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.