The Rocket Sentry (or Rocket Turret) is an automated, static turret used in the Aperture Science Enrichment Center, firing compact rockets fed through a belt. Apparently used within the facility as a defense / security weapon, it is eventually used by Chell to defeat GLaDOS at the end of Portal.
Rocket Sentry turrets consist of what appears to be a Personality Core mounted atop a robotic arm that retracts in the ground, only leaving the top of the Personality Core visible. When the Core's left panel slides, a rocket launcher and sensory device that emits a blue-green laser for targeting are revealed. In addition, the Personality Core "eye" is used to indicate the current status of the turret by color:
green = idle
yellow = locked on target
red = firing the rocket.
The Rocket Sentry, either by the virtue of being mounted to the floor or having a highly effective armor, is unable to be destroyed by Chell. For instance, if one places a portal behind and in front of a Rocket Sentry and make it fire into the portals, the rocket will fly through the portals until it hits the Rocket Sentry, and vaporize without making any damage, with effects similar to when an object is thrown in a Material Emancipation Grid. Only two Rocket Sentries are found within the Aperture Laboratories. One among abandoned offices and activated by a small red button, and another in GLaDOS's chamber. It is unknown if they are used in Test Chambers or other locations. When activating, the rockets slide down towards the barrel before the laser is set up. When locking on, one rocket is removed from the rack and loaded into the barrel, then fired.