GURPS Explosives: Custom Shrapnel and other ways to deal more damage

[MrFix]

Originally Posted by Kromm

When considering fragment lethality, don't overlook that most fragmentation attacks generate multiple hits. You're not looking at trying to get a fragment to kill somebody, but at trying to get two or three or more fragments to kill somebody. For instance, a single 2d(0.5) pi- pellet from an M18A1 isn't going to kill most targets . . . but that's a highly directional device, and at the mentioned 5 yards, the average victim is going to eat seven pellets, which is rather another matter.

As a broad rule for dedicated high-explosive munitions (not just solid ones with bursting charges), at a given TL, explosive damage goes up with the square root of explosive weight, while fragmentation damage goes up roughly with the cube root of (device weight - explosive weight). You can optimize for one or the other by altering the proportions. This is where skill comes in.

So you could get 1-lb. devices like these:

• 1 lb. TNT in a cardboard casing of negligible weight: 6d×2 cr ex
• 0.75 lb. TNT in an 0.25-lb. shell designed for light fragments: 5d×2[1d] or 5d×2[1d+1] cr ex
• 0.5 lb. TNT in a 0.5-lb. shell designed for moderate fragmentation: 8d+2[1d+2] cr ex
• 0.25 lb. TNT in a 0.75-lb. heavy fragmentation sleeve: 6d[2d] cr ex

The above assumes omnidirectionality. Directional devices are better modeled on shot shells.

Thank you very much for this information.

Based on this information, Kalos produced this formula, which appears to be relatively reliable and roughly in line with Kromm's data and High Tech.


(shrapnel material weight in lbs)^1/3 *2 in dice

Running Mk82's weight ratio (192 lbs explosive, 308 lbs shrapnel), I get [13d+2] shrapnel, High Tech puts it at [7dx2].

All that's left is to decide the minimum limit of explosive in the bomb, otherwise firecrackers can launch some serious shrapnel.

Let's say, minimum 25% of the bomb must be explosive filler?

Based on these assumptions, 16 lbs C4 satchel, with explosive filler of 4 lbs and shrapnel filler of 12 lbs, deals 6dx4.73 [4d+2] cr ex damage, or 28d+1 [4d+2] cr ex, or 7dx4 [4d+2] cr ex.

[DouglasCole]

Originally Posted by MrFix

Let's say, minimum 25% of the bomb must be explosive filler?

For the big AP and HC shells I spoke of:

The 2,700 lb shell has about 41 lbs of explosive. (1.5%) But fragmentation isn't really the point of that.

The 1900 lb shell has 150 lbs of explosive. That's about 8%.

If you like round numbers, 10% seems like a proper minimum.

[Rupert]

Originally Posted by Eukie

Blast overpressure can definitely kill you. Fragments are often better at killing, but explosives will kill too.

True, but it takes a lot of overpressure to kill a human, especially in the time frame of a GURPS combat (rather than days later from lung failure).

As for the OP's idea - bear in mind that while more mass in the fragmentation sleeve (or pre-made fragments in the case of a nail bomb, etc.) means more and/or larger fragments, it also means less explosive energy per unit mass of fragments. At some ratio you'll reach a point where more fragments and less explosive actually reduces the fragmentation damage.

Another issue is that in the rules as they are, bigger weapons get more fragment damage, but their effective reach doesn't go up because there's no adjustment to their odds of hitting. Once the fragments get enough damage to one-hit kill someone from a body or head hit (so 4d or more), they don't really get more lethal in the military sense (in the game sense they do, because PCs often have rather higher HP pools and HT, and may well have serious body armour), because all explosives have the same effective 'skill' for their fragments.

I've suggested in the past giving a bonus of +1 to hit per doubling of grenade/shell size, starting from somewhere in the 20-30mm range, or per doubling of fragment damage (as it's linear with shell diameter). As shells and grenades have 10-20% explosive by weight in a normal HE-Frag type, for customised explosive devices you'd probably want to work out the hit bonus for a 'heavy' model (using Kromm's examples), and use that for all sizes of 'sleeve'. Realistically, weapons using very small amounts of explosive (like the big naval AP shells) should have a penalty for their fragments' hit chance, but the fragments should do oversized amounts of damage which is fine for wrecking the insides of battleships but is very poor for killing infantry on a battlefield.

[Icelander]

Originally Posted by DouglasCole

For the big AP and HC shells I spoke of:

The 2,700 lb shell has about 41 lbs of explosive. (1.5%) But fragmentation isn't really the point of that.

The 1900 lb shell has 150 lbs of explosive. That's about 8%.

If you like round numbers, 10% seems like a proper minimum.

I'm not sure that there is so much a minimum explosive payload percentage, as that you can't get the full fragmentation effect for your anti-personnel casing or filling below a certain explosive damage.

There is no doubt a complicated field of study for this in the real world, but for gameable purposes, we should just take care to benchmark against real-world bombs and use common sense. If almost all real world bombs of that size use a certain amount of explosive payload, there is probably not an advantage to using less.

[MrFix]

Also, is there really no connection with power of the explosion and the shrapnel damage? You'd think that, for example, REF 10 explosive would propel shrapnel faster and farther, compared to same weight of TNT REF 1.

With minimum of 10%, 16 lbs satchel charge (1.6 c4, 14.4 shrapnel), the output is 6dx3 [5d] cr ex. This is very close to early WW1 bombs of 22-25 lbs range (6dx3 [4d+2])

[fula farbrorn]

I am kinda curious why Frag is Cutting btw, not questioning it just curious :3

i personally redid the HT bombs and grenades with a RoF

[Anthony]

Originally Posted by Eukie

Blast overpressure can definitely kill you. Fragments are often better at killing, but explosives will kill too.

Sure, blast overpressure can kill you. It's just really horrible at killing soft objects like humans. One of the problems with the GURPS explosion rules is that explosives are too good at directly killing people, and not good enough at destroying buildings (any blast wave sufficient to kill humans with overpressure will also completely destroy ordinary buildings).

[Rupert]

Past a certain point, the fragments aren't going to get much more energy from the explosive because once the casing has broken and split the energy will go into pushing the combustion gasses through the gaps, just as high-powered guns waste energy by having very high pressures when the bullet leaves the barrel, because all the energy still in the high pressure gas just goes into the gas' expansion.

[Rupert]

Originally Posted by fula farbrorn

I am kinda curious why Frag is Cutting btw, not questioning it just curious :3

Because your average piece of shell casing or random junk thrown by an explosion tends to be a twisted and jagged object spinning around some random axis.

i personally redid the HT bombs and grenades with a RoF

That seems unnecessary, given that you then also need to give them an effective skill. Currently they are effectively Skill-15, Rcl 3, RoF ∞. Aside from giving bonuses to bigger shells/bombs, I'm okay with that. What value do you get from giving them a RoF stat, etc.?

[Fred Brackin]

Originally Posted by Icelander

I'm not sure that there is so much a minimum explosive payload percentage, as that you can't get the full fragmentation effect for your anti-personnel casing or filling below a certain explosive damage.
.

The weight of comventional artillery shells before filler may have more to do with firing stresses than design for fragmentation. Or aror pentration for the big APEX.

A rule of thumb for aerial bombs and mortar rounds is that casing is 50% of total weight while you're looking at c. 90% for artillery shells and 95% for APEX.