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I was directed here from Sci-Fi SE to pose this question.

Towards the end of the Aliens (1986) movie, Ripley is forced to override the clearly marked "outer door" control having already opened the inner one with the resultant decompression and loose items being pulled out, etc. This was to hopefully expel the Alien trapped under the machinery.

How realistic really is this effect? Would the instant decompression force not pull her out (without being physically fixed down, locking arms does not to me seem sufficient but I may be wrong) and it seemed to take a relatively long time to evacuate the air in the ship too, from memory it was still being exhausted when she closed the inner door again.

Aliens (1986) - Power Loader and Alien in Open Airlock

Đαrkraι
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AndyF
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I did a quick extrapolation from Organic Marble's data (1.5" diameter hole = lose 1500 lbs of air per hour from 14.7 psi; flowrate proportional to the square root of the pressure differential) and my estimate of the hangar and hatch size in the scene (40x40x10m hangar; 3m square hatch). There's a lot of uncertainty here and the large opening might not produce the same choked flow that a small hole would, but my estimate says half the air is gone in 7 seconds. After 14 seconds about 15% of the air is left, and the effective altitude is ~12km, which this article suggests leaves no more than 12 seconds of useful consciousness -- Ripley's going to black out before the Alien Queen lets go.

In the scene from the film, the airlock is open for about 70 seconds. If the volume of the hangar was 10 times my estimate, e.g. something like 20m x 80m x 100m, half the air would still be left after 70 seconds, and the scene would be more plausible. I'm not quite bored enough to watch the other hangar scenes to make a better estimate of the hangar volume; it's also possible that some air source beyond the hangar (i.e. the rest of the Sulaco's interior, or tanked air) might be feeding into the hangar during this scene.

Would the instant decompression force not pull her clean out (without being physically fixed down, locking arms does not to me seem sufficient but I may be wrong)

If Ripley were blocking the airlock completely, the pressure differential across her body would amount to several tons and she'd be blown out instantly. However, hanging on the ladder, there's some air flowing into the space downstream of her, so the pressure differential isn't as extreme. As discussed in comments below, this force is still at least a few tons; as mighty as Ripley is, she wouldn’t be able to hold onto the ladder, let alone climb out.

Russell Borogove
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    Sounds reasonable given the little that we know. – Organic Marble Jan 07 '20 at 04:49
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    The hangar is probably quite a bit bigger than 40x40x10; each of the dropships scales to about 25-30m, and the space between them looks enough to fit another dropship or two, perhaps three or four if you fitted them straight against the wall. There may be other considerations that complicate the calculation too - e.g. instead of 1 atm of Earth-like atmosphere, they might be running at 0.2 atm of pure oxygen or something like that, not to mention that there probably is a system that replenishes the air in the hangar. The film also probably shows different places at the same time. – Luaan Jan 07 '20 at 11:40
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    FWIW, wikipedia lists the Sulaco as having capacity for eight of those dropships. Whether those ships were aboard and unused, or weren't loaded, the hangar space aboardship must be enormous, even assuming that there are multiple hangars (very likely, why would they put all their eggs in one basket?) that's a reasonable 2 - 4 ships per bay, plus adjacent support garages for ground vehicles like the APCs. – Ruadhan2300 Jan 07 '20 at 13:02
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    Don’t rely on that article. It’s not wrong, but there is an /enormous/ amount of variability person to person and day to day. – fectin Jan 07 '20 at 14:49
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    The volume occupied by the dropships is not occupied by air. – Organic Marble Jan 07 '20 at 16:59
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    @Luaan They wouldn't run 0.2 atmo of pure O2, that is what in the space industry they call "an extreme fire hazard." – Draco18s no longer trusts SE Jan 07 '20 at 18:20
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    @Draco18snolongertrustsSE the amount of O2 available for a reaction (and thus the fire risk) depends on the partial pressure of oxygen, so the 0.2 atm of pure O2 is no more risky than than 20% O2 1atm. – mbrig Jan 07 '20 at 18:55
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    @mbrig Yes and no. partial pressure of nitrogen tends to nitrize surfaces, rendering them passivated to reaction with oxygen. It also helps fill microscopic nooks and crannies with mainly nitrogen, not oxygen (thus reducing the rate of reaction if shit hits the fan, by the transfer of bulk oxygen into the nooks and crannies) So considering only the oxygen, you are right - but the lack of nitrogen certainly increases the fire hazard. Not sufficient enough to warrant bringing huge quantities of nitrogen into space, but still. – Stian Jan 08 '20 at 11:28
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    It would appear from this answer and its comments that the correct answer is "It's plausible. There's not enough data available to say one way or another" – Steve Ives Jan 08 '20 at 13:22
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    It's possible that there is some sort of atmosphere replenishment system, given that there is a 'Evacuate the atmosphere really quickly" system. – Steve Ives Jan 08 '20 at 13:23
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    Datapoint: Max force due to fluid flow across a body is F ~= 0.5 x D x A x V^2. (D in kg/m^3, A in m^2, V in m/s . F in N. Divide by 10 (OK 9.8) for kg. Multiply F x V to get power in Watts. That's a not too bad formula. If you can guesstimate peak velocity it gives you some estimate of forces. | At earth sea level (14.4 psi / 100 kPa) air density = 1.2 kg/m^3. | Alter assumptions to suit. The 'g' term is simply to link Newtons and kg at sealevel and is not related to earth gravity in other contexts. | Say 2 m^2, 10 m/S , around 1 kg/m^3. F=100 N or 10 kg. Not vast. ... – Russell McMahon Jan 09 '20 at 10:08
  • ... Double V and you get 40 kg - care needed. Triple velocity and it's 90 kg. not undoable but a Super Ripley is needed. That's for a 2 m^2 Ripley. Adjust to suit. – Russell McMahon Jan 09 '20 at 10:09
  • Interesting link to https://www.skybrary.aero. I was not aware of the reversal in oxygen flow from the blood into the lungs and always thought one has a minute of two or even three left, as usual when holding one's breath. (Assuming we deal with the shock and the pain.) Which brings me to the obvious question: Could one? Could I hold my breath in a vacuum? Can the lung and airways contain one -- or at least, say, 0.5 -- atmospheres pressure? – Peter - Reinstate Monica Jan 09 '20 at 10:19
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    @StianYttervik Does nitrogen passivation actually happen with molecular nitrogen, though? The nitrogen molecule is extremely stable. – Luaan Jan 09 '20 at 12:12
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    @Luaan Yes, and it remains stable, but it is attracted to surfaces and will adsorb to (most) unpassivated surfaces. I know from personal experience, having investigated the required nitrogen pressure to get a protective surface in an industrial setting, and how to break that surface (you can with other gases - it is a surface tension issue as well) – Stian Jan 09 '20 at 14:20
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    @RussellMcMahon The speed of the air at the choked throat equals the speed of sound. If we assume sea level conditions, that's about 340m/s. That would mean ~11500kg! – alain Jan 09 '20 at 14:57
  • @alain That should deal with momma Alien :-). Flow at throat is as you say, choked - or less - ie it builds to a maximum of sonic. But in this case less seems unlikely. What the velocity is away from the throat with flow channels, obstructions ...., would need to be considered - but it looks like Ripley would have a hard time [tm]. Clipping on a super-nonexisteum tether to a suitable implausibly-strongium anchor point may have helped. – Russell McMahon Jan 10 '20 at 03:49
  • Subsonic flow in the airlock should be inversely proportional to cross section, right? So if the airlock is a little wider than the hatch, maybe area is 1.5x, so v around 225m/s, so in the ballpark of 5000 kgf. RIP Ripley. – Russell Borogove Jan 10 '20 at 04:32
  • @RussellBorogove, roughly, yes, but not exactly because air is compressible. I didn't notice the assumption of 2m^2 frontal area before, which is too high since the air flows in the direction of head to feet. Let's take 0.3m^2 and v = 200m/s as a lower bound, it would then be ~720kgf, still impossible to hold. The drag coefficient was assumed to be 1, which I think is reasonable. It might be quite a bit higher though, because of clothes flapping in the "breeze" for example. Yes indeed, RIP poor Ripley. – alain Jan 10 '20 at 15:41
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As much as I love this movie, it's not very realistic.

Here are some cabin leak numbers for the shuttle Orbiter.

At an initial 14.7 psi cabin atmosphere, a 1.5 inch diameter round hole was projected to give an initial leak flowrate of greater than 1500 lbm / hour.

That would have been catastrophic for the shuttle with the cabin pressure dropping initially at over 2 psi/minute.

And using peterh-Reinstate Monica's hole size of 8 x 8 meters?! That hangar bay would be at vacuum in seconds.

(Source: used to leak the shuttle for a living (simulated only))

Organic Marble
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The outer door of the airlock should open inwards against the air pressure. But opening the door against the huge force of the pressure to the large area of the door is impossible.

If the door is opening with the air pressure, a manual or motorized unlocking would be possible only at a very low pressure inside the airlock. Opening of the door locks at normal air pressure by brute force would result in damages to the door and the locks. Rapid closing of the damaged door would be impossible.

Uwe
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    Looks like the outer doors slid perpendicularly into the airlock wall. – Organic Marble Jan 07 '20 at 16:46
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    @OrganicMarble A sliding mechanism designed for very low pressure difference would not work at normal pressure. A partialy opened door would be damaged by normal pressure. – Uwe Jan 07 '20 at 17:28
  • When I read the above I thought about Canal / River "Locks" used to transfer boats etc from high to low and back again simply as the smaller human manually operated ones open towards the high side and against the low side simply so you could not by virtue of water weight or pressure behind the door accidentally open it until the levels were equalised. The thought on the sliding doors is good too I can see the immense vacuum almost bending them perhaps :o – AndyF Jan 07 '20 at 17:32
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    @AndyF Uwe is 100% correct about real airlocks. They are made so that air pressure holds the (hinged) outer door closed. – Organic Marble Jan 07 '20 at 17:33
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    I find your lack of faith in Wayland-Yutani engineering to be disturbing. Only the most powerful and reliable equipment for our Colonial Marines! – Russell Borogove Jan 07 '20 at 19:40
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    the system is clearly designed to be capable of opening when pressurised. –  Jan 07 '20 at 20:17
  • Regarding the outer door itself I just watched the clip again. Around 1 minute in you can just about see the outer door is or appears to be two doors. Best time I think to catch this is just when they start to open before the camera cuts away to "Newt" and then "Bishop". If the double door is meant to be two separate ones (which from a safety and additional failure redundancy point of view at least makes good sense) or if it is merely a Sandwich type with a gap for insulation effect I could not say:) – AndyF Jan 07 '20 at 22:48
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    The same effect (reversed) can be seen on Earth in a car submerged in water. Just a few feet of depth in water is enough pressure differential to prevent even a relatively strong human from opening the doors. Hence why they advise either breaking the windows or waiting for the cabin to mostly fill up with water (hold your breath) in order to equalize the pressure. – Darrel Hoffman Jan 08 '20 at 04:55
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    They might want the doors to be able to be opened with a pressure differential for various reasons. Evacuation a fire or whatever being one example. The Apollo 1 fire left them unable to open the door from the inside when there was higher pressure.

    Also maybe the whole area is meant to be able to be usable in vacuum for loading and unloading of machinery for repair missions or whatever. Maybe the whole storage area is a huge airlock, that normally would not be opened in a rush like that.

     – Lassi Kinnunen Jan 08 '20 at 06:06