D&D General Is this a fair trap?

[Ruin Explorer]

When this trap was invented there wasn't really the adventure; there was the dungeon. I think this trap would be a part of the dungeon players would be likely to remember.

That's like, half-way there. Would they remember it as a cool thing or a dumb thing, or a really really dumb thing that sucked? You seem to think "memorable = good", and that isn't true except for a bunch of old farts sitting around discussing "the good times" or whatever. Memorable can be bad or neutral. It's probably better than totally forgettable, but this trap is, as written, so dumb it's not even really in the "so bad it's good" territory some traps occupy - it's not Rube Goldberg enough or stylish enough to hit that "Wall of Fame" like the old "stick your hand in here" trap or its ilk. It's not fit for an MST3K of traps.

I actually think in the context of 1E traps this one is pretty forgettable, if I'm honest, apart from it's physics are so profoundly stupid and impossible, even by the standards of 1E traps, that it might trigger an extremely lengthy non-diegetic discussion (or argument, if it kills someone, but it probably won't) between the DM and PCs about how nonsensical it was, and potentially derail a session.

There's not a lot interesting to engage with and it just relies on dubious usage of a couple old-skool monsters.

 

[jgsugden]

Didn't read the 8 pages so far due to lack of time, but: The trap is fair - but it has to be earned. Why would such an elaborate trap exist? Who is feeding the cube and mold?

As long as it makes sense in the story, it is perfectly fine. However, you'd need to have a pretty quirky dungeon maker for it to make sense.

 

[Ruin Explorer]

Didn't read the 8 pages so far due to lack of time, but: The trap is fair - but it has to be earned. Why would such an elaborate trap exist? Who is feeding the cube and mold?

As long as it makes sense in the story, it is perfectly fine. However, you'd need to have a pretty quirky dungeon maker for it to make sense.

Yup. I think conceptually the bizarre-ness of the trap is pretty fun. My only objection is that the physics and rules totally don't work, as written, but you could easily design a similar trap where they did - and yeah you'd want a colourful-as-heck backstory to justify it.

 

[tomBitonti]

I found some of that stuff too. But having had some friends who did doctoral work on rheology and non-Newtonian fluids (both where chemical engineers) I am confident that the maths will be beyond my ability to interpret or apply! (A warning sign for me: as soon as the paper/web page starts talking about shear forces I know I'm not going to be able to follow it!)

Haha, and it gets a lot worse from there . Gels are non-linear fluids. The maths behind their motion gets very heavy with tensors and co and contravariance, and other such matters.

Re: Your reply to my reply. Yeah, there would be an initial splat, I'm guessing a pretty violent one, with the block stopping very suddenly and a narrow edge of GC material being forced upwards along the narrow edge. Afterwards, probably not very much, just a slow ooze of the GC material up around the edge. Maybe nothing at all, depending on how much shear(?) is needed for the GC material to stop acting like a solid. The block might not create enough pressure to get the GC material to flow.

For the initial impact, I'm thinking the impact time will be a lot less than 1/10 of a second. But that just adds more to the idea of there being an initial violent splat. If there were a simple transfer of energy and momentum (basically, making the GC material a big seesaw), we could estimate how quickly the GC material that is ejected is moving. But, I have no sense of how much of the energy and momentum are transferred to the surrounding rock (or back to the rock that was dropped).

TomB

 

[Xetheral]

With 10' x 10' 10' block of solid granite it would most certainly be completely spilled out of the pit.

But not quickly. See below.

You've calculated the static pressure: weight = 170 lb X 1,000 cu feet = 170,000 lb; pressure in psi = 170,000/ 14,400 [the surface area, in sq inches, of the block] = approx 12 psi as you say.

In this trap, though, it is the impulse pressure that seems relevant (qv: I might be able to balance a rock or brick on my head without being squashed; but drop that rock or brick on my head from a balcony and it might kill me!).

Impulse = change in velocity * mass / time of deceleration. Upthread we've established that the velocity of the block at impact is around 8 metres per second. The impact brings it to a stop. The mass in kg is around 77,000 kg. Let's say the deceleration happens over one tenth of a second (in "reality" it probably wouldn't be uniform; I've chosen this figure by Googling some collision web pages and using a generously long figure compared to actual collisions). Then we get impulse = 8 * 77,000 / 0.1 = 6,160,000 N. With a surface area of around 9 sq metres, we get a pressure of around 684,000 Pa. That's about 100 psi.

The more rapid the deceleration the greater the pressure. I don't know what sort of pressure is going to be necessary to splat a Cube. The spongier the Cube and the more deformation it can handle, I guess the greater the deceleration time and thus the less the pressure the Cube suffers. My attempt to Google up the physical properties of jellies wasn't very successful (the webpages and papers that came up weren't ones I could interpret for the purposes of this discussion); so I don't know what a "realistic" set of figures is for a Gelatinous Cube.

Note that shorter impact times necessarily happen over shorter distances. Shorter impact distances displace less gelatinous cube material during the impact. So the higher the dynamic pressure, the smaller the quantity of cube material that will ejected by that pressure. So a super-short, high-pressure impact would violently displace only an extremely thin layer of the top of the cube. After the impact is over, you're left with only the static pressure, causing the remaining volume of the cube to ooze out as the block slowly descends to the bottom of the hole (assuming it doesn't get stuck on the way down).

Also, are we sure the material of a gelatinous cube is still dangerous after the Cube dies? If so, dead cube stuff would be extremely valuable--possibly even a strategic military resource. (Almost as useful as catapulting a live cube, but less dangerous to store and usable in more managable volumes.)

Does dropping 10' on a stone block count as rough contact? I think it's pretty reasonable to suppose that it does. How many contacts does it amount to, for the purposes of 50% chances? I don't know, and I don't think AD&D has a canonical answer to this. Looking up the Rules Cyclopedia, it says that each touch of a torch has the requisite 50% chance, but nothing more. I think the fall of the block must count as more than one touch - maybe one check for each 5' x 5' quadrant?

The deceleration experienced by the mold depends only on the mechanical properties of the gelatinous cube and the impact velocity. The stone block is irrelevant--it just transfers the impact force. So the right question to ask is instead: "Does dropping 10' on to a gelatinous cube count as rough contact?"

 

[AbdulAlhazred]

Where are we get 10x10x10 for the dimensions of the falling block? None are specified AFAICT.

As @Crimson Longinus notes, it's obviously not even remotely possible that it's a 10x10x10 block because it's held up by a single rope, which is not going to hold 50 tons. I read that a 1" rope of the type usually found in D&D can probably hold somewhere up to maybe 8000lbs on a very good day. You're proposing it holds up 112,000lbs, which is, what 14x that? So we can figure that, at most, the block is 1/14th of 10x10x10, so 71 cubic feet. So 4.1x4.1x4.1 maybe? Unless that's not how cube roots work. That's a hell of a lot smaller and 14x lighter - what is going to happen is it will displace some of the goo as it rapidly sinks.

You might claim that could cause splatter, but wait, we've established the pit is 11' deep so it can have the 1' lip described. So that means it has 100' cubic feet of potential displacement before anything goes out of the pit. Which is more than 71' cubic feet. Now the 4x4x4 block is hitting at maybe 6-7mph, so there might be SOME splatter, depending on how viscous the Gelatinous Cube is, but frankly, it can absorb a person walking or running into it at 4-6mph without problems, so I'm guessing even for a much heavier thing with more displacement, it's not going to splatter hardcore. And again, the shape of the pit dictates the spray goes UPWARDS, and then probably comes back down almost vertically. Depending on the exact physics some may go over the edge but won't be much.

You haven't addressed the other various failings of the situation, like the fire killing the Yellow Mold, either.

As for implausible, you're confused.

There's implausible as it unlikely to exist/be set up, which can be excused under certain circumstances or as a genre trope. That's fine, and then there's implausible as in "not believable to function as described". If this trap was implausibly high-tech involving hydraulic pistons or w/e, fine, genre trope. But this is just "dude in 1980 didn't understand basic everyday physics".

But let me revise my "implausible" to "impossible". The trap simply could not function as described. It's so extremely bad, too, that many players will immediately start asking questions about it. If you use it to actually kill PCs, as is intended, those are likely to be quite searching questions and deserving of actual answers.

OK, lets begin again... First of all I went back and did the actual mass calculations. A 1000 cu ft block of stone is 2.83x10^7 cm^3 at 2.5g/cm^3 (typical rock, some is up to 3.0) = 70.5 metric tons. So my guess was 'close enough for government work' (thank you D&D, The Dragon article "How Much Does My Giant Weigh" taught me all this back in 7th grade). So, now, the Internet says a 2" manilla rope has a minimum breaking strength of 24 metric tons, roughly. Thus a braid of say 5 2" ropes would clearly hold our block in place (maybe not to engineering safety margins we would accept, but it should hold with some margin).

Beyond that, we can easily redesign the mechanism such that the block is NOT held by the 'trigger rope' at all. I can think of several variations on such a mechanism without even breaking a sweat, so this is well-within our design parameters. Thus I suggest that the actual trigger rope is probably a simple 1/4" manilla rope. This will also give us a plausible burn-through time, as a braid thicker than a man's leg, no matter what it is coated with, isn't going to burn through without a minute or two delay. Of course maybe that's what you WANT, depends on how you use the trap.

Beyond that, IMHO the description of the trap sounds like the block fits fairly closely to the pit. I guess maybe this is not spelled out, but this is how I would design it! So, yeah, maybe the block is a few inches under 10x10x10, or maybe the pit is actually 10.5x10.5, whatever. I would note this, in case some PC gets out his carpentry kit and starts measuring.

REALISTICALLY, making the release mechanism so reliable that it drops the block precisely into the pit might be pretty tough, I'm not a mechanical/civil engineer, so my opinion on that is not perhaps terribly relevant. In that sense I think the trap is 'implausible' in the real world, but still seems acceptable as a threat in an RPG where Rube Goldberg type devices are common fare. Lets imagine the block is also slotted into the ceiling, this would tend to insure that it starts out in proper alignment. With a clean release, there's a good chance it will fall as intended. This also conceals the Yellow Mold (in the space above the block where the mechanism and pulley system or whatever is located).

I think the suggestion to provide a good number of clues is good. So maybe the PCs can smell a moldy smell. They can clearly see the rope is covered in grease. They can examine the ceiling and see the seams where the block is fitted there (assuming it isn't just hanging in plain sight, which I wouldn't do, but...). Clearly the Gelatinous Cube will also soon be revealed. You might also note to the PCs that there are odd 'splash marks' on the walls of the chamber/passage/whatever, or on the floor if they look carefully (and they will, or they deserve to die).

So, really, this is going to be an interesting challenge, but it is hardly a 'death trap' in a gotcha! sense. It is possible the party will inadvertently set off the trap (IE casually tossing a torch 'into the pit' as a starting move, though I would give a good chance it doesn't touch the rope before it lands on the surface of the cube and goes out). OTOH its quite likely the players will be wary of fire, given the greased rope. In that case they may not get the 'treasure', but they also probably won't set off the trap. The cube itself should be a trivial issue to kill with pole weapons or missile fire once they know about it. Overall it is an amusing trap, but probably not that deadly. However, it might be perfectly capable of wiping out a bunch of newbs on their first dungeon crawl! Probably better deployed AFTER they have encountered one or two less deadly scenarios.

 

[AbdulAlhazred]

Size to Weight Calculator (Rectangular Box)

The size to weight calculator (rectangular box) evaluates the weight of a rectangular box.

www.omnicalculator.com

I put in 10x10x10 for dimensions and used granite, which has a density of 165-172 pounds per cubic foot. I used 170 for nice even numbers. That block came out weighing 170,000 pounds or 85 tons.

Right, I went back and did a full calculation with limestone, and got 70.75 tons. Lets assume the block is undersized by 6" on each side, it would still mass between 50 and 80 tons, and their would be plenty of room for the 'squirt' (especially if we discount the 1' lip idea perhaps). I think the huge momentum of the block, and its sheer density, would cause it to sink fairly quickly. While GC might be fairly viscous, most viscous substances actually FLOW pretty well, particularly gels, which tend to 'break' when they are subjected to enough force (IE their viscosity decreases greatly under sufficient force).

Obviously we have to make some assumptions here, but who cares? The point of the scenario is that the players should realize that the situation is unusual and clearly poses a threat. IMHO they can work out what that threat is, and it is at least plausible enough that you cannot just dismiss it, not in a D&D world.

 

[AbdulAlhazred]

So ... unless the GC material is compressible, if the block is nearly as big as the pit and falls face on, the impact is likely to be quite jarring; nearly instantaneous. The GC material towards the center won’t have enough time to flow out of the was of the cube.

it does seem that a narrow band of GC material might be expelled quite explosively by the initial impact. I’m thinking as a fine mist and dispersing in a wide plume.

In term of dimension and the mass of the stone block, keep in mind that the GC scales at the same rate as the cube. What matters is the height of the cube, not so much its width, and probably the relative density of the rock vs the GC material. Although, the GC material might be close to water. Or not, if there is a lot dissolved in it.

TomB

Aqueous gels are pretty much going to be close to 1g/cm^3, same as water. Limestone is 2.5g/cm^3, granite or basalt could be 3.0. So the rock is a good bit more dense. Channels could be cut in the sides of the pit, the block could be camphered, etc. These minor tweaks would probably significantly alter (presumably enhance) the splash effect. It sure wouldn't be implausible for a GM to rule that everyone within 10' of the pit was covered in 'cube (and thus paralyzed potentially).

I think it is also safe to rule that the mold spores disperse in a cloud everywhere within 10' of the block. Granted that is another assumption, but it seems well-within the parameters that an AD&D 1e or other classic D&D DM should be free to assert, particularly if it is asserted as part of the pregenerated trap description. Remember, this is not some off-the-cuff ruling of the DM! His key says "XYZ" and that is perfectly fair in my book. Rules lawyers can be picky if they want, but this is VERY far from "rocks fall, you're dead."

 

[AbdulAlhazred]

You've calculated the static pressure: weight = 170 lb X 1,000 cu feet = 170,000 lb; pressure in psi = 170,000/ 14,400 [the surface area, in sq inches, of the block] = approx 12 psi as you say.

In this trap, though, it is the impulse pressure that seems relevant (qv: I might be able to balance a rock or brick on my head without being squashed; but drop that rock or brick on my head from a balcony and it might kill me!).

Impulse = change in velocity * mass / time of deceleration. Upthread we've established that the velocity of the block at impact is around 8 metres per second. The impact brings it to a stop. The mass in kg is around 77,000 kg. Let's say the deceleration happens over one tenth of a second (in "reality" it probably wouldn't be uniform; I've chosen this figure by Googling some collision web pages and using a generously long figure compared to actual collisions). Then we get impulse = 8 * 77,000 / 0.1 = 6,160,000 N. With a surface area of around 9 sq metres, we get a pressure of around 684,000 Pa. That's about 100 psi.

The more rapid the deceleration the greater the pressure. I don't know what sort of pressure is going to be necessary to splat a Cube. The spongier the Cube and the more deformation it can handle, I guess the greater the deceleration time and thus the less the pressure the Cube suffers. My attempt to Google up the physical properties of jellies wasn't very successful (the webpages and papers that came up weren't ones I could interpret for the purposes of this discussion); so I don't know what a "realistic" set of figures is for a Gelatinous Cube.

Well, I measured the pressure at the hose bib at my house when I moved in (the building inspector did) at 60 PSI. I can squirt water out of the end of my hose at that pressure and it travels several meters. Now, to actually see what pressure you would have AT THE 'NOZZLE' so to speak, you have to know the pressure of the area through which the cube material is actually squirting, this is NOT the area of the face of the cube! It is the area of the slack between the rock cube and the pit wall at the lower face of the rock cube. Thus this pressure will be MUCH higher than your 100 PSI, at least for a brief time! Even after this, the 12 PSI calculated by @tomBitonti has to be adjusted for the ratio of the face of the cube vs the 'nozzle' areas, which MUST be at least a factor of 10. So we are up to impulse pressures of 1000's of PSI, and static pressures of over 100 PSI, which should be enough to eject the material at least several meters through the air (like a hose with a standard spray nozzle on it perhaps). Now, maybe not much material can spray out at high pressure in a few milliseconds, but it seems to me that you will get a BIG splash and then a longer soaking spray. Watch things like ship launches where large masses impinge suddenly on water, there's quite a lot of spray if it happens fairly quickly, even without a constraining 'box'.

 

[AbdulAlhazred]

I do not, sorry

Gels, like gelatin, normally are only really viscous at low pressures. Their properties derive from a network of linked long-chain molecules suspended in a solvent (water and proteins or starches in the case of gelatin). Once enough force is applied the intermolecular bonds (which are pretty weak) break down and the substance 'breaks'. At that point its viscosity approaches that of the solvent pretty closely. It will still be somewhat more viscous than water, but maybe not much. So if we model a GC as gelatin, then when the block falls on it, it should splash a lot like water. Now, it may be that the bulk of the cube doesn't immediately 'break' and the block sinks more slowly from then on, with significant flow resistance, but there should be an initial 'big splash' IMHO. Still, not an engineer or expert on rheology, so YMMV.