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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

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – called2voyage ♦
    Commented Jan 10, 2020 at 14:57

3 Answers 3

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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.

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    $\begingroup$ 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. $\endgroup$
    – Luaan
    Commented Jan 7, 2020 at 11:40
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    $\begingroup$ 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. $\endgroup$
    – Ruadhan2300
    Commented Jan 7, 2020 at 13:02
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    $\begingroup$ @Luaan They wouldn't run 0.2 atmo of pure O2, that is what in the space industry they call "an extreme fire hazard." $\endgroup$
    – Draco18s no longer trusts SE
    Commented Jan 7, 2020 at 18:20
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    $\begingroup$ @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. $\endgroup$
    – mbrig
    Commented Jan 7, 2020 at 18:55
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    $\begingroup$ @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. $\endgroup$
    – Stian
    Commented Jan 8, 2020 at 11:28
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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))

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    $\begingroup$ related via leak rate estimates: How could the 2018-08-30 Soyuz MS-09 / ISS leak be so slow? and generally related Is the air “sucked out” or “blown out” into space when the hatch opens? $\endgroup$
    – uhoh
    Commented Jan 7, 2020 at 1:57
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    $\begingroup$ Nowhere near 8m. You can see Ripley strapped into the loader in the pic; the hatch is maybe a little more than 3m across. The hangar is huge, though; it carries two shuttles (the size of attack helicopters) plus a whole lot of gear; call it 40x40x10m. $\endgroup$
    – Russell Borogove
    Commented Jan 7, 2020 at 4:00
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    $\begingroup$ I thought 8 ft square but I'm no photometrist. A giant hole in either case. $\endgroup$
    – Organic Marble
    Commented Jan 7, 2020 at 4:06
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    $\begingroup$ It's like the square root of delta-p times a constant. $\endgroup$
    – Organic Marble
    Commented Jan 7, 2020 at 4:08
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    $\begingroup$ OTOH the ship was pretty huge and I don't think the hangar bay was entirely isolated from the rest. The bulk of air could escape the hangar bay within seconds, then the rest of the ship would leak through all kinds of ducts at slower rate still causing a "wind" in the airlock but not the same massive burst of air. $\endgroup$
    – SF.
    Commented Jan 7, 2020 at 6:35
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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.

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    $\begingroup$ Looks like the outer doors slid perpendicularly into the airlock wall. $\endgroup$
    – Organic Marble
    Commented Jan 7, 2020 at 16:46
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    $\begingroup$ @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. $\endgroup$
    – Uwe
    Commented Jan 7, 2020 at 17:28
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    $\begingroup$ I find your lack of faith in Wayland-Yutani engineering to be disturbing. Only the most powerful and reliable equipment for our Colonial Marines! $\endgroup$
    – Russell Borogove
    Commented Jan 7, 2020 at 19:40
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    $\begingroup$ 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. $\endgroup$
    – Darrel Hoffman
    Commented Jan 8, 2020 at 4:55
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    $\begingroup$ 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. $\endgroup$
    – Lassi Kinnunen
    Commented Jan 8, 2020 at 6:06

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