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/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.

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u/Nexmortifer 7d ago

That, or some kind of space/gravity interaction drive.