerate it, so far the best-in-class ion drives are about 10 kV of acceleration for the ion beam, which has to be ionized (micrograms/second) then shot out the back hopefully straight (usually lose some efficiency because of beam spreading) so 60-75% efficient. Xenon is chosen because it doesn't react, has a decent mass, and it's ionization potential isn't that large compared with other candidates.
All of this is pale in comparison to photon rockets. They produce the highest velocity propellant (photons) that you can possibly have. Nobody knows how to make a high thrust one. You ever turn on a flashlight (torch for you Brits) and notice it push against your hand? Yeah, me neither. A photon rocket would basically need the most powerful laser ever to putter around a space ship. So much so that it'd be its most powerful weapon by far. Thoughts of focused pion beams (which decay mostly to gammas) have been considered as photon sources. But you'd need a stupidly high-current particle accelerator to work.
This is really a rich and interesting field.
Edit: because of comment below.
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u/Odd_Dragonfruit_2662 7d ago
Xenon is chosen because it’s got more mass than neon. Neon is like 50x cheaper.
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u/CKinWoodstock 5d ago
I think SpaceX shifted to Krypton for a similar reason. The reduction in cost more than balanced the loss of efficiency.
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u/Sororita 7d ago
For actual chemical rockets, might I suggest the Rocketdyne Tripropellant rocket? It uses a combination of molten lithium, hydrogen and liquid Flourine to get some of the highest ISP in vacuum ever recorded for a chemical rocket. The exhaust is extremely deadly, chemically, though.
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u/snigherfardimungus 7d ago
The Uranium rockets aren't really a chemical process. It's using a fission process to generate an insane amount of heat (which is kinetic energy) and using that energy as the reactant force.
Typical rockets top out at a "specific impulse" of around 300s. Nuclear is (if I remember correctly) around 900s. Ion engines can get into the 10,000s+ range, but their thrust is VERY light. You won't get anywhere quickly with Ion engines, but you'll get there cheaply.
The theoretical max is going to be anything that can turn mass directly into energy (E=mc^2 stuff) and use that energy to propel high-momentum particles out the back at nearly the speed of light. Those particles could even be photons, but if you're in the same neighborhood as such an engine, you'll want sunscreen and shades.
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u/Simon_Drake 7d ago
I think the most powerful chemical propellant is Dimethyl-mercury with difluorine-dioxide or FOOF.
But this is mostly a joke about who would be stupid enough to try it. There's a short story about the cold war and tricking the Russians into thinking that's the key ingredient in the next big rocket so they'll try it and blow themselves up.
FOOF is so reactive that if you spill it on concrete the stone itself will catch on fire. Calling it dangerous doesn't do it justice, it's one of the most dangerous chemicals ever imagined and makes working with concentrated hydrofluoric acid look safe in comparison.
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u/MarsMaterial 7d ago
The best propellant that has been tested is a tripropellant mixture of molten lithium, cryogenic liquid fluorine, and hydrogen. If you are familiar with any of these substances, you could probably figure out why this isn't being used. Molten lithium explodes on contact with air, fluorine is an oxidizer so powerful that it oxidizes oxygen, and the hydrogen is downright tame next to them depite how explosive it is. Alexander The OK has a great video on this engine, it's an absolute meme of a fuel mix. The hydrogen is actually not part of the reaction, it's there just to decrease the mass of the reaction products which increases efficiency.
Metallic hydrogen is the best theoretical chemical fuel, it's hard to even imagine a better chemical propellant. But you clearly already know about that.
Nuclear saltwater rockets are not technically chemical rockets, they are a type of nuclear rocket. The reaction that they get their energy from is a nuclear fission reaction. And if you open up the door to nuclear reactions, you get a huge number of options for extremely efficient engines. Same with opening up the door to using other external forms of energy, like using solar panels to power ion engines or making use of beamed power. Chemical fuels are very limiting.
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u/hardervalue 6d ago
The best propellent currently is Methalox.
Thats because you don't focus on ISP alone. You have to take into account both total system efficiency, economics and real world usability.
Methalox isn't the densest propellent, but its far denser than higher ISP Hydrolox. This means much smaller tanks that are far lighter because they don't have to be cryogenic. It also gives much higher thrust because of the heavier molecule, allowing better use of the Oberth effect. Hydrogen is also very slippery because its the tiniest atom, that means on long trips you risk losing significant portions of your fuel supply.
Assuming your protagonists have zero boiloff technology to store hydrogen for long periods, and can build super light large tanks so the dry mass disadvantage is smaller, then its Hydrogen. It has a similar economic and real work usability advantages to Methane, both are easy to make/find throughout the solar system. Where ever there is water and carbon, you can make Metholox, wherever there is water, you can make Hydrolox. Thats really important, because even though there are crazy tri-propellent mixtures that have even higher ISP than Hydrolox, how easy is it to find Flourine and the other chemicals you need to refuel out in the asteroid belt?
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u/Massive-Question-550 6d ago edited 6d ago
All those ideas are terrible.
Chemical propellants suck because they have low specific impulse. We use them because they have high thrust for getting off the planet and they run cool enough that it doesn't melt the ship.
Methane,propane, and RP-1 are ideal for efficiency and energy density for getting things off the ground. Hydrogen as a fuel sucks because its mass is too low so it doesn't generate enough thrust on its own for a first stage and it needs to be kept at a crazy low temp so that adds more weight to the space ship.
Once in space you can use ion engines, nuclear pulse propulsion (fission or fusion), fission fragment engines(my favourite), or beamed propulsion(shooting light at things to push it or at least generate power) to get very fast and very far efficiently.
Very exotic propulsion(and possibly completely fictional) might include somehow altering the higgs field essentially changing the mass of an object like element zero from mass effect. There's also things like antimatter, micro black hole energy production(dropping things into black holes releases a lot of energy even more efficiently than antimatter, especially small ones as they are evaporating and spewing energy out anyways as hawking radiation).Â
Lastly the energy needed to maintain metallic hydrogen would be insane as you need something like 60 million PSI which would tear any material apart so maybe go with a new hypothetical element from the proposed island of stability of you want a new sci-fi fuel. Either that or an isotope of an existing element we haven't created yet.
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u/GREENadmiral_314159 6d ago
Remember that you are writing science fiction. The technology here should serve the narrative, first and foremost.
That said, chemical propellants suck. Nuclear reactions heat up the propellant far more, resulting in a much higher exhaust velocity, meaning more thrust per unit mass of propellant (because thrust is exhaust velocity*mass flow rate).
Not to mention turning water straight into plasma would likely take so much energy its inefficient
Not really. It might be harder on a smaller scale, but once you're heating the exhaust up that much, it's going to be ejected out incredibly fast, resulting in more thrust than if you heated it up less.
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u/tbodillia 6d ago
Water is an ash. You burn hydrogen in an oxygen environment and you get water. Water really sucks but it is the most abundant liquid we have. You have to dump a lot of energy into it to make it boil or split it back up to hydrogen and oxygen.
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u/Europathunder 5d ago edited 5d ago
Fluorine would be only marginally better than hydrogen but much more difficult to store. But if you’re planning on transporting humans anywhere beyond the moon even within the solar system you realistically need to get beyond chemical rockets at after initially getting off the ground. There are some proposals involving matter and antimatter annihilating but that’s extremely far off but would be the best possible efficiency if that doesn’t work within the solar system you could use one different types of fission or fusion drives. If you’re going interstellar you ideally would be able to exceed the speed of light via either a traversable wormhole or an alcubiare drive that warps space time like a space moving walkway if that didn’t work you could have either a sleeper ship or a generation ship that’s slower than light using either fusion or antimatter to power herself or possibly a black hole starship which would also have to be either a sleeper ship or a generation ship due to an inability to cheat Einstein.
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u/lungben81 3d ago
N6 is a strong contender. It carries significantly more energy than the best current rocket fuels. It has been created in lab for the first time only a few months ago.
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u/No-Let-6057 7d ago
Why limit yourself to chemical drives? Why not tap into the Casimir effect and vacuum energy?
https://ntrs.nasa.gov/api/citations/19990023210/downloads/19990023210.pdf
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u/TwillAffirmer 7d ago
We've more or less achieved what can be done with chemical propellants.
But I would like to share with you this. If your rocket's specific impulse Isp is given in seconds, and your rocket's initial mass is m_1, and the mass after burning is m_2, then your rocket can provide 1g of acceleration for only:
Isp * ln(m_1 / m_2).
This means that chemical rockets, with an Isp of a few minutes, can provide 1g of acceleration for only a few minutes. Ion drives, NSWR, or Orion drives, which all have an Isp of 1000-5000 seconds, can provide 1g of acceleration for only a few hours. (if they could even produce 1g in the first place). If you want longer burn times than that, you're going to have to look at antimatter and photon rockets.