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An airlock on board the Space Shuttle.
An airlock on board the Space Shuttle.

An airlock, air-lock or air lock, often abbreviated to just lock, is a compartment with doors which can be sealed against pressure which permits the passage of people and objects between environments of differing pressure or atmospheric composition while minimizing the change of pressure in the adjoining spaces and mixing of environments. The lock consists of a relatively 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 or different pressures, or both, to minimize pressure loss 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.

Air-locks are used in space flight, saturation diving accommodation, hyperbaric chambers, submarines, some underwater habitats, cleanrooms, and pressurised caissons and tunnels. Depending on the pressure difference, when people use an airlock to transfer from higher to lower pressure, it may be necessary to follow a decompression schedule to avoid decompression sickness. A similar arrangement may be used for access to airtight clean spaces, contaminated spaces,or unbreathable atmospheres without necessarily having a pressure difference. In these cases a decontamination procedure and flushing takes the place of pressure changes.

The procedure of entering an air-lock, sealing it, equalising the pressure, and passing through the inner door, is known as locking in. Locking out is to unseal the outer door after equalising pressure and exit the lock compartment to the ambient environment. Locking on and off refer to transfer under pressure where the two chambers are physically connected or disconnected prior to equalizing the pressure and locking in or out.


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 underwater diving, and a diver or compressed air worker may have to wait in an airlock for some hours, in accordance with an appropriate decompression schedule.


US Navy submarine diving lock out, 2007.
US Navy submarine diving lock out, 2007.

Airlocks are used in

Underwater diving

Main article: Saturation diving

Locks are used for access to hyperbaric chambers and accommodation in saturation diving, where the saturation spread is commonly pressurized for several weeks, and divers and occasionally support personnel must be transferred between the accommodation chambers and the closed diving bell which is used to transport the divers to the underwater workplace and back. A saturation spread will normally include a stores lock and a medical lock, facilities to lock on a closed bell, and a personnel airlock to compress and decompress people. In a "split-level" spread, where divers may live at two different storage pressures, more airlocks may be needed to effectively service both sections. To decompress the current crew from saturation, one or more of the main accommodation chambers will normally be used as the decompression may take more than a week. Transfer to a hyperbaric escape chamber or lifeboat would also go through a lock, but there will generally be no significant pressure change.

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Hyperbaric treatment chambers

Further information: Diving chamber

In any hyperbaric treatment chamber capable of accommodating more than one person, and where it may be necessary to get a person or equipment into or out of the chamber while it is pressurised, an airlock is used. There will usually be a large airlock at the chamber entry capable of holding one or more persons, and a smaller medical lock for locking in medical supplies and food, and locking out waste.

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Compressed air work

Civil engineering projects using air pressure to keep water out of the workplace use an airlock to transfer personnel equipment and materials between the pressurised workplace in a caisson or sealed tunnel and the external normobaric environment. The airlock may need to be large enough to accommodate the whole working shift at the same time. Locking in is usually a quick procedure, taking only a few minutes, while the decompression required for locking out may take hours.

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Space flight

An air-lock is used to decompress astronauts after suiting up in space suits, in preparation for extravehicular activity, and to recompress them on return to the habitable spaces.

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Similar mechanisms

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 kill 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:

See also


  1. ^ "Archived copy" (PDF). Archived from the original (PDF) on October 29, 2013. Retrieved October 23, 2013.CS1 maint: archived copy as title (link)
  2. ^ "Archived copy". Archived from the original on October 29, 2013. Retrieved October 23, 2013.CS1 maint: archived copy as title (link)