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In this thread about Kerbal Space Program 2 authors says they are building gameplay based on technologies that are potentially viable in the near, or ar future. Main topic there is metallic hydrogen.

It ignited a heated discussion on that technology and some people claim it was already debunked and proven impossible. People there cite various sources that says yes or no for this possibility.

So, what do we know now? For real? Is it already proven impossible? Harder than we thought? Is anyone still working on fuel applications?

What I'm looking for is short impartial overview of the current (beginning of 2020) state of knowledge.

About 3 years ago we had a list of questions on that topic, but supposedly more recent research brings some sad news.

Also, new answers to first two would not help us now because we know it was considered and it's not going to change, and oxidizer one assumes it exists and works, and that assumption is what I want to verify.

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  • $\begingroup$ @uhoh linked questions are nice, but 3 years old. Research that supposedly debunked it is, as far as I understood, less than one year old. But I am not sure if I understood. $\endgroup$
    – Mołot
    Commented Feb 27, 2020 at 11:17
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    $\begingroup$ @uhoh question updated with links to old questions and explanation why I believe new one makes more sense than, for example, a bounty. $\endgroup$
    – Mołot
    Commented Feb 27, 2020 at 11:26
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    $\begingroup$ chemistryworld.com/news/…. This article covers a recent probably successful attempt to form metallic hydrogen. Paper: nature.com/articles/s41586-019-1927-3 $\endgroup$
    – Barry Jenakuns
    Commented Feb 28, 2020 at 1:21
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    $\begingroup$ Additional info about the feasibility of solid hydrogen: I remember when working on JWST in the mid-2000 time frame that solid hydrogen was considered for several years, as a coolant. Here is an article about the approach: sae.org/publications/technical-papers/content/2005-01-3041 $\endgroup$
    – user24033
    Commented Mar 5, 2020 at 14:13
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    $\begingroup$ @DanPro: There is no question about the feasibility of solid hydrogen, which has been used as a coolant on several space telescopes (such as WIRE, which launched in 1999...and promptly evaporated all its coolant by ejecting its dust cover while the telescope was pointed at Earth). The question is about metallic hydrogen. $\endgroup$
    – Christopher James Huff
    Commented Apr 22, 2020 at 20:10

2 Answers 2

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This paper, claims that metallic hydrogen at low temperatures might be relatively long-lived at pressures above about $50 GPa$ (with a large margin of error) which is about 500 thousand atmosphere and decay rapidly below that pressure. This does make it seem that tankage for a metallic hydrogen fueled rocket would have to be quite robust, and the consequences of a tank rupture would be quite severe. This is just one paper though -- I don't have an overview.

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    $\begingroup$ It doesn't appear to be worth expending resources on if a practical system is the desired outcome. $\endgroup$
    – Organic Marble
    Commented Feb 27, 2020 at 13:39
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    $\begingroup$ @Mołot That's a pretty severe drawback (the pressure), without any path to amelioration within the laws of physics. Given the relatively small "payback" for a working metallic H system over existing fuels, the risk-reward ratio is lousy. If some extremely unlikely discovery of a different state of H (solid or metallic at reasonable pressure, perhaps) then things might change. $\endgroup$
    – Carl Witthoft
    Commented Feb 27, 2020 at 13:51
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    $\begingroup$ From my understanding there isn't much that could be done since 2017. Unless there are theoretical breakthroughs that make calculations of something that can't be confirmed experimentally, there's no way to predict this with any certainty. And unless diamond anvil cells suddenly get better for some reason, this may not be accessible experimentally either. So in this particular case it is possible and wouldn't be unreasonable if nothing further is known about the (meta)stability of metallic hydrogen since three years ago. Condensed matter theory has no Moore's Law equivalent. $\endgroup$
    – uhoh
    Commented Feb 27, 2020 at 13:54
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    $\begingroup$ Given that a diver's tank is ~3000 PSI (204.1379 atmospheres) how in the absolute heck could you possibly make a tank that can hold a material that is 2450X more pressurized than your average diver's tank...? What in known/proven material science is the current front-runner in pressure vessels? In other words, what's the record on making a pressure vessel? Could this even be accomplished on a micro scale...? Doesn't seem like it... sounds like you'd need a pressure vessel akin to Jupiter. $\endgroup$
    – Magic Octopus Urn
    Commented Feb 27, 2020 at 16:48
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    $\begingroup$ @MagicOctopusUrn it's a diamond anvil cell (en.wikipedia.org/wiki/Diamond_anvil_cell), which only works for very small volumes. I think "what are the limits for a sizeable pressure vessel" would be a good question for engineering $\endgroup$
    – llama
    Commented Feb 27, 2020 at 19:02
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Tentatively I would forward the suggestion that for Metallic Hydrogen with it's high pressure requirements to become significantly useful to us outside of the interior of gas giants, that we will possibly be needing developments presently outside of our immediate grasp in the use of magnetic fields to simulate high pressure environments.

That is a long shot, but probably more likely than the next possibility that occurs to me, which would entail the creation and precise control of mini-singularities to create extremely intense localized gravity wells.

When the limits of materials technology are reached, it will be time to start working with the fundamental forces.

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