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If water could be converted to plasma for a propellant by electricity how much thrust would water produce? What would be the byproduct?

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  • $\begingroup$ If you build a huge engine, you can get much thrust even when using a low energy fuel. $\endgroup$ – Uwe Nov 19 '18 at 21:40
  • $\begingroup$ What exactly does "converted to plasma for a propellant by electricity" mean? Arcjets do make some plasma but the function there is for heating by passing a large current through it. But that just makes it hot, the same way burning makes things hot. The arcjet produces thrust by expansion in a nozzle, and so the propulsive actions are not related to electricity. Since you've got a nice answer for that process, if you'd like to ask about electric propulsion using water, maybe you should ask separately. $\endgroup$ – uhoh Nov 20 '18 at 5:20
  • $\begingroup$ Of course you can see your own edit history: click this: i.stack.imgur.com/rKr6i.png then click this: i.stack.imgur.com/uVxkY.png $\endgroup$ – uhoh Nov 21 '18 at 1:26
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    $\begingroup$ I'm glad to see the question restored to its original form, and the excellent answer now restored as well. $\endgroup$ – uhoh Nov 22 '18 at 2:38
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That's an arcjet.

Like other electrically-powered rockets, the thrust of an arcjet is quite low, but the specific impulse is good. Hydrazine, ammonia, and hydrogen are the most-often proposed propellants for arcjets, but water has been used experimentally as well (paywalled paper, so I don't actually know the thrust value they got). Specific impulse is around 500-600 seconds; thrust would be on the order of 100-200 Newton per megawatt of power input. The exhaust would be water in some form or another -- water vapor, droplets, or some combination.

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  • $\begingroup$ This is how arcjets work: space.stackexchange.com/a/18837/12102 Can you cite a source showing that "water has been used as well"? $\endgroup$ – uhoh Nov 20 '18 at 0:49
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    $\begingroup$ @uhoh sighs in late capitalism yeah, link added, and clarifying that water arcjets are experimental. $\endgroup$ – Russell Borogove Nov 20 '18 at 1:48
  • $\begingroup$ Also I think I slipped a few digits in the thrust value there. $\endgroup$ – Russell Borogove Nov 20 '18 at 1:56
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    $\begingroup$ "Water has been used in an arcjet thruster" was what I was trying to say, not "water arcjet thrusters have been used on flown spacecraft". $\endgroup$ – Russell Borogove Nov 20 '18 at 1:57
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    $\begingroup$ The exhaust would be water in some form or another -- water vapor, droplets => So, not plasma... This doesn't answer the question as stated. $\endgroup$ – Antzi Nov 20 '18 at 2:19

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