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Frosty
2008-05-21, 01:14 PM
Oneof my players recently acquired a Decanter of Endless Water. She has since been thinking a lot, often sporting a strange grin on her face. Then, last session, she asked me about getting a few more of those and possibly fashioning them into some sort of jet-pack.

Is that feasible, or should I just tell her it's stupid?

Azerian Kelimon
2008-05-21, 01:16 PM
Oneof my players recently acquired a Decanter of Endless Water. She has since been thinking a lot, often sporting a strange grin on her face. Then, last session, she asked me about getting a few more of those and possibly fashioning them into some sort of jet-pack.

Is that feasible, or should I just tell her it's stupid?

If the force of the combined geysers is enough, it's perfectly feasible. It's gonna be pretty difficult to maneuver and it's going to cost MUCH more than Wings, though.

Frosty
2008-05-21, 01:17 PM
Perhaps I should reword this to: practical or not?

kamikasei
2008-05-21, 01:19 PM
Perhaps I should reword this to: practical or not?

Fantastically impractical, as it would lack any sort of reliable or usable control system. Magic is great, but engineering and ergonomics it ain't.

AKA_Bait
2008-05-21, 01:20 PM
No, not practical. You could get items that would allow flying for less money which would be easier to maneuver.

Landing would be a female dog with a bunch of decanters strapped to your back.

Azerian Kelimon
2008-05-21, 01:23 PM
Actually, landing would be easy as pie. You just lower the flow one step at a time. Just make sure all the decanters respond to the same word. And you get bonus XP if it is Nildenosaj.


But yeah, Wings are cheaper and easier to land.

Frosty
2008-05-21, 01:28 PM
Well, she *reaaaaaly* wants to make this work. Those with ranks in knowledge (physics) could you please enlighten me as to how she might put this contraption together (preferably with a workable control system?) that can achieve up to Perfect Maneuverability?

Azerian Kelimon
2008-05-21, 01:31 PM
Well, she *reaaaaaly* wants to make this work. Those with ranks in knowledge (physics) could you please enlighten me as to how she might put this contraption together (preferably with a workable control system?) that can achieve up to Perfect Maneuverability?

Epic wealth and dozens of free action triggers activated by mental command. Any less will get you a Clumsy fly speed.

The idea is awesome, but it's more impractical than going Swashbuckler 20.

Citizen Joe
2008-05-21, 01:46 PM
First, you would need about 4 decanters for enough thrust to give the lift you need. Even then, I don't believe you'll have enough thrust for more than low level flight, using ground effects. Your two main lift decanters would have to be mounted about shoulder height and back a little bit, then directed down and forward at about the hips. At rest, with the thrust full bore, you'd hang there with your center of mass in line with the the thrust jets. By adjusting your weight, you can pivot and change from idle to forward and somewhat back thrusting. The next two decanters are directional thrusters and should probably be attached to the forearms, pointing away from the hands. I also recommend a gripping device so that you aren't relying solely on your shoulder strength to keep your arms from flailing wildly.

Note that flying this way is very tiring and would probably take your action for the round. It is conceivable that you could take an action while hovering, but you don't have a stable footing and are likely to be twisting about somewhat.

Lochar
2008-05-21, 01:50 PM
At best you'll get Poor manuverability. Perfect would be mental triggers to each separate decanter, and it's not.

I'm seeing an adamantine box with X+3 decanters.

All but three of the decanters are aligned with spouts pointing down the bottom of the box. They all have the same trigger word for Geyser.

Command is called for geyser. X number of geysers sprout, lifting the box and whoever is wearing/tied to it up.

Different commands are called for them to turn off until you level off.

The other three geysers are set into the box as pointing to your left, right, and behind. Separate commands are given to these to activate when you need to move forward, go left, or go right.

TheCountAlucard
2008-05-21, 01:58 PM
Not to mention, you'd get water everywhere, tick everyone off, and rapidly flood any room you're attempting to travel through. :smalltongue:

SamTheCleric
2008-05-21, 02:00 PM
And she'd just be copying Mario from Mario Sunshine :smalltongue:

Curmudgeon
2008-05-21, 02:02 PM
Unless the thrust is perfectly balanced around the center of mass she'll turn into a pinwheeling bludgeon, smashing repeatedly into things until the decanters break or fly loose. You would need fine gradations in the water pressure to make this controllable, and Decanters don't have that.

I've got a degree in aeronautical engineering from M.I.T., if that helps you accept my evaluation of this hair-brained scheme.

Frosty
2008-05-21, 02:14 PM
So, assuming you can change the Decanters to accept mental commands as free action, what kind of steps could a very intelligent character in a DnD environment (she's got good knowledge skills) do to achieve the kind of Maneuverability say...that Iron Man has from his suit (as seen in the movie)? Or is that pretty much just impossible?

Azerian Kelimon
2008-05-21, 02:16 PM
So, assuming you can change the Decanters to accept mental commands as free action, what kind of steps could a very intelligent character in a DnD environment (she's got good knowledge skills) do to achieve the kind of Maneuverability say...that Iron Man has from his suit (as seen in the movie)? Or is that pretty much just impossible?

Pretty much impossible without better magic than Decanters. A better way to begin would be a continuous item of flight, and then combining that with the spell that improved maneuverability.

But then, why do you need Perfect maneuverability if you have continuous Fly?

Ne0
2008-05-21, 02:19 PM
The geyser effect causes considerable back pressure, requiring the holder to make a DC 12 Strength check to avoid being knocked down. The force of the geyser deals 1d4 points of damage but can only affect one target per round. The command word must be spoken to stop it.

Despite the fact that your player isn't aiming it at someone, to use this decently she would probably need another strength check just to keep hold of one. With three or four of these things, the character would require some very good strength, since sooner or later, there's that natural 1. (Unless in this case, it doesn't mean automatic failure? I've been corrected so many times, I daren't say.)

I'd see it as practically impossible, since:
A) Very poor and unreliable manouvrability.
B) Failing a Strength check ends very messy.
C) It floods the entire area, after a few rounds probably requiring casters to make a concentration check.
D) It'll be worthless in a dungeon. (Unlike wings or magic flying help)
E) You probably won't even be able to attack, Like Citizen Joe mmentioned.
F) Pretty slow movement, I think.
G) Is it even possible? I mean, 30 gallons a round is a lot, but imagine you doing it: It doesn't sound that elegant.

On top of that, it's really expensive, and I don't think it fits in the majority of the campaign settings (so, perhaps not yours either.)

Frosty
2008-05-21, 02:19 PM
She just thinks flying is cool.

Azerian Kelimon
2008-05-21, 02:22 PM
She just thinks flying is cool.

Then, she should pick Overland flight and be done with it.

Emperor Tippy
2008-05-21, 02:27 PM
Pretty much impossible.

She is really a lot better off just going with some custom Boot of Flying/Wings of Flying that have been slightly re flavored.

Take the boots, make them mechanically work exactly like the wings except switch from Command word activation to at will and add in a Prestidigitation and/or Pyrotechnics effect to make it look like flames are coming out of her feet when its activated (if she wants that look).

Call it 60K.

Ne0
2008-05-21, 02:29 PM
Then, she should pick Overland flight and be done with it.

True. For the price of 3 decanters, she can buy 540 hours of flight. >_>

RS14
2008-05-21, 02:34 PM
Well, lets assume that the DC12 strength check roughly corresponds with a medium load for a character who can make such a check on average. A medium load is the first point at which it starts to hamper this character, after all, and it requires a check because it changes direction so readily.
Assume this means 85lbs of force, seeing as this is middle of the range of medium loads for Str 13 and 14 respectively.
For the average human female (Weight=140lbs), two decanters provide enough force to accelerate her upwards at approximately 7ft/second^2; less if she carries a load. Probably three or four decanters are needed for reasonable performance with a normal adventuring load, particularly as the frame will probably weight quite a bit.

Stability will be a very big issue. If she tips over, there will be no recovery. I'd probably see about getting a water or air elemental dedicated to providing thrust to keep the whole contraption upright. She'll also want finer control than just full on and full off, so that might mean a custom magic item. Even then, I would probably require a maximum turn and some distance between down and up movement, as with poor maneuverability.

It's a cool idea; don't be picky about the price; rather, look up other items that grant flight and base the price off of those. It's dangerous and difficult to control, but anyone can use it, it can probably be used as a weapon, and it weighs a great deal. I'd probably give a small discount from, say, a Broom of Flying.

BRC
2008-05-21, 02:39 PM
I'd say just charge her for Wings of Flight, and re-fluff it into a Water Jetpack

Nohwl
2008-05-21, 02:47 PM
Stability will be a very big issue. If she tips over, there will be no recovery.

if she tips over and falls, make sure to add more damage because the decanters are pushing her towards the ground.

Poison_Fish
2008-05-21, 02:54 PM
Obligatory Image Post

http://i7.photobucket.com/albums/y286/Flib/Decantering-1.png

Edit: using my own URL

Nohwl
2008-05-21, 02:56 PM
its not showing anything.

Frosty
2008-05-21, 03:35 PM
if she tips over and falls, make sure to add more damage because the decanters are pushing her towards the ground.

Hmm...either she'd need a lot of decanters pointing in many directions, or one decanter with many exits and she can mentally choose the direction of exit. Bah...this is really complicated.

Azerian Kelimon
2008-05-21, 03:44 PM
Hmm...either she'd need a lot of decanters pointing in many directions, or one decanter with many exits and she can mentally choose the direction of exit. Bah...this is really complicated.

Which is the reason you sneakily take the Wings and refluff them into a waterpack, incorporating the decanter as part of the cost. Which equals a happy player and little work for you.

Bauglir
2008-05-21, 03:49 PM
Possibly adamantine caps that can alter the width of an opening in the top by the manipulation of some control. That should increase the speed at which water flows out with a smaller opening, granting some control. Probably a DC 30 or so craft check to make (if you're making a nonmagical one).

Chronos
2008-05-21, 03:56 PM
Of course it's impractical. It's also really, really cool.

So, assuming you can change the Decanters to accept mental commands as free action, what kind of steps could a very intelligent character in a DnD environment (she's got good knowledge skills) do to achieve the kind of Maneuverability say...that Iron Man has from his suit (as seen in the movie)? Or is that pretty much just impossible?Sure you could do that. If, that is, you mean the first test, where he uses 50% power. On the bright side, you wouldn't need a robot on fire control duty.

Seriously, assuming enough decanters (I'll calculate how many, a little later) and an appropriate harness to hold them, I would allow a DC 20 Balance check to replicate a Levitate spell. If you want to make any horizontal movements, then you need to make a new balance check, at 20 + the base speed you're moving at, repeated each round. Fail any of the balance checks, and you can make a reflex save to turn the decanters off in time. On a successful save, you take normal falling damage; on a failed save, the decanters propel you into the ground for double damage.

A highly-intelligent character who wants to do a better job of it will end up building a separate attitude control system, which amounts to an item that gives a bonus to Balance (either just for this use, or for all balancing in general). Use the standard magic item pricing rules for that.

Frosty
2008-05-21, 04:12 PM
Of course it's impractical. It's also really, really cool.
She seems to think so.


Sure you could do that. If, that is, you mean the first test, where he uses 50% power. On the bright side, you wouldn't need a robot on fire control duty.
As I recall, his first test was actually only at 10% thrust. I can't believe he wasn't wearing a helmet. How did Stark NOT get a concussion?

Triaxx
2008-05-21, 04:41 PM
Actually two Decanter's strapped to her back is just about sufficient to get her up in the air. Manuevering is just a matter of leaning forwards and back. DEX check is all that's required to manuever well, train a skill for perfect.

FMArthur
2008-05-21, 04:44 PM
Is there a way to give the same enchantment as the Decanter to smaller or more suitably shaped items? I think that would be the most workable solution. And water-powered flight sounds too hilarious and messy not to want; I'd even like to have it.

Benejeseret
2008-05-21, 04:59 PM
Perhaps just let it add a major boost to Jump.

Lets her leap very high or very far but does not allow maneuvering nor easy landing.


Or better yet, take the cost of wings, and the cost of a decanter and add them together (150% for the decanter cost I think, foggy on magic item rules) this is now the flying water sprayer. The cost is properly balanced and the effect is what you both want. You can them re-fluff to say the flight is due to the water gush.

Vasdenjas
2008-05-21, 04:59 PM
Wow! It's amazing the number of catgirls who have died here.

If the player thinks it's cool, and it will make them happy, and you have no issue with it, then no amount of physics can stop it. She straps two of them to her back, takes a couple weeks off of adventuring to practice, and viola! She's flying! It uses up her back/cape slot, and gives her perfect manuverability, and has a side effect of after being used, every where she walks, a strange squishy sound emanates from her boots.

Chronicled
2008-05-21, 05:05 PM
Actually, landing would be easy as pie. You just lower the flow one step at a time.

I believe it's more accurate to say "Actually, landing would be easy as pie. Landing ALIVE would be unbelievably tricky."

It's like the old bomber's saying: "Our bombs are 100% accurate. They are guaranteed to always hit the ground."


Anyways, I agree with the advice to give her an item of flying and reflavor it. If your players are having fun, you're doing it right.

FMArthur
2008-05-21, 05:07 PM
On the plus side, your DM gets to describe some very angry and wet peasants whenever you fly around (and land) in the city.

Frosty
2008-05-21, 05:13 PM
Hmm, I wonder if we can add a use-activated Prestidigitation set to activate every time you turn on the streams.

Don Beegles
2008-05-21, 05:16 PM
I think a major problem that everyone seems to be forgetting is that you would be extremely limited in your height unless the decanter can increase its flow essentially ininifinitely. Which it can't. It can produce a stream 20 feet long and 1 foot wide, so to even get 20 feet, she would have to have probably 6 or 7 decanters, because the flow would disperse and lose pressure as it fell. I figure you would probably be able to get 12, maybe 15 tops, before you started needing ridiculous numbers of the things. The best you would get is hovering, not jetpacking.

And, to say it again, when she with one of her other ones, she'd just torque over and fall, rather than starting moving in the direction she wants too. It's an amazingly cool idea, but it's also unfortunately unworkable.

EDIT: If she is obsessed with the idea of flying with water, and can be content with hovering, she could find her local wizard and have him make her new magic items that can modulate the direction and flow of the water. It'd be expensive, inefficient, and just plain silly, but that doesn't make it any less awesome.

Maerok
2008-05-21, 05:19 PM
I think it would work as a jumppack (Gordon Freeman suit upgrade in HL1) , not a jetpack (a la Boba Fett).

Chronos
2008-05-21, 05:50 PM
I think a major problem that everyone seems to be forgetting is that you would be extremely limited in your height unless the decanter can increase its flow essentially ininifinitely. Which it can't. It can produce a stream 20 feet long and 1 foot wide, so to even get 20 feet, she would have to have probably 6 or 7 decanters, because the flow would disperse and lose pressure as it fell. I figure you would probably be able to get 12, maybe 15 tops, before you started needing ridiculous numbers of the things. The best you would get is hovering, not jetpacking.It's just a question of thrust. If thrust equals weight, you'll hover, no matter what height you're at. If thrust exceeds weight, you'll accelerate upwards. The thrust won't depend significantly on how high up you are: Ground effects will only be significant at heights comparable to the diameter of the nozzle.

Frosty
2008-05-21, 05:58 PM
It's just a question of thrust. If thrust equals weight, you'll hover, no matter what height you're at. If thrust exceeds weight, you'll accelerate upwards. The thrust won't depend significantly on how high up you are: Ground effects will only be significant at heights comparable to the diameter of the nozzle.


Correct. Similar to how Space Shuttles can go upwards and don't need to stay near the ground for their thrust to work.

Grey Watcher
2008-05-21, 06:32 PM
Didn't they try a water-powered jetpack on MythBusters? As I recall, it was a complete disaster and there wasn't really a way to make a rig that could actually be worn that'd put up enough pressure to launch a human. A Decanter might mean that you have enough water, but you still can't manage enough pressure (unless I'm grossly mistaken on the rules for Decanters).

Frosty
2008-05-21, 06:34 PM
If you make the nozzle smaller, doesn't the pressure and force of the ejection increase, since the Decanter always sprays the same amount every round?

Worira
2008-05-21, 06:44 PM
Didn't they try a water-powered jetpack on MythBusters? As I recall, it was a complete disaster and there wasn't really a way to make a rig that could actually be worn that'd put up enough pressure to launch a human. A Decanter might mean that you have enough water, but you still can't manage enough pressure (unless I'm grossly mistaken on the rules for Decanters).

That's fairly irrelevant, since they weren't using magic.

seedjar
2008-05-21, 10:29 PM
If you make the nozzle smaller, doesn't the pressure and force of the ejection increase, since the Decanter always sprays the same amount every round?

The question remains as to whether the water's flow is constant or just the force that moves (creates, evokes) it. The SRD does make it sound more like the former, but that seems inconsistent with, for example, the "no crushing" rule of magical size changes. If it's merely a matter of nozzles, you can create a decanter that sprays a practically arbitrary distance, or (in the fashion of a water drill) a cutting tool that is worlds more powerful than any 9000 gp weapon out there.
Also... maybe this has been mentioned already, but isn't Iron Man a poor example to model flight in D&D? That suit goes a fair shake faster than any other flyer in D&D, but it handles like a boat. It's basically a rocket with a dude inside. Sure, works at low speeds, but by D&D standards it isn't great.
~Joe

The Fat Kid
2008-05-21, 10:52 PM
Before I go killing Catgirls I just want to say that I agree with Vasdenjas, in that it isn't really gamebreaking so let your player have fun.

That said, a decanter of endless water sprays 30gallons/6s in a stream one foot in diameter. So it will output a stream that is (.5ft^2*pi)*.85 (five gallons) feet every second. So the water is ejected at .85 feet per second or .26 meters per second. Five gallons of water weighs ~18.9 kg. So the decanter will output ~4.5 newtons of thrust which is almost exactly one pound of force. So for a jetpack that would be able to move a character with any speed or agility, you're looking at about 1-200 decanter's per direction of travel you want. so on the order of five or six hundred.

(yes I am fully aware that this prevents the stream from being twenty feet high and makes the strength check make no sense. Apparently physics doesn't work in DND)

Edea
2008-05-21, 10:56 PM
I'd say just charge her for Wings of Flight, and re-fluff it into a Water Jetpack

Ba-da-boom, QED. You can even have the Decanters in the description, maybe specially modified for the task or something.

Solo
2008-05-21, 11:01 PM
Ba-da-boom, QED. You can even have the Decanters in the description, maybe specially modified for the task or something.

What happens to the water? Does it just disappear after reaching the ground, or does the jetpack flood the place?

Edea
2008-05-21, 11:09 PM
Disappears into a fine and mechanics-irrelevant mist...unless you want her flooding everything, which WOULD be awesome.

Frosty
2008-05-21, 11:13 PM
(yes I am fully aware that this prevents the stream from being twenty feet high and makes the strength check make no sense. Apparently physics doesn't work in DND)

When does physics ever work in DnD? And I would not mind her flooding stuff if she needs to.

Keld Denar
2008-05-21, 11:19 PM
Until a couple of Nyriads with a few Elder Water Elementals gate in and gank your party for blatent abuse of planar nexi. Unlike the firey temper of those on the Elemental Plane of Fire that burns hot and disperses quickly, a water creatures judgement is as steady, predictable, and unstoppable as the tide. Caveat Emptor.

Collin152
2008-05-21, 11:23 PM
Until a couple of Nyriads with a few Elder Water Elementals gate in and gank your party for blatent abuse of planar nexi. Unlike the firey temper of those on the Elemental Plane of Fire that burns hot and disperses quickly, a water creatures judgement is as steady, predictable, and unstoppable as the tide. Caveat Emptor.

Ah, but do not the tides recede?

Learnedguy
2008-05-21, 11:31 PM
I say you screw the rules and let her do it. She seems to like the idea of flying around with a pair of decanters, and who are you to stop her:smallbiggrin:?

Just require her to have 2-4 of those buggers strapped to her arms and back and then have her make a simple dex check to decide maneuverability (a low check equals poor, a decent check medium etc) at the beginning of her turn.

She'll love you for it:smallamused:

Keld Denar
2008-05-21, 11:32 PM
Ah, but do not the tides recede?

They do, but then relentlessly renew their assault with each passing day. Yea, kind of like that.

Frosty
2008-05-21, 11:33 PM
I'll probably do that. Should being in Plate Mail confer any penalties for her?

Learnedguy
2008-05-21, 11:45 PM
I'll probably do that. Should being in Plate Mail confer any penalties for her?

Well, in case you're using my idea then yes I suppose. She's doing dex checks, so armor penalty should probably apply.

averagejoe
2008-05-21, 11:51 PM
Before I go killing Catgirls I just want to say that I agree with Vasdenjas, in that it isn't really gamebreaking so let your player have fun.

That said, a decanter of endless water sprays 30gallons/6s in a stream one foot in diameter. So it will output a stream that is (.5ft^2*pi)*.85 (five gallons) feet every second. So the water is ejected at .85 feet per second or .26 meters per second. Five gallons of water weighs ~18.9 kg. So the decanter will output ~4.5 newtons of thrust which is almost exactly one pound of force. So for a jetpack that would be able to move a character with any speed or agility, you're looking at about 1-200 decanter's per direction of travel you want. so on the order of five or six hundred.

(yes I am fully aware that this prevents the stream from being twenty feet high and makes the strength check make no sense. Apparently physics doesn't work in DND)

I went from the other direction (metaphorically), assuming that the output and distance were correct, and ignored dimensional considerations.

I calculate 30gal~113.5 liter=.1135 m^3. .1135m^3*992kg/m^3=112.6 kg, so we have about 112.6 kg/6sec=18.77kg/s. Now, it's spray is 20ft long. I assumed that this was straight up in the air, because otherwise it would depend on the elevation of the bottle. So, that would require a starting velocity of v(i)=sqrt(v(f)^2-2*a*d)=sqrt(2*9.8m/s^2*6m)=59.7m/s. 59.7m/s*18.77kg/s=1,121.3 newtons. Online it says that an average woman is 61 kg, 61*(-9.8)=891.8 newtons, so one bottle would provide enough thrust to get her off the ground (albiet inefficiently) with enough to spare for gear assuming the stream is maintained, though you'd probably want more than that so she's not all slow. Or, you could just average mine and The Fat Kid's results, because all of these variables aren't consistant with each other. Or, just ignore this altogether, because it's unlikely to come up anyways. You can just set the amount of force as, "enough." Also note that you need less force to manuver side to side than you do to get off the ground, since you don't have to overcome gravity (although you may have to overcome wind, etc.)

I would personally suggest strapping a bunch to her back and, instead of using them to fly, use it for a boost of speed in order to charge/ultimate bull rush of doom someone.

By the way, to answer a question above, a smaller nozzle size wouldn't really do anything for these purposes. It would increase the pressure (force/area) but it wouldn't increase the overall force, which is what you want. Getting hit with a higher pressure stream would hurt more, because the force would be more concentrated, but it wouldn't knock you back any further.

Edit: I wouldn't impose a penalty for being in heavy armor, except maybe to account for the extra weight. I doubt it would take more dexterity than, say, handling a sword.

Keld Denar
2008-05-21, 11:51 PM
Having the decanters strapped/bolted/soveren glued to the plate mail would give an extra degree of stability, IMO, since it is strapped at multiple points on your torso, shoulders, and arms. This would both relieve the stress that much thrust would produce, as well as allow you to use more of your mass for directional control. A traditional "jetpack" would put so much stress on your shoulders and arms alone that it would probably shred you from effort. This allows a more even distribution of the force along your trunk the stongest collective group of muscles in your body.

EDIT:
Destroying the moon isn't so easy. It has millions of HP, and you still have to get there. How are you going to get there? Fly with your water jet pack?

Also, how appropriate is it that while debating physics, They Might Be Giants: The Sun Song comes on my winamp. If you haven't heard it, look it up. It'll make your head spin, in a good way!

Collin152
2008-05-21, 11:51 PM
They do, but then relentlessly renew their assault with each passing day. Yea, kind of like that.

The conclusion is obvious. Destroy the moon.

Solo
2008-05-21, 11:58 PM
Maybe we should just strap 2 decanters to her legs and 2 to her arms, and let her use the arm decanters to blast people with....

averagejoe
2008-05-22, 12:06 AM
Maybe we should just strap 2 decanters to her legs and 2 to her arms, and let her use the arm decanters to blast people with....

She'd also blast herself, as it were. Newton's third and all that. The legs would probably be workable, though.

Frosty
2008-05-22, 12:10 AM
Maybe we should just strap 2 decanters to her legs and 2 to her arms, and let her use the arm decanters to blast people with....

Just because she's going to be flying around in Plate mail doesn't make her Iron Man :smallamused:

Keld Denar
2008-05-22, 12:11 AM
Just because she's going to be flying around in Plate mail doesn't make her Iron Man :smallamused:

Right, she lacks the appropriate anatamy. Iron Woman doesn't have the same ring to it...

xirr2000
2008-05-22, 12:11 AM
Oneof my players recently acquired a Decanter of Endless Water. She has since been thinking a lot, often sporting a strange grin on her face. Then, last session, she asked me about getting a few more of those and possibly fashioning them into some sort of jet-pack.

Is that feasible, or should I just tell her it's stupid?

It's stupid. I'm not going to do the math, it's stupid.

Solo
2008-05-22, 12:12 AM
But she is in a suit of iron, as opposed, say, a gold-titanium alloy.

Collin152
2008-05-22, 12:13 AM
She'd also blast herself, as it were. Newton's third and all that. The legs would probably be workable, though.

Newton's laws scarecely come into consideration when the laws of thermodynamics are cheerily thrown out a window, and the law of conservation of mass is trampled upon daily.

Cuddly
2008-05-22, 12:13 AM
If you want her to do it with a decanter, sure. Either let her use a single, souped up decanter, or perhaps several small decanters, complete with tiny stabilizing ones and stuff.

Frosty
2008-05-22, 12:14 AM
Right, she lacks the appropriate anatamy. Iron Woman doesn't have the same ring to it...

Well what if her call sign became Iron Maiden? Gah! Stop giving me ideas! This will only encourage her to push for more :smallyuk:

Solo
2008-05-22, 12:16 AM
What have we created?

Frosty
2008-05-22, 12:17 AM
But she is in a suit of iron, as opposed, say, a gold-titanium alloy.

I'm sure Full Plate is made out of some sort of tempered Steel.

averagejoe
2008-05-22, 12:18 AM
Newton's laws scarecely come into consideration when the laws of thermodynamics are cheerily thrown out a window, and the law of conservation of mass is trampled upon daily.

Yes, but in those instances a wizard did it. Or possibly a sorcerer. There was probably some sort of magic involved at some point. In DnD, the laws of physics pretty much always apply unless the rules specifically state otherwise. Anyways, this item specifically lists a knockback effect, one that would be much harder to resist without good ol' friction keeping you company. Or, like, the ground.

Edit: She could take ranks in profession: cook and become the iron chef. :smallwink:

Nohwl
2008-05-22, 12:21 AM
you should have the armor start to rust after a month or so.

Keld Denar
2008-05-22, 12:23 AM
Newton's laws scarecely come into consideration when the laws of thermodynamics are cheerily thrown out a window, and the law of conservation of mass is trampled upon daily.

But....the Laws of Thermodynamics has regeneration or so, so it can recover from the impact that results from defenestration. You know, even though the concept of regeneration completely mauls the Laws of Thermodynamics and leaves them a horribly mangled shell of what they used to be. Where does the body obtain enough energy to produce replacement cells as such a staggering rate!!!!

EDIT:
Yea, I just used Defenestration in a sentance...I'm just that good.

Cuddly
2008-05-22, 12:26 AM
Cells? You are really overthinking this.

Dr Bwaa
2008-05-22, 12:32 AM
Yes, but in those instances a wizard did it. Or possibly a sorcerer. There was probably some sort of magic involved at some point. In DnD, the laws of physics pretty much always apply unless the rules specifically state otherwise. Anyways, this item specifically lists a knockback effect, one that would be much harder to resist without good ol' friction keeping you company. Or, like, the ground.

To be fair, we are talking, here, about a bottle--no, multiple bottles--of infinity. That somehow are able to propel this infinity at a very specific rate or rates, forever. Please tell me how that abides by any laws of physics :smallsmile:

Also, the actual thrust from these things cannot, I don't believe, be calculated strictly (within reason, you'll see why): because Thrust is dependent on the velocity of the water with respect to the decanted, which we could figure out in the real world with the 20-foot range: but in D&D, a 20-foot range means a jet of water that reaches 20 feet in any direction at all, unaffected by gravity, and then suddenly ceases/fall/whatever. There is no listed velocity, so going by how mechanics in D&D appear to work, the velocity, and therefore the thrust, are likely infinity. Which leads me to this answer: yes, you can fly. Yes, you can go to the moon. No, you can't stop, ever :smallsmile:

averagejoe
2008-05-22, 12:42 AM
To be fair, we are talking, here, about a bottle--no, multiple bottles--of infinity. That somehow are able to propel this infinity at a very specific rate or rates, forever. Please tell me how that abides by any laws of physics :smallsmile:

Were you listening to the part where I said a wizard did it? The game has a rule which states "these bottles never run out of water and always eject it at a certain rate." So, the laws of physics aren't followed, but a rule said otherwise.

Chronos
2008-05-22, 01:52 AM
That said, a decanter of endless water sprays 30gallons/6s in a stream one foot in diameter. So it will output a stream that is (.5ft^2*pi)*.85 (five gallons) feet every second. So the water is ejected at .85 feet per second or .26 meters per second. Five gallons of water weighs ~18.9 kg. So the decanter will output ~4.5 newtons of thrust which is almost exactly one pound of force. So for a jetpack that would be able to move a character with any speed or agility, you're looking at about 1-200 decanter's per direction of travel you want. so on the order of five or six hundred.The problem with this calculation is that it assumes that the stream is a foot in diameter at the nozzle, which would make it a rather silly thing to call a "decanter". That'd be more like a "bowl of endless water". I think it's much more reasonable to assume that it has a very narrow opening, like a decanter ought to, but that the stream spreads out to about a foot wide shortly after leaving the nozzle. In this case, it's better to use the maximum height to calculate the effect, which gives us only one needed.

Though I'd still recommend two, since it'd be almost impossible to balance a single one.

And why are folks using the weight of a human female? This is clearly a gnome, we're talking about here. Or at least, if she's not a gnome, she should be.

Talic
2008-05-22, 02:04 AM
Strap a dozen onto the back of a wagon. It's both propulsion and a way to mire down all the wagons behind you!

Also, we can't claim that the Newton's 3rd law doesn't apply if the basic premise of the thread is using the 3rd law to generate thrust.

Jothki
2008-05-22, 02:06 AM
I hate to say this, but the concept is probably fundamentally flawed. In order to generate thrust, you would have to actually split up an object's mass, applying an equal force in opposite directions to the two parts. The Decanter is effectively generating mass out of nothing (or shooting mass out of an object not connected to the person/Decanter system, the physics will work out the same), so there won't be any force put on the bottle.

Talic
2008-05-22, 02:14 AM
The geyser effect causes considerable back pressure, requiring the holder to make a DC 12 Strength check to avoid being knocked down. The force of the geyser deals 1d4 points of damage but can only affect one target per round. The command word must be spoken to stop it.

The SRD disagrees with you. Force is generated.

Thus, we can show that this is a magical effect which transports water to the material plane (flavor is from the Elemental plane of Water, I believe), and propels it at high speed, generating thrust.

Frosty
2008-05-22, 02:16 AM
Chronos, technically, this is a Warforged female Paladin of Freedom with Adamantium Body, so we might need a few more Decanters than expected.

Seffbasilisk
2008-05-22, 02:40 AM
Assuming adamantium doesn't rust, and that her body is stable enough, have her encounter an 'eccentric' (they're only insane if they're below a certain wealth level) who wants to design such a suit. Crazy maxed out ranks in craft and knowledges. Go for 6-8 decanters and have to spend at least a week in down-time being outfitted, then give her Clumsy maneuverability. Have the gnome have hundreds of pages more of notes, but the first time she takes off, it destroys the notes, and pleases the gnome enough that he switches to a new project.

You can turn it into an entire quest to find someone else to help work on it. Each person she finds might be able to improve it one step. Maneuverability, speed...eventually someone working in a Use-activated prestadigitation to have the water gysers evaporate once thier thrust is actuallized.

Talic
2008-05-22, 04:24 AM
Wow. Fighting crime, faster than a speeding bullet... Able to water large fields in a single pass! Able to kill them equally fast (nothing beats salt water for salting the land)!

Belial_the_Leveler
2008-05-22, 04:32 AM
Think outside the box;

1) Buy decanter of endless water.
2) Buy scroll of heat metal and permancy plus an adamantine tube.
3) Fix decanter to one end of the tube.
4) Cast heat metal then permancy on the tube.


Now you have a steam rocket. Enjoy.

Azerian Kelimon
2008-05-22, 07:10 AM
Also, keep in mind this is not Iron Man, Iron Maiden, or Sparta. Nay...

THIS! IS! THE-PAIN-KILLER! (http://www.youtube.com/watch?v=JAagedeKdcQ)
*fas-ter-than-a-la-ser-bul-let...*

Frosty
2008-05-22, 01:09 PM
To clikc on the link, or to ignore it...the CHOICES!!

Citizen Joe
2008-05-22, 01:36 PM
It seems clear to me that she intends to have the decanters built into her own frame as jet boots. What is worse, she'll probably figure out a way to make the water holy. Imagine the destruction caused on a horde of undead as she hoses down the entire area, leaping to their undead master.

Knaight
2008-08-07, 09:11 AM
Thats just awesome. But if building them into the frame is an option, then were looking at a whole bunch more options. You could put them all in metal casings, then implant about 3 of them in the back in a triangular pattern, pointing backwards and slightly down, one in each foot, and 3 in the chest, again in a triangular pattern. Or possibly create a ring of them at waist level, firing down and outwards, with the chest and back jets in as well. Foot Jets would also help.

hewhosaysfish
2008-08-07, 10:19 AM
Does it have to be a jet-pack? Some sort of hang-glider with Decanter's providing thrust would seem address some of the issues of stability and steering.

Lochar
2008-08-07, 10:44 AM
*Knaight has cast the dreaded spell, Thread Necromancy*

Arise thread, arise and do my bidding.

Knaight
2008-08-07, 08:57 PM
I got here through a link, had no idea what page it was on. Was the whole jetpack thing even resolved?

Frosty
2008-08-07, 10:35 PM
It became a non-issue. The character died in an incident involving experimenting with Reverse Gravity.

holywhippet
2008-08-08, 01:35 AM
It became a non-issue. The character died in an incident involving experimenting with Reverse Gravity.

She fell off the face of the planet?

For the original query, I wouldn't allow a character to do something so metagamerish unless their character background justified it. In this case I'd expect ranks in knowledge: engineering and some kind of craft skill ranks.

Kiara LeSabre
2008-08-08, 02:44 AM
I would have just asked, "First of all, if you really want to do this, why aren't you asking if you can custom-create Decanters of Endless Air?"

Because ... it just strikes me that all concerns about the jetpack idea itself being impractical ... air as propulsion would also have been more practical than water?

Chronos
2008-08-08, 03:21 PM
It became a non-issue. The character died in an incident involving experimenting with Reverse Gravity.OK, this really sounds like my kind of character, now. As if there was any doubt before.

Frosty
2008-08-08, 04:05 PM
She fell off the face of the planet?

For the original query, I wouldn't allow a character to do something so metagamerish unless their character background justified it. In this case I'd expect ranks in knowledge: engineering and some kind of craft skill ranks.

IC-ly, she has a Half-Gnome friend who is somewhat of a mad scientist type. The player justified it by saying that the mad scientist talked to her about the idea and he needed a test subject...


Because ... it just strikes me that all concerns about the jetpack idea itself being impractical ... air as propulsion would also have been more practical than water?

Not a bad idea. Any physics majors here disagree? oh btw I love your avatar.


OK, this really sounds like my kind of character, now. As if there was any doubt before.
:smallbiggrin: Let me ask you: How are you picturing her death?

Kiara LeSabre
2008-08-08, 05:01 PM
Not a bad idea. Any physics majors here disagree? oh btw I love your avatar.

Thanks! Oh, there's a Hello Kitty Online now, too. I'm tempted to try it, but I fear I'll be disappointed to learn that I can't level up my kitty and then run around ganking newbies with her. :smallfrown:

sikyon
2008-08-08, 05:27 PM
Any stability issues are best adressed with how rockets adress stability issues:

Fins. Dynamically stable fins. Adjustable to change direction.

You don't need vectored thrust or perfectly balanced mass or any of that horse rubish. Just strap a few dynamically stable fins onto yourself with some steering controls to change their vectors and you are good to go. LRN2 1900's people.

Edit:

Thrust: 6 gallons/second, 3.3ft/1 seconds. that's 1 m/s, 22.7 liters/second. That's 22.7 kg*m/sec^2/decanter.

So in the absence of gravity, 22.7 kg will have an acceleration of 1 m/s^2. (conservation of momentum)

acceleration due to gravity is 9.8 m/s^2

Therefore, in order to be able to lift off the ground, for say a 68 kg person, you need ~30 decanters of water.

not feaisable.


I'm not sure about the math, I may have left something out. but I sincerly doubt it is enough.

Frosty
2008-08-08, 06:35 PM
I'm tempted to try it, but I fear I'll be disappointed to learn that I can't level up my kitty and then run around ganking newbies with her. :smallfrown:

My kinda girl :smallcool: Although ganking newbies isn't very nice.

Do you play WoW or something?

Deth Muncher
2008-08-08, 06:58 PM
Any stability issues are best adressed with how rockets adress stability issues:

Fins. Dynamically stable fins. Adjustable to change direction.

You don't need vectored thrust or perfectly balanced mass or any of that horse rubish. Just strap a few dynamically stable fins onto yourself with some steering controls to change their vectors and you are good to go. LRN2 1900's people.

Edit:

Thrust: 6 gallons/second, 3.3ft/1 seconds. that's 1 m/s, 22.7 liters/second. That's 22.7 kg*m/sec^2/decanter.

So in the absence of gravity, 22.7 kg will have an acceleration of 1 m/s^2. (conservation of momentum)

acceleration due to gravity is 9.8 m/s^2

Therefore, in order to be able to lift off the ground, for say a 68 kg person, you need ~30 decanters of water.

not feaisable.


I'm not sure about the math, I may have left something out. but I sincerly doubt it is enough.

This point, of course, being moot, because as was stated, the character died due to Reverse Gravity experiments. Now, I'm going to bet that within 12 hours, Roland will show up saying something to the effect of "Please, don't resort to Thread Necromancy. If you'd like to start another thread about Decanter Jetpacks, that's perfectly fine. Thread closed."

sikyon
2008-08-09, 09:57 AM
This point, of course, being moot, because as was stated, the character died due to Reverse Gravity experiments.

The point of this thread was originally a decanter jetpack, just because OP's situation changes does not mean we have to stop discussing the issue. The OP does not own the topic. It does not start or end with the OP. If there is real, meaningful discussion going on then by all rights a thread should continue. Since alot of good ideas have been presented in this thread, i think that it's reasonable to continue it.


Now, I'm going to bet that within 12 hours, Roland will show up saying something to the effect of "Please, don't resort to Thread Necromancy. If you'd like to start another thread about Decanter Jetpacks, that's perfectly fine. Thread closed."

http://www.giantitp.com/forums/announcement.php?a=1

ericgrau
2008-08-09, 10:21 AM
Given that someone with str 14 can stop the force on average, we're talking at most 175 lbs. of force.

Okay, so you use multiple geysers to lift someone off the ground, then what? 30 gallons per round is 5 gallons per second, across a 1 foot wide stream. But let's be generous and say the decanter has a 3" wide mouth, to increase the velocity. 5 gallons per second = 0.67 cubic feet per second. The area of the mouth is about 0.05 square feet. So the exit velocity is about 13.6 feet per second, or 82 feet per round. Your forward velocity can't be any higher than that. So at most you could get about 80 feet per double move, or 40 feet per move. But, practically speaking, you'll probably be lifting around half your maximum load and you'll only get 20 feet per move. And you'll need to slowly accelerate up to that speed over the span of maybe 2 rounds (as a guestimate). No matter how many geysers you have, your speed is limited in this way. More geysers just help you carry more weight and get up to speed faster. And I was being generous with the geyser's mouth. In reality I bet it would be 25-50% slower.

The thing would be incredibly unstable. You might be able to fix multiple decanters securely to a frame, spread out, and with some kind of control mechanism to help. You'd be able to hover in place and move in all directions, but momentum and reversing direction would be a problem. Precise movement would be nearly impossible, except at very low speeds.

sikyon
2008-08-09, 11:15 AM
So the exit velocity is about 13.6 feet per second, or 82 feet per round. Your forward velocity can't be any higher than that.

Bzzzttt thanks for playing!

That's not the way force works.


But let's be generous and say the decanter has a 3" wide mouth, to increase the velocity.

Doesn't even matter. Force = pressure * area. A smaller mouth means higher pressure, but the same amount of force.

Dervag
2008-08-09, 12:15 PM
It became a non-issue. The character died in an incident involving experimenting with Reverse Gravity.Why am I not surprised?

Test pilots and Literal Genie (http://tvtropes.org/pmwiki/pmwiki.php/Main/LiteralGenie) magic do not mix, and this warforged clearly had the soul of a test pilot.


Unless the thrust is perfectly balanced around the center of mass she'll turn into a pinwheeling bludgeon, smashing repeatedly into things until the decanters break or fly loose. You would need fine gradations in the water pressure to make this controllable, and Decanters don't have that.

I've got a degree in aeronautical engineering from M.I.T., if that helps you accept my evaluation of this hair-brained scheme.If any person here has seen Iron Man, remember the trouble that Stark had using his repulsors to fly? The tests where he kept getting hit with the fire extinguisher?

This would be like that, only worse.


Hmm...either she'd need a lot of decanters pointing in many directions, or one decanter with many exits and she can mentally choose the direction of exit. Bah...this is really complicated.Yeah. If she wants to control her motion, she needs something like a vectored thrust system. Something like the Harrier jump jet. Only the Harrier has computer control, which takes a huge load off the pilot.


Actually two Decanter's strapped to her back is just about sufficient to get her up in the air. Manuevering is just a matter of leaning forwards and back. DEX check is all that's required to manuever well, train a skill for perfect.If you "just lean forwards and back," and don't do it perfectly, you will end up spinning around in the air and smacking into things, like Stark in his tests. Note that he never managed to fly effectively until he got into his suit, where he presumably had computer control to help him with the precision.


Wow! It's amazing the number of catgirls who have died here.I blame the evil, evil person who assigned them to test-pilot water jetpacks. Those things are lethal.


If the player thinks it's cool, and it will make them happy, and you have no issue with it, then no amount of physics can stop it. She straps two of them to her back, takes a couple weeks off of adventuring to practice, and viola! She's flying! It uses up her back/cape slot, and gives her perfect manuverability, and has a side effect of after being used, every where she walks, a strange squishy sound emanates from her boots.What concerns me is that I think this would be extremely difficult to control. It's not as easy as walking on stilts, and that takes considerable practice to do correctly too.

I mean, Rule of Cool is great, but the character should have to pay for the coolness.


I think a major problem that everyone seems to be forgetting is that you would be extremely limited in your height unless the decanter can increase its flow essentially ininifinitely. Which it can't. It can produce a stream 20 feet long and 1 foot wide, so to even get 20 feet, she would have to have probably 6 or 7 decanters, because the flow would disperse and lose pressure as it fell. I figure you would probably be able to get 12, maybe 15 tops, before you started needing ridiculous numbers of the things. The best you would get is hovering, not jetpacking.This is not the case. The idea is to use the flow from the decanter as rocket thrust- the decanter(s) squirt(s) so much water down that the character is propelled upwards.

The system will work anywhere. You could even use it in the vacuum of space. If you can control it at all, it will work anywhere.


Having the decanters strapped/bolted/soveren glued to the plate mail would give an extra degree of stability, IMO, since it is strapped at multiple points on your torso, shoulders, and arms. This would both relieve the stress that much thrust would produce, as well as allow you to use more of your mass for directional control. A traditional "jetpack" would put so much stress on your shoulders and arms alone that it would probably shred you from effort. This allows a more even distribution of the force along your trunk the stongest collective group of muscles in your body.Why? There are plenty of people who can support their own weight using their upper body strength. A jetpack only has to exceed your own weight.


Well what if her call sign became Iron Maiden? Gah! Stop giving me ideas! This will only encourage her to push for more :smallyuk:Iron Lady? She could take over a medium-sized country... though probably not using this system. It just isn't that effective a weapon.


What have we created?I don't know, but I expect to see it pinwheeling comically through the sky.


Chronos, technically, this is a Warforged female Paladin of Freedom with Adamantium Body, so we might need a few more Decanters than expected.Yes. Yes we definitely are. Also, this means she's less likely to be severely injured when she inevitably starts crashing into things.

Knaight
2008-08-09, 12:19 PM
Well, for a Warforged where vacuum of space works anyways, thats actually pretty much ideal, without friction you would be looking at ridiculous speeds.

Kami2awa
2008-08-09, 12:56 PM
Is it likely that a scholar in a medieval world would know enough physics to do it? Or, in a world where magic is a reality and everything seems to run by positive and negative energy and the four classical elements, that the laws of nature are similar enough for it to ever work?

Kiara LeSabre
2008-08-09, 01:35 PM
My kinda girl :smallcool: Although ganking newbies isn't very nice.

Do you play WoW or something?

I was being a little over the top there. I wouldn't really gank newbies. :smalltongue:

I don't, but I used to play DAOC long ago. The closest I come to MMOs now is Second Life.

ericgrau
2008-08-09, 02:15 PM
Bzzzttt thanks for playing!

That's not the way force works.



Doesn't even matter. Force = pressure * area. A smaller mouth means higher pressure, but the same amount of force.

BS. We're talking exit velocity here on a jet engine, which you appear to be ignorant of. Once you start moving as fast as the water you get zero pressure/force from the water. Hence you cannot possibly exceed that speed, and usually you go a bit slower than that speed.

Otherwise every jet engine would have a wide nozzle or giant propeller, and turboprops would be the fastest planes around. In reality the exact opposite is true: jets with smaller outlets have a higher top speed but lower thrust, and that's what we use on all the fast jets. It's also why a good sports car with 1/100th the horsepower of an F16 can beat it on the runway. Next time get your facts straight and don't bash regardless. If anything the latter only hints that you're compensating for a lack of the former, besides being bad form.

TheDarkOne
2008-08-09, 03:24 PM
BS. We're talking exit velocity here on a jet engine, which you appear to be ignorant of. Once you start moving as fast as the water you get zero pressure/force from the water. Hence you cannot possibly exceed that speed, and usually you go a bit slower than that speed.

Otherwise every jet engine would have a wide nozzle or giant propeller, and turboprops would be the fastest planes around. In reality the exact opposite is true: jets with smaller outlets have a higher top speed but lower thrust, and that's what we use on all the fast jets. It's also why a good sports car with 1/100th the horsepower of an F16 can beat it on the runway. Next time get your facts straight and don't bash regardless. If anything the latter only hints that you're compensating for a lack of the former, besides being bad form.

I think you should get your facts straight, the idea isn't anything like a jet, much more similar to a rocket. The only differences are the decanters won't get lighter as they use up fuel, and they don't eject their fuel based on combustion. (Despite being called a "jet pack") So the way you're thinking about it isn't really valid.


Any stability issues are best adressed with how rockets adress stability issues:

Fins. Dynamically stable fins. Adjustable to change direction.

You don't need vectored thrust or perfectly balanced mass or any of that horse rubish. Just strap a few dynamically stable fins onto yourself with some steering controls to change their vectors and you are good to go. LRN2 1900's people.

This will not work except at high speed. How do you think the fins stabise thigns? It's from the forces generated by the air going by them at high speed. If you're almost stationary you will get no benifit from a fin.

This idea will have massive control problems, the only way to stabilize it would either an active control system, or good pilot reflexes. (and obviuosly some way of adjusting the direction and magnitude of the force easily and with a good range..)

sikyon
2008-08-09, 03:38 PM
BS. We're talking exit velocity here on a jet engine, which you appear to be ignorant of.

What you appear to be ignorant of is the fact that this situation is nothing like a jet engine. Jet engines have a maximum speed because they intake fluid. There is no fluid intake in a decanter of endless water. You need to start applying rocket physics, not jet engine physics.


This will not work except at high speed. How do you think the fins stabise thigns? It's from the forces generated by the air going by them at high speed. If you're almost stationary you will get no benifit from a fin.

True. Gyroscope?

Frosty
2008-08-10, 12:35 PM
I was being a little over the top there. I wouldn't really gank newbies. :smalltongue:

I don't, but I used to play DAOC long ago. The closest I come to MMOs now is Second Life.

Why don't you play Dark Age of Camelot anymore? Got bored?


This idea will have massive control problems, the only way to stabilize it would either an active control system

What do you mean by this?

sikyon
2008-08-10, 01:40 PM
What do you mean by this?

He means that you need to be able to carefully change the direction of the decanters or the force of the decanter, from moment to moment. This is typically impossible.

I have a solution though: Immovable rod on the harness to "hover". Use the immovable rod to calibrate a gyroscope for direction changes (immovable rod absorbs angular momentum) to get a gyroscope started/change orientation. Gyroscope keeps you stable at low speeds, fins keep you stable at high speeds.

snafu
2008-10-08, 02:57 PM
Apologies for more necromancy, but I found this thread and was reminded of some calculations I did on another forum last year. I admit it, I love magitek. Suppose we could do this by magic, then working through otherwise normal physics what would that mean? It has nerd appeal on so many levels.

In this case, we may have a death ray on our hands.

The magic: A decanter of endless water is a device which creates water and sprays it into the world at 30 gallons per round.

30 gallons per 6 seconds is 19 litres per second assuming a US gallon. So every second the decanter fires nineteen kilograms of mass. The speed at which it does so depends upon its bore: force the water through a narrow nozzle and it must emerge at a higher speed in order to satisfy the 19 litres per second specification.

Let the nozzle diameter be 64mm, typical of a firehose, reasonable for a table decanter of water. Then 19 litres per second, divided by the area through which the water passes at the mouth of the decanter (pi * r^2), gives 5.9 metres per second. And hey, that's about 20 foot, just as the item description says.

The kinetic energy, 0.5 * m * v^2, is 331 joules for that one-second burst, so 331 watts of power for a continuous stream.

Recoil? Momentum is 5.9 kilogram metres per second. Assuming that the momentum must be conserved in our world (i.e. we can't dump the momentum back through the gate into the elemental plane of water) then a typical human massing 70kg, with his whole weight behind it, would be pushed backward at 8.4 centimetres per second, every second. That's a fair old force, but not unreasonable to hold down with strength or practice; DC 12 strength check, again as described.

Good. So our theoretical decanter matches the standard magical item fairly well so far. Now let's suppose we can produce these things with narrower necks. Perhaps we could fit it with an adjustable nozzle such as we have on our garden hoses.

Let the nozzle diameter be 20mm. Then to pass 19 litres of water in 1 second, the speed must be 60 metres per second. Power: 35 kilowatts. That's 47 horsepower. Recoil would accelerate a 100kg mass such as a large fighter backwards at 1.17g. You'll want to have this mounted on something, not held in your hand.

Let the nozzle diameter be 2mm. One tenth the radius is one hundredth the area, is one hundred times the muzzle velocity. So to pass 19 litres of water in 1 second, the speed must be 6 kilometres per second, not far short of orbital velocity. One hundred times the speed is ten thousand times the energy and hence power: 350 megawatts, a bit less than the engines of two Nimitz-class aircraft carriers. Recoil: 100kg mass accelerated at 117g, and is well on its way to interstellar space.

Now, let's talk damage. The regular 64mm decanter does 1d4 damage per round to a target. The 2mm decanter has 1024 times less area, so 1024 times faster flow. So the target is hit by the same amount of water, moving at 1024 times the speed. Damage, 1024d4.

Except it seems to me that it's not momentum that hurts you as much as kinetic energy. Would you rather be hit by a car at 30mph, or another car half the mass at 60mph? Exactly. Kinetic energy goes as the square of the speed. Same mass of water in same amount of time, carrying far, far too much energy. Damage... 1,048,576d4.

Per round.

Oh, and I figure the expanding cloud of steam would level all non-reinforced structures out to a radius of maybe 20m. Based on figures I found on a study of the blast effects of nuclear weapons.

Wonderful. Just pray the DM doesn't ask awkward questions like 'so, when you do that, what's the pressure inside the decanter?' and then work out the likely velocity of the shards of decanter that gouge through your face when the whole thing blows up.

And of course none of this works if it's the pressure that's fixed by the magic, not the rate of water production; then tightening up the nozzle would just leave you with a rather pathetic dribble.

Now, go build one of these beasts and kill some catgirls for me.

Roland St. Jude
2008-10-08, 10:48 PM
Apologies for more necromancy ...

Sheriff of Moddingham: Please don't.

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Bringing a thread back from “the dead.” If a thread has fallen to page three and hasn’t been posted in for a month and a half, don’t post to it. Start a new topic if you want to discuss the subject.