If a movie is set on a space ship or space station, at some point there is a good chance that someone or something will be blown out of an airlock. Or, some character, depressed or guilt-racked, will attempt to eject themselves from an airlock. Or someone will threaten to or attempt to “space” or “float” another character. Being blown out an airlock is a very common theme in sci-fi, to the point of having become a cliché. See here for numerous examples.
If players try to follow this too well-trodden path, a wise GM may choose to incorporate some more realistic design features for the airlocks.
Lock the Lock!
The first objection to blowing something out an airlock is that opening an airlock to vacuum will not be as violent and dramatic as it is usually depicted. We are dealing with a relatively small volume of air, at only one atmosphere pressure.
In addition, any realistic airlock is likely to have design features that prevent it from being suddenly opened. One of the most obvious is that airlocks are likely to be locked under normal circumstances. Smaller vessels, such as those crewed by a single family of belters, may not bother with such precautions, but they are likely to be standard practice on most commercial and military vessels. The last thing you want is passengers being able to open airlocks to dispose of, or escape from, unloved spouses! Locking may include some physical system, such as a keycard. You don’t want a cyber-hacker to be able to remotely open your airlocks! Keycards can be stolen, so the user may also have a keycode that must be punched in when the keycard is used.
Airlock doors may be designed so they cannot be opened if there is a significant pressure difference on either side. You do not want the internal door opened if the airlock is depressurized. You do not want the outer door opened if the airlock is not depressurized. Doors might be designed so that a significant pressure difference physically locks them in place. Such a system may be mechanical, preventing remote or high-tech tampering. Such could be achieved by having the inner and outer doors open inward. When there is a vacuum on one side of the door, the door is held shut by a force of 14.7 pounds per square inch (which for a door of around two square yards is equivalent to 19 tons of force!). Since the volume of an airlock is likely to be designed to be as small as practical, an inward swinging door may not be desirable. Possibly the doors will be of the sort that retract inwards and then slide to one side. (Inward retracting doors may be incompatible with the minimal volume requirements of a kwiklok).
When docked to another vessel or station, equal pressure on both sides of the pressure plug doors will allow both inner and outer doors to both be opened, facilitating loading operations.
If the design of airlocks (and physics), prevents you from blasting the contents out into space, what is to stop you depressurizing the interior, then opening the inner door?
The interior of an airlock is likely to have controls for pressurization and depressurization. If you are on your last gasps of oxygen, you do not want to wait for the bridge crew to operate the airlock for you. Comms may be down, or the bridge unmanned. Logic suggests that the airlock interior controls would be able to override commands from other control stations. If you are in an airlock you will want to be able to prevent anyone depressurizing it without your consent. Similarly, if your suit is low on oxygen you should be able to prevent anyone preventing you from cycling the airlock. Thus the interior controls will probably include an override that will repressurize a lock and keep it pressurized.
This feature means that you cannot depressurize an airlock without the occupant’s consent or cooperation. Providing that they are conscious, and understand the controls, they can countermand any attempts to empty the air.
If you can empty the airlock, there is no helpful Hollywood blast of air to throw the unwanted contents into space. They will probably stay in the airlock unless there is someone there to throw them out!
Suppose a would-be suicide attempts to use the internal controls to depressurize the airlock that they are in? This would be a relatively prolonged process and would take considerable willpower to go through with as the pressure and oxygen level falls. It would also be fairly easy to counter. The override described above can also be engaged from outside the airlock, and can be locked.
For humanitarian reasons, the interior controls of an airlock have priority with respect to pressurizing the airlock. The interior controls can countermand any attempts to empty the airlock by using the override. The reverse may be the case concerning depressurization. The occupant cannot empty the airlock if the bridge or someone outside the airlock wishes to prevent it and activates the override. As well as preventing suicides, this feature prevents unauthorized exit from the vessel.
As may be deduced from above, any airlock is likely to be connected to alarm and monitoring systems. Any attempts to use an airlock will be drawn to the attention of crew and/or the vessel’s AI.
The above comments have been made with space scenarios in mind. Many of the suggested features would also be applicable to undersea airlocks. The primary difference is that the pressure differential is reversed. Exterior pressure will be higher than interior, and may be of the order of many atmospheres, depending on depth. Sealock doors are therefore likely to be outward opening, as we see on modern submarines. The dense atmosphere on the surface of Venus means an airlock on Venus would resemble a sealock in operation.