The idea of potentially using Metallic Hydrogen as a replacement for smokeless powder in current day rifles as a propellant has often been thrown around. What these question don't ask is how well it would work.

So to get to the question: How well would Metallic Hydrogen work as a propellant?

To start with some assumptions:

  • Metallic hydrogen is stable at STP

  • The engineering difficulties for containing the rabid expansion inside the weapon has been solved through tougher and lighter materials etc.

To have a more defined question to answer, how much Metallic Hydrogen is needed to propel a 9mm (9x19), 5.56mm (5.56x45), 7.62 mm (7.62x51) and a 12.7mm (12.7x99) to the velocities that they typically display in modern firearms (in grams).

Calculations and the formulas would be appreciated so that I and any world builder who would like to use metallic hydrogen as a propellant have the nessacary resources.

  • $\begingroup$ You could call your gun the "Cujo". +1 $\endgroup$
    – Willk
    Apr 29, 2021 at 23:22

1 Answer 1


Here's a crude, back-of-the-envelope estimate:

This source ( https://www.nasa.gov/pdf/637123main_Silvera_Presentation.pdf ) estimates the recombination energy of metallic hydrogen at 216 MJ/kg. For comparison, the same source estimates the energy for TNT at 4.2 MJ/kg and from https://en.wikipedia.org/wiki/Dynamite#Form, one megajoule is roughly equivalent to the energy of a stick of dynamite, so metallic hydrogen is pretty energetic stuff. Converting 216 MJ/kg to more reasonable units gives us 216 kJ/g.

The muzzle energy (https://en.wikipedia.org/wiki/Muzzle_energy) of various projectiles can be looked up easily. For example, a NATO 7.62x51 round seems to have a muzzle energy of about 3.5 kJ.

We can find an estimate for energy efficiency of about 33% for small arms sourced from https://en.wikipedia.org/wiki/Physics_of_firearms#Firearm_energy_efficiency, meaning that we need to triple the muzzle energy to get the necessary propellant energy. (Yes, the estimate is for a combustion firearm but it will serve for illustrative purposes. The actual efficiency would depend on the specific mechanics of this hypothetical gun.) Doing the math, (3.5kJ * 3) ÷ 216 kJ/g gives us roughly 0.05 grams of metallic hydrogen as the minimum that delivers the necessary energy.

  • 4
    $\begingroup$ Interesting (and +1). Given that conservation of mass means that 0.05 g of metallic hydrogen will turn into 0.05 g of hydrogen gas (with a much lower volume than the gas produced from conventional propellant) a much shorter barrel is implied. I may ask a follow-up question regarding barrel length and efficiency impact if @Seraphim doesn't. $\endgroup$ Apr 27, 2021 at 23:48
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    $\begingroup$ @KerrAvon2055 You would almost certainly need to "cut" the hydrogen with some inert material, even just some air, behind the bullet. I tried calculating the initial acceleration of the projectile, and got.....silly values. like 10^12g acceleration. The volume change of the hydrogen from metallic to ultraheated plasma is just absurd $\endgroup$
    – PcMan
    Apr 28, 2021 at 2:10
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    $\begingroup$ @KerrAvon2055 The breakup of the metallic hydrogen should proceed at the speed of sound, or faster. This is 35km/s or more. You end up with hydrogen gas at 14000K, at something like 650 gigapascals, although my gaspressure calculators get the giggles when I feed them that temperature and density, complaining about "exceeds parameters". There is a reason we do not use high explosives as propellant for rifles.((the only references to hydrogen gas under those densities come from secondhand info from early NIF fusion experiments)) $\endgroup$
    – PcMan
    Apr 28, 2021 at 2:35
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    $\begingroup$ I think this crude, back-of-the-envelope calculation is the best we can provide when talking about technologies that doesn't exist. Wood, coal, gasoline or butter are all substances with higher energy density than the propellents used in ammunition, but they can't be used for that purpose because they don't defragrate, or only do it in impossible-to-replicate conditions inside a projectile case. Nitroglycerine and other high-explosives are also more powerful, but again can't be used due to its high brisance. Just handwave your projectile around the values provided in this answer. $\endgroup$
    – Rekesoft
    Apr 28, 2021 at 9:10
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    $\begingroup$ @DavidHobs There is no oxidizer needed; all that energy comes from the phase change from metallic hydrogen back to gaseous hydrogen. The pressures needed to create metallic hydrogen are so extreme that a tremendous amount of energy gets stored. $\endgroup$ May 5, 2021 at 15:57

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