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The X-3 ion engine is the most powerful ion engine ever made, and puts out an astounding 5.4 N of force, using 103 kW. It's fuel is inert xenon gas, ionized and heated to a plasma. Now, what would happen if someone used hydrogen gas instead of xenon to "overpower" it? Would that be useful someday, for interplanetary travel?

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    $\begingroup$ 1: Depends on the engine. Deets? 2: Shrapnel, which probably wouldn't hit anything and would eventually fall into a star. 3: Depends on the nuke and asteroid - somewhere between an intact, irradiated asteroid and irradiated shrapnel $\endgroup$
    – Punintended
    Commented Feb 26, 2019 at 21:00
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    $\begingroup$ One question per post, please. Voting to close as too broad. I also suggest you give some context to your questions to avoid them being closed for other reasons. Your question #1, for example, is meaningless without more knowledge about the type of engine. Since you're a new contributor, I suggest you spend some time reading questions on the site that are highly upvoted to get an idea of what a good question is considered to be by the community. $\endgroup$
    – Organic Marble
    Commented Feb 26, 2019 at 21:21
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    $\begingroup$ Also the nuke one has been kind of asked before (independence day style): space.stackexchange.com/questions/19091/… Knew I saw that somewhere before :). $\endgroup$
    – Magic Octopus Urn
    Commented Feb 26, 2019 at 21:27
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    $\begingroup$ I've added some information to your question and clicked the reopen vote. You can vote to reopen as well. It would be better though if you added some explanation for why you are asking. For example, you need to know the level of the vacuum and the size of the chamber and type of pumps to understand the effect on the chamber, but maybe you only want to know about the effects on the engine? By the way the @ should only be used when you are trying to send a flag to a specific user. There is a user named everyone but they have not commented yet. $\endgroup$
    – uhoh
    Commented Feb 27, 2019 at 1:20
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    $\begingroup$ I also voted to reopen, but this question still needs work. Agree with @uhoh, what is behind this question? What's the normal flow rate and propellant for this engine? If you are asking about a totally random occurrence, there will be no way to answer the question because no one will have ever thought or cared about it. Please edit to explain why you are asking this. Context is important, otherwise this seems like a completely random thing. "What would happen if a water buffalo ran onto the shuttle runway?" $\endgroup$
    – Organic Marble
    Commented Feb 27, 2019 at 1:42

2 Answers 2

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While @Hobbes points out you get less momentum per atom or per unit charge for lighter ions, assuming charge is +1 and acceleration voltage is constant, that's not the whole story.

The momentum of a particle accelerated to a kinetic energy $E = qV$ is:

$$p = \sqrt{2mqV} = \sqrt{2mE}.$$

So Hydrogen atoms give $\sqrt{131} \approx 11.4$ times less impulse or "kick" per atom, or per unit charge from your high voltage supply.

However that hydrogen atoms weights 131 times less than an atom of Xenon. So per kilogram of propellant, you get $\sqrt{131} \approx 11.4$ times more impulse or "kick" per kilogram, so if everything else were equal, hydrogen would be a much higher Isp propellant than Xenon!

Hydrogen is second the most difficult atom to ionize after helium, but the difference is not huge. Starting with H2 gas, you need about 4.5 eV just to break a hydrogen molecule into two neutral atoms, plus 13.6 eV to ionize each one. Xenon is an easier gas to ionize at only 12.1 eV.

The reason ionization is important to consider for a spacecraft is because the mass of an ion engine is tied up in the power supply and plasma-confining magnets necessary to produce the high current of medium-energy electrons in the plasma that ionize the atoms during energetic collisions.

So going from xenon to hydrogen gives you over a factor of ten in Isp, and the next step would be to lower your spacecraft mass by using a low ionization potential material. That's a separate question.

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    $\begingroup$ Would there be any advantage to ionizing molecules rather than atoms? They could be about as heavy as you like, ionize about as easily as you like, and selected to store well. $\endgroup$
    – Greg
    Commented Feb 28, 2019 at 11:51
  • $\begingroup$ @Greg I suppose it depends on the molecule. I'm not familliar with any cases beyond diatomic things like iodine and hydrogen, see What are the parameters of the new Iodine electrical rocket engine developed by RSC Energia? and also MARS-CAT; What is a Cubesat Ambipolar Thruster and how does it work? Why don't you consider asking a new question about propellant stored as larger molecules, I think there could be a good one in there somewhere! $\endgroup$
    – uhoh
    Commented Feb 28, 2019 at 12:07
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    $\begingroup$ @Knudsen "Any attempt..." may not be exactly true; there are other ways to bring hydrogen along, and some people bring up ways to harvest it though that's a whole nother ball of wax. Speaking of wax, What are ways to store hydrogen for electric propulsion without a heavy pressure vessel? $\endgroup$
    – uhoh
    Commented Mar 14, 2019 at 1:57
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    $\begingroup$ @Greg a molecule might break down and leave residue on the grids when it hits them (as a small proportion of the ions do). A noble gas does a lot less damage. Mercury was used in early ion engines (better ratio of mass to ionization energy and very dense to store) but apart from being annoyingly toxic, it gunged up the grids. $\endgroup$
    – Steve Linton
    Commented May 16, 2019 at 20:10
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    $\begingroup$ Also, for a given degree of ionization, a lighter ion has a higher charge-to-mass ratio than a heavier one, and will, thus, accelerate more for a given voltage. In this respect, hydrogen totally rocks; the charge-to-mass of ionized hydrogen is fully twice that available from the ions generated by any other readily-available fuel (in large part because ordinary hydrogen is almost entirely hydrogen-1, which just so happens to be the only stable nuclide without any uncharged neutrons massing it down), and you only need to remove one electron from each atom to get it that way. $\endgroup$
    – Vikki
    Commented Nov 4, 2021 at 2:43
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Using hydrogen instead of xenon would make the engine less powerful. The engine works by accelerating ions. When you replace heavy xenon ions with very light hydrogen ions, thrust is reduced a lot.

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  • $\begingroup$ Is there a question about using heavier ions than Xenon? $\endgroup$
    – Magic Octopus Urn
    Commented Feb 27, 2019 at 15:37
  • $\begingroup$ I don't think so. $\endgroup$
    – Hobbes
    Commented Feb 27, 2019 at 15:43
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    $\begingroup$ Radon is useless. It decays too fast for pretty much any space mission. $\endgroup$
    – Hobbes
    Commented Feb 27, 2019 at 18:00
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    $\begingroup$ @Hobbes while looking for anything related to Radon my google searches were pretty much absolutely flooded with "Radon mitigation in your home!" Good to know though, thanks!! I guess Xenon is just in that "sweet spot". $\endgroup$
    – Magic Octopus Urn
    Commented Feb 27, 2019 at 19:27
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    $\begingroup$ Yes. Xenon is the heaviest stable gas. $\endgroup$
    – Hobbes
    Commented Feb 27, 2019 at 20:18

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