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It is thought that Metallic hydrogen would make a better rocket fuel that ones we currently have. I wonder, what are the waste products of its combustion? Would it be correct even to call it combustion?

Edit Due to my ignorance of physics, my question was poorly formed and confused. From reading popular science news, I came away with an understanding that metallic hydrogen might be a "game changer" as a practical energy source for human society. I don't know the principles or proper terminology involved.

My intended question was, if metallic hydrogen does prove to be a viable energy source in a scenario where it can be cheaply produced, what would the "waste product" of its use as a fuel be?

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    $\begingroup$ I'm voting to close this question as off-topic because it is a question about chemistry, not physics. $\endgroup$ – sammy gerbil Aug 26 '17 at 9:56
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    $\begingroup$ @sammygerbil actually it's physics rather than chemistry - the proposed method of propulsion relies on a phase change from the metallic state to the gaseous one, not a chemical reaction. (I realise of course that I can only say this in light of knowing the answer.) $\endgroup$ – Nathaniel Aug 26 '17 at 11:11
  • $\begingroup$ Waste products? Under oxygen... Lots of dihydrogen monoxide mixed with oxygen dihydride and hydrogen hydroxide in a 1:1:1 stoichiometric mix I guess. $\endgroup$ – Julian Moore Aug 26 '17 at 11:26
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    $\begingroup$ @sammygerbil no, the OP is asking about the proposed use of metallic hydrogen as a rocket fuel, and is asking whether it's correct to call it combustion. The answer is no, it isn't correct to call it that, because it's a phase change rather than an oxidation reaction. $\endgroup$ – Nathaniel Aug 26 '17 at 11:31
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    $\begingroup$ To emphasise a point from Nathaniel's answer: metallic hydrogen is not an energy source, in the same way that batteries are not an energy source. It is a way to store and transport energy, but that energy needs to come from somewhere else. (Until we figure out a way to mine metallic hydrogen from the jovian inner mantle, of course.) $\endgroup$ – Emilio Pisanty Aug 26 '17 at 15:43
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It is not currently known whether it's possible to use metallic hydrogen as a rocket fuel. However, if it is possible then the idea is simply to let the hydrogen turn into a gas. This isn't a chemical reaction, it's just a phase change like boiling water, so the waste product would just be hydrogen, likely in the form of a very hot plasma rather than $\mathrm{H}_2$. Combustion is a kind of chemical reaction, so this wouldn't be combustion.

Most phase changes would be completely useless as a rocket fuel. One could imagine filling a rocket up with ordinary compressed liquefied hydrogen and just heating it up and releasing the pressure to produce gas. This would produce thrust, but nowhere near as much as reacting it with oxygen, and it wouldn't be practical. However, metallic hydrogen is much more compressed than ordinary liquid hydrogen, and this causes the electrons not to be bound to the atoms any more. Because of this, letting metallic hydrogen turn into a gas (or a plasma) releases vastly more energy than letting liquid hydrogen turn into a gas - so much so that going on to then react it with oxygen wouldn't result in a meaningful gain in thrust.

The problem is that all that energy has to be put into the hydrogen when you compress it, so making metallic hydrogen is really really difficult. (The last time I checked, one lab had claimed to make a tiny amount, but it was controversial.) What we don't know is whether it's stable. That is, we don't know if it stays in the metallic state when you release the pressure, or if it just turns back into a gas immediately. If it's not stable then it's probably impossible to use it as a rocket fuel, because you'd need a really heavy tank to maintain the pressure.

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  • $\begingroup$ The rocket can have two fuel stores: one for hydrogen, the other for oxygen, can't it? Are you making a distinction between fuel and oxidizer? We don't know the OP's intent in that regard, so I think your answer should also address the possibility that an oxidizer is carried. $\endgroup$ – garyp Aug 26 '17 at 12:30
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    $\begingroup$ @garyp I addressed this in the answer: "Because of this, letting metallic hydrogen turn into a gas (or a plasma) releases vastly more energy than letting liquid hydrogen turn into a gas - so much so that going on to then react it with oxygen wouldn't result in a meaningful gain in thrust." If you are using metallic hydrogen as a rocket fuel, you are not oxidising it. I don't know the OP's intent, but I'm reading it charitably and doing my best to educate. $\endgroup$ – Nathaniel Aug 26 '17 at 14:02
  • $\begingroup$ Yes. I apologize for reading your answer too quickly. $\endgroup$ – garyp Aug 26 '17 at 17:14
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Combustion: Rapid chemical combination of a substance with oxygen, involving the production of heat and light.

Based on the definition above, burning anything by reacting it with oxygen is combustion, including metallic hydrogen.

For rocket fuel, the hydrogen is used as a propellant because of its phase change from solid to gas - not burning (i.e reaction of oxygen and the metallic hydrogen does not occur). No chemical reaction is occurring here, and hence, it cannot be considered as 'combustion'. The expanding hydrogen gas comes out of the nozzle at huge speeds, and due to Newton's 3rd Law, the rocket goes up. The waste product would be hydrogen gas.

Such rockets are very hypothetical, and I'm skeptical if we'll ever produce metallic hydrogen economically and in any considerable quantity.

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  • $\begingroup$ -1 because although this is true, metallic hydrogen would not be reacting with oxygen (or indeed anything) if it were to be used as a rocket fuel. $\endgroup$ – Nathaniel Aug 26 '17 at 11:12
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    $\begingroup$ @Nathaniel The question uses the term and asks about combustion though. While, the OP may have confused combustion with the reaction of the rocket fuel, it's unfair to downvote a perfectly valid answer to a question on combustion $\endgroup$ – Beta Decay Aug 26 '17 at 11:25
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    $\begingroup$ The OP is using the term combustion incorrectly. The question is about the use of hydrogen as a rocket fuel, which doesn't involve oxygen and isn't a form of combustion. The OP asks whether it's correct to call this combustion, and the correct answer to this is no, it isn't a form of combustion. The waste product is hydrogen gas, not water. $\endgroup$ – Nathaniel Aug 26 '17 at 11:39
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    $\begingroup$ I've removed the downvote, since the answer now addresses the issue I had $\endgroup$ – Nathaniel Aug 26 '17 at 14:07
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    $\begingroup$ @JMac I must say I find all this weird snarking about my answer very weird and snarky. The question is about liquid hydrogen as a rocket fuel, and I just think it's more educationally helpful to explain what that would actually entail than to take the question literally and explain something else that has nothing to do with rocketry. $\endgroup$ – Nathaniel Aug 26 '17 at 14:14

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