Explosion damage

I'm somewhat unsatisfied with the standard GURPS rules for explosion damage. This is an attempt to patch things up a bit.

Basic explosion damage

Standard GURPS rules indicate that explosion damage should be linear in energy. This is a Bad Thing, since no other type of damage seems to be linear in energy; a square root relationship seems to prevail elsewhere, and the DR of armor is generally linear in the armor's thickness (which also fits well with damage being proportional to square root of energy).

Therefore, my house rule is that the ground-zero damage done by an explosion (at 1 meter range or less) is proportional to the square root of the energy released. I have selected a damage of 6d*8 for 1 kg of TNT; this is somewhat more than the standard amount in GURPS, but I think it matches reality better. To find the damage done by an explosion, first find out how many kg of TNT it's equivalent to (this is the mass of the explosive, multiplied by its REF). Take the square root of that number, then multiply by 48d and round off as convenience dictates.

Note: Most explosive warheads are not 100% explosive by mass. Of the ammunition types in GURPS Vehicles (table on p. 112), only HEC warheads should get their entire mass counted as explosives. Ordinary HE warheads and others that do the same amount of damage (C*X/16,000 in that table) should be counted as 75% explosive by mass; the rest is fragmented casing or similar auxiliary stuff. APEX ammo is only 50% explosive by mass.
Formula: GZD = sqrt(mass*REF),
where GZD stands for "Ground-Zero Damage", mass is in kg, and REF is the Relative Explosion Force (see High-Tech or MA Lloyd's Vehicles addenda in the GURPSnet ftp site).

The effects of range

Instead of the weird exponential-decay effect of range in standard GURPS, I prefer the following method:

In atmosphere, the "lethal range" of an explosion seems to scale as the cube root of the energy in the explosion. This fits well with a simple physical consideration: If an explosion goes off in a homogenous medium, the total volume subjected to at least some given level of "damage" should be a sphere whose volume is directly proportional to the energy involved. Since damage scales as the square root of the energy, this means that damage should be inversely proportional to the 3/2 power of range. Yeah, you'll need a calculator with a "power" button to compute this; who doesn't have such a calculator in this day and age?
Formula: DAM = (GZD)/(range)^1.5

In space, the damage done by an explosion is pretty much entirely transmitted by the flash of heat and light; there is no intervening medium to absorb damage along the way, so the energy density is inversely proportional to the square of the range. Damage should then be inversely proportional to range.
Formula: DAM = (GZD)/(range).

Effect of target size

The above formulae apply to human-sized targets. Targets that present a larger area are hit by more energy, and should take more damage. Since incoming energy is directly proportional to surface area (all other things being equal), it seems to me that damage should be multiplied by some factor proportional to the square root of the target's surface, normalized so that a human gets x1 damage; to account for variation in proportions etc., there's no point in grading this scale any more finely than in integers. Say the average human has a surface area of 20 sf (a little less than 2 m^2). Then, the following little table yields damage multipliers for larger objects (numbers have been rounded off to get "nicer" categories).
Surface area (sf)		Damage multiplier
  Below 5				x0.5
    5-50				x1
   51-125				x2
  126-250				x3
  251-400				x4
  401-600				x5
  601-850				x6
  851-1100				x7
 1101-1450				x8
 1451-1800				x9
 1801-2200			       x10
And so on, the progression should be fairly obvious. This should be precomputed for every vehicle and large critter. Back to GURPS page.
Last modified: Sun Mar 2 14:31:05 MET 1997