[Ultra Tech] Variable Solid Propellant Firearm
 

I guess I'm on a bit of a UT gun kick at the moment. Over in my thread about increasing firearm power with TL, Anthony suggested modern smokeless powders may be at or near the maximum specific energy for a stable monopropellant. I'm not sure I buy that, but it got me thinking about options to simply add more propellant without calling for long cartridges, and I thought of this monstrosity.

The idea is that, as with a liquid propellant firearm, you have a magazine of bullets and a bottle of propellant - but instead of the bottle consisting of two chambers with separate liquids (mixed and injected into the firing chamber just before shooting), it's instead filled with smokeless powder. You set how much you want to use, it dumps that amount in the firing chamber, and the bullet is loaded in and then pressed back to basically pack down the powder (if necessary; I think modern cartridges typically have the powder loose in there, so maybe you could just leave the powder loose in the firing chamber). The powder is then ignited (by electricity, or by a percussion/blasting cap of some flavor that is loaded in from a magazine when the bullet is, but on the opposite end of the chamber), propelling the bullet with the force you dialed in earlier.

This would give you the variable performance of LP (so you can use suppressed shots against close/unarmored targets, both making less noise and conserving propellant; use extra propellant to boost your performance against distant/armored targets; or use the nominal amount of propellant for those in between) without needing to worry about mixing issues (from what I understand, one of the big hurdles for LP is the random formation of air bubbles when mixing the liquids together, which makes performance too variable from shot to shot to have any degree of accuracy). It also gets you the enhanced magazine size of LP, as your magazine contains only the bullets and you've got a separate propellant bottle. And, if you need a really powerful shot (and your weapon is up to it), you can potentially have more propellant than what a standard-sized cartridge could hold.

Does this sound potentially feasible? I wouldn't expect this to be able to manage a very high RoF - maybe full auto isn't an option, and such weapons can only have up to RoF 3? I'm thinking the same statistics as LP would probably work (it should probably be cheaper than LP, but I'm not sure a mere +50% is sufficient for LP anyway, even if the latter becomes workable); indeed, there probably wouldn't be much need for these weapons in a setting where LP is an option.

Re: [Ultra Tech] Variable Solid Propellant Firearm
 

Artillery uses powder that is separately stored from the projectile to allow controlling velocity, but if you aren't doing indirect fire I'm not sure how useful it is.

Re: [Ultra Tech] Variable Solid Propellant Firearm
 

Aside from the first round fired, your description appears to be dumping raw powder into a hot chamber. It would not take many shots before the chamber would be hot enough to immediately ignite the powder.

Artillery pieces take great care to ensure that there are no hot embers in the breech and that the metal is not hot enough to ignite the powder.

Re: [Ultra Tech] Variable Solid Propellant Firearm
 

My guess is that a mechanism to automatically measure, load and ram solid propellant for each shot at a speed fast enough to allow semi-automatic or automatic fire would be inherently mechanically more complex than squirting in a liquid propellant, which is why liquid propellant was proposed. In contrast, the US Army managed to achieve up to 350 rounds/minute even with the limited tests they did with binary liquid propellant, suggesting it would be useful for things beyond slow-firing artillery rounds.

The technology is certainly not there yet due to the ignition failure reliability problems cited, but it's the sort of thing that a half-century or so of research (e.g., TL9+) could fix -- one imagines better computers to study the fluid dynamics, advances in materials processing, etc.

As mentioned, many indirect-fire artillery pieces are already variable-velocity solid propellent, but they do this with pre-measured charges. In some cases, e.g., big naval guns, a degree of mechanical assistance is used. The most sophisticated is the modern electrical-ignition uni-charge system used in modern designs, which successfully beat out liquid propellant in the army contest for a new large-caliber artillery design. But by itself, that wouldn't be suitable for automatic or semi-automatic repeating firearms.

However, one can imagine a future nano-engineered(?) "smart propellant" that is designed to burn at different pressure levels depending on the electrical impulse trigger it received. Each round would have the same propellant charge, but receive one signal and only some of it ignites. That way you could have caseless or regular ammo that doesn't need any sort of mixer. You won't save any weight, but the ammo weight is usually minor compared to the projectile if you're using caseless or plastic cases anyway.

Advanced chemical propellents certainly exist, but I do think that there is a tendency keep things fairly static rather than advancing them every TL; if people are conservative about guns they're even more so about propellants. One of the things I learned when researching weapon design systems is that most small arm cases are not as densely packed with propellant as they could be. (This is maybe 40-50% density for pistol ammunition, about maybe 80% density loading for rifle ammo and somewhat higher for high-velocity cannon ammo). But a lower density tends to improve reliability and reduces ammo cost (simpler tolerances etc). In general, you can more or less treat most TL6-8 propellants similar except in some very high-end examples. ETC technologies are actually often stated as helping better enable the transition to more advanced solid propellants, as they mitigate some of the issues encountered.

The general data I saw was that current R&D in the US aimed ultimately to increase muzzle energies by as much as 25% without increasing barrel wear through new technologies that reduced the heat of ignition, etc. Buzzwords were propellants based around thermoplastic elastomers (TPEs) using novel nanostructured energetic materials often in advanced layered geometries with outer layers of slow burning propellant and inner layers of fast-burning, thus producing sustained pressure curves similar to the advantage gained by ETC. It is also noted that ETC is synergistic with advanced propellants and by optimizing them you can get even better performance. A 25% objective increase in muzzle energy equates with about 10-11% increase in damage and both 1/2D and Max range, and about a 5% increase in ST and Rcl. numbers. The Rcl. mod is mostly invisible.
It remains to be seen whether this objective could be achieved without extra temperatures that cause barrel erosion.

Much research on advanced propellants has instead focused not on boosting pressure and thus energy and velocity but rather on keeping numbers the same, but making propellants more stable, or which reduce barrel wear to increase lifespan. (This is especially important for warships, where you don't want to go around replacing 5-inch gun barrels at sea if you can't help it), as well as developing industrial processes that are "greener" and propellants that can be more easily disposed of or a longer-lasting when stored. While kind of boring in game terms, as GURPS doesn't really track barrel life much except in some optional rules for machine guns, these logical elements have taken up about as much of the advanced propellant research as making more energetic powders.

I'd probably say a more conservative TL9 is about x1.1 for advanced propellants alone, solid or liquid, or just assume they're the same value but a little less is used, and it's folded into the weight saving for caseless to get the "twice as many shots with caseless" simplification (it's probably closer to 1.7-1.8x as many shots otherwise). Then about 1.5-1.6x as effective energy with solid advanced propellant + ETC (or possibly with TL10 liquid propellant) which in game terms would simplify as about 1.25-1.3x muzzle velocity and damage, giving a nice "+1 per die" modifier. After that, I'd probably just say "go mass drivers" at 1.5 to 2x the muzzle velocity and 2-4x the energy, but you might shoehorn advanced liquid propellant + ETC into there somewhere in between, though skipping that entirely is also viable.

Re: [Ultra Tech] Variable Solid Propellant Firearm
 

It sounds like a TL4+5 Lorenzoni. That would be a seriously divergent tech path. :)

The first change that occurs to me is to use neither a liquid (much less two) nor a dry powder. Instead you go for a viscous but squeezable gel. Your gun squeezes out a desired quantity of that directly onto the back of the bullet and you set it off with an electric arc.

This would avoid the mixing problems of binary liquid or even liquid monopropellant with ambient air. It also avoids the handling problems of a dry powder. Your gel doesn't mix with anything and it doesn't rattle around either.

Re: [Ultra Tech] Variable Solid Propellant Firearm
 

Artillery is where the real-world is interested in using LP for as well, but semi-cinematic heroes at least would be able to get use out of the ability to ramp power up or down to match the threat.

Something I failed to spell out is that this presupposes weapons that can reliably avoid their barrel getting hot enough for something like this to happen - through cooling mechanisms and/or using propellant that isn't susceptible to having this occur - which is already kind of necessary for caseless ammunition to be an option.

One thing I had heard was that the random nature of the mixing may not be a surmountable problem, which this suggestion presupposes to be the case. As I noted, if LP is an option, this probably wouldn't be available. That said, if you feel this would indeed be more complex than LP, it may be appropriate to boost the cost to 2x rather than LP's 1.5x; if LP becomes available in the setting, expect VSP to rapidly die out (there will still be some people using VSP - or even purchasing new ones, if available - but absent artificial constraints they'll be a minority). VSP might have a higher LC than LP in some settings, if we presume VSP cannot be made fully-automatic, such that VSP is more readily-available to the civilian market (mostly for hunting, I'd imagine).

Honestly, part of me feels this could be an option for ETC weapons - basically, you can shift how the burn is regulated - perhaps you could interfere with the burn to produce a subsonic projectile (provided you aren't too far from the sound barrier to start with), enhance it to get +1/die to damage, or anywhere in-between.

As the rest of your post is about advanced propellant options, I'll respond to it in my other thread. I will note here, however, that the idea of mixing powders for enhanced performance makes VSP even less likely, as you aren't going to be able to arrange the powders as needed to get that optimal burn profile. I certainly appreciate all the input, thanks!

I was thinking this could potentially have been an option for a black powder weapon, and now that you mention the name, I believe I came across that before (which may have been part of the subconscious inspiration for this idea). It wouldn't necessarily be divergent, just adapting an old concept to a modern weapon.

I like this idea. Provided whatever method you used to turn your propellant into an extrudable gel results in something that burns cleanly and reliably, without much loss of density and specific energy, this sounds like it would be a preferable option to both LP and my proposed VSP - no need to control the mixing implies it may be lower complexity (and thus lower cost, but I'd be disinclined to reduce it below 1.5x), although it would probably need a dedicated power supply to run the mechanism that squeezes out the gel (which I'm envisioning being something like a miniature caulking gun). What would you call it? Extrudable Propellant (ExP)? Gel Propellant (GP)? Spicy ToothPaste (STP)?

Re: [Ultra Tech] Variable Solid Propellant Firearm
 

One of the big benefits of cased ammunition is that when the case ejects it takes a lot of heat with it. Non cased ammo firearms will overheat much more easily.

Re: [Ultra Tech] Variable Solid Propellant Firearm
 

AFAICT, this is a large factor in why polymer-cased ammo is considered more likely to be 'the wave of the future' than caseless ammo is... and why left-handed women don't like to wear low-cut tops while shooting a gun that ejects to the right.

Re: [Ultra Tech] Variable Solid Propellant Firearm
 

That's been the conventional wisdom for a while but now that polymer-cased ammo has started being used one of the benefits seems to be that using a thermal insulator rather than a conductor sends more heat out the barrel and the gun runs cooler.

This is still cased ammo but you might have to actually use caseless ammo in significant quantities to ee what happens.

Re: [Ultra Tech] Variable Solid Propellant Firearm
 

Interesting, I didn't know that. Thank you for sharing that.