Dust Explosion - Grain, flour, wheat

[Icelander]

A PC in a fantasy campaign set at TL4*, a PC mage who has access to a lot of fire magic and a fair amount of flying magic wants to explore the possibilities of burning and/or exploding a huge number of people in a besieging army around the walled city he and his men are occupying.

This PC commands some fifty mages, most of whom are very low-powered on a Mass Combat scale. Magery levels are mostly 1 (ca 60% of them), with a minority of Magery 2-3 and maybe eight mages who are Magery 4+. These mages have a way to fly magically, but most of them are not powerful enough to cast such a spell themselves, but instead use one of several Wands that can be used to cast up to 50 such spells before being depleted.

All of the flying wizards also have at least one Wand (from 5-25 charges) that can make a ranged attack, mostly Fireballs** that can cause significant injury within 10' or so and wound out to 30' radius.

Now, the PC wants to increase the destructive power of these flying wizards against a mostly earth-bound foe. Low-flying wizards casting fireballs are cool and all, but not only would they be taking huge risks flying that low over enemy lines, they could also expend all their fireballs without taking out all the enemy. There is a very large army of 30,000 men coming, with the cavalry vanguard and elite light troops already outside the city walls preparing the assault.

With trial and error, our PC has already found out that flying high and dropping stuff on ground-bound foes is very effective. On the other hand, such indiscriminate dropping of sharp and/or heavy things from the air not going to make much of a dent in the numbers of 30,000 enemies marching to take the city the PCs are defending, especially as those 30,000 enemies are going to either take the city or be broken against the walls within the next 24 hours.

The flying wizards don't have the flight magic for more than one or two sorties within that time. Also, lingering above the enemy lines dropping stuff on them eventually leads to them summoning flying creatures to kill the mages. Any height where the mages can actually see valuable targets and hit them with fireballs or dropped stuff is also a height where ground-bound enemy spellcasters can take them out and/or summoned flying creatures can easily catch them.

The city where the PCs are preparing their defence used to be the primary port city of the invading enemy armies. Military supplies for three field armies, more than 70,000 men, were kept in warehouses there, after being shipped in from the country where the enemy comes from. There is also food for the population of the city and for tens of thousands of labourers working for the invading armies. That means a lot of tons of grain (so many thousands), some of which has already been ground and made into flour.

So the flying mages can get as much grain as they want. And more of that grain than they could carry up in the air, even in several sorties, is available as finely ground as they'd like.***

Is dropping a ton or two of flour into the air around enemies they want blown up before fireballing them even a viable strategy?

The way the PC sees it happening is that his mages carry up into the air as much flour, still in bags, as they can carry. Then they drop it all on the foe, hoping that it will hit the ground hard and a lot of it will be spread to the sides and float around in the air. The PC then thought he'd get a summoned djinn to spread the flour around at ground level before hitting the area with as much fire as possible.

How much flour do you need to spread into the atmosphere for a proper dust explosion?

Working with no more than maybe two tons of flour, how large an area could the PC realistically aim for?

What are guidelines for the damage of such a dust exlosion, if I can get one to happen?

*With modifiers, the most significant one being that conventional gunpowder doesn't work for some arcane reason and smokepowder, the alchemical alternative, is a couple of orders of magnitude more expensive.
**6d burn ex/2.
***There were enough nobles and commanders in the city for there to be a lot of demand for top-quality unhealthy white bread and other baked goods.

[Anthony]

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).

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.

[Icelander]

Originally Posted by Anthony

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.

Originally Posted by Anthony

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.

[Nereidalbel]

Two tons of flower? What else can your mages and/or army grind into dust? If you can grind things fine enough, almost anything is explosive, even aluminum and wood. Do note that sawdust should be readily available, and probably a bit cheaper than flour.

Here's some data on various explosive dusts.

And here's 4 kilograms of flour exploding.

[Bruno]

I don't buy them needing to be at a low altitude to do bombing runs. It's not going to be about precision anyways, unless you're seriously trying to take out commanders or standard bearers or whatever. If your target is a massed army, you're fairly likely to disrupt columns even if your dropped junk goes between two soldiers. People become very concerned about things being dropped on their heads...

That said, you'd do better to drop things more interesting than just rocks.

I understand the urge to try and get the flour to go ka-WOOOSH, it's pretty hardcore. However, it's a hazard associated with grain silos because those are enclosed spaces. You can get it when transferring grain in a train yard, but it's a lot less common.

Oil? Animal fat? Alcohol?

[The Colonel]

Dust explosions can be nasty, but seem especially hard to arrange directly - the Flakwaffe apparently put a lot of effort into trying to develop a thermobaric weapon based on coal dust, but never had much success - getting the mixing right is apparently a struggle.

[Icelander]

Originally Posted by Bruno

I don't buy them needing to be at a low altitude to do bombing runs. It's not going to be about precision anyways, unless you're seriously trying to take out commanders or standard bearers or whatever.

Well, that's one way for 30-50 guys to do something that manages to actually affect an army of 30,000.

The 1,000 or so 'heroes' spread around the enemy army are the important ones. They include the spellcasters that will breach the walls and the superhero warriors who will lead their men through quite a lot of TL3+1 mechanical artillery and good archers and even musketeers. Without the heroes, a lot of the slave battalions will refuse to go into action at all and even the better quality units will probably not be able to force a breach defended by the PCs and their professional mercenary reserve.

Originally Posted by Bruno

If your target is a massed army, you're fairly likely to disrupt columns even if your dropped junk goes between two soldiers. People become very concerned about things being dropped on their heads...

Disrupting some columns is fairly trivial, but not likely to make much of a difference to the final result. Killing a thousand men in seconds might cause the enemy to stop and reevaluate, but killing a hundred men all over a whole field with 30,000, over a period of several minutes, is probably not going to stop any attacks.

Disrupting the momentum of assaults as they happen is more promising, but requires at least accuracy within 5 yards of the target or so. The 5,000-8,000 elites who will actually make the assaults of the city will attacking the walls that the defenders are standing at while the most important battle is happening, after all.

Originally Posted by Bruno

That said, you'd do better to drop things more interesting than just rocks.

There's a lot of arrows, crossbow bolts, darts, javelins and scorpion bolts.

Those are a lot more bulky to carry and throw than flour bags and need to be dropped with rather more care for each one. I figure you could carry far less weight of them while flying. I don't think many of the mages could carry more than say 100 such items each for a sortie, most probably fewer (depending on arrangment of quivers and such). That's maybe 25 lbs. each, instead of maybe 50 lbs. for a bag of flour each flying mage. Those flying with dragonwing harnesses might carry 200 lbs. of flour, but added strength doesn't really help them with the bulk issue for sharp things.

Also, to drop arrows so that they fall properly would seem to require actual Dropping skill and decent Flight or Aerobatics skill, while getting a bag of flour up into the air and then causing it to fall somewhere within a fairly large radius might not. So even if 50 mages could fly up and drop a single bag of flour, I'm guessing that only 30-40 of them are capable of dropping arrows with any effect.

So, we could deploy maybe 3,000-8,000 sharp pointy things weighing 0.25-2 lbs. each if we went with that instead of maybe up to two tons of flour.

How many of the sharp things will hit a soldier when dropped in bunches from a height of thousand yards in the dark?

I'm guessing that even when you hit a column, very few of the arrows actually hit an individual soldier. And that many of the mages will not hit even a column of soldiers 12x30 yards in area from a height of 1,000 yards.

Originally Posted by Bruno

I understand the urge to try and get the flour to go ka-WOOOSH, it's pretty hardcore. However, it's a hazard associated with grain silos because those are enclosed spaces. You can get it when transferring grain in a train yard, but it's a lot less common.

I realise that if the flour caught on fire while out in the open we would get a very inefficient 'explosion', as opposed one one in an enclosed space. I am trying to determine if it would be not worth doing, however.

Even if we got a REF of only 0.01 (flour isn't really an explosive, though it burns), one or two tons means it would still be the equivalent of 2-4 lbs. of TNT spread over a fairly large area.

Originally Posted by Bruno

Oil? Animal fat? Alcohol?

Alchemist's Fire and Thayan (Greek) Fire are both available. It's just that the PCs expect to expend their entire supply without stopping the enemy and are looking for ways to make their firebombing more effective.

[Anthony]

Originally Posted by Icelander

Well, that's one way for 30-50 guys to do something that manages to actually affect an army of 30,000.

Bear in mind that all realistic options amount to "and the 30-50 guys get killed while causing modest damage to the enemy". The normal use for a force that small is to do popup attacks that distract the enemy and get them off going on sniper hunts instead of something more useful. If you want a result other than that, you're going off on cinematic tangents and realism may be moot.

[Nereidalbel]

Out of curiosity, are any anachronistic metals known to alchemists/metallurgists? And by that, I mean things that weren't discovered in the real world until long after TL4 was done with.

[Icelander]

Originally Posted by Anthony

Bear in mind that all realistic options amount to "and the 30-50 guys get killed while causing modest damage to the enemy". The normal use for a force that small is to do popup attacks that distract the enemy and get them off going on sniper hunts instead of something more useful. If you want a result other than that, you're going off on cinematic tangents and realism may be moot.

If each of the 30-50 guys were flying a F-35 Lightning II, I'm not all that sure this would be true. If the 30-50 guys are combat engineers setting off several days' worth of mines and explosive traps where the enemy is trying to get through, I'm sure it isn't. Powerful devices of war which require only a few humans to operate are not only realistic, they are real. In this case, the setting is a fantasy one, with most positions of political and military power occupied by what amounts to superheroes.

The 30-50 guys in this case are mages who will be flying. That's not realistic, I admit, but it doesn't have to be cinematic. Most of the 30-50 guys are not really superheroes, but are instead fairly realistic humans, other than having Magery 1-2 and a few spells. That, in turn, makes them able to use magical objects that allow them flight at up to Move 40 for less than twenty minutes a day, while carrying up to Heavy encumbrance (which, however, slows them). They also carry at least one magical weapon each, with most of them having something like a Wand of Fireballs (6d burn ex/2, Range 75/150; 5-15 charges left).

I realise that this is a far cry from a modern fighter jet. But if we treat their supernatural abililities in a realistic fashion in the setting, they still might be able to have an impact on the battlefield that is only marginally related to their small number. It doesn't require cinematic rules, only the fact that they have a capability that the other side can't match.

The enemy can arrange for flying opposition, but did not expect to have to do so and do not have much tactical expertise at directing summoned flying creatures to secure the air above the battlefield. So far, anything flying that the enemy has brought has succumbed to PC-related injuries. I realise that this might change, but the PCs are planning for air supremacy*, at least for a brief window of 15-20 minutes.

If the flying mages fly low enough to cast spells at the enemy, they will be in arrow, sling and spell range from the other side and will quickly die. At 500+ yards, survival becomes theoretically possible. At 1,000+ yards and not lingering long enough for an organised response of summoned creatures to emerge, the risks might even be tolerable. At least enough so that the PCs have decided to risk a flying sortie shortly before or during the enemy evening assault on the city.

The flying mages can carry up to 2 tons of weight while still being able to fly at a reasonable rate and maintain control fairly well. Bulk considerations might lead to a much lower load, of course. I know that 2 tons of Alchemist's Fire would be very effective as an air-to-ground weapon, but there isn't that much available of it. And in any case, it already comes in artillery shell form, which means that what the PCs have of it will be deployed on the enemy anyway.

The PC in command suggested trying to use some of the massive amount of grain there to cause a dust explosion among the enemy army. It seemed the best source of finely ground powder and a TL3 port city didn't seem to offer anything more destructive per weight to drop from a height of 1,000 yards on an army. Now, the flour obviously won't be destructive at all unless he can create perfect conditions for a dust explosion in a given area. 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.

The character in question has TL4 versions of Alchemy; Engineering (Combat); Explosives (Demolitons); Explosives (Fireworks); Expert Skill (Natural Philosophy); Intelligence Analysis; Mathematics (Applied); Mathematics (Surveying); Naturalist; Tactics (Air-to-Ground); Weather Sense at skill levels 13+. His Scrounging skill is 17. He has spells such as Measurement, Tell Position and Predict Weather.

A djinn ally can be a great help in spreading the flour around, with the ability to turn into a magical tornado and generally a fairly precise control over the atmosphere around him. He can't do all the work himself, but if 2 tons of flour were haphazardly thrown to the ground from 1,000 yards in the air, he could probably undertake to move things along a lot closer to ideal distribution of flour to air. Under combat conditions, of course, it won't be anywhere near ideal, but that just means that less of the flour actually burns/explodes.

Under imaginary perfect conditions, 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? I have to admit that I have no idea. Are we talking 100 lbs. or 100 tons? More?

*Because if they don't manage to secure it, the odds are pretty good that the army with the far larger number of people and the much higher Troop Strength will win.