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The Aperture Science Sentry Turret, or simply known as sentries or turrets, is a Personality Construct-based gun platform used throughout the Aperture Science Enrichment Center in Portal and Portal 2.
The Aperture Sentry Turret has a single red eye/sensor, a sleek appearance, and predominately white color scheme which is common in most Aperture Science technology. Sentry Turrets are commonly seen serving as guards in the maintenance areas of the Enrichment Center, and during testing as an obstacle to the Test Subject. They appear to vary in size, from half the size of the player-character to many times larger; the majority are all of the same size, but it is never shown how large they are.
Sentry Turrets often speak in a sweet child-like, artificial voice similar to GLaDOS and a lavatory bowl within the Enrichment Center. They have a strangely polite demeanor, which is in direct contrast with their deadly behavior. They will even forgive Chell for knocking them over. In one scene in Portal 2, an Oracle Turret is seen stuck inside the Aperture Science Pipe Network, who attempts to get Chell and Wheatley's attention. At Wheatley's encouragement, Chell ignores it, but the Sentry Turret politely says "thanks anyway" with a hint of shattered hopes.
The Sentry Turrets are armed with two dual machine guns, which can only be armed once a target comes within visual sight. Upon visual acquisition of the target, the Sentry Turret will come to full alert, and the machine guns will emerge. They will begin to fire at any visible targets (usually Test Subjects, intruders or dummies). When no targets are in visual range, the Sentry will scan the area in a sweeping motion with its laser targeting beam. Upon seeing that there are no more targets, the Sentry Turret will return to sleep mode after three seconds. These weapons, the machine guns, along with its antenna are normally hidden inside the body, during the Turret's "sleep mode".
Aperture-Brand Resolution Pellets, the bullets for a Sentry Turret's guns, are simply dumped into its hollow body, called the Shell Redistribution Chamber; this allows for an unusually large supply of ammunition for such a small device (infinite). Aperture-Brand Resolution Pellets are the only type of ammunition that is recommended for use with a Sentry Turret. In addition, bullets are not "fired" in the traditional sense. They are propelled, shell and all, with a spring-loaded mechanism called the Projectile Acceleration Cylinder, which is guarded by the Aperture Turret External Barrel.
The Personality Core of the Sentry Turret is located within the outer shell, right above the guns. It is fitted with a Fire/No Fire Incident Resolution Chip, presumably to help the Turret distinguish when or when not to fire, an Empathy Generator and an Empathy Suppressor. Apparently, the Empathy Suppressor is of greater influence, as simply standing near a Sentry Turret may cause its Empathy Suppressor to accidentally activate, greatly reducing the Sentry Turret's feelings of empathy toward its foe.
The consumer version of the Sentry Turret is available in hundreds of different designer patterns and colors, including "Forest" and "Desert", as well as "Table" and "Evening at the Improv", which were named on the spot by Cave Johnson ("What idiot picked this?"). 
Detailed in a Portal comic strip, the inception of the Sentry's strange disposition for singing and using melodious tones came from Aperture Science consumer product testers using the weapon as a possible use for protecting children from criminals. The test is a success but the baby begins to cry every time so they decide to include soft tones and lullabies. The second test is then deemed a success with the sleeping child.
Sentry Turrets, when idle, conceal their weapons under the two plates on either side of their body. When they spot a target it takes a moment to deploy its weapons to fire. A Sentry Turret's weapons are very powerful and are able to kill a human, such as Chell, after a few seconds of continuous fire. ATLAS and P-body, while sturdier than a human, still cannot endure lengthy sustained fire before falling apart.
However, Sentry Turrets can be easily spotted by the visible laser beam emitted from their eye at all times, making it easier to avoid them. When knocked over, Sentry Turrets fire frantically in all directions in an attempt to hit what knocked them over, before permanently deactivating. Like all other objects within the Enrichment Center, the Sentry Turret will disintegrate (and produce a soft "owowowowowow..." sound) if it passes through a Material Emancipation Grill.
In Portal, the primary method of defeating the Turret is to disable it via knocking it over (echoing the method used in Half-Life 2 against combine turrets). This can be accomplished in various ways: by picking it up from behind and dropping it in an odd position, knocking it over with a weighted cube, hitting it with another object (including another turret), shooting a portal at its feet and directing a High Energy Pellet into it (which knocks it over, but does not destroy it). Additionally, turrets seem to be unable to damage most objects directly.
Portal 2 introduces two new means for the player to disable the Turrets. One is to direct a Thermal Discouragement Beam onto it (usually via a portal and/or Weighted Pivot Cube), causing it to heat up and explode. Another is to cover it in Repulsion Gel or Propulsion Gel (via the Perpetual Testing Initiative), causing it to bounce with increasing force and shoot blindly until it either shatters or falls away.
In Portal 2, it is revealed that the Turrets' lateral plates also conceal a sound-producing mechanism (unidentified, but possibly an electronic sound synthesizer), which they can 'play' to make music by means of opening and shutting the plates at their sides, not unlike the 'stops' on a clarinet. Each size/class of Turret plays a different tone: the Standard Turret plays, indeed, a clarinet or woodwind-like note, the flat-topped Turrets another, slightly lower, and the gigantic 'Animal King' Turret a deep bass note.
- The turrets from the first Portal game are essentially reskins of the Combine Sentry Gun from Half-Life 2 and its episodes: Portal reused several assets from Half-Life 2, including the Aperture Science High Energy Pellet being the Combine's Energy Ball. This is most obvious in the original Portal trailer, where the turrets use the exact same alarm sound effects. This was replaced by the voice lines in the finished game.
- For Portal 2, which was built using a later build of the Source engine, the turrets use more original sound effects, but the same alarm sound from Half-Life 2 plays during the encounter with the four turrets who start the final turret opera. The "Turret Wife Serenade" also samples the same sound effect.
- In contrast to Portal, Portal 2 Sentry Turrets will not attempt to shoot at enemies if glass is between them, while still tracking the enemy with the laser pointer. A Hard Light Bridge will prevent the turret from detecting enemies.
- During Portal 2's chapter The Escape, Chell passes through the production line for the commercial version of the Aperture Science Sentry Turret. The developer commentary states that originally, they would have had the Sentry Turret built and boxed, then passed into a machine directly below. This machine would unbox and disassemble it, then recycling the parts, meaning that the Sentry Turrets are never actually shipped. They also would have included a massive pile of all the unrecycled packaging material, giving Aperture the most inefficient recycling system possible.
- Some of the Sentry Turret's sounds were not used in the game, but can be found in Portal's Game Cache File, including "Hello" from Portal 2 teaser trailer and "Your business is appreciated," played when flushing the lavatory bowl in the Relaxation Vault.
- In Portal 2, Sentry Turrets will fire at Wheatley's monitors, which is vital to get the Smash TV achievement. One will have to grab them then aim them at the screen.
- Two unusual, non-defective turrets appear in Portal 2. The first is the "Animal King," used as part of a display to demonstrate some kind of non-human force dominating humanity. It also plays the bass tones during the opera at the end of the game. The second is the Soprano Turret, which sings the opera. The Soprano Turret is a common reference in the opera; "It's not over until the fat lady sings", in that the game is not over until the "fat turret" sings.
- The Sentry Turrets are powered by a compact fusion core as seen on the Defective Turrets, since they lack any casing.
- The turret box states that one of the blue turrets was bought on January 25, 2010 by a person with an unreadable name. The reference to Michael Jackson can be seen on the posters.
- In Portal 2, turret boxes indicated that turrets inside are blue. However, the machines in Turret Manufacturing build and box white turrets.
- Despite the fact that the bullets fired from the Aperture Science Turret are propelled using a spring, the guns produce a muzzle flash and a noise; this is unusual considering that there is no gunpowder, or explosive propellant being used. It could be properties re-cycled from Half-Life, however.
- In Half-Life: Alyx, a graffiti image of a turret can be found in game.
List of appearances
- Portal (First appearance)
- ApertureScience.com (Non-canonical appearance) (Not before Christmas 2007)
- Portal: Still Alive (Non-canonical appearance)
- Portal ARG
- PotatoFoolsDay ARG
- Portal 2: Lab Rat
- Portal 2
- The Final Hours of Portal 2
- Half-Life: Alyx (cameo as graffiti)
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