This comment points out that any advantages of using low-mass ionized atomic hydrogen (i.e. protons) for electric propulsion if you had a fixed acceleration voltage and plenty of electrical power would be wiped out by a huge mass penalty because of the need for a pressure vessel.

Are there ways to store hydrogen and make it available at the low feed rates necessary for a plasma-based thruster that would not need such a heavy pressure vessel, perhaps some kind of hydrogen generator?

Let's assume electricity is not the limiting factor, I'm just asking about storage of hydrogen sans heavy pressure vessel and making it available at a rate required for a thruster for multi-U cubesat.

  • $\begingroup$ If plenty of electricity is available how about a refrigerator--store it as LH2? Anything else is going to eat up substantial mass. $\endgroup$ – Loren Pechtel Mar 14 '19 at 4:08
  • $\begingroup$ @LorenPechtel "I'm just asking about storage of hydrogen sans heavy pressure vessel and making it available at a rate required for a thruster for multi-U cubesat." As long as the mass isn't larger than that of a pressure vessel, for the purposes of this question, it's fine! Go ahead and post a refrigerator-based answer if you like, you can keep it in the shade behind a shade if you like. $\endgroup$ – uhoh Mar 14 '19 at 4:13
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    $\begingroup$ There is a long article in wikipedia about hydrogen storage $\endgroup$ – Uwe Mar 14 '19 at 9:18
  • $\begingroup$ @Uwe it's gotta work in space and fit in a cubesat not a car; which ones look most promising to you? $\endgroup$ – uhoh Mar 14 '19 at 9:45
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    $\begingroup$ If we use a chemical storage methode we need a material which reacts with hydrogen. But all possible reactants are much heavier than the bound hydrogen. Hydrogen is the element with lowest mass and the next one Helium is unable to bind to hydrogen. The next element lithium is about 7 times as heavy as hydrogen. We replace the heavy pressure vessel by a heavy chemical compound. $\endgroup$ – Uwe Mar 16 '19 at 0:18

This may seem strange, but certain compounds have greater hydrogen density than liquid hydrogen and thus can potentially pack more hydrogen per unit volume than the liquid. Liquid hydrogen weighs in at 70.8 kg per cubic meter, while the hydrogen content of magnesium hydride (one of many materials being evaluated for this purpose), for example, comes out to about 111 kg per cubic meter (i.e. 7.66% hydrogen at a total density of 1450 kg per cubic meter, both from the Wikipedia article just cited). Thus metal hydrides or similar media are being considered by for getting the hydrogen into compact form.

A more comprehensive review is to he had simply by looking up "Hydrogen storage" on Google.

  • $\begingroup$ So for situations where you only need a very low mass flow rate, a solid source at a reasonable temperature might be better than a tank of liquid H2. That's pretty amazing! $\endgroup$ – uhoh Mar 14 '19 at 12:26
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    $\begingroup$ Forgot to add the link on the first attempt. $\endgroup$ – Oscar Lanzi Mar 14 '19 at 12:33
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    $\begingroup$ But if I had say a few hundred grams of LiH2 in a container on a cubesat, how could the satellite extract the hydrogen? How might this be possible? Do you heat it or squeeze it or shake it? $\endgroup$ – uhoh Mar 14 '19 at 12:47
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    $\begingroup$ Lithium hydride is LiH. You'd generally need heating at some point in the "hydrogen cycle" with these compounds, either to form or decompose the compound. Getting the process to run reversibly without excessive temperature is one of the things we want. $\endgroup$ – Oscar Lanzi Mar 14 '19 at 12:53
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    $\begingroup$ okay so there must be other materials as well, and you've just mentioned this one as an example. Got it, thanks! $\endgroup$ – uhoh Mar 14 '19 at 13:01

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