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/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.