D&D General Is this a fair trap?

[AbdulAlhazred]

Whilst I don't buy the physics you're going on, even in this scenario, the spray would be pretty much entirely vertical. It would hit the ceiling then spray back down, so you'd probably get the 5' squares on the two shaved sides, and nowhere else.

I certainly agree that if we could 'TrapBusters' it, that we might well find it doesn't work at all, or it works in one of several ways, possibly one that none of us have accurately described. Though I suspect that if you were to QA test it enough times and could go through various permutations of block size and shape, pit parameters (lip, no lip, etc.) you'd probably be able to come up with a design that slimed some people.

IMHO the interesting question IS the diversity of opinions. I assume we are all reasonably clever adult humans with at least an ordinary appreciation of how everyday things work, and yet we are pretty far from a consensus on what would happen if the trap was triggered. SURELY even a bunch of Skilled Play veterans would face some diversity of opinion here, maybe someone like @Rob Kuntz would comment on that! The point being, the question of the poll, 'is it fair?' might rest on that.

Again, IMHO, experienced players, such as myself, would not assume that the trap was ineffective, would conclude it IS a trap, and then proceed to assume on that basis that it requires either disarming or bypassing. I think I would assume its mode of operation at least well enough to TRY to disarm it, though TBH I'm not sure how you actually do that... Killing the 'cube is easy enough, but then what? I'd be suspicious that doing so IS the trigger, or A trigger at least. Surely a long sequence of searches and cautious experiments would follow first contact.

Given all that, I still say the trap is 'fair', its existence is obvious. It is possible to reason out what would set it off, and at least SOME of the consequences of doing so seem fairly obvious (at least in intent). The yellow mold part is of course the real biting part, and that IS concealed. As I said up thread, if someone bothers to smell the room I'd describe it as smelling moldy.

I also liked the idea of famous trap makers and their signature trap types. That sounds pretty cool, and an area where thieves would clearly be in their element. I'd also note that one way to resolve the "how does it work?" question vis-a-vis the physics would be to allow an INT check for a PC to correctly gauge the degree of splash, etc. Regardless if this is literally physically correct, it would at least unlock the DM's judgment on this critical factor and tell you what the 'blast radius' of the trap is.

Here's a follow up question. What if the trap, as described, actually worked in a different way entirely? I'm not proposing a specific 'other way', but is it fair to present something that seems to work one way and then it turns out it works a whole other way?

 

[Xetheral]

Yeah, if the flow rates are beyond a certain point. So, I previously approached this as a momentum transfer, with the assumption that (at least as a first approximation) all the momentum of the falling block was transferred to the cube.

Since the block is falling downwards and the displaced dead cube stuff is going upwards, the net downward momentum transfer to the structure (and the ground below) must exceed the original downward momentum of the block by some factor X. There's no reason to assume that X=2, which is what would be required for the magnitude of the upwards momentum of the dead cube stuff to equal the magnitude of the downwards momentum of the block.

However we can approach it in a completely other way which is probably even better. The block can be thought of as a hydraulic cylinder with an area of approximately 90 ft^2, and the reacting cylinder would then have an area of 10 ft^2. With a ratio of 9:1 the pressure on the reacting side is 9x that on the acting side (it is just like a lever with a 9:1 ratio, basic hydraulics). We established that the PASSIVE pressure on the cub was on the order of 14 PSI, so the passive pressure must be 140 PSI, roughly. This is still enough to eject material with considerable velocity, and only accounts for the block's weight, not its initial downward momentum.

Surely during the deceleration phase the pressure MUST be considerably greater. Here @pemerton calculated the pressure at 100 PSI at the moment of impact, which would imply 900 PSI outflow. Your average power washer is putting out no more than 250 PSI, tops. I think a 10 ft^2 outflow at 900 PSI is going to be a pretty big outflow! Granted, it is probably only happening for a very brief time. Pemerton stated 1/10th of a second for his pressure calculation, so it is a short, but high volume, squirt.

Honestly, if the average speed during deceleration is 4 m/sec and it is 0.1 sec long, then the travel must be .4 meters, which means roughly 5 m^3 or about 50 ft^3 of material was displaced, at decreasing pressures. That should be enough to thoroughly wet anyone or anything within a few meters of the trap. It would certainly be on the order of having a bucket of slime dumped on you!

Pressure is definitely a better way to look at it, but the limitations of choked flow still apply. If the outflow area isn't large enough, the block will decelerate even faster because it can't displace the necessary fluid. And the faster the deceleration the shorter the distance and the less initial outflow.

I have no idea if a 0.1 sec deceleration is reasonable, but amount of displaced fluid in the initial splash is heavily dependent on that assumption. So your bucket description could be correct, but it's also possible it would instead just be a fine mist. Also, as @Ruin Explorer points out, the geometry matters quite a lot in determining the fluid vector and thus the radius of who gets splashed.

I also think that the GC slime has to have SOME effectiveness, if only for a very brief time. After all, contact with it is paralyzing, and it certainly doesn't have time to change its bulk chemical composition much in the brief instant it is being splashed... I guess you could suppose that the effect is actually requiring a reaction of some sort involving some complicated dynamic chemical process. More likely the cube stores some of the 'toxin' in its body somehow, or a couple of simple precursors that form a 'binary toxin'. I'd be happy to rule that the save you got was at a bonus. The effect would also probably end fairly quickly, though for the purposes of the trap that probably doesn't matter MUCH.

That seems reasonable! Although the difference between being engulfed vs being splashed/sprayed/misted probably warrants making that save at a very large bonus.

 

[Benjamin Olson]

It's an overly elaborate fun-house dungeon type trap. It is thus "unfair" and should be, as that is the genre of trap it is. Dropping it in a context with no other elaborate traps where players aren't going about thinking they are in some battle of wits with the designer of the dungeon it is probably unfair (though given that players' sensory perception is DM dependent, it really depends how it is described). Drop it is the middle of the Tomb of Horrors and it is par for the course. If it is in the middle of, say, a kobold warren with no internal logic as to why someone put it there or how they maintain it then I would consider it unfair. Make it that whatever more powerful being bosses the kobolds around is forcing them to set aside treasure for him in this pit and have it be that if the adventurers interrogated any of the kobolds rather than fireball the lot of them the secret or hints would be revealed and it becomes a fair trap again.

Basically the answer is: context.

 

[Ruin Explorer]

Here's a follow up question. What if the trap, as described, actually worked in a different way entirely? I'm not proposing a specific 'other way', but is it fair to present something that seems to work one way and then it turns out it works a whole other way?

Yes it is and I've done it, more than once, however, IMHO, you have to not be a jerk about it. Traps are physically real objects (or they're magic as all hell which means vulnerable to Detect Magic lighting up real bright and detailed), and too many trap designers in 1E and even 2E and later editions forget that like, these are not existing in "idea space" (or "katana space" - that being wherever Duncan MacLeod of the clan MacLeod draws his katana from). Way too many traps just don't have the mechanism they'd need (even like, in a simple way) to actually make them function. To me, and I don't mean to dictate on this, but to me, that's the worst kind of BS. So I always check all the traps in any adventure I intend to run to ensure they aren't naughty word in that way.

But yeah, if you have a "double-play" trap that's fine IMHO so long as if they really think about it, or make an effort to double-check, there's some way to discover that at least something hinky is going on.

Personally when I come across traps in someone else's adventure which I intend to run I check the following:

1) Does the trap even work, as written? About 80% do. In the remaining 20%, they range from missing some obvious thing which will make it fail (or totally trivial to avoid) to outright "didn't understand physics". I have run ones that didn't work and just had them fail where it contributed to the vibe of a dungeon/adventure. If they don't, I usually fix them though, trying to retain the concept/style.

2) Does it make a single lick of sense that the trap would be there? This is like 80% in 3PP adventures, and er... more like 50-70% in WotC and TSR adventures. I have no idea what is up with the latter (and to be fair I am not talking 5E much, mostly earlier editions but definitely including 4E where it was often more like 30-50% in early 4E WotC stuff. Is there some plausible explanation or does it being implausible help in some sense with atmosphere or something? If the answer is no, I decide - either I remove the trap, or I come up with some sort of explanation as to why it is there (which has actually come up a surprising number of times).

3) Is the trap actually going to make the adventure more interesting? Again we're at like 70-80% on 3PP adventures and er... a lower figure for WotC and most TSR ones. WotC in 3E and 4E really had a lot of "this trap obviously just exists because there was a trap quota" traps. If the answer is no, I either just remove it, or replace it with one that's actually interesting.

Again, IMHO, experienced players, such as myself, would not assume that the trap was ineffective, would conclude it IS a trap, and then proceed to assume on that basis that it requires either disarming or bypassing. I think I would assume its mode of operation at least well enough to TRY to disarm it, though TBH I'm not sure how you actually do that... Killing the 'cube is easy enough, but then what? I'd be suspicious that doing so IS the trigger, or A trigger at least. Surely a long sequence of searches and cautious experiments would follow first contact.

I can't speak for everyone, but my main party, or me, would immediately zap the rope from a distance because it's suspicious as hell. No-one just leaves a useful rope to get out of a pit, or suspiciously vertical in a pit. I just don't see any scenario that rope doesn't get zapped or shot with a flaming arrow from at least 30' (or some poor familiar or the like is sent in 5E, if the angle isn't right). That would be hard to defeat because the trap designer couldn't guess where the party would be when they did it.

Of course if you'd used this on me when I was 10-12, my PC would be dead at the bottom of that pit right now.

One thing I feel is a bit underused in adventures is fired traps or traps which have partially failed due to age. The latter can be especially fun because you have a really terrifying trap that is quite dangerous but also fails in some way that makes everyone sigh with relief.

 

[Lanefan]

Hell, maybe we should do a Mythbusters of old-skool traps? I.e. first we analyze them exactly as written, and probably a lot of them fail, but then we figure out how we could make them work.

Mythbusters would only look at the traps that involved things blowing up, though, which might limit the sample size a bit...

 

[Ruin Explorer]

Mythbusters would only look at the traps that involved things blowing up, though, which might limit the sample size a bit...

Late-season Mythbusters, yes, but early and middle-season they'd have tried it! Trapbusters would probably have a 10x10x10 granite block made in like S2 that they used to try to replicate a bunch of traps through the seasons.

Oh god, I just thought of a whole RPG campaign where the PCs were basically Mythbusters, trying to replicate the myths of various heroes of legend in their world, recording it all with some kind of device. Like one "episode" they need to test if dragonbreath can melt full plate so are having to coax/bargain with/trick a dragon to get him to breath directly on it. I guess the PCs would be both the producers and stars in a sense. I think that idea actually has some legs.

 

[AbdulAlhazred]

Late-season Mythbusters, yes, but early and middle-season they'd have tried it! Trapbusters would probably have a 10x10x10 granite block made in like S2 that they used to try to replicate a bunch of traps through the seasons.

Oh god, I just thought of a whole RPG campaign where the PCs were basically Mythbusters, trying to replicate the myths of various heroes of legend in their world, recording it all with some kind of device. Like one "episode" they need to test if dragonbreath can melt full plate so are having to coax/bargain with/trick a dragon to get him to breath directly on it. I guess the PCs would be both the producers and stars in a sense. I think that idea actually has some legs.

Awesome.

 

[tomBitonti]

So this was interesting:

How do you get velocity? With ultrahigh-pressure water.

Ordinary tap water is filtered and fed into a waterjet pump rated to pressures of 60,000 to 94,000 psi (~4100 to 6400 bar). The pressurized water runs to the cutting head safely contained within the plumbing to maintain pressure. At the cutting head, the water to pass through the jewel orifice and the supersonic waterjet stream is created.

All the pressure is exchanged for velocity when the water passes out of the jewel orifice. The higher the pressure, the higher the stream velocity.

TomB

 

[Stormonu]

Something I found, but not the result I expected. (At 1:10)

 

[AbdulAlhazred]

So this was interesting:



TomB

Sure, and that is why a small orifice will produce a higher velocity jet, because the pressure is pushing a smaller mass with the same force, F=MA, or in this case A = F/M. Pressure is the force. It is really just a restatement of the way hydraulics work.