Okay, I've got a bit of a science question for those more knowledgable than myself. (It actually has to do with a Trek story that I'm writing, but I figure on getting some real science figures if possible. )
I've got a closed shell, at least 650 meters long and 300 meters in diameter, filled with a breathable atmosphere at normal pressure... (The concept is an enclosed, pressurized drydock for starships, so that engineers can work without spacesuits.)
The question is, if the doors open with the air still inside, what kind of danger would the explosive decompression pose to the structure itself? I'm thinking that it would (at the least) act like a kind of rocket, propelling the structure in the opposite direction of the escaping air. But if you're considering such a huge volume of air (more than a cubic kilometer), would the sudden escape actually pose a risk of breaking the shell itself? I'm thinking of unequal forces or something of that regard... but I'm not familiar enough with the principles to know for certain.
If anyone knows more about this, could I have a few pointers? Posted by E. Cartman (Member # 256) on :
How large are the doors? And what is the tensile strength of the shell's hull material?
Posted by Shipbuilder (Member # 69) on :
Doesn't really matter. If your structure is sufficiently designed to contain the pressures inside, it shouldn't cause a problem with all the flow exiting through the door. "Theoretically" the velocity of the flow at the door frame would be 0 anyway.
Posted by TSN (Member # 31) on :
Well, how strong is the shell? If it's strong enough, it won't break. If it's not, it will. If I were going to work inside the thing, I would hope it would be strong enough.
As for movement, IIRC, the force of the air going out would create an equal force pushing the structure the opposite way. However, depending upon the mass of the structure, and what motion it already has, the effect may or may not be negligible.
Posted by Wraith (Member # 779) on :
Also you'd have to secure any loose objects... (not really answering the question, I know but still). Heavy items of eqipment could cause some damage, I'd imagine.
Posted by OnToMars (Member # 621) on :
C.
Always pick C when you have no idea.
Posted by MinutiaeMan (Member # 444) on :
Okay...
Since this is a Trek setting, I guess we can say that it's transparent aluminum or something equivalent.
I figured the doors would be as large as the end of the shell itself, since a ship needs to fit in, and it would be rather counterproductive to have a ship that could fit inside but can't get through the doors.
I suppose the structure would be strong enough -- after all, most doorways don't break when there's a hull breach or something; in all the shows we just see the air rushing through the available space.
I'm mainly wondering because it's such a large volume of air, and is sealed against a vacuum. I guess it's like letting the air out of a balloon -- it might pop outright, or it might just let the air out. I'm thinking that some damage could be done given such a huge volume of air, even if it's a metal frame.
Posted by Topher (Member # 71) on :
This sounds more like a chemistry q than a physics q... Effusion, etc...
Posted by Shipbuilder (Member # 69) on :
I'd say more fluid dyanamics (flow) and structures (pressure vessel design).
Posted by Masao (Member # 232) on :
No answers, just some thoughts
Movies would have us believe that an airplane cabin pressurized to 1 atm would suddenly be sucked out (along with the entire contents of the cabin) when a hole the size of a dinner plate is suddenly opened to the outside air at 30,000 feet. The effects are shown to be even worse if the outside atmosphere is zero pressure.
But does this really happen? Intuitively, the pressure of 1 atm (14.7 pounds per squre inch or 101,325 pascals, or newtons per square meter) is applied equally to the entire surface of the structure. When a hole is opened, the air is pushed out at a pressure of 1 atm. Movies show the effects to be much worse, but in airplanes I think that has a lot to do with the fact that the plane is moving at a couple hundred miles per hour. If it's vessel is not moving relative to the outside, how bad is it?
Posted by Sol System (Member # 30) on :
No, the air wouldn't escape, since a force field would hold it in ala shuttle bay....
Posted by MinutiaeMan (Member # 444) on :
Thanks, Sol! I'll look at that page...
Ritten, I suppose a forcefield is possible (after all, this is a fictional universe!) but I think that given the TNG-era technology, flying an entire starship through a forcefield is kinda unlikely. Or at the very least, impractical due to power requirements.
(Besides, I'm actually TRYING to create problems here. It's really just a small point in the story, but the point is that the ship inside the drydock is getting its hull repaired/upgraded and they're using a specialized enclosed/pressurized facility to speed up the process. The problem, though, is when an emergency occurrs and they need to leave the dock ASAP. I'm trying to consider the potential consequences of rapid depressurization -- forcing the ship to hold off the launch until it's safe to open the doors. But I'm not too attached to the idea, so I wanted to try to get some of the facts to make it more believable.)
Posted by Peregrinus (Member # 504) on :
So what's wrong with a larger selective-permeability force field like the ones they used on the shuttlebays in TNG?
--Jonah
Posted by Harry (Member # 265) on :
That it would probably need a lot of energy on such a huge scale.
Posted by Sol System (Member # 30) on :
Well, is one of those selectively permeable forcefields more or less energy hungry than your standard shield setup? Because we've never heard of shields in their usual configuration being power hogs, and they are by definition as large as the ships that generate them.
Posted by Shipbuilder (Member # 69) on :
Yep, I'd say the forcefield would be a standard in this case. Now you're gonna need something exiting/entering the pressurized area to get the door open, then something to cause the whole station to loose power (and redundant systems can't come online)...sounds like sabotage to me Posted by Ritten (Member # 417) on :
and the plot thickens....
Posted by MinutiaeMan (Member # 444) on :
Yeah, a forcefield MIGHT be considered standard... but there's a huge difference between a shuttlebay barrier and a drydock barrier that measures hundreds of meters in diameter. That's likely to be an exponential increase in energy required...
Posted by Wraith (Member # 779) on :
Well, presumeably the forcefield would only need to be up for a few minutes at a time, as the ships enter and exit; the rest of the time a physical structure would be in place.I don't think the power expenditure for this amount of time would be prohibitive.
Posted by Timo (Member # 245) on :
If you want trouble, I'll give you trouble!
You want to hold the horses even though the stall on fire? Well, structural damage to the dock shell need not be the issue here. Rather, the ship inside might have to wait until the air around is thin enough that it won't buffet the ship and make her bang onto the walls when it rushes out. If the ship is your standard highly asymmetric design, with large aerodynamic surfaces offset from the center of gravity, and isn't tightly tied in place and isn't running position-keeping engines... Then the danger of banging to the walls could be very real, even if the "wind forces" are minor.
Alternately, you could say that hydrogen-spewing starship engines and oxygen-containing atmosphere don't mix, since there will always be a moment during engine activation when the concentration of hydrogen is in the explosively combustive range.
Or the doors might open asymmetrically, so the "exhaust jet" would send the dock spinning and create all sorts navigational hazards for the departing ship and for the surroundings.
And depending on the severity of the distress, you could play on the real-world issue of costs. The dock shell might have been built to split open in emergencies, but is this one dire enough to justify the costs of putting it back together again?
Or let the person in charge agonize over some personnel outside, on some scaffolding that is on the way. Downplay the atmosphere aspect, but retain it as one of the factors that keep your hero from simply blasting out.
Timo Saloniemi
Posted by Kosh (Member # 167) on :
quote: . I'm trying to consider the potential consequences of rapid depressurization -- forcing the ship to hold off the launch until it's safe to open the doors. But I'm not too attached to the idea, so I wanted to try to get some of the facts to make it more believable.)
I don't know how realistic it was, but in "Cause And Effect" (TNGseason5) when they blew out the shuttle bay, the ship moved
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