I'm currently designing the armory for my far-future hard-scifi setting. Technology is quite advanced and for the purpose of this question assume that liquid metallic hydrogen (LMH from now on) is actually metastable, and diamonoid material tanks capable of holding it are a thing. The technology was developed for spacecraft propulsion as LMH is the best chemical rocket fuel (actually uncertain about how to call it, since no chemical reaction is used, just a phase change; any clarification would be appreciated). All technological problems concerning the LMH part of the question are to be considered already solved.

The actual design would work as follows.

  • a diamonoid material shell containing the LMH is inserted into the barrel; it is near the edge of the pressure it can hold

  • it is propelled down the barrel, currently I'm thinking that pressurised hydrogen gas would serve me best

  • a laser in the back of the barrel is fired to heat the shell so the pressure inside it increases and the diamonoid material is weakend; the bullet is now armed and while it will still survive travelling though air, a direct impact will break it

  • the impact on the target breaks the bullet, the rupturing of a high pressure pressure vessel, the highly energetic phase change to H2 gas (I read once that the rocket exhaust of LMH would be at 7000K) and a number of chemical reactions initiated by the hot, reactive hydrogen will wreak havoc on the target; I'm uncertain about what exactly would happen on impact, if someone would explain the process which is likely to occur in detail, it would be nice

I'm aware that this isn't really a real plasma weapon in the spirit of star wars, but a ludicrous grande launcher. Yet considering the nature of the explosion some plasma might be involved. Whatever the case, the marketing department decided to call it a plasma weapon, so it is a plasma weapon. Furthermore I'm not trying to make a practical infanterie weapon here. I'm aware that enemy snipers would make it a sport to shoot your ammo supply. However this weapon will be great if my protagonist has to show of his manliness on a monster infested jungle-world. Or if you are not in the mood to dispose of the body and don't mind major scorch marks. Additionally the gun, or lets be real here, the portable light atelery piece, should have a feature prohibiting it to be shot at a point less than 20m from the user.

Is the design plausable? Could I improve it somehow? Will it be as effective as I think?

  • $\begingroup$ So basically it’s a hot burning napalm bomb ? And comes with a fuel that only exist under extreme pressure? $\endgroup$ Commented Aug 24, 2019 at 11:44
  • $\begingroup$ It sounds like, you want to use exotic materials like "liquid metalic hydrogen" and "diamond pressure vessel" to achieve rather mundane effects (or energy densities) which can be much more easily achieved otherwise (either by mass driver, or using some nuclear material). Diamond strong in compression, but definitely is not the best in tension. For exotic High density energy material I would suggest metastable 178m2Hf which can possibly achieve energy density of atomic bomb (en.wikipedia.org/wiki/Hafnium_controversy) $\endgroup$ Commented Jan 9, 2020 at 17:01
  • $\begingroup$ I think it could be done with less exotic materials. Also if the shell is destroyed only on the impact area then it'll work similarily to a shaped charge, instantly jetting its payload into the target (and launching the projectile off-course so a perfect hit is still preferable). $\endgroup$
    – Demigan
    Commented May 9, 2020 at 11:57
  • $\begingroup$ Probably relevant: en.wikipedia.org/wiki/Light-gas_gun $\endgroup$
    – neph
    Commented May 10, 2020 at 1:50
  • $\begingroup$ "Whatever the case, the marketing department decided to call it a plasma weapon", For thing like this I hate the departament of Marketing $\endgroup$ Commented Oct 27, 2021 at 2:03

3 Answers 3


desert eagle gold with tiger stripes

It is totally impractical and so perfect for your purpose.


You poked the obvious holes in your crazy scheme yourself. Lean into them! You know exactly the man who must have a weapon which is such an ostentatious cadenza of high technology in the service of violence. A man oblivious to cost but very conscious of appearances. Probably he has several capable hirelings with more practical weapons nearby, in case his attempts to bring down Xenotyrannosaurus with his modified rocket tech do not go well.


The temperature of 7000K is pretty much a plasma by definition, so heat and various EMP effects will be felt by the target. I would suggest the mechanism you describe for launch is probably both too complex and unlikely to work well in practice; the most likely result is you will also be caught in the fireball (since the round is likely going to drom on the ground after exiting the barrel).

Something more on the lines of a mass driver would make sense, especially since the LMH core would react to and interact with the moving magnetic fields of the weapon, you could accelerate the round without over stressing the casing, but give it enough kinetic energy that the casing will fracture on impact. This might actually cause a few other issues - the round will pass through soft targets without detonating, while failure due to manufacturing flaws, mishandling by the troops and so on will cause the round to prematurely detonate in the barrel.

The fireball and EMP will be very short lived effects (milliseconds), while the high velocity of the plasma will mean any flame front from the Hydrogen reacting with Oxygen will take place at a great distance from the initial explosion once the Hydrogen has cooled enough to chemically react with the Oxygen in the air (or any other reactive materials). This will be a very minor issue since it will be so diffused and moving so rapidly, the eye might get a flicker as the flame front ignites and then dissipates in a tiny fraction of a second. Most of the damage will actually be caused by the shockwave as the hydrogen plasma expands at hypersonic speeds through the air.

This seems like an extremely elaborate and specialized weapon, and it is difficult to see what advantages it would have over a much cheaper chemical explosive round or grenade. Indeed, even the ability to launch at massive velocities using a mass driver is not unique to the use of LMH, a simple tungsten bar would act as a kinetic energy round and an explosive round would have very flat trajectory and the possibility of penetrating into targets using a delay fuse.

Perhaps you should think about other elements in the setting to understand why such an elaborate weapon would make sense (i.e. what are the targets, what sort of effects would you want to achieve and what sort of economic and logistical capacity does the user have to make and supply the ammunition?) Once you answer that, then everything else will fall into place.

  • $\begingroup$ Why would it drop on the ground after exiting the barrel? My idea was to use a hydrogen gas air-pressure gun. I used to own an air pressure gun which we used to kill small game. Hydrogen has a very high speed of sound and seems thus functional for my purposes. The railgun part seems like an unnecessary complication. (then again, this weapon class is an unessecary complication) $\endgroup$ Commented Aug 24, 2019 at 21:13
  • $\begingroup$ Does the hydrogen plasma have to cool down into the gas state in order for chemical reactions to occur? Or why does it have to cool. Thanks for the analysis. $\endgroup$ Commented Aug 24, 2019 at 21:15

As @Thucydides said, your design is far too complex and dangerous for a rifle-like implementation, it would make much more sense as an artillery-style weapon or like an RPG.

Artillery Style:

If you've watched Clone Wars, you would know about the droid tanks in S1E14 with the biomatter destroying weapon, which was attached to the tank, kind of like a suppressor on a rifle. And for propulsion, mix cesium into the metallic hydrogen and fire it like a railgun (cesium is affected by magnets). This piece of the weapon would be placed on specialized versions of the tanks. The metallic hydrogen would be stored in the aforementioned diamondoid tanks/shells laced with cesium or some other metal, which would be loaded into the tanks. This also solves the problem of safety as the tank would have a magnetic field, shielding it from metal projectiles and the plasma. This solution would also propel the projectile much further and at greater speeds. With your design, if the enemy were to concentrate their fire on the opening of the weapon's muzzle, the diamondoid material would shatter, vaporizing the shooter.

RPG Application:

In this case, the diamondoid tank would be loaded into an RPG/Missile launcher, with the barrel of the weapon containing a powerful magnetic field, the head of the rocket is also attached to an SRB, which will assist in the arming process. Here is a picture that should make it more intuitive: RPG Design

The LIDAR sensor helps guide the rocket and the SRB is laced with cesium. First, the arming apparatus is attached to the head, then the head is attached to the SRB with the help of the arming apparatus. Then a code is inputted to the arming apparatus by twisting the head, like a safe. The arming apparatus scans the SRB, and will only continue if the SRB is accepted, then, the apparatus energizes the cesium, preparing it for launch. Then the rocker is loaded into the launcher where the targeting system connects to the user's HUD and will only launch if the target is more than 30m away from the target - this can be overridden in dire situations with a special code that can only be inputted remotely. The head also has a magnetic field that will prevent from being hit and exploding before it reaches the target. (P.S. the HUD part is optional if you don't have that in your world).

  • $\begingroup$ You could also use EMPs in the artillery part to prevent the opponent from using magnetic fields. In the RPG application, this unnecessary as you would not use cesium in the LMH as it has no purpose there. $\endgroup$ Commented May 9, 2020 at 9:09

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