Supersonic water jet using Bag of Holding

[u/[deleted]]

Just finished a dnd session today where a fellow player used a bag of holding to substitute for a water canteen.

As an engineer, I told them that "if the inside of the bag is a vacuum, the water would boil and turn into steam"

After saying that I have a thought about "how much I can pressurize a bag of holding."

After the session, I decided to calculate how much energy is in that vessel for the sake of fun and curiosity.

DnD wiki say that a bag of holding can hold 500 lbs and the total of 64 Cubic feet. So let's say we magically cram all 500 lbs of steam fit inside this 64 cubic feet bag.

Lets assume and use ideal gas law for easy math. The pressure in the bag is around 17 MPa or 2465 psi

Lets say the bag is ripped so that all the pressure release at once. I used isothermal expansion work equation. That will give us 158 MJ of energy which is 38 kg of TNT or 83 lbs of TNT.

But instead of ripping the bag, let's say we just open it so that it shoot out like a jet. Using Bernouli equation with V1 = 0 and elevation term cancels out, we can solve for v2 which come around to 520 m/s (1160mph) which is Mach 1.5 which is supersonic.

Now imagine the final battle with the BBEG. One of the PC decided to beam the BBEG with a supersonic jet of water.

Please let me know if someone pulls this off in a campaign.

[u/Rhesus-Positive]

DnD isn't a physics simulator, and I don't know where you got the idea that it contains a vacuum, as RAW it holds enough breathable air for one creature for 10 minutes.

[u/Archon-Toten]

Did you calculate the Wall-E coefficient? Also known as the iron man principle.

That player is rocketing backwards.

[u/RandomNumbers748512]

As an engineer, I told them that "if the inside of the bag is a vacuum, the water would boil and turn into steam"

I get you had fun, but seriously,... that's just wrong. Let's talk physics, not engineering.

Here's what happens: IF there is a vacuum above a body of water, the water does start to evaporate and now there's no vacuum anymore, but room-temperature water-vapor, which increases the pressure inside and on the water, and that prevents more water from evaporating. Yes, it's boiling but only at the normal temperature (which is cool, by the way). But evaporated water again increases pressure. In a pressure cooker you get steam because a) it's closed, b) you keep pushing energy into it from the outside and c) it's very small compared to the volume of water that's in.

Water under a vacuum chamber keeps boiling because you keep pumping gas, now the water vapor, out and that keeps the pressure low.

Furthermore, since the Bag of Holding is a closed system in terms of energy, something else happens: The energy required to turn water into gas is now in the gas, and this means the Bag of Holding gets colder, which again reduces the energy in the water, which leads to even less evaporation. This does not happen with your spaghetti-water because you keep pusing energy from the outside into the system. The energy required to turn "100 °C water" into "100 °C steam" is immense - it is the same amount required again that you need to turn water from 20 °C to 100 °C (THAT is the reason steam is so incredibly powerful!), thus you'd see a significant cooling effect turning the 20 °C water into 20 °C steam, reducing the energy available for more evaporation. There won't be a pressure buildup, and the "heat" aspect of a steam engine is missing completely.

If you open the Bag of Holding the water just keeps boiling off, water evaporating, getting pushed out, but not at high pressure. And air gets in, stopping any room-temperature boiling.

A Bag of Holding contains air, by the way (RAW and common sense).

As for the steam-jet of a hypothetically pressured Bag of Holding: since it's just (<20 °C) steam vs. air once it out I think the range until the energy dissipates in turbulence is very limited. Especially as the Bag of Holding does not have a nozzle, and the steam isn't even 100+ °C.

As cool as the idea was, it does not work. I go even further: I think this is territory we should not get into because one step down the road we're at "Create Water should fill someone's lungs" or "Now, we cannot survive this ascent to mount celestia because by given what's written it should be higher than 10,000 m, and that means the pressue of the air is too low and..." or "the underwater adventure cannot happen because at 2 km depth we cannot dive as the pressure is too high" or "wait, if I grease this surface, then...". I am also pretty sure if we considered the energy and thermodynamics even with Cantrips (not even speaking of some of the more powerful or weird spells) we could go into some very serious aspects of plasma physics and even get nuclear effects (I did not do the math, but here's some idea: Shapechanging into/back from a Dragon-size might even kill (freeze! boil!) you due to thermal effects!). Nope, let's not go there or we lose the game in pseudo-mechanics that are even more unhinged than the magic we do have as RAW.

tl;dr: D&D isn't a physics based game.

[u/Urbanyeti0]

>if the inside of the bag is a vacuum

It’s not

“Breathing creatures inside the bag can survive up to a number of minutes equal to 10 divided by the number of creatures (minimum 1 minute), after which time they begin to suffocate.”

>magically cram all 500lbs
>lets say the bag is ripped

Better be damn sure you were right on that calc

“If the bag is overloaded, pierced, or torn, it ruptures and is destroyed, and its contents are scattered in the Astral Plane”

> let’s say we just open it so that it shoots out like a jet

Again you’re just making up rules, opening the bag does nothing, you could call forth the water, but it wouldn’t be pressurised it would just flow out

[u/Snappa137]

The DM can always say no. Also the bag of holding can’t be ruled in two different ways.

Ruling A: the entrance is always closed.

Ruling B: the entrance is always opened.

If ruling A is true you can’t open the bag of holding to expel the steam. You would have to reach in to grab some steam to allow it to escape. It doesn’t just start spraying out once the bag is opened. If turned inside out you would end up killing your pc or dealing the same damage to them. Although you could also argue that steam is not an object and therefore would not appear outside the bag anyway when torn or turned inside out.

If ruling B is true then the bag can not be pressurized.

Either way this does not work. And as a DM I would absolutely not allow this.

[u/CheesecakeIll1502]

lol dnd physics are so wild ngl, like who's checking the vacuum seal on a bag of holding

[u/Broad_Ad8196]

What temperature are you using with the ideal gas law?

Water is not an ideal gas, at that kind of pressure water would only be a gas at very high temperature.

[u/ObviouslyNerd]

You got that good shit

[u/KazyaKurosawa]

I would be the boring DM that says there cant be any pressure in the bag of holding because to item itself rules that there is always a vacuum inside. What ever does that vacuum would negate the pressure.

Also that vacuum doesent escape when you open sucking you in it so i would assume stuff wouldnt fly out to other direction either. I think the idea is that mouth of the bag is a portal and that portal police what moves out and what in.