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I'll just point to the following two questions and their associated answers for background:

For the recently reported production (January 2017) of metallic hydrogen in the laboratory - what is the evidence exactly?

Are there predictions that hydrogen could remain metallic at ambient pressure?

A comment then led me to the BBC news article Claim made for hydrogen 'wonder material', quoted extensively in the second question. One line in the discussion about the possibility of hydrogen remaining in a metallic state after removing the initial, incredibly high pressure (about 5 million atmospheres or 500 GPa):

The US space agency is also fascinated by the material. Already super-cold liquid hydrogen makes for a very powerful rocket propellant, but the dense metallic form of hydrogen promises to deliver really colossal levels of thrust that would enable huge payloads to be lifted off Earth.

So I have to ask: Has (theoretically) metastable metallic hydrogen been considered for use as a propellant?

I am guessing that if hydrogen could remain metallic at ambient pressure, the density would be significantly higher than liquid hydrogen, overcoming one of the disadvantages of LH2 - giant, heavy tanks. But is there thinking that the thrust would also be significantly enhanced?

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  • $\begingroup$ The thinking is that solidification of hydrogen is highly endothermic - a lot of energy is input to create the stuff, so when it reverts out of the solid metallic state, all that energy is released, and could be harnessed to contribute to thrust. $\endgroup$
    – Anthony X
    Commented Jan 29, 2017 at 0:52
  • $\begingroup$ @AnthonyX OK that makes sense, roughly speaking it would be like stored mechanical energy (the way that wood is "stored sunlight" as Feynman analogizes). Sounds profoundly dangerous to me. It would then also be a mono-propellant - heat up or "tickle" a tiny chunk and it would suddenly expand as a hot gas. Gee, imagine a powdered monopropellant feed system! If there's anything written about metallic hydrogen as a propellant, that could be the basis of an answer here. $\endgroup$
    – uhoh
    Commented Jan 29, 2017 at 1:16
  • $\begingroup$ References linked in this answer have some bearing here. $\endgroup$
    – uhoh
    Commented Jan 29, 2017 at 2:33
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    $\begingroup$ Found a link to support my previous comment. A scientist involved with the experiment is quoted as saying: "It takes a tremendous amount of energy to make metallic hydrogen, and if you convert it back to molecular hydrogen, all that energy is released, so it would make it the most powerful rocket propellant known to man, and could revolutionize rocketry." phys.org/news/2017-01-metallic-hydrogen-theory-reality.html $\endgroup$
    – Anthony X
    Commented Jan 29, 2017 at 14:17
  • $\begingroup$ @AnthonyX yep - thanks! So I've asked this follow-up question. $\endgroup$
    – uhoh
    Commented Jan 29, 2017 at 15:21

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Considered, yes. See e.g. this study from 2010:

We discuss the applications of metastable metallic hydrogen to rocketry. Metastable metallic hydrogen would be a very light-weight, low volume, powerful rocket propellant.

The main attraction of metallic hydrogen is its Isp:

metallic hydrogen has a theoretical Isp of 1700 s.

And this paper from 2008:

If one assumes that metallic hydrogen is stable at usable temperatures and pressures, and that it can be affordably produced, handled, and stored, then it may be a useful rocket propellant. Assuming further that the available specific energy can be determined from the recombination of the atoms into molecules (216 MJ/kg), then conceptual engines and launch vehicle concepts can be developed.

NASA is looking into this in its 'Early stage innovation' program.

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  • $\begingroup$ There is a lot of physics mentioned in that Early Stage Innovation page! Now I understand "atomic" is used to to differentiate from solid molecular H2 "ordinary" ice. Some people also use "solod metallic hydrogen" to dofferentiate from a "liquid metallic" state. But the big news (to me at least) is the way that they mention to prodice it - by injecting electrons. That's something worth reading more about. Thanks! $\endgroup$
    – uhoh
    Commented Jan 29, 2017 at 13:22
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We have to wait. The generation of metallic hydrogen should be reproduced by other laboratories and the stability at low pressure should be shown. Is it possible to produce not only very small samples but also large volumes necessary for rockets? How to tank a rocket with solid metallic hydrogen? How to keep it stable for hours, days, weeks at the launch site and in the rocket? How to pump it into the combustion chamber? How to convert it from the solid state in a liquid or gaseous state? How to use it for the cooling of the combustion chamber?

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  • $\begingroup$ So is the statement by the BBC incorrect? I think an answer tom the question as asked might address that, and address the reason why metastable solid metallic hydrogen + LOX would or would not potentially produce more thrust than LH2 + LOX. While I see you've put a lot of thought into it, this is an answer to a question I have worked hard not to ask! $\endgroup$
    – uhoh
    Commented Jan 29, 2017 at 10:34
  • $\begingroup$ The statement of the BBC is not incorrect, the do state that the experiment was not reproduced by other laboratories yet. $\endgroup$
    – Uwe
    Commented Jan 29, 2017 at 12:58
  • $\begingroup$ The statement that I have quoted in my question! $\endgroup$
    – uhoh
    Commented Jan 29, 2017 at 15:06

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