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Decompressing spaceships: A physics question

OneWingedBird

OneWingedBird

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This has been busting my bean for a while so I figured it's about time I asked it.

Every so often a sci-fi movie comes out that has a bit in it where the characters are on a spaceship and the airlock door gets opened, then everyone strugles around in the high wind created by the escaping air until someone makes it to the nice big red button that closes the door again. (think Aliens)

I'm convinced it shouldn't happen like this, my guess is that it would be more like taking a cork out of a bottle, a quick Phtm! and it's all gone.

Does anyone know how fast decompression would really occur?

I have a good grasp of science generally but my physics is lousy...
 
i sopose it would be down to the relative size of the opening to the volume of excapeing air... a small hole i would expect to evacuate in spectaclar "Goldfinger" fashion but a large hole would rekon go pop...but what sort of sizes and volumes i havent and idea.
 
just thinking on it a moment... in this situation the air may well act more like a bottle of shaken up coke.... it not evacuateing in compleate gases but as a decreasing pressure throught the ship. And disapateing to the vacum, rather as co2 bubbles out of a fizzy drink.
 
The air would be under pressure, but the hole through which it was being released is relatively small. So I guess it would take a while for all of the air to make it's way out. Best analogy I can think of is a litre bottle of coca-cola, which you shake up and then punch a hole in with, say, a screwdriver (or perhaps something smaller).
 
How about when the hole is about the size of a car, I'm thinking big generic sci-fi airlock door type of thing.

Ah, where is Fortis, he seems to be the physicy/mathy sort?
 
The flow rate of air through the air lock would be dependent primarily on the pressure differential between the spaceship and space and the size of the orifice. It's basically a pressure vessel depressuring into a constant pressure of zero - if you can supply some dimensions (the volume of the spaceship, the door size, for example) then it's fairly simple to plug it into some equations and figure it out. :goof:
 
Um, for hypothetical purposes, lets say that the ship is 300x50x50 metres, and the airlock is 5x5 metres.

I did try running this one past the guys at Insultingly Stupid Movie Physics but they're horribly busy...
 
"I blew it out of the goddamn airlock! :hmph: "

"Are you sure dear? That doesn't sound terribly plausible, Now say the airlock was 10x10..."

There used to be an 'educational' programme on BBC 2 a while back that examined 'Hollywood stunt science' co-hosted by the chap that played Kryten in Red Dwarf.

Interestingly enough, they concluded that Bruce Willis' firehose escape/re-entry to the Nakatomi building was just about plausible but he may have suffered slightly more than cut feet...:D
 
BlackRiverFalls said:
Um, for hypothetical purposes, lets say that the ship is 300x50x50 metres, and the airlock is 5x5 metres.

I did try running this one past the guys at Insultingly Stupid Movie Physics but they're horribly busy...
Click to expand...

Relative to the size of the ship, that airlock is not particularly big - I guess air can only escape from just one place at a given rate, which is dependant on the overall air pressure.
 
Okay dokey - this is a tough one - definitely not as straight forward as it seems

After a good rake about I've come up with the info below.

If anyone wants to check out the whole page, which also covers explosive decompression of the human body go to http://www.sff.net/people/Geoffrey.Landis/vacuum.html


How Fast Will A Spaceship Decompress If It Gets Punctured?

The decompression time will depend on how big the hole is. For a fast estimate, you can assume that the air exiting through the hole will travel at the speed of sound. This gives you a quick (and only roughly accurate) rule of thumb: if you put a one square centimeter hole in a one cubic meter volume, the pressure will drop by a factor of ten every hundred seconds, and this time scales up proportionately to the volume, and scales down proportionately to the size of the hole. So, for example, a three-thousand cubic meter volume will decompress from 1 atmosphere to .01 atmosphere through a ten square centimeter hole on a time scale of a sixty thousand seconds (seventeen hours).
The seminal paper on the subject is by Demetriades in 1954: "On the Decompression of a Punctured Pressurized Cabin in Vacuum Flight."
More accurately, for laminar viscous flow (that is, near atmospheric pressure), using Prandtl's equation in the limit Po/P is zero, and assuming a simple aperture (a pipe of zero length), the gas flow conductance is Cvisc= 20 A liters/second (for A in square centimeters) and hence the pressure drop is:
P = Pinitial exp (-0.02 tA/V)
where V is volume in cubic meters, t is time in seconds, and A is area in square centimeters. As the pressure decreases the flow changes to molecular flow, and the depressurization rate decreases by about a factor of two. This is for air at 20 C; for the case of pure oxygen, the leak rate is about 10 percent slower.
For reference, when the pressure drops to about 50% of atmospheric, the subject will be entering the region of "critical hypoxia"; when the pressure drops to about 15% of atmospheric, the remaining time of useful consciousness will have decreased to the 9-12 seconds characteristic of vacuum.
Professor Andrew J. Higgins of McGill University posted a more detailed answer to the question of how fast a spacecraft will decompress through a given size hole on Usenet. With his permission, I have copied his response here: http://www.sff.net/people/Geoffrey.Landis/higgins.html

I'm sure the answer to Blackriverfalls question is in here somewhere, but I'm buggered if I can be bothered to work it out myself.

Best of luck y'all
 
Finally got around to this....

I have access to simulation technology which can approximate this kind of scenario. Using the dimensions BRF gives above, I took a 5x5m hatch and linearly opened it over five seconds. The results pour cold water on any Hollywood scenario.
Upon opening the hatch, the air obviously rushed through it into the cold, heartless vacuum of space. It does it at a hell of a speed, one minute you’d be sat in James Whitehead’s Space Bar reading the electropaper when Rodney the Barthing accidentally leans on the airlock button resulting in a gust of wind that accelerates to about 400 mph within the first second. After that, the air velocity maxes out at the speed of sound, around 700 mph, due to a well understood phenomenon called “choking” or “choked flow” which is the maximum velocity that the air can flow through the airlock. So at winds of that kind of speed, it’s unlikely you’ll be horizontal, holding onto a handrail with one hand and your love interest with the other, yelling at her to hold tight. In fact it’ll probably rip all your clothes off for starters as that is what happens in high speed break-ups in commercial air crashes, which would add an interesting dimension to the movie, but you’d probably already be in the void.
The ship would take about ten minutes to completely depressurize, though if someone knows at what atmospheric pressure it becomes impossible to breathe I can tell you when this limit is reached.
So no, it doesn’t go “pop” but the Hollywood version is cobblers.
If anyone knows any better feel free to comment! Seems like pretty realistic results to me.
 
EEEEK sounds very plausable... and nasty.



so would Goldfinger realy have been sucked through that tiny window?
 
Unfortunately I can't simulate the passage of a fat German through a small hole, though there are probably websites for that. :cross eye
 
How about an instant 2cm hole (impact with debris) which would be a far more common occourance... Then ther's the Pod-to-airlock routine in 2001...

Damnit DD, we need answers! :D
 
I can't find the link but IIRC a person will pass out at about 50% atmospheric pressure, though they will be showing signs of 'altitude sickness' at much higher pressures.
 
OK - but what then happens to the "air" (i.e. hydrogen, oxygen etc).

Does it just float around in the void as a mass? Or would it all get dissipated to a molecular level? Would it eventually be attracted to a star or perhaps planet via gravity?

Physics is not one of my strong points:)
 
Interesting question; the atmosphere would rush out of the spaceship as a particular speed, perhaps seventy or a hundred miles an hour; the individual particles (molecules, dust specks or ice) would then continue to travel on that trajectory, spreading out because of the initial chaotic reactions;

there will be certain effects like the gravity and electrostatic force of the gas particles themselves which will attempt to compress the gas cloud, but these are not very powerful effects.
The gas cloud will spread out, individual particles moving apart until they reach the same density as the local interplanetary (or interstellar) medium.

Depending on the velocity and direction of the particles, some will be attracted to the surfaces of planets or moons.

Mostly though, if a spaceship is in orbit when it decompresses, the particles will remain in orbit, spreading out into a thin cloud or even a ring.
 
The good news is that at that rate, the air from a 50 x 50 x 300 spaceship would take 90s to exit completely, or 45s to reach half pressure (ignoring the effect of the flow reducing as pressure decreases), so you're got a while to close the airlock.

It would certainly blow away any aliens hanging around pretty effectively though, but make sure you're tied down before you try this trick or you'll be out there floating in space with them.
 
Eburacum45 said:
Interesting question; the atmosphere would rush out of the spaceship as a particular speed, perhaps seventy or a hundred miles an hour; the individual particles (molecules, dust specks or ice) would then continue to travel on that trajectory, spreading out because of the initial chaotic reactions;

there will be certain effects like the gravity and electrostatic force of the gas particles themselves which will attempt to compress the gas cloud, but these are not very powerful effects.
The gas cloud will spread out, individual particles moving apart until they reach the same density as the local interplanetary (or interstellar) medium.

Depending on the velocity and direction of the particles, some will be attracted to the surfaces of planets or moons.

Mostly though, if a spaceship is in orbit when it decompresses, the particles will remain in orbit, spreading out into a thin cloud or even a ring.
Click to expand...

So, on a theoretic level, if one were to release enough "air" near a large gravitational source (say, planet) could one create a breathable atmosphere, similar to Earth, which then hangs around in orbit as a ring around a moon/planet?

So, the inhabitants of a ship in orbit at the right place would be able to step outside and breath in the lovely air!

Obviously, I am talking theoretical for this purpose. I am sure the practical considerations of how much "air" would need to be pushed out to make this anything other than a negligible amount are ridiculous.
 
BlackRiverFalls said:
I can't find the link but IIRC a person will pass out at about 50% atmospheric pressure, though they will be showing signs of 'altitude sickness' at much higher pressures.
Click to expand...
It's probably academic really, but the 50% mark is reached after 110s. Though breathing in a 700mph wind would be quite a feat. It's probably not asphyxiation but the effects of rapid depressurisation that would kill you. Or a swiftly moving chair in the face. :D The pressure in the ship falls exponentially over ten minutes before it's all over.
 
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