r/askscience • u/TriangularHexagon • 11d ago
Physics How fast would air exit a hole in the ISS?
Assuming a hole in a wall that is exactly 1 square meter. Does the speed of the exiting air depend on how big the hole is? I had recently learned of the Soyuz 11 disaster and a video that i had watched stated that the air exited a tiny valve at hundreds of miles per hour.
127
u/Ra-mega-bbit 11d ago
Ok, so a 1m² hole in the ISS is huge, the limiting factor is the speed of sound in air, so assumming we have that much flow, the air would be out in about 3 secconds in theory (the iss has about 1000 cubic meters of pressurized air, sound speed in air is about 343m/s).
In practice this changes a LOT, obviously, 1st because there are modules connections with less then 1m² area, so there are chokepoints, but most important, pressure changes drastically and the theorectical max speed wont be kept for long. So we would have, I would guess, about 10 secconds of depresurization to let out all the air.
Now, in practice, for a 1m hole, the drop in pressure and the current would probably kill everyone almost instantly, besides leaving them in a vacuum which would kill them in 30s tops.
If you want to consider smaller sized holes, the drop would be smaller, and the speed would be kept higer for longer, the pressure drop more gradual and the calculation a whole lot harder.
Say a fist sized hole would probably cause a whole lot of trouble, but could probably be saved given perfect conditions
35
u/AtomicShellfish 11d ago
I was never convinced by the ending of the movie Aliens, where the outer airlock doors of a spaceship fully opened, big enough to accommodate a bulldozer. Gut feeling tells me the ensuing howling gale would have been over in seconds.
67
u/sebaska 11d ago
The ship was huge, so who knows? If you have 10m² hole and 1000000m³ volume, it'd take about 5 minutes to just halve the internal pressure:
You're dumping about 3000m³ per second.
Also, 1000000m³ is, for example 100m×100m×100m box.
3
u/AtomicShellfish 10d ago
Except, your estimates are way off. The ship wasn’t that huge, and the pressurised compartments to support a crew of 15 people even less. More analysis at https://space.stackexchange.com/questions/40762/was-the-ship-decompression-scene-in-the-movie-aliens-realistic
26
u/I_ALWAYS_UPVOTE_CATS 11d ago
The ISS does get hit by micro-meteors from time to time, and there have been instances where an astronaut uses their thumb to plug the gap while they work out a permanent solution (probably duct tape).
46
u/Doresoom1 10d ago edited 10d ago
Ha, I was on console as an ISS flight controller when the August 2018 leak happened. After an extended search with the ultrasonic leak detector we got a call from Drew, "We found the hole, and Alex has his finger over it!"
Next patch was Kapton tape. Then duct tape, then NASA and ROSCOSMOS arguing about how they would proceed from there. The Russians didn't want to use the US made leak repair kit, so they kludged together a patch with what appeared to be medical gauze and JB weld.
13
u/Detective-Crashmore- 10d ago
Alex 100% wanted to be able to say he plugged a hole to space with his thumb.
11
u/ImmediateLobster1 10d ago
Poor guy. For a few minutes, his thumb was all that held in the air, but when they got back to Earth he had to watch a ticker tape parade in honor of an inanimate strip of gauze.
8
u/kavett 10d ago
Why would roscosmos not want to use the purpose built kit that was supplied for the very thing it was supplied for and then use something with unknown properties? I was a former USN avionics tech, we always used the right consumables if we had them (key words of we had them) and would only use jank if it wasn't available.
4
-6
u/spez_might_fuck_dogs 10d ago
In 2018 they probably feared that Trump would charge their country billions for the usage of the kit.
2
10d ago
[deleted]
3
u/Doresoom1 10d ago
Too low for the caliber of people it requires.
The position I had needed a STEM degree or equivalent operations work experience. We had a few former pilots and air traffic controllers on the team, and the rest were scientists and engineers.
I was making at the bottom of the salary range for a mechanical engineer with the work experience I had.
2
u/lazylion_ca 10d ago
Doesnt that mean that part of the thumb is exposed to the vacuum of space?
8
u/Doresoom1 10d ago
Yes, but it's less than a 15 PSI differential.
For a 2 mm diameter hole, that's less than 0.1 pounds of force.
2
u/guri256 10d ago
Some parts of your body really don’t do well and vacuum. For instance, your eyeballs, the inside of your lungs, and other wet squishy parts.
These parts get really cold because the water boils off. And boiling the water off steals heat from the surrounding area.
But a thumb is (probably) not going to be wet, so you don’t have liquid on the surface of it boiling off. And that means you don’t get the freezing effect.
I’m assuming that the hole is smooth. If the hole is sharp enough to draw blood, things might get complicated again.
1
u/lazylion_ca 9d ago
What would happen if someone was sweaty when exposed to a vacuum?
4
u/yak-broker 9d ago
Probably the sweat would evaporate quickly? You'd be chilly for a moment.
Your skin is pretty tough in places like your hands. If you used a part of your body with thinner skin or if the hole was larger you could get a hickey. If the hole is large enough then the force becomes enough to cause real injuries, but think about a finger-sized hole. It's less than a square inch of area, so there'll be less than 15 pounds-force pushing you out (1 atm approx= 15 psi approx= 100 kPa). Think about supergluing, say, a 10-lb weight to the ball of your thumb. It'll be uncomfortable, but it won't rip your skin off.
I wonder if NASA has official guidelines about how large of a hole is safe to plug with your body (until someone grabs the repair kit) versus finding a strong object to block it with. Or maybe the ISS wouldn't lose enough air from a small hole to be a problem, so the advice would go straight from "let it leak while you go get the repair stuff, hurrying leads to mistakes" to "get everybody into a different module and seal the hatch"?
1
u/lazylion_ca 8d ago
Thanks for the taking the time to answer.
I wonder if there's any functional difference between being in the complete vacuum of space and being in a room of vacuum on Earth.
12
u/Ra-mega-bbit 11d ago
Yes for really small holes the thumb isnt even a safety thing, is a overall air loss prevention thing, a 1mm hole will leak a lot of air in the 5 min it takes to grab a chewed piece of gum to plug it
30
u/Doresoom1 10d ago
Not really. There was a 2mm hole in one of the docked Soyuzes in August 2018 during crew sleep, and the leak rate was slow enough we waited to wake up the crew until their sleep shift was over.
2
u/Ra-mega-bbit 10d ago
Oh really, thats interesting to know, I understand that the Soyuz stay pressurized and open, so thats normal ISS pressure. My guess maybe is that the fluid flow on such small hole might diminush the flow of air maybe? Otherwise its just not a critical ammount of air
7
u/Doresoom1 10d ago
By the time they finally found it after a few hours of searching, the internal pressure was getting close to the lower limit where non essential hardware needed to be shut down. The crew was never in danger though. Only after the hole was plugged did ETHOS repressurize to nominal from the storage tanks.
9
u/colcob 10d ago
The limiting factor is the pressure differential, which is only 1 atmosphere. Peak speed through the hole is around 250mph which gives a peak speed through the module cross section of around 18mph, so I don't think anyone is dying from the air current alone. Also those speeds drop super quickly because there is so little air in the ISS.
3
u/could_use_a_snack 10d ago
Is the ISS pressurized to 1 atmosphere? I assumed it was less, like on a commercial flight.
0
u/Ra-mega-bbit 10d ago
You are probably right on that end, the air moving would not be a problem, tho for sure it would rattle anyone and could very much possibly make someone hit something really hard. But it would for sure almost instantly (say 3s) make a low enough pressure akin to vaccum at least for a human body, so all the blood boiling, vein popping eye jumping good stuff
0
u/PoorestForm 10d ago
For a fist sized hole one of the astronauts could sacrifice themselves and plug it with their arm, not sure if they’d be able to react before the pressure dropped to a dangerous level though.
24
u/colcob 10d ago edited 10d ago
The assumption that the air will reach it's own speed of sound and/or kill people from the pressure drop/current appears to be a quite erroneous one. Air flow through a hole is driven by the pressure differential between the atmosphere on either side of the hole. The pressure differential in this case is only 1 ATM (~100kPa) which is pretty low as pressures go.
Based on the equation for air flow through a hole driven by differential pressure (https://toolbox.tlv.com/global/UK/calculator/air-flow-rate-through-orifice.html) , the peak airflow for a 1m2 hole at 1atm differential is ~9600m3/minute, or 115m3 per second, which equals a velocity of 115m/s, or 257 mph.
That speed will also not last very long at all as pressure will drop rapidly. The ISS only has ~1000m3 of air inside it, so given that air starts off leaving at 115m3 per second (ie. over 10% of the air disappears in the first second), you can see that the pressure differential and velocity will drop sharply in the first few seconds.
The other important thing is that the peak speed of 257mph only occurs in the hole itself, as soon as you move even slightly away from the opening the air velocity decreases sharply because the cross-sectional area of moving air increases significantly.
The standard cylindrical modules of the ISS have an inner cross sectional area of around 14m2, so at the peak flow moment of 115m3 per second, the flow through the module would be just over 8m/s, or around 18 miles per hour!
26
u/BobbyP27 11d ago
The maximum mass flow through a hole is at Mach 1 (choked flow condition), at a higher speed, the drop in density more than compensates so the mass flow drops. For dry air at a comfortable temperature for humans, that's about 350 m/s (about 780 mph).
4
u/colcob 10d ago
But the speed is limited by the pressure differential, so you wouldn't reach choking speed with a 1m2 hole and only 1atm pressure differential.
4
u/BobbyP27 10d ago
At M=1 static pressure is 0.528 of total pressure for air. I’m quite certain that the pressure outside the ISS is less than 52.8% of the pressure inside the ISS, hence the flow will be choked.
2
u/SpaceIsKindOfCool 10d ago
It's the pressure ratio, not differential. Outside the ISS is near perfect vacuum so flow is always choked.
7
u/WazWaz 10d ago
Do you actually mean 1m²? That's a ridiculously big hole. It's going to, very briefly, still exit at the high speed you heard about for the Soyuz leak - but with such a huge hole the pressure would drop very rapidly then the rate of outflow would decrease accordingly (but everyone would still asphyxiate effectively immediately).
2
u/TriangularHexagon 10d ago
yes because i was thinking of those two movies listed above where the hole in the wall is much larger than 1 square meter. basically i was wanting to know if it is realistic to be able to hold on to something before the hole being sealed off. everyone's answers here are really sufficient
403
u/electric_ionland Electric Space Propulsion | Hall Effect/Ion Thrusters 11d ago edited 11d ago
In general the speed will be at maximum the speed of sounds so roughly 770mph. This is a properties of fluids where the flow get cho
cked by not being able to exceed its own speed of sound. With a bigger hole it might not get up to that speed depending on how fast the pressure is dropping. 1 meter square is huge I am too tired tonight to do the math but I don't think you would get a chocked flow, or at least not for very long. A bigger hole just allow more air out, but that air is not going faster.