What would be the "waste product" of metallic hydrogen combustion? 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?
 A: 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. 
A: 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.
