An airlock is a device which permits the passage of people and objects between a pressure vessel and its surroundings while minimizing the change of pressure in the vessel and loss of air from it. The lock consists of a small chamber with two airtight doors in series which do not open simultaneously.
An airlock may be used for passage between environments of different gases rather than different pressures, to minimize or prevent the gases from mixing.
An airlock may also be used underwater to allow passage between an air environment in a pressure vessel and the water environment outside, in which case the airlock can contain air or water. This is called a floodable airlock or an underwater airlock, and is used to prevent water from entering a submersible vessel or an underwater habitat.
Before opening either door, the air pressure of the airlock—the space between the doors—is equalized with that of the environment beyond the next door to open. This is analogous to a waterway lock: a section of waterway with two watertight gates, in which the water level is varied to match the water level on either side.
A gradual pressure transition minimizes air temperature fluctuations (see Boyle's law), which helps reduce fogging and condensation, decreases stresses on air seals, and allows safe verification of pressure suit and space suit operation.
Where a person who is not in a pressure suit moves between environments of greatly different pressures, an airlock changes the pressure slowly to help with internal air cavity equalization and to prevent decompression sickness. This is critical in scuba diving, and a diver may have to wait in an airlock for some hours, in accordance with decompression tables.
Airlocks are used in
- aviation, certain airplanes are equipped with airlocks for skydiving, and/or emergency exits.
- spacecraft and space stations, to maintain the habitable environment when persons are exiting or entering the craft.
- hyperbaric chambers, to allow entry and exit while maintaining the pressure difference with the surroundings.
- submarines, diving chambers, and underwater habitats to permit divers to exit and enter.
- torpedo tubes and escape trunks in submarines are airlocks.
- cleanrooms, protected environments in which dust, dirt particles, harmful chemicals, and other contaminants are excluded partially by maintaining the room at a higher pressure than the surroundings.
- hazardous environments, such as nuclear reactors and some biochemical laboratories, in which dust, particles, and/or biological agents are prevented from leaking out by maintaining the room at a lower pressure than the surroundings.
- pressurized domes such as the USF Sun Dome where pressure loss would cause collapse of the structure.
- electron microscopes, where the interior is near vacuum so air does not affect the electron path.
- parachute airlocks, where airfoil collapse due to depressurization can result in dangerous loss of altitude.
- fermentation vessels, where a fermentation lock allows fermentation gases to escape while keeping air out, such as in breweries or wine-makers.
- In cold climates, two doors arranged in an airlock configuration are common in building entrances. While not airtight, the double doors minimize the loss of heated air from the building. A similar arrangement is common in hot climates, where it is used to keep interior spaces cool. Revolving doors may be used for the same purpose.
- Some jewelry stores and banks have airlock-like doors to slow the escape of thieves.
- Butterfly farms and aviaries usually have an airlock-like entrance to prevent the exit of inhabitants and entrance of predatory species.
- Planetariums may have "light-locks" to minimize outside light, protecting visitors' sensitive dark adaptation. These pairs of doors also reduce outside sound.
Airlocks in fiction
A four-door airlock (with three interior chambers) was proposed by science fiction writer H. Beam Piper in his novel Uller Uprising. The atmosphere inside the fictional structure was human-breathable, while the outside atmosphere was highly toxic. Only one door of the airlock opened at a time, and the middle chamber of the three would always contain a vacuum to minimize traces of the exterior atmosphere reaching the habitat.
In the 1979 spy film Moonraker, James Bond dispatches the villain Hugo Drax aboard his own space station by first shooting him in the chest with a cyanide dart, then pushing him out an airlock into space ("Take a giant leap for mankind.").
In the 2014 science-fiction film Interstellar, airlocks are featured several times, beginning with the initial journey into space. In the film's climax, Dr. Mann attempts to maroon Cooper and Brand by stealing their spacecraft, the Endurance. When Mann tries docking his own ship onto the Endurance, he does so imperfectly. Despite this, he decides to board the Endurance from his own spacecraft regardless of the circumstances. In the midst of an arrogant monologue, he opens the airlock door. Due to the pressure change caused by the vacuum of space, the airlock explodes, killing Mann and critically damaging the Endurance. As a result of this, Cooper and Brand are forced to pursue the damaged Endurance in their own ship, as it is their only chance of survival. Due to the explosion, the circular ship is spinning out of control toward the stratosphere of a nearby planet. In a last ditch effort to save the mission, Cooper attempts to match the RPM of the Endurance with that of his own spacecraft. Due to the intense g-forces being subjected to Cooper and Brand, they must rely on their versatile AI known as TARS to accurately dock the ship with the Endurance while spinning.
In the 2015 science-fiction film The Martian, airlocks are used in the Hab, a habitat and base of operations on Mars, as well as on space-faring vessels. Mark Watney, an astronaut stranded on Mars improvises a farm in the Hab where he is living. Subsequently, the failure of an airlock and the depressurization of the environment kills the potato crops he was growing. Additionally, the crew of the Hermes vessel tasked with rescuing Watney deliberately breached an airlock to produce reverse thrust, in order to slow their vessel down enough to intercept Watney's capsule.
In Star Trek, Star Wars and some other works of fiction, conventional airlocks may be replaced by forcefields which hold in air while allowing solid matter like spacecraft to pass through. Airlocks of this type usually have pressure doors as a backup.
Airlocks are commonly used in science fiction as a form of execution, often referred to as "spacing" or "airlocking". Battlestar Galactica, Babylon 5, The Expanse and the Honorverse make frequent use of or reference to it. Airlocking was also used as a means of expelling an antagonist into space in the film Alien and one episode of the television series Space: 1999, "The End of Eternity". This last example displays the plot complication of having to lure the formidable alien being Balor to the airlock and to maneuver him alone inside it.
Airlocks are also used in the following video games:
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