Plug door
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A plug door is a door designed to hold higher pressure on the side it opens into - the shape of the plug door (typically like a slice of a wedge base) prevents it from being opened when there's a difference in pressure across the door.
Non-plug doors rely on the strength of the locking mechanism to keep the door shut, whereas a well-designed plug door relies only on the strength of the wall around it and the material the door itself is made from.
[edit] Uses
[edit] On aircraft
The plug door is often seen on aircraft with pressurized cabins. Due to the air pressure within the aircraft cabin being higher than that of the surrounding atmosphere, the door seals itself closed as the aircraft climbs and the pressure differential increases. This prevents the opening of a plug door on board a pressurized aircraft.
However, in the event of a decompression, with there no longer being a pressure differential, the doors may be opened, and as such most airlines operating procedures require cabin crew to keep passengers away from the doors until the aircraft has safely landed.
[edit] On spacecraft
This style of door was used early in the US Space Program on the "Block I" Apollo Command Module; however, after the Apollo 1 fire that killed the three astronauts during a ground test, NASA decided to return to outward opening doors, using plug doors only for the CM docking hatch, and the two hatches on the Apollo Lunar Module (as a fire in the low-pressure atmosphere was easily extinguishable, the Apollo 1 fire occurred in a high-pressure pure oxygen atmosphere). Currently, plug doors are used only on the outer hatch doors on the Space Shuttle and International Space Station as well as on the hatch between the Orbital Module and Descent Module on the Russian Soyuz spacecraft.
