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Old 11-04-2015, 03:10 PM   #21
Anaraxes
 
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Default Re: Dust Explosion - Grain, flour, wheat

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Originally Posted by Icelander View Post
how many pounds of flour do we need to distribute in a low-hanging cloud that is at least 50 yards in radius for it to be explosive?
Varies by as much as a couple of orders of magnitude depending on conditions.

0.023 - 0.0253 ounces / cubic foot (minimum, coal dust)

40 g - 4000 g / cubic meter, 0.5 - 1 ounce / cubic foot (optimal), 0.15 - 0.3 cubic inches dust / cubic foot volume (minimum), 1.5 - 3 in^3 / ft^3 (optimum) (OSHA safety guidelines)

55 g / m^3, 0.05 oz / ft^3 (corn (maize) dust, minimum, Safety Data Sheet)

The phrases { Minimum | Optimum | Upper } Explosive { Limit | Concentration } and acronyms (MEL, etc) seem to be useful search terms.

Last edited by Anaraxes; 11-04-2015 at 03:16 PM.
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Old 11-04-2015, 03:13 PM   #22
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Default Re: Dust Explosion - Grain, flour, wheat

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Actually getting an explosion of interest is probably outside of the technical reach of the PCs -- it requires fairly constrained ratios of air to combustible material, and convincing large amounts of dust to spread out exactly right to produce that explosion is not trivial (it can happen by accident in a dry enclosed building, but you'll note that the vast majority of granaries do not explode).
In preparation for trying this, the PC took some courageous assistants and some bags of flour into an empty stone warhouse for several hours to use Measurement spells and his engineering and mathematical skills to determine the ratios of air to combustible materials.

Granted, replicating those ratios under combat conditions is impossible. But the range between minimum explosive concentration and maximum explosive concentration seems to be fairly wide, so it's not like there isn't a margin of error. Also, causing just 10% of the grain dust they throw down to actually explode in a proper dust explosion would probably rate as a wild success, given that this would equal the explosive force of more than half a ton of TNT among the enemy troops.

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My tendency, given the abilities you describe, is a ritual Create Fire (with that many mages, you can manage around a 100 hex radius), a ritual Shape Fire on the result, and then send the resulting gigantic ball of flame out to roll over the enemy.
In setting, ritual magic requires a Style Perk for the specific type of ritual magic practised and is fairly rare and controlled. None of these mages have access to it.

The enemy does have ritual magicians, but have been using this capability mostly to victual their armies on a series of forced marches that preceded this assault. They will also use it to open gates and breaches and try to use it to ruin morale, bowstrings, torsion artillery and smokepowder among the defenders by causing rainstorms with vindictive lightning strikes that will herald their attack. What ritual magic the PCs possess is mostly devoted to counter weather-magic.
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Old 11-04-2015, 03:27 PM   #23
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Default Re: Dust Explosion - Grain, flour, wheat

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Why not both? You've got plenty of flour, after all, and the more you can send over, the better your chances of getting a boom.
Yes, but the more flour you drop close to or even on your own walls, the better your chances of any dust explosion not being confined to the enemy ranks.

It may be possible to jury-rig 5-lb to 15-lb ballistae and various types of scorpions to shoot small bags of flour in the time alloted, but probably not for ranges anywhere close to what they get with bolts, lead balls or rocks. So chances are that the flour would be uncomfortably close to the defenders.

The enemy left a significant number of magical stone and lead projectiles for ballistae and magical bolts for scorpions. Some have longer ranges than normal, most are more accurate and some even split into many smaller projectiles. All of these would probably be more useful than jury-rigged flour bombs from the artillery, as long as the enemy is actually within range. Even non-magical bolts or lead spheres are probably better from these smaller devices.

As noted earlier, the big catapults are designed for harbour defence and situated poorly for landward defence. The enemy is taking care to attack where they are out of range from those or they at least have to shoot over some of the city and their own walls to hit. And even then, the improvised flour bags would probably be hitting very close to the walls, if they weren't bursting over them.

And there's also the roleplaying aspect that one PC, the one in a junior position, commanding only a small experimental unit of apprentice mages who use flying magic in battle, is the one who wants to do this. His superiors, PC and NPC, didn't actually forbid him from doing it, but neither did they show any special indication of wanting to give him scarce resources to accomplish it.

In fact, he was pretty much told: "Sure, whatever, it's your unit. Drop anything you like on the enemy if you've run out of Alchemist's Fire. Anything you can commandeer for your purposes is fine with me, it's a whole city we've captured and we haven't even started inventorying the loot. We gave you a rank; go and talk to some ships' pursers or quartermasters and bully them, that's what rank is for. Just give me support when they make their attack, kill as many of them as you can and force the rest to keep looking for threats in the sky. And don't kill our men, you young fool!"
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Old 11-04-2015, 03:45 PM   #24
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Default Re: Dust Explosion - Grain, flour, wheat

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In this case, the setting is a fantasy one, with most positions of political and military power occupied by what amounts to superheroes.
In which case you're still talking 30-50 against 1,000. It is certainly possible for one sufficiently cinematic or well equipped character to take on a thousand much worse equipped characters, but there simply doesn't sound like there's a large enough divergence in capabilities to make up for the numbers.
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It doesn't require cinematic rules, only the fact that they have a capability that the other side can't match.
Why is the other side incapable of matching? In any case, the obvious option for the opposition, given GURPS Magic, tends to involve weather magic.
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So I'm looking for guidelines on how much flour you'd need to have a chance of a dust explosion inside, say, a 10'-30' high cloud of flour that is at least 50 yards in radius.
According to OSHA, you need 40-4,000g/m^3 of flour dust to have an explosion, with an optimal value of 0.5-1 ounce/cf (500-1,000 g/m^3). Unfortunately, simply dropping flour will mostly produce clumps of flour on the ground, very little of it will actually wind up suspended -- you basically need a bursting charge that will cause your dust packets to break apart a short distance above the ground and come sifting down, and even then it will require anti-clumping agents or most of it will still come hailing down as lumps of flour. It might well be possible to invent a spell that will solve that problem, but it's inventing a new spell, not using an existing spell.

Your other problem is that, well, there's no way the PCs can actually design and implement any of this stuff in a useful time scale. Historically, the Nazis worked on coal dust bombs (using liquid oxygen), with testing as early as 1943, and still not deployed by 1945. Using the GURPS invention rules, it's probably an Average or Complex TL 7 technology that is totally new, which means a TL 7 character would be rolling at -15 to -19 on the concept roll.
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Also, causing just 10% of the grain dust they throw down to actually explode in a proper dust explosion would probably rate as a wild success, given that this would equal the explosive force of more than half a ton of TNT among the enemy troops.
Your most likely results are, in order:
  • Failure to ignite.
  • Non-explosive ignition -- you produce a fireball comparable to flame hexes.
  • Low pressure explosion -- you get shockwave formation, but not enough pressure to cause serious injuries.
  • Weak explosion -- something like 1% of theoretical max yield.
Also, half a ton of explosives really isn't that much; even if you managed it, you might get a hundred casualties out of it.
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Old 11-04-2015, 04:04 PM   #25
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Default Re: Dust Explosion - Grain, flour, wheat

Have they considered poisoning the flour and making it easy for the enemy to steal it?
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Old 11-04-2015, 04:17 PM   #26
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Default Re: Dust Explosion - Grain, flour, wheat

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you basically need a bursting charge that will cause your dust packets to break apart a short distance above the ground and come sifting down, and even then it will require anti-clumping agents or most of it will still come hailing down as lumps of flour. It might well be possible to invent a spell that will solve that problem, but it's inventing a new spell, not using an existing spell.
The djinn tornado is meant to accomplish that fact. Now as I think of it, though, if you've got that djinn to spread stuff through the air, you might be better off using oil than flour. Carry the biggest kegs of it you can, pour it over the target. When it hits the whirlwind, it'll be at least partially aerosolized, then drop a fireball on it.
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Old 11-04-2015, 04:22 PM   #27
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Default Re: Dust Explosion - Grain, flour, wheat

Grain explosions seem to happen on still, hot days. Shape Air might give you enough control to get a nice matrix of particulates. A magician could actually practice beforehand, creating a small dome of still air over a sack of flour, churning it gently, then sparking for effect.
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Old 11-04-2015, 04:29 PM   #28
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Default Re: Dust Explosion - Grain, flour, wheat

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In which case you're still talking 30-50 against 1,000. It is certainly possible for one sufficiently cinematic or well equipped character to take on a thousand much worse equipped characters, but there simply doesn't sound like there's a large enough divergence in capabilities to make up for the numbers.
The actual numbers are around 30,000 enemy, about a thousand of them superheroes*, with maybe 8,000-10,000 of the enemy being disciplined heavy infantry that actually have the morale, armour and cohesion to take a breach defended by discplined heavy infantry backed by musketeers and artillery. All of the enemy are hungry and tired, having operated at above and beyond human endurance for more than a week and long outpaced any supplies. Only drawing on the magical powers of their superheroes at far beyond the rate of at which they can replish have they been able to sustain their operating tempo at all.

On the PCs' side, around 3,000-5,000 sailors, 3,500 infantry of various types, but heavy on the professional, disciplined side, and some 4,000 cavalry, 7,500 infantry and maybe twenty dragons that they hope will arrive before morning. None exactly fresh, but all of them regularly fed, with good access to supplies and water, and only the kind of tired you get one or two days into a well-managed operation.

The concern is that the ca 10,000 heavy infantry that will assault through multiple magically created breaches and magically opened gates will succeed at overruning the ca 3,000 infantry that can be opposed to that at the critical point before the PCs' side is ready for the decisive battle at dawn, with all their armies having joined together.

And the PC in command of an experimental unit of flying mages is convinced that it is down to him and him alone to prevent it.

In actual fact, the battle may be won or lost without his involvement. It will help if his men annoy the enemy, of course. Killing 200+ enemy by dropping stuff on them would be great. It would help even more if they can tear the heart out of a thousand man unit of heavy infantry of the enemy reserve, massing into a column in preparation for exploiting a breach, the violence of the explosion stopping the assault for several hours as the ardour of any but the most fanatical enemy unit is momentarily dampened.

*Most of them just a bit tougher and more skilful than ordinary men, maybe capable of inspiring men to follow them with more than usual fervour and giving them small combat bonuses for short periods of time.

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Why is the other side incapable of matching? In any case, the obvious option for the opposition, given GURPS Magic, tends to involve weather magic.
Until very recently, most mages in the enemy country did not have anything to do with the military. The army was for occupying ground and keeping the peace, not TL4+magic combined arms operations. Superpowered champions duelled about anything important and while they could have flying chariots or change into birds, they usually didn't know the first thing about actual aerial warfare.

And most mages avoided any of this unpleasantness, as well they should, as the revolution of many combat mages against the ruling theocracy several centuries ago showed what mages who stepped out of their scholarly role were capable of doing.

It isn't that the enemy can't manage flying spells, if desired. It's that they have no useful tactical doctrine or training for flying units among most of the superheroes that happen to be along. They do have units of flying chariotry, griffon riders and Royal Guards who turn into man-sized Sunhawks, but none of them happen to be in this army. Many of them have already fallen in other battles, as their mobility led them to be sent to succour fleets that have been destroyed, the other field army that was broken by dragons or the cities that have been taken from the sea.

So any enemy that do use magic to fly will not have skills like Aerobatics, Airman (the aerial equivalent to Soldier), Dropping, Flight and Tactics (Aerial or Air-to-Ground). While the experimental unit that the PC commands has these skills at very low levels, they do at least have several months of training in them.

And then there's the fact that while the enemy could probably get around a 100 of their men flying, only around ten of those would match the speed and maneuverability of the flight magics used by the flying mages on the PCs' side.

And none of these ten novice fliers would come close to matching in agility and speed the PC commander himself, who is using magic that deploy wings made from a Great Wyrm Red Dragon, is the son of an ifrit prince and has soulbonded with a spirit of elemental air.

Mind you, on the ground, probably around 200 of the enemy champions could take him one-on-one. In the air, he's a modern fighter jet facing 1914-1915 WWI scout planes.

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According to OSHA, you need 40-4,000g/m^3 of flour dust to have an explosion, with an optimal value of 0.5-1 ounce/cf (500-1,000 g/m^3). Unfortunately, simply dropping flour will mostly produce clumps of flour on the ground, very little of it will actually wind up suspended -- you basically need a bursting charge that will cause your dust packets to break apart a short distance above the ground and come sifting down, and even then it will require anti-clumping agents or most of it will still come hailing down as lumps of flour. It might well be possible to invent a spell that will solve that problem, but it's inventing a new spell, not using an existing spell.
They won't invent any spell. But would a djinn, a spirit of air, that can transform himself into a tornado, be able to distribute the grain dust or flour?

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Your other problem is that, well, there's no way the PCs can actually design and implement any of this stuff in a useful time scale. Historically, the Nazis worked on coal dust bombs (using liquid oxygen), with testing as early as 1943, and still not deployed by 1945. Using the GURPS invention rules, it's probably an Average or Complex TL 7 technology that is totally new, which means a TL 7 character would be rolling at -15 to -19 on the concept roll.
They will design nothing, no. There is no way for them to get a technological device that will yield consistent results in a safe way that are competative with TL6 explosives.
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Old 11-04-2015, 04:35 PM   #29
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Default Re: Dust Explosion - Grain, flour, wheat

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Have they considered poisoning the flour and making it easy for the enemy to steal it?
Ironically, yes. Some days before this took place, soldiers working for the PCs left a defensive position* to the enemy. Before they left, they were instructed to destroy anything of use to the other side. That included some 200 tons of grain.

The commander of the scout unit was busy poisoning the grain when this PC arrived. It was pretty clear to the PC that the enemy would probably not allow everyone to gorge on it at once, before using magic to check it for purity, and poisoning it would therefore only serve the purpose of killing poorly discplined impressed troops that were serving as peltasts and got to it first. The PC therefore felt it was more cruel than actually militarily useful and would lead to a cycle of atrocities that didn't contribute to either side winning the war. So he used his fire sorcery to burn it all instead.

*As part of a redeployment that eventually led to them being able to make an amphibious assault that took this city.
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Old 11-04-2015, 04:47 PM   #30
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Default Re: Dust Explosion - Grain, flour, wheat

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The djinn tornado is meant to accomplish that fact. Now as I think of it, though, if you've got that djinn to spread stuff through the air, you might be better off using oil than flour. Carry the biggest kegs of it you can, pour it over the target. When it hits the whirlwind, it'll be at least partially aerosolized, then drop a fireball on it.
Why not both? Any flour that doesn't burn in the initial run is still a flammable, somewhat sticky substance when the cloud of oil ignites. If the first firestorm doesn't kill 'em, the second probably will!
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