Consider a futuristic world where space travel and colonisation of the solar system's planets is an everyday reality. In space vessels, the airlocks are used quite often to execute people. It's fairly easy: shove a person in, push a button and they're gone. They call it "spacing".
How would you make airlocks safer? How would you reduce their usefulness for murder?
A couple ideas:
If there's vacuum on the outside of the airlock, then the outer door can only be activated from within the airlock itself. There's also an override within the airlock to prevent the door from being opened.
The airlock security system identifies when people are in the airlock and requires that everyone have on a functional spacesuit before unlocking the outer door.
Space Navy Code D.E.E.P/6 para 4
I think you're possibly looking at this from the wrong direction. You're proposing a scenario where the death penalty is a legitimate punishment. You say the air lock is used "quite often" as a method of execution. In such a situation I don't understand why you would want the method to become less effective in doing its job. This would be like injecting a homeopathic dose of KCl into a death row inmate.
If you simply want to change the practice of execution to a different method, you amend the above naval code to read "...the airlock shall not under any circumstances be used as a means of carrying out the penalty of death." You simply make a regulation that specifies what methods of execution will be allowable.
Naval officers in general are honourable men and women: they often come from military families and are decent, patriotic folks. They are also trained from academy days to take the rules and regs seriously. If you make it a rule that the air lock can't be used to execute someone, then it won't be used for that purpose. Officially.
Same goes for any non-naval vessel. You just make it illegal (generally) or against corporation regs to use the airlock as an execution device, assuming corporations have been granted the juridical right to execute a person in the first place. Anything you do to the airlock itself that would prevent its use as an execution device is also going to prevent its more legitimate functions.
Space is a dangerous environment and keeping people alive requires a lot of things to go right. You can put safety measures and backups in place to reduce the number of accidents, but they will do little to stop premeditated murder and even less to stop executions. (Here I define an execution as a premediated killing by the local authorities, e.g. a ship's command staff.)
For that matter, the executioners could always gum up the air vents in a storage room and send their victims there. That takes longer and leaves more of a mess, but it would circumvent any airlock safeguards.
Remove the executioner-just-outside-the-airlock button entirely. Like a Death Ray, it has only evil applications in the script.
Require the use of multiple buttons simultaneously --one on the bridge and one inside the airlock-- to cycle down to vacuum. But only one button is needed to cycle up to pressure.
Have the outer door open conveniently inward so the corpse cannot be freely ejected to space. You gonna kill somebody, you gonna stick around and clean it up.
Decompressing airlocks by opening of the outer doors is undesirable as you will lose some of your precious oxygen to the vast emptiness of space. Besides, pressure equalization by opening the hatches may be explosive in nature, as the rate at which the pressure equalizes is uncontrollable. Explosive decompression might cause damage to equipment as well as severe injury or even death.
The airlocks hatches shall therefore either open inward (similar to what you'll see on submarines or decompression chambers) or have a pure mechanical interlock which prevents opening the hatches unless the airlock pressure has already been equalized.
During normal operation, the airlocks will be equalized by using an equalization compressor which will evacuate the air from the airlock and store it on a tank. The equalization compressor shall be equipped with emergency push buttons both on the inside and the outside of the airlock, which will stop the compressor and open a valve to bleed the air back into the airlock from the air tank.
There will be mechanical valves for emergency equalization of pressure between space and inside the airlock, as well as from within the airlock and to the interior of the spacecraft. The mechanical valves will require specialized tools for operation. The valves for equalizing between space and within the airlock can only be operated from the outside or from within the airlock. Similarly, the valves for equalizing between the airlock and the spacecraft interior may only be operated from within the airlock or the spacecraft itself.
Obviously someone from the outside will have the theoretical possibility of bleeding out the airlock against the will of the crew of the space craft, but this is mostly a hypothetical situation which does not warrant any particular countermeasures unless it first proves to become a issue.
The requirements for safe design and operation of airlocks will be a part of the SOLIS-code (Safety Of Life In Space) sanctioned by the International Space Organization (similar to International Maritime Organization) which is an organization consisting of the different governmental space agencies of nations with vested interest in space operations. The codes will require periodic inspection of the safety functions of the space craft. The national space agencies will have the authority to inspect and to place any space craft that does not meet the SOLIS-codes under arrest until the spacecraft is up to standard. The governmental agencies can also blacklist space crafts and impose sanctions on the space craft owners in case they attempt to bypass the inspections by never docking in a regulated space port.
Multiple locks.
Mass can be valuable in space. A series of airlocks would facilitate reclaiming air by having a series of small rooms that together constitute the lock.
Let us call them ship, A, B, C and space. On closing the wall between ship and A, atmosphere from A is pumped back into B. As the pressure difference increases, the pump must work harder. A then opens to B. After the wall between A and B closes, B is pumped into A, probably with the same pump. Then B to C and then when C opens to space there is very little remaining atmosphere to lose.
You could push someone into room A, and pump down the pressure in room A and make their ears pop but they would not be obliged to leave room A when you opened the partition to room B. The condemned would just stay in A, glaring at you through the window, breathing steamy breath on it, drawing pictures in the steam, labelling the pictures with an arrow and "YOU". The pictures are unflattering.
Side benefit of locks like this is that they can be used as a decompression chamber. Just as a diver ascending to fast can get the bends, if persons accustomed to a higher atmospheric pressure came onto a ship with a lower pressure, those people could get the bends - decompression sickness. A stepwise lowering of pressure allows gradual outgassing of gasses dissolved in the blood, and avoiding the bends.
That’s kind of like asking how do you prevent the plugs, switches and other electrical equipment in your house from being used in an electric chair or any guns you may own from being used for a firing squad.
Presumably an advanced, spacefaring culture would make use of a standard airlocks with redundant safety features ie multiple interlocks, restricting unauthorized use, etc. If said culture also had capital punishment on the law books with ‘spacing’ as an authorized method of execution, they would probably have a specific location and equipment specially designed for the purposes of carrying out the executions, similar to what modern death houses at prisons today do.
That’s not to say that someone might misuse an airlock to commit crimes, or make use of one to commit suicide, etc.
For safety reasons the airlock will be designed so that the controls for each door will be adjacent to that door, so that the outer door cannot be opened by someone who is within the inner door, and vice-versa.
The airlock is used only when personnel are entering or leaving the ship, and during normal operations this must be authorized by the officer in command of the ship. Consequently the door controls are inoperative unless activated from the bridge.
Furthermore, no door can be opened unless the atmospheric composition is the same on both sides. This is to prevent mixing atmospheres of different compositions (so that properly-suited personnel can be transferred to a ship with a different atmospheric composition), as well as to prevent the venting of atmosphere to space.
The danger of fire is addressed by prevention: The ship is constructed from materials that do not combust under normal conditions.
If spacing becomes a lawful punishment, it will take place by putting the condemned into the airlock, closing the inner hatch, pumping out the atmosphere therein, and then opening the outer hatch, with perhaps a mechanism to push the condemned out.
The reliability of the systems, and their vulnerability to sabotage, will always be an issue.
As multiple answers have demonstrated, electronic, mechanical or computer "fail safes" have the potential to create issues and problems of their own, up to disabling the ship itself. You certainly don't want to do this with such a critical piece of equipment, and indeed if this was installed in the factory or shipyard, hackers would soon be looking to defeat these measures to protect themselves and the ship.
If the issue is judicial executions, such as the death penalty from a "Captain's mast" or the crew quelling a mutiny, then there are either legal safeguards built into the process, or common sense limitations. Spacing a person ejects valuable oxygen from the ship, as well as the person themselves. A body might be more profitably sent to the recycling staton where it is reduced to component atoms and fed into the hydroponic gardens or something similar. space is a high energy environment, and ships using nuclear fission or fusion for propulsion will have lots of on board energy. In a pinch, the ship's engineering division could create a large parabolic mirror to capture the available sunlight to do the job (if the ship is in free fall, the mirror can be arbitrarily large and made of a sheet of aluminum or similar material a few molecules thick).
So judicial executions would be done in such a manner as to preserve the ship's stores of life support, and even replenish them by conserving and recycling the bodies. Depending on the circumstances, non judicial murders may be done with the same reasoning in mind. Captain Bligh isn't going to be cast adrift with the loyal crew, they are all going into the recycler, one way or another. Indeed, the extra time, effort and energy needed to drag a victim to the airlock and set up a cycle to space a person might be enough to deter most criminals, and crimes of passion are going to take place "on the spot" where the aggrieved party gets their hands on some sort of killing implement or uses their bare hands.
So the reason to not use the airlock to "space" a person revolves around maintaining the integrity of the life support system, and the extra difficulties that using the airlock would cause most criminals who are contemplating committing murder.
The exception, of course, is when HAL 9000 is the one planning to kill you using an airlock....
"Dave, I need you to get something for me...over there by the airlock..."
Trying to make an airlock that can't be used to kill anyone is like trying to make a knife that can't cut someone's throat. It's not impossible, but doing so will compromise its effectiveness.
Instead, have a logging device that shows exactly what is in the airlock and who is activating it. Automatically transmit all recordings home. That way, if someone uses it for murder they will get caught.
It's just not a good way to execute someone. It's slow, gross, ineffective at getting rid of them, inefficient, and undramatic.
How it goes in the movies:
Bad Guy tricks Hero into the airlock, then slams the door, trapping the hero inside. Bad guy outside the airlock pulls the big red lever with the black and yellow danger-chevrons around it, and the far door quickly opens to space, sucking the hero out to their certain doom, where they freeze and/or explode.
This is dramatic! Audiences love this stuff.
How it would go in reality:
Bad Guy tricks Hero into the airlock, then slams the door, trapping the hero inside. Hero seems to have some kind of plan, and starts to don an EVA suit.
Bad guy looks around for a way to open the far door, and sees nothing. He reopens the door, overcomes the Hero, and pulls them out of the EVA suit to tie them up. By chance, he sees the procedure manual for operating the airlock, grabs it, and reads it, cursing, realizing he needs to be inside the airlock to start depressurization.
Following the manual, he begins the pre-breathe protocol, ten minutes of exercise elsewhere in the station, while breathing pure oxygen. After a further 40 minutes of breathing O2, during which he watches an episode of his favorite TV series, he climbs into the tiny airlock with the Hero, and initiates depressurization which sounds an alert throughout the station, but nobody comes running because reasons.
Over a period of 30 minutes and another episode, the pressure drops from 14.7 PSI (1 atm) to 10 PSI (~2/3 atm), at which point an alarm sounds telling him to suit up.
This is equivalent to moving from sea level to an altitude of 10,000 ft in half an hour. The Hero will start to find it difficult to breathe, and may suffer signs of altitude sickness at this point, such as pulmonary edema (fluid in the lungs), cerebral edema (swelling in the brain), and general hypoxemia and hypoxia. He is likely to pass out during this period unless the Bad Guy gives him oxygen too.
Bad Guy, still breathing oxygen, puts on the EVA suit, glad that the hero is well trussed up in these tight confines or he'd be in big trouble. He clips his safety harness to the ring by the airlock, and waits the final 60 minutes for the airlock to completely evacuate, binge-watching two more episodes with subtitles because he can't hear through the suit, let alone through the vacuum that's forming as he watches the pressure slowly drop.
Even with a breathing mask, Hero definitely passes out and dies at some point, his skin blackens with bruising, he swells up and maybe starts bleeding and/or prolapsing from some orifices. Other bodily fluids start exploding out and floating around the airlock, and Bad Guy is glad he's in the suit. They fluids boil, spattering globules as they bubble, and as the fluids boil and gas off, what's left drops in temperature until it freezes in little ice-gobbets, sparkling shards of gore.
Eventually the pressure has dropped all the way. Bad Guy waits expectantly, but the hatch doesn't open, so he walks over and pushes on the hatch. It still doesn't budge. He fiddles with the latches, which seem to work fine, but it won't push open.
He checks the manual and finds that airlock doors ALWAYS open inwards, so that: locking failures cannot cause the doors to pop open under air pressure; and it's impossible to open the door when pressurized, since it would require literal tons of force to open. Sci-fi always gets this wrong, because it's non-dramatic, and because they feel that hatches should open outwards like on tanks.
He pulls, it opens, but there's only a barely perceptible puff of the last remnants of the air going out. Hero's bloated and messy corpse just drifts where it was, up against Bad Guy's facemask in the cramped quarters, both of them surrounded by a halo of gross boiling globules.
Bad guy flails at it and shoves it toward the hatch, but it bounces off and has to be kinda weightlessly wrestled out, all the various bodily fluids going freaking everywhere.
Eventually it's out, and he recloses and latches the door, and begins the repressurization process. This time, he waits the whole time in the suit because he knows it's going to stink out there.
Partway through the next episode of his show, he hears a splatting thud and looks through the airlock viewport to see that the corpse has flown in a slow, graceful loop and splatted back against the closed airlock, losing more fluids which boil and freeze almost instantly. It's drifting away again, though this time in a much smaller circle, having lost in the impact most of the energy with which he threw it out of the airlock. This is because they're orbiting every 15 minutes, and the body is in only a very slightly different orbit, just slightly more elliptical.
Bad Guy is now faced with two unpleasant options. He can either shed the suit here and try to touch as few floating particles of corpse-slime as possible as he escapes from the airlock back into the station, or he can wear the suit out of the airlock and remove it there... but the suit's covered in the stuff too. Either way, the air in the station is now contaminated with the stuff and when he goes through the door, particles will be following him. The scrubbers are good, but they aren't amazing: there'll be droplets of this stuff dried all over the place forever, and the air will from now on always stink of dead Hero dude.
Damnitt, next person he kills will be going into a suit before spacing them, and he'll just cut the air pipes. In fact, screw spacing them, he'll kill them and put them in the trash bags with all the rest of the trash.
In fact, thinking about it, the next person is going to be whoever comes with the resupply vessel, and that might try to come in through that airlock with the hero-corpse floating outside of it... he probably needs to go drag it back inside and stuff it in a trashbag, so's not to tip them off. Crap.
This is... not dramatic. Audiences will not love this stuff.
There're a few cases that designers need to consider when creating an airlock in a spacecraft or space station... or in a novel.
One person accidentally or deliberately cycling the airlock while a second person is in there for cleaning, equipment check, just returned from EVA, etc. Addressed by having the controls inside the airlock; a very slow cycle time measured in minutes or hours; and no doors that lock on only one side. [Edit: this is how it is in real-life craft]
A single person triggering the airlock on their own while in it (suicide, unauthorized EVA, etc). Audible and visible alerts when depressurization starts: "Alert: Port-forward airlock depressurizing. No scheduled EVA at this time." Allow depressurization to be canceled from the rest of the station. [Edit: this case is not covered in real-life craft: depressurization is done from the airlock, venting the air into space until pressures equalize]
One or more people (including rescue personnel) outside the station needing to get into the station when someone has accidentally or deliberately locked the door, overridden the airlock controls, etc; and where all crew inside the station may or may not be incapacitated. Allowing depressurization to be started from outside the ship should address this. [Edit: this case is not covered in real-life craft: the inner door is kept open except during EVA,so you can't "come in from outside".]
A single person needing to get out through the airlock when nobody else in the crew is capable of functioning. Controls in the airlock resolves this case. [Edit: this is how it is in real-life craft]
One or more people needing to operate the airlock from inside the craft, while nobody is in the airlock, is low, and MIGHT NOT be worth factoring into spacecraft design. They can just suit up and go into the airlock to operate it. [Edit: not possible in real-life craft]
One or more people getting spaced by multiple others coordinating together (murderous mutiny or execution). Probably not worth considering, could never be made foolproof anyway. [Edit: not covered in real-life craft, because you can't space someone who's not incapacitated already.]
Hostile boarders: probably not worth considering. Even if space pirates are really a thing in your universe, not even locks and door keys are needed. Space pirates will just magnetically clamp onto a ship and cut through the hull, rather than waste time waiting for some slow-ass airlock. If the victims are lucky, they might pressurize the area around the hole they cut, so people don't immediately die. [Edit: in real life craft, the inner airlock door is always open.]
Everything needs to follow some basic design tenets:
[Ref for some of the numbers: https://spaceflight.nasa.gov/station/eva/outside.html]
Advanced AI
In an era of advanced every-day space travel, ships would have advanced AI to do any number of ship operations including:
It is likely that any ship in this era would have almost all systems, if not ALL systems, automated and controlled by advanced computer or software.
Just like airlines of today there are systems that were handled by Flight Engineers, now a redundant position, and which now are handled by computer - in space mathematical calculations, issues and problems are much more complex than humans can handle and thus computers and AI would be in place, accompanied by appropriate robotics if necessary.
This AI would ensure the safe passage of its occupants to the destination - and simply would prevent the use of airlocks if it endangers the crew or passengers. Multiple redundant safety systems would be in place, making it impossible for abusing any onboard ship system.
Implement the following logic:
Ok this still leaves the possibility that somebody climbs in a spacesuit, together with the person to be killed (which is already tied up), puts him/her in a spacesuit and opens the door there.
Put a cancel button inside the airlock
The airlock can't be opened if the cancel button gets pressed. Make sure there's a 15-30 second (minimum) delay so that anyone inside can have a chance to press it. After that, the cancel has to be manually reset from inside the airlock before the airlock can be depressurized.
Could the victim be tied up and put in the airlock? Sure. But if they are tied up, they can also be strangled, so you're not really preventing anything in that scenario.
Maybe a O2 sensor for checking if there is sufficient air? This could allow the airlock to open if there is a sufficient amount of O2.[FLAW 1]
Precaution 2: Button is inside [FLAW 2]
3: Heat sensor inside and out of the airlock. [FLAW 3]
4: Suit sensor [FLAW 4]
5: Access code required [FLAW 5]
6: AI [WARN 1]
7: Automatic atmospheric generation [FLAW 6]
Flaws and warnings:
WARN 1: AI can backfire and do it themselves
FLAW 1: Can be faked. (See precaution 7)
FLAW 2: Signal can be sent externally (TIP 1)
FlAW 3: Can be faked. (TIP 2)
FLAW 4: Hat made out of suit.
FLAW 5: Access code can be added (Can be faked, TIP 3)
FLAW 6: Can be terminated, Can trap. Would pop eventually anyways
Air lock doors, like the canal lock doors they are based on, only open towards the side with higher pressure. When the lock is pressurized, a door 2 square meters in size would take 20 tonnes of force pulling it open to break the seal, and there is no need to install actuators that will do that for standard operation.
To execute someone by throwing them out the airlock, the condemned would be carried into the lock by two suited guards, the lock would be depressurized by pumping the air into the ship, so as to not waste it, a tether would be attached to the now unconscious prisoner, the outer door opened, the body thrown out.
And then reeled back in, doors close, the lock repressurized, and the corpse taken to the organic waste recycling center like the rest of the dead. You don't want to waste that either. It's bad enough that you wasted some water in that spacing execution.
As many here point out, all reasonable fail-safes can be overcome. If they can't they are likely to dangerous.
I think the what is needed is not technical but cultural, religious or ritual practices around the use of the airlock that are socially and psychologically significant.
The original meaning of the work 'sacred' didn't imply 'valued', just that it was outside of the realm on everyday, or 'profane'. So in the ancient Greek world, prisoners needed to be made sacred before they could be executed, because normally killing a person would be murder.
Airlocks and their use needs to be embedded in culture as significant ritual practice, so that stepping outside of this is a psychologically difficult transgression against norms.
I think there's hint of this in different cultures ideas about guns. Guns are actually plentiful in australia and switzerland, but for most people, they are culturally considered 'sacred' objects, to carry a gun in everyday life, except for a specific activity (eg, hunting, professional, sporting) is culturally shocking. Australians aren't cooler headed that Americans, it's the cultural, not legal barriers to possessing and holding weapons that is responsible for the low numbers of gun deaths. The famous Australian 'gun buy back' that has reduced gun deaths, may actually be more about the cultural message it sent that the reduction in gun ownership, which seems to have been temporary:
https://www.gunpolicy.org/firearms/compareyears/10/number_of_privately_owned_firearms
Have the airlock on the outside mechanically unable to open without a keyed dock physically connected to it. If you want to do maintenance - a pod like vehicle, akin to the starfury forklift from babylon 5 makes more sense than an EVA suit on its own in a larger deep space habitat.
Basically make it impossible to open these airlocks into space.
If people try to live in space any length of time, there will be hanky panky, and before long, rug rats. Well, baby's first airlock should be designed by somebody who has thought about it a minute.
Airlock checks for human flesh. If it sees human skin or smells human presence (CO2, pheromones, etc.) then obviously that is a huge red flag to biohazard sensors that are designed to spot a single Andromeda Strain monomer. Under those conditions it's not opening without a full manual override, including all the usual frenetic safety klaxons and alerts and automated head counts by station sensors that implies.
Airlock does not go along with silly requests easily. If you want both doors open at the same time, cue the klaxons and head counts.
Airlock is meant to work with suited astronauts and suitable packages. If it spots anything that is likely to be a mess in vacuum conditions - like a half-eaten container of strawberry jam - it's going to check with Mommy.
Airlock likes to have a notion of what's going on. It may not know where the kid is going, but it wants to have basic indications from suit telemetry that he didn't forget his galoshes or the extra oxygen bottle. There's a plan for most things, and it responds to variations in the plan by inquiring with the right people.
