I'm building a moderately hard scifi future setting that focuses on space and land warfare and I'm trying to decide what would be the most realistic from an engineering standpoint of artillery tech to add.

I've ruled out lasers, railguns, and coilguns as being ubiquitous weapons.

However if the setting is gonna mainly stick to chemical propellants, is there any real physical possibility to create chemical propellants, hundreds, if not thousands of times more powerful than what we have today (and much higher barrel pressures too)? I'm excluding antimatter as a propellant.


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    $\begingroup$ welcome to worldbuilding. Mind that a nuclear bomb cartridge uses nuclear reactions, not chemical reactions. So it doesn't fit your will of sticking to chemical propellants. I also see two questions in your post, while we prefer a "one post, one question" scenario. Please edit your post to comply with our standards. You can learn more in the help center and taking the tour. $\endgroup$ – L.Dutch Jan 18 at 6:25
  • $\begingroup$ Out of curiosity, what made you rule out lasers and magnetic weapons? In general, the more information you can give us about your requirements and your thought process, the more in-depth our answers can be. $\endgroup$ – Cadence Jan 18 at 6:26
  • $\begingroup$ @Cadence No energy source thats portable enough to be put on an AFV has been discovered and the large ones that exist have a too high maintenance and build cost, except for large space ships. $\endgroup$ – A.boj Jan 18 at 6:31
  • $\begingroup$ en.wikipedia.org/wiki/Girandoni_air_rifle its a lethal air gun or you could use hydrogen it has a higher compression level. also if you want to talk to someone use their name @A.boj like i just did $\endgroup$ – Creed Arcon Jan 18 at 6:31
  • $\begingroup$ @Cadence Also in the case of space ships in my world, reactors are preferred to be used entirely for propulsion/life support/computers, making railguns/coilguns/lasers massive dead weight and power sinks that prevent a space ship from accelerating and maneuvering quickly enough. $\endgroup$ – A.boj Jan 18 at 6:33

Create - maybe. Use - no

Let us look at the SS109 5.56mm x 45mm rifle cartridge. A complete round of ammunition (projectile, case, primer and propellant) has a mass of approximately 12 grams. The propellant mass is less than 2 grams, and does not completely fill the cartridge case. It would be trivial to double the propellant charge in a SS109 cartridge (which would have the same effect as using a more powerful propellant) for an increase of only 10% in ammunition weight, but this would require a stronger cartridge, chamber, barrel, recoil system etc. The real limitations are on the materials that the firearm is made of, not the propellant.

Gunpowder does not technically detonate, it deflagrates, resulting in a "pushing" rather than a "shattering" effect. In high quality propellants, each individual grain of powder is carefully shaped to optimise the burn rate for the type of firearm it is to be used in - which is why there are different powders for shotgun, pistol and rifle ammunition. Pistol powders, for example, are designed so that each grain will complete its combustion into gas (to propel the bullet) before the bullet has finished traversing the relatively short barrel of a pistol. Rifle powders, conversely, are designed to continue burning until the bullet is close to reaching the end of the (much longer) rifle barrel. The type and quantity of propellant are designed to impart the maximum possible velocity to the projectile without exceeding the safe working pressures of the cartridge case, chamber and barrel.

Using a higher energy propellant or replacing the propellant with an explosive with a high velocity of detonation (eg C4) will not make the bullet travel faster, it will instead make bits of the firearm travel quickly in a wide variety of unpredictable directions that will distress, injure and/or kill the firer and those standing near them.

Examining much larger weapons - examples such as the Rheinmetall Rh-120 with a muzzle velocity of 1800 metres per second are approaching the maximum feasible muzzle velocity that can be achieved with practical chemical propellants. Ah-ha, you say, an application for a super-propellant. Unfortunately, there is a theoretical limit to muzzle velocity for any conventional chemical propellant projectile weapon. As was discussed in user6511's answer to this related question, the maximum possible muzzle velocity of a conventional chemical propellant weapon is limited by the speed of sound of the working fluid. While incremental improvements might be achievable, it is generally recognised that chemical propellants are approaching their absolute limits, hence the current research into railguns and similar technologies.

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    $\begingroup$ "make bits of the firearm travel quickly" reminds me of this quote: "A slipping gear could let your M203 grenade launcher fire when you least expect it. That would make you quite unpopular in what's left of your unit." (PS Magazine, 1993) $\endgroup$ – CaM Jan 18 at 15:01
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    $\begingroup$ Here's an example of someone who used pistol powder in a rifle: bulletin.accurateshooter.com/2015/01/… $\endgroup$ – jamesqf Jan 18 at 19:41
  • $\begingroup$ @jamesqf Ouch - that's a really good example of why you don't want to exceed safe chamber pressures. $\endgroup$ – KerrAvon2055 Jan 18 at 21:12
  • $\begingroup$ Amen, also bear in mind that a HUMAN can take only so much recoil. Ant tank rifles had the nasty habit of breaking collarbones. Adding mechanism to cope with more recoil means more weight, etc $\endgroup$ – Gustavo Almeida Jan 18 at 21:50
  • $\begingroup$ @Gustavo Almeida: And thus you have the development of things like bazookas, recoilless rifles, shoulder-launched wire-guided missles, &c $\endgroup$ – jamesqf Jan 19 at 19:33

If you want the highest muzzle velocity, you have to use light-gas guns. However, they have drawbacks: they are bulkier, more complex and thus more expensive, and potentially more fragile.

The basic principle for maximum muzzle velocity is similar to rocket exhaust velocity: the fundamental limitation is the speed of sound in the expanding material. Hotter materials have higher limits, as do lighter molar masses (lighter molecules). Adding enough heat to make a difference would be impractical, at least for most conventional gun-like weapons, so you have to search for lighter combustion products. And the lightest ones are from gaseous (at room temperature) reactants - hence the name.

At this point, you can look at chemical rocket fuels as constraints are the same, and for the same reasons. Hydrogen/oxygen is the best, but hydrogen is atrociously bulky, stupidly cold, tends to escape any container, weaken metals and is generally not worth it. If you are OK with cryogenically stored fuels, methane-oxygen or ethylene-oxygen is a good compromise. In general, look at chemical rocket fuels, look for specific impulse (Isp) which is proportional to exhaust velocity and search for the compromise you want between practicality and efficiency.

Now, if you want much higher velocities than that, you can use a piston light-gas gun. The entire gun apparatus is essentially single-use and they fire small projectiles, so you won't be able to carry many, but you can reach more than 10 km/s. They may have their uses.

Theoretically, you could use a detonation gun based on the principles of the pulse detonation engine - as with the engine, detonation allows you to reach a faster exhaust/muzzle velocity than with déflagration. However, I am not aware of any firearm using this principle, and there may be significant problems with the concept.

A way to extend range and possibly have a supersonic (or even hypersonic) projectile without a giant, complex cannon is to put a ramjet or (let's be crazy) scramjet on your projectile, assuming advanced mass-production techniques allow you to do so at reasonable prices. The cannon accelerates the projectile enough for the sc/ramjet to start being effective, and the projectile itself accelerates further, and continue to do so for some of the flight duration. Use solid fuel ignited when firing to have an entirely passive, no-moving-parts motor.

(An alternative to chemical combustion is electrically flash-heating the propellant, similarly to an arcjet It is currently studied for tank cannons, though I couldn't find the source at the moment)

(Also, theoretically, the most powerful chemical explosive/propellant would be metallic hydrogen, but this is still very hypothetical.)

  • $\begingroup$ I really like the metallic hydrogen. Good plausible scifi. $\endgroup$ – Willk Jan 18 at 20:41

You want to use an electrothermal-chemical gun. The basic idea behind it is to improve the performance of chemical propellants by controlling the burn with a plasma charge.

An ETC gun can achieve higher muzzle velocities than chemical-only propellants, using less electrical energy input as coilguns and railguns. This property makes the ETC gun the better choice for mobile vehicles.

  • $\begingroup$ Welcome to Worldbuilding, Josh Kurkland! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$ – Gryphon Jan 18 at 22:36

If I'm not mistaken, we don't have a theoretical limit on how much energy can be released by a chemical reaction, so we can't rule out the possibility of explosives tens or hundreths of times more powerful that we have. But there are many other concerns you'd have to address:

  • Stability: usually, the more energetic a chemical compound is, the less stable. We already know of explosives ten times more powerful than those we are currently using, but only as experimental chemistry: they are so unstable they can be triggered by looking at them. Normally, you start with a promising compound, then you try to make it useful - as in "surviving its use".
  • Efficiency: our current most powerful explosives are not used as propellants for bullets. There's several reasons for that: price, stability (see point above: some chemicals are "safe" to be carefully stored, but can malfunction if shaken, hit, wet or subjected to temperature changes) but most of it, because you need a deflagration to propel a bullet, not a detonation. In a detonation, the force of the explosive will be delivered to every adjacent surface to the explosive, effectively destroyin the gun rather than propelling the bullet. You need a slower reaction to push the bullet, but slower reactions produce slower speeds. The only way to make for a slow reaction and large muzzle speed is a long firearm - just like liquid fuel rockets achieve higher efficiency than solid rockets: they manage to push for longer.
  • Usability: a bullet fired at twice the speed, all other parameters equal, will exert four times as much recoil to the shooter. If you plan to use your unobtanium explosive to boost handguns and rifles beware that recoil would render unusable to humans.
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    $\begingroup$ A bullet fired at twice the speed with exert two times as much recoil. Recoil is an application of the conservation of momentum, not of conservation of energy (which does not even apply). $\endgroup$ – AlexP Jan 26 at 21:51

There are many good answers here, but the advancements that need to happen are not just in the propellant so much as the gun-barrel. We still use gunpowder largely because it explodes slowly and cooly enough to fill the area behind the bullet with gas without destroying the chamber or barrel.

To open up the floor to stronger propellants, you probably need graphene. It distributes force from an impact better than any other known material, it is ten times as hard to fracture as steel for its weight, and it has a melting point of ~4125K. The only current limitation is that no one has figured out how to make it at industrial scales yet, but it is not unreasonable to think it could become a mature technology in the near future with as many companies as are researching it right now.

Make the barrel several times stronger, and you can replace gunpowder with a higher-yield explosive (of which there are many to choose from).


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