[Ultra Tech] Variable Solid Propellant Firearm

[David L Pulver]

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.