Let’s say metallic hydrogen becomes an economically viable resource that we are able to either make ourselves or mine from a gas giant, how would we use it as a power source for vehicles both planetary and space based?
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1$\begingroup$ You're asking multiple questions here, some of which are quite broad, something discouraged here (have a read of the help centre for more information). Prune it down a little. Your final question would be a reasonable start, for example. $\endgroup$– Starfish PrimeCommented Oct 21, 2019 at 17:00
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$\begingroup$ @Starfish Prime Fixed, thanks! $\endgroup$– Boo RadleyCommented Oct 21, 2019 at 17:17
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1 Answer
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"It depends".
If metallic hydrogen is metastable at STP (or at least pressures that can be realistically engineered by humans) then all you have to do is to warm is up to about 1000K and it'll go foom vigorously as it turns into a gas, which you can then blow out of a conventional rocket nozzle. You could make a single-stage-to-orbit rocket without the need to muck about with beamed power or hazardous nuclear engines because so much energy will be released by this process. You could, in theory, use the same stuff as a heat source in a gas turbine or similar, providing thrust or driving a generator as necessary. It would also make a brilliantly dense fuel storage medium for other hydrogen-driven engines (fusion rockets, for example) where conventional cryogenic hydrogen tanks might be rather inconvenient.
The really interesting stuff might actually be if it turns out to be a room-temperature superconductor too, which might actually be more valuable than a super rocket fuel. It is also possible that it might make a very good charged-particle radiation shield, if it has more hydrogen nuclei per unit volume than alternatives like polyethylene (its density is lower than polyethylene, but there are no carbon atoms, but I'm not going to work out interaction cross-sections). And assuming that being hit by cosmic radiation doesn't make it explosively sublime. Maybe its best not to risk that one...
If it isn't metastable (and research suggests that even if it remains metal at STP it'll spontaneously break down due to quantum tunelling, which would make it quite a dangerous material to work with), then you're faced with the problem of making a storage tank capable of withstanding 25Gpa (or about 250000 atmospheres) of pressure. This is likely to be so big and heavy and awkward and expensive that it simply won't be worth the effort and will remain merely an item of scientific interest.
Probably you'll have to wait til someone actually manages to make macroscopic quantities of the stuff to find out. Mining it from gas giants is likely to be forever implausible, due to those punishly deep gravity wells.
answered Oct 21, 2019 at 17:21
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$\begingroup$ Would metallic hydrogen, if put in something like a gas turbine engine, be more efficient than conventional fossil fuels? Have we figured out an energy density for metallic hydrogen yet? $\endgroup$ Commented Oct 21, 2019 at 17:35
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2$\begingroup$ Much depends on "how stable is metastable". If a tank of metallic hydrogen is significantly more likely to go kaboom than a tank of gasoline, its use would be limited. $\endgroup$ Commented Oct 21, 2019 at 17:41
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2$\begingroup$ @BradleyKnauer about 216MJ/kg, which is better than any combustible fuel, and you could burn the resulting hydrogen gas if you felt the need which might get you an extra 140MJ/kg in a separate cycle. This is unrelated to efficiency, which is (more or less) a property of the engine. $\endgroup$ Commented Oct 21, 2019 at 17:47
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1$\begingroup$ @BradleyKnauer I imagine it could be treated like any other solid fuel. Maybe you could grind it to a very fine powder and then blow it around like a fluid using gas-phase hydrogen (or some other propellant). $\endgroup$ Commented Oct 21, 2019 at 17:49
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1$\begingroup$ @BradleyKnauer sounds very alarming, quite frankly! There isn't an obvious benefit there... firearm limits are more related to barrel and action metallurgy, and the squidgy meatbags who have to carry the things, rater than the propellants, I think. $\endgroup$ Commented Oct 21, 2019 at 18:00