Decanter of Endless Water Steam Rocket

[TeslaJr]

I was browsing the web looking for ways to increase the speed of my party's airship, and I came across this doc (https://docs.google.com/document/d/14zilT4WGOyHM0AfpG4-GmD2FkgDg1HZ9HC1cTleQHds/edit) that proposed using a DoEW to create a rocket. I know trying to apply real world physics to DnD is a nightmare, but I was curious if anyone out there has tried, because it would be pretty awesome to have a rocket propelled airship.

[Barstro]

I believe that it cannot by done by RAW; something about "no such thing as pressure" or similar argument. If you did say that pressure existed and try to apply physics, you are up against Newton's Third Law (but see Mythbusters showing an interesting outcome of using a fan to fill the sails of a boat.)

Reality; it doesn't work.
In-game; Make it do what you want, but keep the rate constant to make things simple.

[VoxRationis]

I believe that it cannot by done by RAW; something about "no such thing as pressure" or similar argument.
Reality; it doesn't work.
In-game; Make it do what you want, but keep the rate constant to make things simple.

That magic item specifically has things in its entry about pressure from the highest setting. You have to make a Strength check of 12 to avoid being knocked down (and you don't have to have the stream facing a solid object for you to have to make the check), which implies that the water is flowing with the equivalent force of Strength 14 (+2, plus 10 for the base DC, equals 14). Strength 14 has a light load limit of 58 pounds, a medium limit of 116 pounds, and a heavy load limit of 175 pounds. Presumably, this means that the decanter on its own can lift, slowly, as much as 175 pounds. That's a pretty expensive rocket; you'd do better with carpets of flying.

[Fouredged Sword]

The closest I have done to this is a castle that acted as the headwaters of a decanter of endless water river. It was a small river, but the logistics of being able to place a small river at the top of a hill and irrigate your farmland + power waterwheels on command is something most mid-evil kings would sell half his army for.

[Shalist]

Per the RAW, the decanter doesn't care size, and could knock a planet out of orbit as easily as knocking a gnome over. If you assume medium creatures as a baseline though, then (disregarding any actual math) the decanter would be roughly equal to a ‘windstorm’ (51 – 74 mph) wind (http://www.d20srd.org/srd/weather.htm#tableWindEffects) in terms of force, which could 'check' a huge/gargantuan flying object (move it 1d6*5'/round, avg 17.5'/rnd).

Per D 'n D logic, you could argue that 4 decanters yields the next 'size' of wind, and 16 decanters the size after that (hurricane- and tornado- force respectively). Thus 16 decanters could 'knock back' (1d6*10'/rnd, avg 35'/rnd) a flying gargantuan object, and 'check' a flying colossal object.

Another recommended house rule is that for every 1d6*10'/rnd of movement acquired in this manner, the maneuverability drops 1 notch (due to the greater difficulty in turning and what not); and if it ever drops below 'poor,' the object simply rockets about completely out of control (like a loose firehose (http://www.youtube.com/watch?v=2Ktcz3WX2Sc)).

[Barstro]

Hell, if it specifically says it has pressure, sounds fun.

[Kioras]

It puts out 30 gallons of water in a round, or 5 gallons a second, in a 1 foot stream. It ejects ~40lb of water each second without having any internal weight.

I have no idea on the math involved, but it should be possible to use a few of them to make some sort of either steam rocket, (multiple alternating chambers filling with water, heated by a perm'd wall of fire), or all else fails, a pretty interesting steam powered potato gun/cannon.

[Kraken]

Wall of magma works better then wall of fire for a steam engine, due to walk of magma explicitly merging with existing stone just fine. So you can make a wall of stone container for your wall of magma, then run the decanter's geyser mode in one end, and out a nozzle on the other end to generate thrust.

[Shalist]

Using math:

Given:

Volumetric flow rate (V*): 5 gallons/sec => 0.668 ft3/sec
Mass (m): 41.6 lb
Displacement (s): 20 ft
Acceleration (a) -32 ft/sec2

1) How fast is the water moving when it leaves the nozzle--err, decanter:
Initial velocity (v) = (2 * a * s).5 = (2 * 32 ft/sec2 * 20 ft).5 = 35.78 ft/sec

2) How big does the decanter need to be for 5 gallons to pass through it at that speed each second?
Area (A) = V* / v = (0.668 ft-3/sec / 35.78 ft/sec) * 144 in2/ft2 = 2.69 in2 (incidentally, this gives decanters a diameter of 1.85 inches).

3) Given that the water shoots 20' straight up, what is its pressure?
Pressure (p) = 0.434 s (http://www.engineeringtoolbox.com/pump-head-pressure-d_663.html) = 0.434 * 20 = 8.68 lb/in2

4) Given the pressure, and the area it is acting on, calculate the force being exerted on the decanter:
Force (F) = A * p = 2.69 in2 * 8.68 lb/in2 = 23.3 lb

Acceleration would just be force / mass, 'course. For the sake of simplicity, you could say that you reach maximum speed (due to wind resistance) after a single round, so a ~1000 pound flying object with a single decanter moves ~15' in the 1st round, and ~25-30' every round after that.

Dunno about the math of converting all that into superheated steam, or hydrogen/oxygen gas electrolysis (http://en.wikipedia.org/wiki/Electrolysis_of_water)), though.

[Abithrios]

One thing to note is that the stream of water shooting out of a decanter of endless water is described as having a volume of 30 gallons per round and being a foot wide, which means (by volume/area/time) that it moves out of the bottle at about five feet per round. Multiply the mass per second by the speed to get the force, in this case, about a pound of force. Clearly, they did not run the numbers when they designed it.

On the other hand, that does not matter, as you can force the water to go faster by putting it through a smaller area. For example, if the 5 gallons/second are forced through an area half as large, they will have to travel twice as fast, so will impart twice the force.

This will, however, produce more pressure on the part of the device near the source (where it is coming out wide and slow). To calculate the pressure, start by neglecting the initial velocity of the water and the pressure of the atmosphere--we will be going much higher than that. Next, the pressure is proportional to the mass density of water (62.43 pounds/foot3) and the square of the final velocity, vfinal.

P=62.43*vfinal2/2

vfinal=sqrt(P/31.2)

Force=vfinal*41.7 pounds(mass)/second=41.7*sqrt(P/31.2)

How much pressure is safe for the inside of the device? The decanter does not say it stops working if submerged deep under water (under very high pressure), so I would base the maximum value of P based on the mechanical stresses of the device. If you want, you can look up the internal pressures inside modern (or possibly even historical) machines to see what is a reasonable value (use units of pounds per square foot and put it in the equation above to find the force in pounds), but I do not feel like going to that much effort, so instead I will base my number off of something in the Dungeon Master's Guide.

Characters take 1d6 points of damage for every 100 feet they are below the surface of the water. If you make your device out of steel, then the hardness should negate almost 200 feet of depth worth of pressure, 195*62.43 pounds/square foot would be the pressure, so the force would be 823 pounds. The velocity would be about 20 feet per second, and the diameter of the beam would be about 2.4 inches. If you have 63 of these engines, you could equal the thrust of a single engine from a Boeing 747.

On the other hand, you could make it out of adamantine, ignoring up to 395 feet of depth worth of pressure, and increase the force up to 1172 pounds. The water would come out at a speed of 28 feet per second and the area would be about 2.1 inches across. It would only take about 44 of these to equal the 747's engine.

For reference, such a Jumbo Jet has four engines, can take off weighing over 700,000 pounds, and move at over .8 mach.

Also, I would not want to stand in front of the water sprays, but determining the mechanical effects of such an occurrence is left as an exercise to the reader.

[Kraken]

In D&D, you can just cast hardening on something and make it stronger than adamantine, or to make adamantine even stronger. So I don't know that pressure is going to be a concern with a base material of wall of stone + hardening, both of which have no cost beyond their initial casting.

[Abithrios]

In D&D, you can just cast hardening on something and make it stronger than adamantine, or to make adamantine even stronger. So I don't know that pressure is going to be a concern with a base material of wall of stone + hardening, both of which have no cost beyond their initial casting.

Good point. In that case, you could take the pressure as high as you want. I neglected viscosity and turbulence in my calculations (both are beyond the scope of my knowledge to estimate). If you have too much pressure, then both might become severely nontrivial. Also, I imagine that weird things might happen if you start to approach the speed of sound in water (.9221 miles per second = 4868.7 feet per second).

[animewatcha]

Any chance of signature spell or something being usable in there so that the water can have the color of brown when decanter is attached one's rear. Or yellow for one's crotch?

[Spore]

Any chance of signature spell or something being usable in there so that the water can have the color of brown when decanter is attached one's rear. Or yellow for one's crotch?

Surprisingly, Prestidigitation can do that.

[Fouredged Sword]

One could build it out of Riverine and laugh as it can sustain any amount of pressure as it is immune to physical damage. Save cost by making it foil thin, as it won't break even then. One could ramp up the internal pressures to ludicrous levels by leaving a pinprick hole in the side of the pressure vessel. Warning, the narrow blade of water will act like a water cutter. People use those to cut 1ft thick metal plate, so be careful walking behind your craft.

[Taverick]

If yer looking to improve the speed of your airship, there are some interesting ways in the arms and equipment guide. One such item is the Lightning Turbine. At 90k it propels the vessel at a speed of 90 or doubles it's speed, specifically saying whichever is GREATER. Also mentions it's used in combination with weather control magic, so if you can generate enough lightning to power it (which is doesn't state how much so ask yer GM) you have a pretty fast ship there. It also has the added benefit of making the ship ignore weather based movement penalties. Handy item no?

[cakellene]

I'm confused, where is the steam aspect if using a stream of water from decanter as propulsion device?

[Talya]

Hmm. Are there any magical items that provide an infinite supply of heating?

Filling a high pressure boiler with an endless supply of water water and heating it until it boiled, with the steam exhaust pointed out the back side of the airship would work as a jet engine. The problem is you'd run out of wood/coal used to heat it. You may be served better just firing the decanter itself out the back of the airship and using it as a water jet.

[Lightlawbliss]

steam aspect: heat water into steam to increase pressure in same space, resulting in higher velocity on exit and greater thrust for the same amount of water. However, some of these ideas would require an insane amount of heat to make the water turn to steam with some even reaching "impossible without getting creative".

[Fouredged Sword]

I think the idea is to use a permanent wall of fire as the hot side of your engine. The wall can't be extinguished, so you can sink it into a pool of water to create a great deal of steam.

[Lightlawbliss]

I think the idea is to use a permanent wall of fire as the hot side of your engine. The wall can't be extinguished, so you can sink it into a pool of water to create a great deal of steam.

a submerged wall of fire would be a wall of steam.

[gomipile]

I'm confused, where is the steam aspect if using a stream of water from decanter as propulsion device?

Wall of Magma instantly turns water into steam, as mentioned above in this thread.

[Kraken]

Wall of magma also has the benefit of explicitly being okay to use with permanency.

[Fouredged Sword]

Not to nitpick, but so is wall of fire, CL 12 2000exp.

http://www.d20srd.org/srd/spells/permanency.htm

[John Longarrow]

I'd recommend electricity over "Heat". Same effect, more controllable.

I used to have a link to a website all about using capacitors to get wierd stuff to explode due to internal resistance when current was passed through it. One goal was to blow holes in steel with Jello. Having a one way valve made out of a wall of force with a combustion chamber (think pulse jet) could give a massive push when you strip the electron shells from water and force out a jet of plasma... 40lbs of plasma...

[NoACWarrior]

If you have a divine caster use darkfire and just touch the steam drum to boil the water. You could also use flaming sphere instead at a cost of 48000 continuous. But a permanency wall of fire is more cost effective.

[Haldir]

YOu just need to turn the water to steam. If you have access to DoEW, you have access to a heating element. Set up the turbines to spin when the steam moves a piston and you're good to go. Since the energy is technically coming from whatever you're using to heat to the water, the RAW ruling on the Decanter is null, as is an objection regarding presevation of energy, being an external source.

Coal used to move huge freight trains. If you have a good method of generating lift already, I'm sure you can use steam to get a turbine spinning at jet-speed. The energy is there and you have spells to direct it as you see fit.

[Kraken]

Well, the best way to get lift is definitely the suspension spell from shining south, due to it have both an incredible duration (1d4+1 days/level) and ability to lift weight (1,000 pounds/level). So cast suspension on a rock, and will the rock up and down to lift your ship. Though depending on the size of the ship, you might need several things with suspension cast upon them, which would annoyingly cause several actions to be necessary to change altitude. Hopefully you could get it done in two castings, then you'd still be able to raise/lower the whole thing as a full round action. Then all you're left with is needing thrust, and frankly once you're at altitude you could simply rely on sails if you wanted.

[cakellene]

Wall of magma also has the benefit of explicitly being okay to use with permanency.

Aye, saw that bit. Was just commenting on vast majority of posts are discussing a water jet and not steam rocket.

[draconomial]

Stormwrack says that submerged fire spells become are steam instead of fire. For example: You can't cast a fireball into the water from outside it, and vice versa. And any fireball you cast from within water does the same amount of fire damage, but is a propelled ball of steam instead.

This is just my opinion, but I don't think a submerged wall of steam would turn water around it into steam, while a wall of magma would.

[Talya]

Stormwrack says that submerged fire spells become are steam instead of fire. For example: You can't cast a fireball into the water from outside it, and vice versa. And any fireball you cast from within water does the same amount of fire damage, but is a propelled ball of steam instead.

This is just my opinion, but I don't think a submerged wall of steam would turn water around it into steam, while a wall of magma would.

You don't cast the wall of fire into a tank of water. You cast a wall of fire, permanencied, into an empty tank of air. You then attach the decanter of endless water, and fill the tank. Only the tank won't fill - the wall of fire will vaporize the water as it hits the wall. Pressure will build. Valves release steam in a controlled way to direct the ship (or spin a turbine to do the same.)

[RavynsLand]

if your DM allows open source stuff, just use The Quintessential Gnome to make the airship do whatever you want. Give it cannons too. Sky's the limit.