In this answer there is discussion of some research on the potential, future use of a solid metallic hydrogen phase that would be metastable at ambient pressure as a rocket propellant. The work is futuristic of course, but there is interest.

There is more information in the following questions and their answers:

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?

...and the BBC news article Claim made for hydrogen 'wonder material', quoted extensively in the second question.

Is the thinking is that it would be used as a monopropellant? Is it possible that the much heavier LOX needed for a conventional LH2/LOX engine would not even be needed?

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    $\begingroup$ heed my prediction. It will be exactly as all ultra-high-energy monopropellants: ludicrously explosive. $\endgroup$ – SF. Jan 29 '17 at 18:43

Yes, the fuel would be used as a monopropellant because as soon as you excite the metallic hydrogen, it will be just H molecules which will quickly bond to each other to Form H2, creating mass amounts of energy. If you convert it back to molecular hydrogen, all the energy required to produce it is released. No burning would be necessary, it would just be the metallic hydrogen, which keep in mind would be denser than regular compressed cryogenic hydrogen, so it depends on how much you carry if it would weigh more or less, but no oxidizer would be needed. This source also explains some other key properties of metallic hydrogen if you are interested.


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    $\begingroup$ I haven't. I will do that now, in fact I am quite new I didn't see that $\endgroup$ – user140052 Feb 15 '17 at 5:15
  • $\begingroup$ @uhoh Can the source be a video? $\endgroup$ – user140052 Feb 15 '17 at 5:19
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    $\begingroup$ I always learn from him, he is very helpful in these matters. $\endgroup$ – user140052 Feb 15 '17 at 6:02
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    $\begingroup$ I will look through them yes. $\endgroup$ – user140052 Feb 15 '17 at 6:10
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    $\begingroup$ @uhoh: ad 1) yes. ad 2) it will cool pretty fast. Still, per titular question, oxidizer would only reduce energy density and specific energy of the metal hydrogen used in bipropellant. You'd squeeze more energy per kg of hydrogen but less per kg of bipropellant. OTOH I wonder about using it in air-breathing engine. Big gain of thrust and good cooling simultaneously. $\endgroup$ – SF. Feb 15 '17 at 8:48

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