From Halopedia, the Halo wiki
An airlock is a device which allows passage between two areas of drastically different pressures and/or atmospheric composition, while preventing the exchange of gases and pressure. An airlock consists of a small chamber with two airtight doors in series, which do not open simultaneously. They are most commonly used on spacecraft, where they allow safe passage between the pressurized interior of the ship and the vacuum of space, though they can also be found in enclosed habitats on worlds with toxic atmospheres, for example.
On UNSC spacecraft, hangar bays, lifepod or HEV launch areas are typically equipped with airlocks, used to prevent pressure loss from exposure to vacuum either due to a hull breach or the deployment of a lifepod or HEV. Halcyon-class cruisers, in particular, employ them as redundancy features for survivability during combat with Covenant warships. The UNSC Hopeful had an airlock large enough to cycle a fleet of dropships simultaneously.
The Covenant typically employs energy barriers in place of physical airlocks. Certain UNSC facilities also employ energy barriers for this purpose, presumably having reverse-engineered Covenant or Forerunner versions.
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