I haw some ideas, how it could appear in story.
(btw, i understood, why it would not even be considered as something worthing for story, as only in 2017 they where able to get stable solid sample, which was lost when test installation broke due to immense pressure, and it is unknown, wether it still lies somewhere in lab or degraded).
Earth:
During research of took down alien FTL vessel new proposal is mailed from dr. Konnor - he proposes to give new try for long forgotten research for metalic hydrogen creation. Previous attempts to get metalic hydrogen where too expensive to rely on its usage - resource for gear, used for process, was very pour, as even diamond with thin aluminium oxyde layer was destroyed after first successful attempt. But as soon as alien materials are capable to withstand necessary pressure, cost should decrease drastically. Once technology is mastered, it should provide much safer alternative to antimatter - both for fuel and weapon application.
After reasearch (makes available MH production units):
New unit for MH production is developed (or could be done in separate research, unlocked by this one). It is completely automated and requires only one worker per unit to maintain it working (of course, he must have a workplace in factory). Unit itself must be produced, costing among others some amount of alien materials. The single unit is able to produce 100g of MH per day (i just guessed these digits). It is more expensive, than known fuel in current use, now available for our own production, and while our powers are limited, it needs careful application. There are ideas about enhanced rocket engine, capable to use MH granules as fuel. Also just like antimatter, it is good to be used as explosive part in weapons.
Why aliens still did not introduce it on battlefield? Probably, they really worry, that less developed enemies, they usually target, may easily reproduce this tech, unlike to what is used in plasma and particle weapons, we already discovered, and keep it for really bad days. It is very possible, that our squads will face it at alien home, used agains them in worst ways.
Alien homeworld:
It seems, aliens don't have problem with production of any details from MH. New types of grenades and projectile weapons are discovered, used by aliens.
Alien MH grenade: much more powerful than previous variant, dealing both heat and concussive damage.
Needler MH ammo:
- slightly thicker needles with MH core
- Each needle is as dangerous as single shot from plasma weapons.
Gardian needler:
- uses MH as part of coils, reliably secured from possible sparks and external heat.
- shoots less amount of them, but more accurately.
- secured by fingerprint scanner. On attempt for unauthorized use MH coils will be detonated, resulting in formidable explosion, more powerful than grenade (just like multi-tool like weapon in 5th element
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