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I've been on a guns question binge lately, and this is no exception. Given my previous questions about vanishing projectiles and explosive alkali metals inside bullets, I started thinking of other chemical reactions, except this time not in the form of the projectile, but rather the propellant.

Similar to how baking soda volcanoes or airbags work, using a catalyst and substance that react violently to each other utilizing valves that let only a certain amount mix could provide a very effective and quick propellant. The more explosive the mixture, the faster the projectile could be flung.

This obviously leads me to my main question:

Would such a system work for a chemically powered gun, and what mixture would I need to use to achieve it?

Note: Per the comments, I realized I made a mistake. I'm not looking for fire-based chemical reactions, but there does need to be a reaction occurring, preferably with liquids.

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    $\begingroup$ In what way are you NOT just describing an ordinary gunpower-based gun? $\endgroup$ – ApproachingDarknessFish Sep 9 '16 at 21:53
  • $\begingroup$ @ApproachingDarknessFish HA! I guess I should clarify that I'm not looking for fire-based activation. I'll change that now. $\endgroup$ – Anoplexian - Reinstate Monica Sep 9 '16 at 21:59
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    $\begingroup$ @Anoplexian This is going to be tricky, because any reaction which provides a lot of energy fast is going to start generating some of the properties we associate with fire. Are we trying to avoid chemicals which get hot? Chemicals which burn (redox)? Checmicals which decompose? $\endgroup$ – Cort Ammon Sep 9 '16 at 22:24
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    $\begingroup$ And I missed the hard-science tag, which doubles down on how important those questions are. For example, C4 explosions are not combustion based (they're a decomposition), and neither are nuclear devices. However, they may share the traits which you colloquially call "fire based" $\endgroup$ – Cort Ammon Sep 9 '16 at 22:41
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    $\begingroup$ @Anoplexian That "fire-based" reaction you are talking about is oxidation. You have only two options when it comes to generating gas to push your projectile and that is oxidation, or decomposition. But why bother with liquids? There already exists case-less cartridges. Have your gun pile on disks or blocks of the solid propellant after the projectile, and there you have it. I fail to see the advantage to this in a firearm though, because you now you need to test its ballisics for all possible charges. $\endgroup$ – MichaelK Sep 11 '16 at 8:57
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I was thinking hydrazine, but when I went to research it discovered that the U.S. Military had beat us to it.

You will want to refer to this wikipedia article on Bulk Loaded Liquid Propellents

https://en.wikipedia.org/wiki/Bulk_loaded_liquid_propellants

Bulk loaded liquid propellants are an artillery technology that was pursued at the U.S. Army Research Laboratory and U.S. Naval Weapons Center from the 1950s through the 1990s. The advantages would be simpler guns and a wider range of tactical and logistic options. Better accuracy and tactical flexibility would theoretically come from standard shells with varying propellant loads, and logistic simplification by eliminating varying powder loads.

In general, BLP guns have proven to be unsafe to operate, and they have never entered service.

Fuels tried:

  • solution of ammonium perchlorate in ammonia
  • mixture of 63% hydrazine, 32% hydrazine nitrate and 5% water
  • propylene glycol dinitrate with a stabilizer
  • 90% nitric acid and a proprietary hydrocarbon
  • mixture of hydroxylammonium nitrate, isopropyl ammonium nitrate and water
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What you're suggesting - assuming you're restricting yourself to liquid propellants - sounds like you're basically doing the same thing as a rocket engine, but using the reaction to drive your projectile rather than propel your rocket - so presumably, what's good for rocket fuels would be good for your propellant, and you want something with a high specific impulse.

I'm not a chemist, but I think hydrogen peroxide + a (might be platinum?) catalyst is one possibility.

If you're allowing solid fuels, then you're back to existing gunpowder tech.

You might also be interested in the Gyrojet - speaking of rockets :) The gyrojet was a gun that basically fired small rocket projectiles, rather than detonating stuff behind a bullet. Advantage was a lighter weight weapon (less force to contain in the barrel) and a lower recoil (since the rocket was still accelerating after it left the barrel). Disadvantage was that the projectile wasn't at max velocity for close targets.

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  • $\begingroup$ TaoFlederMaus shot the Gyrojet, and it was not impressive. youtube.com/watch?v=cJAXpyt8-oQ That shouldn't stop fiction, though... $\endgroup$ – RonJohn Aug 18 '18 at 22:28
  • $\begingroup$ There's a balance to be had between high specific impulse and high thrust. Rockets can afford to take minutes to accelerate, so high thrust is much less important than high specific impulse. Guns must accelerate their projectile in just the length of the barrel; taking a few microseconds or so. High thrust seems more important in this situation. Seems like a min-max problem: "What length of barrel and propellant combination should I use" might be a good follow up question. $\endgroup$ – Ryan_L Jan 2 at 1:06
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If you do not want any form of combustion, i.e. a chemical reaction that produces heat and expanding gases, then the only other option is perhaps a compressed air gun, like an air riffle or a paintball gun.

If liquid propellant is the requirement, then CO2 guns, including pellet and BB guns, as well as paintball guns, use canisters of CO2 which are compressed into liquid form. I am not sure if combining any such compressed, liquified propellants would cause greater expansion, providing more thrust, without some form of chemical reaction.

As far as I am aware, any kind of chemical reaction that would aid in the process would have to be exothermic, i.e. fire-like.

Alternatively you could choose another type of gas such as nitrogen or a noble gas such as helium, but I am not aware of any advantage this may provide as far as thrust, nor much if any advantage to do with environmental concerns, i.e. CO2 is already very stable and is not likely to produce immediately threatening reactions, such as fire, with other compounds in the environment.

Otherwise

More on liquid high explosives.

Astrolite is the trade name of a family of explosives, invented by chemist Gerald Hurst in the 1960s during his employment with the Atlas Powder Company. The Astrolite family consists of two compounds, Astrolite G and Astrolite A. Both are two-part liquid-state high explosive mixtures, composed of ammonium nitrate oxidizer and hydrazine rocket fuel.

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Well, I have been wondering the same thing actually and I have come across multiple problems along the way. Pressure building up in the gun can result in an explosion due to too much pressure in an enclosed area. Also add that some of these chemical compounds can be dangerous to your team and/or environment. When you look at the big picture it really is useless depending on what kind of round you are using. If you were to just project a solid piece of metal then why not just use gun powder. But once you start using explosives or poisons of some sort, you really should consider using the possibility of chemical propellant. For example, Using a nuclear round (I know this sounds ridiculous to some) and regular gun powder will ignite the round just from the initial force, if you use chemicals to build gasses up it will be less harsh on the round and less chance of it igniting. Sure this is not accurate but it is a possibility. Conclude that it's not a "Need" but a want or wish for it to happen but the logistics say that it is dangerous and ineffective as a weapon.

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Since it is not entirely clear what sort of reaction you are looking for, I will suggest "electrothermal" guns, an experimental technology which uses plasma to ignite or otherwise vaporize a propellant.

Current ETC technology uses plasma igniters to more efficiently ignite a propellant charge, so in a sense it is still gunpowder technology, but using a different method of ignition to increase performance. To meet you requirements, some variations of ETC technology use powerful plasma ignitors to vaporize liquid propellants, including simply turning water or a water/aluminum slurry into a plasma to push the projectile down the barrel. This is closer in concept to a steam canon, except the steam is replaced with an energetic plasma. The muzzle flash must be truly impressive.

Many of the liquid propellants noted in the other answers would probably work as well as or better using electrothermal ignition technology, simply because it should provide a more reliable and consistent ignition, and a longer "burn" to maximize efficiency.

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two words: RAIL GUN. Ok, I know that with existing science it's totally impractical at this point because the current requirements and such make it so, but still... A magnetically propelled steel slug moving at 20 times the speed of sound.

I do wonder what the purpose of an alternate propellant would be. Are you throwing an object with the notion of just hurting someone? Are you trying to crack fortifications? Is it for a totally multipurpose weapon?

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Menthos + Cola. Allow the gas pressure to build up, then use it to propel a projectile.

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