11-05-2015, 08:36 AM | #41 | |
Join Date: Jun 2013
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Re: Dust Explosion - Grain, flour, wheat
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Let's say the djinn is able to take those 2 tons of flour and make a 15 yard radius of more-or-less optimally dispersed flour in a 2 yard cylinder. That's 1080.86 m^3, or 810.645 kg. That's the equivalent of a little over 7000 lb of TNT. Let's go instead with a suboptimal dispersion that only manages the equivalent of 2,500 lb of TNT (note this means only a little better than 15% of the flour properly detonated). That's 6dx100 damage at the epicenter, and (as a thermobaric explosion) you divide by distance in yards to determine damage elsewhere. Against most foes, 15d injury is pretty much guaranteed death, 10d is likely death, 5d is possible death, 3d is likely incapacitation, 2d is possible incapacitation, and 1d will cause lower-quality troops to feign death or flee. As this is an explosion, Large Area Damage Reduction (LADR) will typically be around 50% (for partially armored targets) and 75% (for fully armored targets) of Torso DR. See the following chart. Code:
LADR 15d 10d 5d 3d 2d 1d 0 40 60 120 200 300 600 1 39 58 113 182 262 466 2 38 56 107 168 233 381 3 37 55 102 155 210 323 4 37 53 97 144 190 280 5 36 52 93 135 175 247 6 35 51 89 127 161 221 7 35 50 85 120 150 200 8 34 48 82 113 140 182 9 34 47 79 107 131 168 10 33 46 76 102 123 155 11 33 45 73 97 116 144 12 32 44 71 93 110 135 13 32 43 68 89 105 127 14 31 42 66 85 100 120 15 31 42 64 82 95 113 If the mage successfully pulls this off, expect him to invest in acquiring some large quantities of flammable dust (sawdust, flour, coal dust, etc) and making heavy use of it in future engagements. Unless the enemy figures out what he just did, however, and opts to dedicate a large deal of their magical oomph to countering it (which is still a strategic victory for the PC, as the enemy will be diverting a lot of resources). EDIT: I should also note that using this may be extremely demoralizing to the enemy, at the very least requiring some penalized Fright Checks by the less-disciplined troops. If the enemy is likely to realize what you did, it may be worthwhile to load some ballistae with flour bags afterall. Not so you can repeat the action (I'm assuming the djinn is only going to be available for 1 summoning during the battle), but because the enemy doesn't know you can't repeat it - after seeing half a brigade vaporized after being showered with flour, getting hit with flour oneself will likely call for a Fright Check to avoid an unplanned retreat. Last edited by Varyon; 11-05-2015 at 08:45 AM. |
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11-05-2015, 08:46 AM | #42 | |
Join Date: Sep 2007
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Re: Dust Explosion - Grain, flour, wheat
A lot of volume depends on the assumption about the shape, certainly. I went with 4m just because that's the Magic standard for Area spell height. 2m should be good enough for people, I agree.
Luckily, we have a literally magical dispersion method that could presumably handle any shape we like. Quote:
How did you get this value? |
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11-05-2015, 09:07 AM | #43 | ||||
Join Date: Mar 2006
Location: Iceland*
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Re: Dust Explosion - Grain, flour, wheat
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A lot of the flour is going to be going up to at least 60'. It will start drifting downwards from there, hopefully mixing well with the air, but as the highest dust drifts down, some of the dust that was spread around at lower altitudes will be clumping and settling on the ground. So waiting too long is just as bad as not allowing enough time for the dust to mix with air. As a rough guess, at the moment of explosion we should probably expect the cloud to be a lot higher than 6'. We might as well assume 60'. That will just be one part of the incredible inefficiency that will result from trying to do something that requires carefully controlling conditions in combat. If it even works. *So as much of the volume as possible is instead devoted to covering a larger ground area, packed with more people. Quote:
On the other hand, for an explosion at ground level, ranks of men between you and the epicenter count as cover DR. Not perfect cover, granted, but enough so that an explosion in the middle of a group of men will cause a lot fewer casualties than analysing each of them subject to the blast in an Infinite Featureless Space would suggest. If the epicenter of the blast is above ground level, however, their cover DR from each other will be much less. Note that it's probable that the enemy reserve, situated to exploit any breach in the walls or gate, will number more than a thousand men. I just mentioned that number because it's the equivalent of a regiment among the opposition. In fact, there will probably be some five regiments in reserve, but not, of course, packed into a neat circle for him to destroy all at once. Getting just one of them in a column of companies would be great. Quote:
However, even with extremely good rolls, I'm doubtful that the PC will get even 15% of the grain dust to explode. And he'll never get it to explode in a neat flat 6' tall cylinder cloud that is shaped exactly like his target, even to the point of not 'wasting' explosive dust with volume above the targets. Frankly, I think he'd be pleased with anything that makes a big boom, even if he doesn't get more than 300 casualties or so. After all, the alternatives that he has been weighing look like they'd yield maybe 200 casualties and nowhere near as concentrated or as shocking. As long as this causes a lot of disruption to the heavy infantry moving to exploit a breach, it's all good. Quote:
There will just be the one djinn. It's a summoning scroll, captured from the stores of an enemy high priest.
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11-05-2015, 09:10 AM | #44 | ||
Join Date: Mar 2006
Location: Iceland*
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Re: Dust Explosion - Grain, flour, wheat
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However, constraints of deployment speed* pretty much dictate that the djinni will take a whirlwind form that is as large as he can, so the height of the cloud will probably be closer to 60', at least in parts. *Aside from everything else, it's no good to have a perfect explosive dust cloud in a location where all the enemy have long since marched through and taken your city. We'd want to give the djinni less than a minute to work and the less time he needs to create conditions that appear conductive to a dust explosion, the better. I'm assuming that the PC will use Measurement spells every second to determine the progress, ordering the flaming once he's satisfied that several random hexes within the cloud appear to show the right air-to-dust mix. Quote:
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Za uspiekh nashevo beznadiozhnovo diela! Last edited by Icelander; 11-05-2015 at 09:15 AM. |
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11-05-2015, 10:12 AM | #45 |
Join Date: May 2013
Location: Ellicott City, MD
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Re: Dust Explosion - Grain, flour, wheat
I got my value for TNT equivalence with this calculator, and this file gives some heat of combustion values.
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11-05-2015, 10:50 AM | #46 |
Join Date: Jun 2013
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Re: Dust Explosion - Grain, flour, wheat
For REF, I simply used Nereidalbel's information of 1 lb flour = 4 lb TNT at optimal mixing.
Now, with the way thermobaric explosives work in GURPS, we don't really need to figure out the dimensions of the detonated flour cloud - rather, we just need to know how much flour it contains at the optimal mix, and where the epicenter is. I don't think we have any way to properly estimate the first, but the second would probably be based on a roll by the PC - essentially an attack roll probably using Combat Engineering, with scattering as usual. Let's assume 15 yards between the epicenter of the blast and the army beneath. At that height, cover from others caught in the blast should be negligible. An explosion of 100% (2 tons) of the flour is going to deal around 6dx125 cr ex3. Every -2 SSR to the amount of flour that actually blows up is -1 SSR to damage. So, an explosion of 1% of the flour - which is -12 SSR - corresponds to 6dx12.5 cr ex3 - which is -6 SSR. Have another chart. This time I'm assuming the foes have LADR 12. In this case, the listed yards aren't yards from the actual epicenter (which is somewhere up in the air), but yards from the point at ground level that is directly below the epicenter. Code:
% 15d 10d 5d 3d 2d 1d 1 N/A N/A N/A N/A N/A 8 2 N/A N/A N/A 7 12 19 3 N/A N/A 4 13 19 25 4 N/A N/A 9 18 23 30 5 N/A N/A 13 21 27 35 10 N/A 9 24 34 41 52 15 5 15 31 43 52 64 20 10 20 37 50 60 75 25 14 23 42 57 68 84 30 16 27 46 62 75 92 35 19 29 51 68 81 100 40 21 32 54 73 87 107 45 23 34 58 77 92 113 50 24 37 61 82 97 120 |
11-05-2015, 10:54 AM | #47 |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: Dust Explosion - Grain, flour, wheat
Using energy values for TNT equivalence assumes perfect shockwave formation and all fuel being consumed in the initial blast, both of which are unlikely to be the case (most dust explosions are actually deflagration, not detonation, and continue burning for some time after the initial blast). For example, a tanker truck explosion involves around twenty tons of flammable material with a heat of combustion about ten times the detonation energy of TNT, but it isn't equivalent to a tactical nuclear weapon.
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11-05-2015, 01:16 PM | #48 | ||
Join Date: Mar 2006
Location: Iceland*
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Re: Dust Explosion - Grain, flour, wheat
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Assuming that we get any detonation at all, more likely than a cataclysmic explosion is a series of irregularly shaped smaller explosions in those pockets where the mix happened to be good enough and, in between, a lot of rapidly burning dust that doesn't quite merit the term 'explosive'. Lack of a big BOOM does not mean harmless, however. I'm not holding out hope for any 6dx100+ damage here, but I'm hoping that for anyone actually inside the cloud of grain dust or flour, there will be decent burning damage over the next couple of seconds.
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11-05-2015, 02:45 PM | #49 |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: Dust Explosion - Grain, flour, wheat
Oh, that's certainly the case; you'll get a quite dramatic fireball, which will consume all available oxygen in the area and produce flame temperatures of more than a thousand degrees. It looks like you actually can't fully burn more than about 0.1-0.2 kg/m^3 of flour, oxygen shortage will starve your flame, but that's still a fireball you don't want to be in the middle of.
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11-05-2015, 02:50 PM | #50 |
Join Date: Jan 2005
Location: Gothenburg, Sweden
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Re: Dust Explosion - Grain, flour, wheat
Won't the djinni be SOL?
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Tags |
explosion, forgotten realms, high-tech, relative explosive force |
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