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Explosions in GURPS 4th edition 18 January 2014

Explosions in GURPS confuse many people. Here's how they work, broken down at greater length than was available in the rules.

A full description of explosive damage takes the form

Xd Y ex [Z]

for example, "2d cr ex [1d-1]" or "1d burn ex". If you're using a packaged explosive like a grenade or an explosive bullet, this should already be supplied. X is the amount of blast damage, Y is the type of blast damage (usually cr), and Z is the fragmentation (if any).

There are two basic effects from explosions: blast and fragmentation.

Blast damage is applied directly if a target is in contact with the explosive as it goes off. If a charge has been tamped or shaped, multiply damage by 1.4 (see High-Tech pp.182-183 for much more on demolition techniques and special charge shapes). If the charge is completely contained by the target (or vice versa), as in the case of someone throwing himself on a grenade or an explosive charge placed in a gap in a structure, use maximum possible damage (effectively ×1.7). If the charge is inside the target, like an explosive bullet, treat it as an attack on the Vitals, with ×3 wounding modifier (×4 if it's in the head); if the target has No Vitals or Homogenous, just give it the usual x1.4 for a tamped charge. If the attack has an armour divisor, it only affects the target it strikes.

Blast damage also affects nearby objects and people. The safe distance to be from a blast is 2×X yards; if you're inside that, divide damage by (3×distance in yards), rounding down. Use torso armor to protect against this. You can't dodge the blast, but you can use Dodge and Drop (p. B377) to increase your distance from it by a yard or get into cover if there's any available.

If you're in vacuum, the divisor is 10 rather than 3: the shock wave doesn't carry far without some sort of atmosphere. Underwater, just use the range in yards without dividing it. Some exotic weapons have different multipliers too: fuel-air explosives divide by 2, and for nuclear and antimatter weapons it's 1.

Fragmentation damage potentially reaches out to 5Z yards: a [2d] fragmentation is dangerous to everyone within 10 yards. A target in contact with the explosion is automatically hit by one fragment. The fragments attack everyone else like an infinite-RoF automatic weapon with Rcl 3 and skill 15, modified by range to target, target's posture (except against overhead bursts), and target's size modifer. (So if you make the roll by 3, two fragments hit; by 6, three fragments; etc.) Again, no defences apply other than not being there when it goes off (Dodge and Drop). Roll hit location randomly; locations behind cover are protected by it. Fragmentation damage is almost always cutting.

Even if there's no fragmentation listed, you may get incidental fragmentation damage if there's loose or easily-broken material at the blast site: [1d-4] for earth, [1d] for loose scrap metal, and most other things will be somewhere in between.

If you don't have the full explosive description available, you'll need to work it out from the amount and type of explosive. The type of explosive gives you a REF, Relative Explosive Force. REF 1 is TNT, REF 2 is twice as much bang for the weight, and so on; 2lb of REF 1 explosive behaves exactly like 1lb of REF 2 explosive. You can find REF values on p. B415, p. 183 of High-Tech, and p. 88 of Ultra-Tech. Multiply REF by the weight of the explosive in pounds, take the square root, and multiply by 12. That's the basic number of dice of damage the charge will do, the X value above.

You can reverse this, of course: if you want a basic blast damage of X, you need a (REF × weight) of (X/12)².

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