Explosion rules?

[Avianmosquito]

Is there a handy rule for standard damage and spread of explosions in d20 based on their yield? I ask, to see what it would take to contain a particular creature of mine whose home is an active caldera. To contain her a party would need to not only contain that caldera in case of an eruption but likely the explosive release of the connected magma chamber. (They might as well be trying to seal in an atom bomb, by the way.) If I had some way to get firm numbers on how much damage that would do, that'd be great.

[tyckspoon]

There is not. Set the numbers at whatever either makes sense to you or would make a sufficient challenge to your party to either pull off or dissuade them from that particular course of action and seek another solution. (Be careful about how you do this, btw - you don't necessarily want to set a precedent for your players about how explosions work unless you *want* them to solve their next 10 dungeons by setting off a bomb in the entryway chamber and then arguing that the ensuing pressure wave should salsa-fy everything in the dungeon. Also note that strictly RAW, D&D objects are absurdly durable against energy damage, which means the caldera is either surprisingly easy to contain or the eruption has to do a crazy amount of damage to burst through any serious attempt at plugging it.)

(Geologically speaking, I'm pretty sure what would happen if they plugged the caldera sufficiently is the magma would simply seek another route of release, probably bursting out and creating a new lava flow at a stress point somewhere on the side of the volcano. A good Know: Geography check, consultation with an appropriate sage, or careful divination would let them know that, and possibly give a pretty good idea of where it would happen.)

[haplot]

Standard 3.5 rules, 1 stick TNT or small flask of nitro or small barrel of blackpowder = 2d6 in a 5 ft spread and reducing 1 dice per 5 ft away from explosion point.

This was taken from the d20 modern book and d20 past book.

You just have to then find the size of the explosion you want and multiply the numbers, remembering that one stick = 1 lb.

So blocking the caldera would require them to enough explosive damage to effectively cover at least half the area of the caldera so move enough stone and rock into it, assuming you use a shaped explosion.

NB placed explosives ignore object hardness and require know (architure) or demolisons (spelling?) (prc sabator skill) dc 15 per ton.

[Elkad]

Standard 3.5 rules, 1 stick TNT or small flask of nitro or small barrel of blackpowder = 2d6 in a 5 ft spread and reducing 1 dice per 5 ft away from explosion point.

This was taken from the d20 modern book and d20 past book.

You just have to then find the size of the explosion you want and multiply the numbers, remembering that one stick = 1 lb.

So blocking the caldera would require them to enough explosive damage to effectively cover at least half the area of the caldera so move enough stone and rock into it, assuming you use a shaped explosion.

NB placed explosives ignore object hardness and require know (architure) or demolisons (spelling?) (prc sabator skill) dc 15 per ton.


Which makes Mt St Helens explosion (24 million tons of TnT equivalent) in the 100billion d6 range.
And the falloff is where it really comes apart.

Assuming Wolfram Alpha didn't fail me.
Damage limit (the last d6) is 3.75 million miles from the center of the blast.
At 60d6 to "break" 1' of moon rock (assuming that damage is lost as it applies), it's only 126 million d6 at a quarter-million miles to vaporize the Moon. Barely a taste of the 90billion d6 we have to work with still.
Earth is about 8x that. Only a billion d6. It's powder too.

[Telok]

Some time ago I was working on importing a functional version of blackpowder into D&D and I ran across something from a mining and blasting site. The danger zone of an explosion doubles (*2) for every ten fold increase (*10) in the mass of the explosive. It's due to the relationship between the volume and the surface area of a sphere. So don't use a linear drop off on damage radius.

[Avianmosquito]

Some time ago I was working on importing a functional version of blackpowder into D&D and I ran across something from a mining and blasting site. The danger zone of an explosion doubles (*2) for every ten fold increase (*10) in the mass of the explosive. It's due to the relationship between the volume and the surface area of a sphere. So don't use a linear drop off on damage radius.

It's 8x, actually. It's inverse cubic progression, that is progression based on volume. Twice radius is 8x volume, so it takes 8x as much power. So according to the dynamite=2d6 thing, an explosion worth a billion sticks of dynamite would be about 2000d6 and have about a 20,000 foot max radius. That 478 kilotonnes having a 3.79 mile MSD isn't too far off, either. According to Nukemap the MSD would really be about 7.75km, or 4.87 miles. The question is, if it's not linear, how would you calculate that fall off without it being stupidly complicated? I doubt a blast that big is going to come up in-game outside of volcanoes, but I wouldn't want to spend an hour on it regardless. Maybe cube the distance between you and the edge? If you're 18,000 feet from a 20,000 foot blast, it deal 0.1%, so 2d6 damage? Probably a save DC of 11, instead of... Well, by that logic at ground zero the save DC would be 1010. I don't know. It's not relevant to any campaign currently planned.