This actually applies to all of 40k, but I found one particular funny example while test-running a scenario between some marines and freedom fighters blasphemous heretics.

Now, we all know promethium rules. 1d10 damage and a 4m explosion per kilogram. These heretics rigged up promethium stolen from a fuel depot, stored it in a half dozen 50 gallon drums, and rigged it to blow. I was just going to make it a save-or-die thing, but I decided to see what the actual damage was.

Now, in america we use a dumb system of measurement, so after some conversions, I discovered that a 50 gallon drum holds a whopping 208 litres. Sounds like quite a lot, but I figured that promethium was fairly dense. So I looked up the density of something very similiar, napalm.

Napalm has a density of about 750kg per metres cubed, or 750 grams per cubic metre, or .75 kg per litre.

So, .75 times 208 is 156. So in a 50 gallon drum there are 156 kg of promethium. Or, in game terms, enough to incinerate a Tyranid Hierophant in a fireball over a kilometre wide.

Lol.

So, clearly we either need to severely reduce the power of promethium or put a cap on the maximum blast power and damage. Any ideas?

Use small craft for better delivery of those bombs.

Pretty sure that explosives like that get a reduced effect as mass (and blast radius) increase. I'm hardly a specialist, but A) the blast is going to be unfocused because not all of the material reacts simultaneously and B) I'm pretty sure that since the volume of a sphere is four thirds pi R cubed, that the strength of the explosion (and the area of effect) is going to be based on the cube root of mass used, as opposed to a linear progression. In other words, 159 kg of promethium would deal 5d10, not 159d10. Similarly, the blast radius would be 22m (5.4 times 4m), not 1+km.

Still awesome, but less game-breaking.

Physics and three dimensional geometry. It matters.

If we have any physicists who really understand this stuff who could come up with a better model than I without introducing needless complications, feel free to jump in.

While I am a physicist, explosions and explosives was never my speciality.

I do own a book on the physics on nuclear weapons, but never got around to actually reading it.

So, what I write here should be taken with a grain a salt, as I'm trying to dredge useful information out of memories I haven't really used in years.

Right, normal explosions tend to have a 1/r^2 behaviour, as we're really looking at the area of the blastwave at the given distance (this btw, is why indoor detonations are so nasty, if the walls are strong enough to hold, they scale closer to 1/r.

Now, fuel explosions behave very differently, and indeed some of you may be familiar with FAE-devices. In which case, feel free to correct me, because I have only superficial theoretical and absolutely no practical understanding of such devices.

The idea here is - as mentioned - that not all of the fuel will detonate at the same time. something will detonate first, and this serves 2 purposes:

1) Spreading the rest of the fuel over a larger area/volume (both, technically, as it will obviously be spread through a volume, but since fuel and especially fuel containers tend to be heavier than air, it'll be a somewhat flattened effect).

2) Setting off these secondary effects, which will likely be more flame and less explosion that the original detonations, as the fuel containers are likely to have been ruptured.

This serves to create a "fireball" effect, wherein a large area is affected in a somewhat homogeneous manner, as opposed to the above-mentioned 1/r^2

It's mainly useful against infantry I believe, though this is purely speculative.

But if you want to hear physics crying, go read through the Rogue Trader scenario Frozen Reaches, wherein we have a "legendary" gas giant hiding in an astroid belt. I like to show that one to physicists who're also roleplayers. Most of them giggle, though a few of them look like they'd rather be crying.

It's like saying a nuke has the explosive force of x amount of tnt. It's true, but you're never going to be able to deliver that explosive potential targeted to that specific point.

Well, of course promethium explosions wouldn't scale like that realistically. I was merely pointing out a funny exploit a player with an ignorant GM could use.

Coincidentally enough, I've taken quite an interest in FAEs (although the broader term is Thermobaric Weaponry) ever since encountering them in Ace Combat 6.

I'll spare you the details, but despite what one might think very little damage is lost when compared to conventional explosives, and is fully capable of leveling buildings and destroying vehicles. It has something to do with it causing a prolonged blast wave, although I won't pretend to fully understand what that means. All I really know is that they've been occasionally been mistaken for small nukes. (Mostly because of the shockwave and mushroom cloud)

But I digress, unless the players are capable of designing a thermobaric weapon themselves, the damage of a large batch of promethium should have a set cap of some sort. What Annaamarth suggested seemed to be on the right track, although I feel like 5 kilos of prom doing the same damage as 159 kilos a little odd. However, a 50 gallon drum of promethium effectively having blast (20) seems fitting.

Bear in mind that those five kilos would each be blast 4, and deal 1d10 damage. 5d10 blast 20 is more catastrophic by far.

Edit: although, having seen Tenebrae's explanation, I could make a case for

Damage= ((mass in kilos)^1/2)d10

Blast ((mass in kilos)^1/3)

But that's slightly more complicated.

I just hope our missionary NEVER READS THIS!!!

Edit: although, having seen Tenebrae's explanation, I could make a case for

It's just me trying to dredge stuff out of my long term memory.

Well, of course promethium explosions wouldn't scale like that realistically. I was merely pointing out a funny exploit a player with an ignorant GM could use.

Coincidentally enough, I've taken quite an interest in FAEs (although the broader term is Thermobaric Weaponry) ever since encountering them in Ace Combat 6.

I'll spare you the details, but despite what one might think very little damage is lost when compared to conventional explosives, and is fully capable of leveling buildings and destroying vehicles. It has something to do with it causing a prolonged blast wave, although I won't pretend to fully understand what that means. All I really know is that they've been occasionally been mistaken for small nukes. (Mostly because of the shockwave and mushroom cloud)

I was purely guessing based on a not very detailed description.

Maybe it's time I read up on them

Bear in mind that those five kilos would each be blast 4, and deal 1d10 damage. 5d10 blast 20 is more catastrophic by far.

Actually, I'm 99% certain that the profile for promethium says 1d10 damage and 4m radius per kilo used...

I just hope our missionary NEVER READS THIS!!!

Well, just remember that if he does you have a premade argument for not destroying hive stacks with a barrel of promethium.

Unfortunately, he might decide to create thermobaric weapons instead.

To get an idea of how scary they can be, take the Firebomb, AKA the Molotov Cocktail. It is in many ways a very crude variation on thermobaric weapons.

Now, the firebomb weighs .5 kg. A 12 oz glass bottle (a close approximation to whatever is used in the Imperium) weighs a very rough average (based on the contents of my recycling bin) of 250 grams. In addition, the fuse is likely a strip of cloth thouroughly soaked in gasoline/promethium, which I would approximate at 50 grams.

This leaves us .2 kg of promethium as the actual fuel, enough to fill the 12 oz (.354 litre) bottle a little less than half full.

I promise the math is almost done now, just bear with me through this last stretch. 1 kilogram of promethium deals:

1d10 E Pen 0 blast (4) Flame

.2 kilograms of promethium when crudely atomised (in a bottle suddenly broken open) and ignited deals:

1d10+3 E Pen 6 Blast (3) Flame

As you can see, the levels of energy released are significantly different. For 20% of the fuel you increase the damage by a significant amount (160% if we assume the average roll is 5), a blast wave that will ignore all but extremely well protected and sealed armour (power armour, storm trooper carapace, etc.). While it has 3/4 the blast, again it only used 1/5 the fuel.

Now, of course these comparisons won't scale exactly. For example, even the larger modern Thermobaric warheads won't destroy tanks, although they can tear off optics, damage engines, and potentially bake the crew if the tanks crew compartment isn't properly isolated.

I will now apologize for the length of this post, and if I've given anyone nasty ideas for weapons to be used by/against players.

Although it does make one wonder why the Imperium bothers with valuable Nova shells on their tanks...

That is a fair point, I read the whole post and it does paint a very interesting picture. Thermobaric weapons would be ridiculously effective against Orks especially, as it would kill the spoor as well as the xenos themselves...

A regular armsman (S30) can carry 36kg of promethium without encumbrance. That's a blast radius of 144m, longer than the range of many weapons, and certainly far longer than the range at which they're likely to hit anything (i.e. point blank).

10,018.8 metric tons of promethium would scorch the entire surface of the earth - how much do you think a transport ship could carry?

I made this thread once before. I just banned stacking explosives in my games. If they wanna blow **** up, they can use meltabombs.

Oh, and for discussing physics in relation to a silly game like Rogue Trader, Bzro, that's 300 points from Gryffindor.

^ this guy did not read the rest of the thread.

Per my suggestion of scaling based on the cube root of mass, 36 kilos of promethium would deal 3d10 damage blast 13- I'd say that's fair for an incendiary satchel charge. A clever Tech-use/Demolitions check might let you convert it to a thermobaric charge, in which case I'd allow the scale to go to square root, for 6d10 blast 24, at which point we are now competing with a lascannon for damage, no?

Okay, that's 500 points from Gryffindor.

*shrug* Meh, I never really liked those books anyway. Stick with your meltabombs, I like my method just fine.

^ this guy did not read the rest of the thread.

Per my suggestion of scaling based on the cube root of mass, 36 kilos of promethium would deal 3d10 damage blast 13- I'd say that's fair for an incendiary satchel charge. A clever Tech-use/Demolitions check might let you convert it to a thermobaric charge, in which case I'd allow the scale to go to square root, for 6d10 blast 24, at which point we are now competing with aa lascannon for damage, no?

That's an interesting idea. No one has ever really made thermobaric weapons that don't get the charge as far away from the user as fast as possible. Especially not one that big.

The Thermobaric RPGs made by the Russians have a lethal radius of 10m. (Almost exactly 10m. Thermobaric weapons are very controllable). And those only have a 2kg payload.

On the other hand, this is 40k. They're using promethium, not specially manufactured FAE materials. And they probably have very little idea how to design one properly.

As an improvised thermobaric charge, I'd say it's fairly well represented. You may want to add a special rule to take into account the fact that cover is next to useless against that kind of weapon though.

Okay, that's 500 points from Gryffindor.

Also, I think the fact that we're doing all of this math simply because we want to makes a pretty strong case for us being Ravenclaw, not griffindor.

LOL Physics?.... In a game? LMAO ROFL LOL !!!!! .......

I hoped this thread was about bullcrap like gravity waves or solar systems that still exists after its star gone supernova, or navigating in void close to stellar bodies, acceleration and the like. (Gas giant losing mass after destruction of its star WTF? I explain it that the Whisperer with its dark-energy, zero point energy generators is countering the planets gravity with antigravity, hence venting its outer atmosphere in the void)

But I also learned that when using explosives like prometheum use sqare root of mass, and should be fine. A truckload of prometheum should make a fireball 50m wide, enough to destroy a wall of embassy and killing lot of people inside.

I don't think you need to reduce the power of it at all or adjust ****, it's such a rare resource it's not a gamebreaker that it can do that. Lets be honest just about the worst thing that can happen in the setting from the player perspective short of the passage collapsing is either Winterscale or Chorda gaining total or near total control of the planet where this crap comes from. The only thing that keeps either of them down is each other and their perpetual Gears of War style war over infinite energy/money.

So I think the important thing to understand though about lighting off a barrel of this crap is that you may as well be lighting off a barrel of gold bullion.

I hoped this thread was about bullcrap like gravity waves or solar systems that still exists after its star gone supernova, or navigating in void close to stellar bodies, acceleration and the like. (Gas giant losing mass after destruction of its star WTF? I explain it that the Whisperer with its dark-energy, zero point energy generators is countering the planets gravity with antigravity, hence venting its outer atmosphere in the void)

But I also learned that when using explosives like prometheum use sqare root of mass, and should be fine. A truckload of prometheum should make a fireball 50m wide, enough to destroy a wall of embassy and killing lot of people inside.

Agreed.

Completely.

I don't think you need to reduce the power of it at all or adjust ****, it's such a rare resource it's not a gamebreaker that it can do that. Lets be honest just about the worst thing that can happen in the setting from the player perspective short of the passage collapsing is either Winterscale or Chorda gaining total or near total control of the planet where this crap comes from. The only thing that keeps either of them down is each other and their perpetual Gears of War style war over infinite energy/money.

So I think the important thing to understand though about lighting off a barrel of this crap is that you may as well be lighting off a barrel of gold bullion.

Anyway, I always assumed that promethium was fairly common (scarce availability, so like hunting rifles), and tended to be composed of whatever was common enough on the planet of origin. I figured thats why all flamer weapons have pilot lights when the purified Promethium fuels used by the Imperial Guard supposedly ignite on contact with oxygen (and are also self oxidizing so they can burn in space and underwater... wut).

Regardless, while valuable, it's not that difficult to get. If we used the straight linear scaling, then a 435 kg (such a weird way to measure a liquid/gel) charge would detonate with the force of the Little Boy nuclear bomb, with a blast radius of 1.74 kilometers. And that aquisition test would have a modifier of +0.

If that's the case, then screw atomics. I'm just going to fire massive promethium charges at people.

Also, while the lighting off a barrel of gold bullion is actually slightly accurate, the imperium frequently makes machine guns that fire explosive scientific miracles hundreds of times a minute, and we somehow have massive supplies of shells the size of buses to fire out of our space guns.

"But if you want to hear physics crying, go read through the Rogue Trader scenario Frozen Reaches, wherein we have a "legendary" gas giant hiding in an astroid belt. I like to show that one to physicists who're also roleplayers. Most of them giggle, though a few of them look like they'd rather be crying."

When running Frozen Reaches, I changed the Damaris system. I made the star an A type supergiant (I think, I might have gone for O or B, I'd have to look it up). The inhabited planet was in the biozone, but the biozone was some wacky distance from the star (again from memory, I'm thinking 200-300 AUs). The gas giant was then some 1000+ AUs from the star, hence why it was undetectable, asteroid belt or not. By using this type of star system I could then justify a potential mining bonanza.

FFG scenarios all need to be retconned. Taken at face value they are all completely silly. I like their situations, though, their plotlines. I just have to find a way to make sense out of their lack of science knowledge.

<snip>

I assumed you were making a joke about a gas giant being undetectable because it was obscured by a bunch of little rocks each a bazillionth the size of said gas giant, all thousands of miles apart.

I didn't cry, I just remade the system to fit the story. I liked the story so I made it work.