After all that discussion about grenades and thus something hit me.

Grenade rules add damage because the blast reverberate on wall back to explosion origine. If room is small enough the blast could reverberate multiple time causing very high damage.

Then I realise that the shockwave cant reverberate more than once on the wall, because the instant he reach back origin point it meet with another shockwave incoming from the exact opposite direction with the exact same strenght and CANCEL each other. Of course, for ease I am considering a square room with explosion origin being the center of the square.

Think it is time to errata the thing to limit reverberation to only one time.

Assume a spherical cow, all the complexity is gone! :/

You know... you could assume that if you stuck a person, in an elevator with a grenade... they deserve to die... Just saying.

I mean as a future DM if my players ever put an NPC in that position that isn't a freaking robot, I'm just gonna explain the fine-mist shooting through the closed doors.

If it's a robot I'll explain the inevitable sound of the elevator dropping X stories screeching all the way down, and thunk... *dust cloud*

Also just an FYI but nuclear weapons used focused explosives to compact raw nuclear material to start the reaction. These specially crafted explosive charges are build to direct their force inward, they don't "negate each other". In an elevator the force would direct outward from the device, attempting to escape the space, be limited by the lack of available space and compact and destroy what is within the space. The force and damage per area of a contained explosion is usually bigger than one that isn't.

You may want to read up on interference. Whether they cancel each other out depends on the wavelength and the difference in traveled distance of the "two" waves. Also the shock wave possibly only has one crest. Reflecting that crest will not cause interference.

"chunky salsa just hit me" I'll get a mop.
There is one small problem with your theory though, beyond what others have said aboput frequency- think of that pressure as two trucks speeding towards each other. Sure they will stop and lose all their kenetic energy when they hit each other, but what is caught between them will absorb ALL of the energy.

But, it they should not drive back and hit that poor thing again...

Yep, the objects retain force, when they strike each other they transfer their force (which is running opposite to each other) violently into the opposing object. Those objects are compressed by the opposing forces, thus the crumpled can effect when a car slams into another. Remaining force is actually radiated out from the collision point, thus expressed by interference, and the radial pattern you see when waves of water collide and express the remaining force outward from the collision point.

Now I remember why I failed physics. My brain hurts now.

Actually, they should… if they're air molecules (just for example). They hit each other, and bounce off again. Pressure waves are different, and just pass *through* each other, I thought. You don't get cancellation like OP mentions unless they're opposite, and they wouldn't be.

There's a good example of how force is transferred, a prime example are custom made "wax slug shells" that are being tested by a group. Basically fill a standard shotgun shell with wax to bind the smell pellets together. When the "wax slug" is fired initially it will travel the distance to the target, strike, enter the target then the friction will begin to apply splitting the shell into its component pellets. What you get is an absolute mess as each pellet transfers more resulting force into the flesh. Essentially the equivalent of shooting a standard shotgun shell point blank with a target without having to be point blank.

There is another version of this for high powered rifles that breaks up upon entry into a wall... splatters whatever is behind it in super-hot shrapnel.

The biggest flaw in the physics of the chunky salsa rule is that the resounding force loses no strength from its impact with the wall. Just the kind of perfect elasticity to have Newton spinning in his grave (entropically of course).

Yeah, but it's complicated enough already. If you wanted to, you could could hitting barriers themselves as an extra 1m of distance, for example. Honestly, it doesn't make the chunky salsa rule any better to do so, and the crazy edge cases are still pretty crazy.

OK, better explosives rules. Gotcha.

I kinda liked the idea that someone half-suggested in the other thread: just give a flat bonus/multiplier for coarse situations like 'small fully-enclosed space' (DV+6, DV*3, I dunno), 'half-enclosed' (DV+3, DV*1.5), etc. You'd have to play around and see if it works at all, but it would at least be fast and simple (half the battle).

The other three-halves of the battle is making grenade/rocket rules *match* demolition/explosives rules in power, effects, and price.

heh... if you want to complicate things further.... When an explosion occurs, especially in an enclosed area all that oxygen is burnt off creating a temporary vacum, now was the players in a hermeticly sealed vault did you say??
In an open area it actually means you get slapped twice... once by the shockwave of the explosion and once by the inrushing air....

That'd be a hell on an explosion. Not a hand grenade, but sure.

In the situation expressed in the OP, in which the subject is exactly in the middle in order for the returning shock waves to meet at the subject, the returning shock waves would add to each other, not cancel.

In implosion-type nuclear weapons, the triggering explosions never get the chance to meet each other. Fat Man (the Nagasaki bomb) had 32 outer explosive charges that hit a second layer of explosives that hit a third layer of explosives that hit a layer of aluminium that pushes a layer of beryllium-polonium and a layer of plutonium, with a now near-perfect inward spheric shockwave. The plutonium is crushed to reach critical mass while the compressed beryllium-polonium layer release a burst of neutrons to ignite the nuclear fission. At that point, what remains of the triggering explosions can abandon all hope of ever meeting each other one day because there now is a nuclear explosion starting between them.

Exactly, pbangarth, but after adding at the exact middle, they'd continue 'through' each other, wouldn't they?

Actually the rules only deal with explosions in confined spaces... I'm not certain simply setting a grenade off by the ground or next to a wall is supposed to cause chunky salsa.

As soon as any hole in the enclosure is created the thing is supposed to stop bouncing and spread out normally.

Though I guess... it depends... an elevator even with the door open is still pretty confined...




Also, high explosives don't burn off the oxygen, they include their own oxidizer... (they'll even go off in a vacuum). Some fuel air explosives will burn off the oxygen in an area... but even that is more of a problem in enclosed spaces.

Well, the blast first checks the barrier; if the barrier breaks, there's no reflection (and in fact causes shrapnel in the outward direction).

Yeah, if you start saying 'the ground or a wall isn't okay', you have to decide where the line is… and why. The 'why' is going to tell you that everything reflects (if it can), so that's out. But then you've got some nutty results to contend with, even *without* thinking about multiple passes of reflection: a grenade in the corner of a strong room is now doing a 800% blast (at least, it's spread over 1/8th of the sphere)? Or is it 400% (which is still horrific)? Get out your protractor and prepare for TPK (or, easy defeat of that big bad you wasted time writing a backstory for).

You forgot the linear vs. quadratic falloff issue.

Guess you could always just consider it volumetically... if a normal grenade covers 300 cubic meters... and the blast is confined in a 100cubic meter space... triple the damage.

Still doesn't deal well with something like a dead end hallway though... except for channeling the blast out and into the PC's (evil GM laugh)...

Yes, they would, just as waves in water. As was mentioned above, too, there would be some inelastic absorption in the walls, even if they held and didn't buckle, and the distance to power curve is not linear. But shit, who wants to do that kind of calculation?

Why did I have to give my physics textbook to my old GM as a RPG aid and not get it back before I moved?

To be fair, the chances of the shockwaves rebounding on the point of orgin are near insignificant.

I do find it funny that frag grenades use this use when the majority of their damage is not caused by a shockwave.

Ok...

I was on the impression that if an explosion occur in the middle of a symetrical square room, then every force vector got a symetrical one going the other way.

Your water ring metaphor is interesting. What happen if I throw a stone in the middle of a pond, the wave will go from the center then hit the border and then go back toward the center, when they meet there they collide and the extra energy is dispersed vertically. They dont get through each other.

Now, what happen in the case of a tridimensionnal pond ? lets talk a sphere as it is more easy to comprehend than a cube but I am unsure the result are that much different. All the shockwave go away from explosion center then they bounce, lose energy then go back toward the center where they meet each other. The explosion origin being the center of the sphere, each and every shockwave meet his opposite. As we are now in 3rd dimension there is no vector toward wich the extra energy can escape so I guess all the enery concentrate on the point of explosion where it disperse as heat.

Am I right ?

If 2 exact charge of 20 meters radius explode at the exact same time when set 10 meters apart, does energy from charge A reach charge B origin point or does both shockwave cancel each other ? my understanding of reative armor make me think they should cancel each other but then my knowledge in Physics aren't all that good.

I think you are overcomplicating this... either way it makes a mess.

Anything that takes 2 explosive charges in an enclosed space is something you want to run from.

No, they do not cancel each other. You're also wrong about the pond; nothing is dispersed. The vertical 'point' you're seeing is the temporary addition of the waves, after which they do keep going (through/past each other). They're not meeting 'opposites', just counterparts. The opposite would be a *hole* (trough) in the water's surface.

Reactive armor doesn't cancel shockwaves, it deflects incoming objects.

Definitely worth mentioning again, TheOOB, but I guess the idea is that the shrapnel itself is bouncing around? I agree with you: it seems like that would be a much lesser effect.

Probably because you failed physics...

Nope.

http://i48.tinypic.com/2vs58j8.png

The yellow circle is an early wave, the orange the wave as it first touches the walls, and the red is the reflection.

If the room were perfectly circular then you'd get the kind of wave interference you are thinking of (sort of*).

*Shockwaves are a kind of compression wave, like all sound waves and in a perfectly circular room, the wave would bounce back and forth. Visually it'd look like a circle expanding to the extent of the room, then getting smaller again, until it's a small point, then expanding again (assuming it had enough force to do so). But as the "peak" of the wave travels ahead of the "valley," even at the center of the room you'd feel the wave each time it passes through.

Disclaimer: I'm at work so I'm not able to confirm the following theory, I may be leaving something out, and its been a while since I've done these sorts of calculations so I've forgotten most of the math involved...but there are a few things I'm remembering that we have left out as well. If anything following is incorrect, I will not be insulted by a polite correction. and so...

Even at the center of a spherical room, as the compression waves return and collide on all sides you'd get an immense build up of pressure at the center. But, a standard explosion isn't a single blast wave that has the same start/stop moment in time, and then only meets with its own rebounding forces. It is an accelerating force over a period of time. If the room is small enough that the blast wave isn't sufficiently dispersed as it travels outward, reaches the extent of the room while still accelerating, Then outward pressure will build as the wave is rebounding. This is why a tamped charge will do more damage to a wall then if its just sitting near. Or why a grenade will blow out all the walls of a room if its surrounded, but barely damage the same wall at the same distance if it was outside the building instead. The increase in pressure, exerts more force on the structure from a single direction.

So in other words, if your in a small enclosed room with a grenade going off in the center, you will still probably be very dead, but the rebounding blast wave is going to collide with additional waves from a continuing push from the explosion. The increase in pressure will be on all sides of you as the pressure in the entire room builds possibly subjecting you to overpressure. Additionally, If the walls don't blow out, that extra pressure will turn to heat. Basic physics shows us that air pressure and temperature are directly related. I don't know if any of you ever did the experiment in school with burning a piece of paper just by increasing pressure in a jar but its the same sort of principle. So now if the explosion didn't burn you, the air itself in the room is now on fire.

As a side note, the idea of rebounding wavelengths canceling each other is extremely unlikely even in a perfectly spherical room because even in high quality explosives they tend to detonate with slight asymmetry. Which is why its so hard to make a nuclear bomb.


There's probably more I could add, but that will do for now.

so...let me get this straight....all in all what your trying to say is....



squish


that about sum it up?

Oh, I remember what else i was going to add....Rules suggestions!

If you change chunky salsa to make it less extreme and more abstraced, you could work it similar to burst fire rules.

Increase damage of blast by +1~3 per nearby barrier, depending on how deadly you want it or...if you want to add a little granularity/flavor... depending on the barrier rating of the nearby barrier. So if your in a drywall closet and a grenade is tossed in, door closed, you stage damage up by +1/wall +ceiling for +5 damage. If its something heavier like brick, do +2 or +3. If you wish to include the floor thats up to you but it should only apply if the detonation is directly above/below you. This way, even in an ideal situation, the grenade is never going to do more damage than +18 damage if you use +3 per barrier. Thats still pretty up there in damage, but if your stuck in a small brick room with a grenade on top of your head, I don't think you should be surviving that anyway.

And if you set it +2, you could stand to increase the base DV of grenades a little to make them a little more threatening without starting a TexMex party evertyime you threw it at a target near a few walls, and would be a heck of a lot easier to calculate on the fly then seeing how much damage is going to drop off as it rebounds of wall a, then wall be, then wall c, then re-rebounds off c, then a....you get my picture.

now...if it can be done it will probably end up on "the list" which is like, 125 pages now, but...is there a spell in SR4 somewhat like the D&D Forcecage?


cause if there is...well....grenade, forcecage....Texmex party...

Sorta, more that in unlike in a situation where your in a hallway where there is no rapid over-pressurization (because it can go down the ends of the hallway freely) and you take more damage from rebounding shockwaves as the continuous accelerating forces are pushed out the sides away from the rebounding forces.

In an enclosed area, the shockwaves, or really blastwaves, are impacted and somewhat restricted by the continuous competing pressure wave from the explosion. So you don't take continuous rebounding shockwaves through your body in the same way because the pressure in the room builds more uniformly...not really sure where i was going really...I guess that you start to get to a point where you trade damage from one source for another and it should start to fall off after a certain point.

Tex-Mex party?

You can use really strong Physical Barrier spells to help, AFAIK.

well....yeah...gotta feed to ghouls somehow right? and who doesnt like chunky salsa? who says it cant be a tex-mex blend? ....throw a pepper punch grenade in with the frag?

Hmm...so can you shape a physical barrier? I dont have the books and havent really played with the spells to much.

One thing to note: Grenades very probably aren't supposed to reflect off the floor, because they contain very small amounts of R12 explosive, which is still too little to get the DVs. So the floor - or first surface it hits - is already in the final DV.

The thing about frag grenades reflecting more than explosive is obviously bullshit, but it's a GAME world, things don't have to work like in reality.

I would really not think about it too much, use the rules as written, but only allow each "vector" to reflect once. So the return vector isn't reflected again.

That also deals with super-grenades under cars and stuff. I've run it before that a grenade under a car almost certainly killed it, no matter the armour, because the explosion is reflected in the tight space until the armour breaks. Now obviously present day armoured vehicles are already designed to deflect the blast vector by putting everything in a V-shaped hull. (See MRAP vehicle or the DINGO.) IRL a hand grenade does very little to an armoured vehicle, even right underneath it, there just isn't enough explosive. With single-reflection, this is now recreated. But even so, a regular explosive under a car can be quite dangerous, because here the ground isn't ignored - so you should always get double DV straight upwards.

Does SR4 still have the rule about the increasing force transfer in water? Non-compressible media carry pressure waves much faster and lose less force along the way. IIRC in SR3 every explosive under water was automatically rated as an AV explosive.

Vykos, I don't have the specifics, but the barriers do interact as you'd expect with explosive blasts. You might need multiple instances, but you could be creative: e.g., 'close the door' on a hall, vehicle, or elevator with just one flat barrier.

Brainpiercing, that's possible, but it means that airburst grenades should have much lower DVs! (This was brought up earlier, in the other thread though.) Good points about cars/armored vehicles (would be nice to have explicit rules, basically), and I could have sworn the blast/underwater rules you mention *are* in SR4.

Hmm...so quick read on break i found that you can make a dome shape with the Physical Barrier spell, but it also says that anything the size of a molecule or less can pass through, would that mean then that the force of the explosive would disperse, but the shrapnel wouldnt?

Hm. Sorry I was totally wrong then! We need to know if/how much the barrier absorbs kinetic energy (of larger fragments). People talk about using barriers as damaging obstacles (car crash), but it matters if the shrapnel would bounce off, or just stop and fall. I forgot about the fact that barriers don't stop air, and therefore arguably ignore blasts. Hm. I feel like that point needs some more debate. I'm not sure if Physical Barriers are *supposed* to ignore blasts (for one thing, that'd make them immune to them), etc. Either way might require a clarifying house rule.

Indeed, lol. Well, they are 1 armor 1 barrier rating per hit, soooo....

barrier calculation wizard...i choose you!

My thought on chunky salsa was to take the nearest barrier, if the P damaged the barrier you get a blow out. Otherwise it rebounds 180 degrees (opposite direction), adding damage.


Examples of how this may work: Mr. Salsa is attack by a grenade from Mr. Chip.

Asssuming the grenade has the following stats: 10P +2AP, -1 per meter. Grenade lands underneath Salsa. Salsa is in a 5X5X4 meter room. The room has a barrier rating of 15 (AFB-but not sure what material would have that barrier rating is-sounds right for something). Mr Salsa takes 10P + 4P (distince to cieling, bounces off due to being less than the barrier rating and back to Salsa, who is not a dwarf) +2P for the second rebound off the floor for a total damage of 16P. Still nasty and this is how I run it (may not be RAW).

Or does the calculation go as follows: Do only one rebound off each surface, so it would be 10P+(5PX4) (for each wall)+10P (for the floor) +6P for the cieling for a total of 36P. Note that in this case the walls would take 12P and hold as they would not be affected by the 2 of the shockwaves. The cieling would hold has well, only taking 12 P (6 Initial +6 from the shockwave bouncing off the floor). The floor would take 12P as well, 10 initial +2 for the shock wave from the cieling.


Am I missing something?

The best way to get a Tex-Mex mix is to throw some CAS and Aztec soldiers into a room then set off the 'nade.

Yes.



That's not how that works. The explosion attempts an attack on the barrier (8 DV against a barrier rating of 15* -> barrier takes damage -> barrier destroyed) and if and only if the barrier survives does the shock rebound.

*Note: There is no such thing as a barrier rating. Barriers have armor and structure. Your standard interior wall has 2 armor and 3 structure (per 10cm of thickness**). A brick wall has 12 and 11. In the above example a brick wall would be sufficient to absorb the blast and reflect it. A drywall...wall is not.

"Reinforced Materials" (ie. security doors and bulletproof glass) are insufficiently strong to withstand a 0 meter blast from a frag grenade (8+2 armor, 9 structure, on average there will be 0.33 boxes of structure left, assuming no rebounds from other directions).

"Structural Material" (ie brick) will be severely weakened (3.66 boxes left, on average)

"Heavy Structural Material" (concrete) would be damaged (7 boxes)

"Armored/Reinforced Material" (reinforced concrete) would scratched (10.66 boxes)

"Hardened Material" (blast bunker) would be largely immune (0.66 boxes taken).

**Structure only. Every "this value" of damage taken, a 1x1 meter hole 10cm deep is created.

Are structures, like vehicles, immune to stun damage? That's why flashbangs are stupidly good at chunky salsa.

Correct.