sentries are only placed in certain places like backwater stations or off the line installations or cargo depots but never front line protection that falls to the meat in the grinders, the fighters and other ships.
however if you want a shielded sentry...
$Name: GTSG Alastor
$Short name: TSGun3
$Species: Terran
+Tech Description:
XSTR("GTSG Alastor sentry gun platforms can be dropped by warships to cover a retreat, or they can be used to strengthen firepower around strategic locations. While Alastors alone cannot hold a position against more than a few fighters, complementing them with a warship and several wings of fighters presents an exemplary defense against all but the most determined foes. Two Bessmer-Kohn cannon turrets on opposing sides of the platform give the Alastor a full field of fire. The turrets are controlled by the latest neural-net cyber-AI to track multiple targets simultaneously.", 3066)
$end_multi_text
$POF file: Platform2T-01.pof
$Detail distance: (0, 250, 500)
$Show damage: NO
$Density: 1
$Damp: 0.2
$Rotdamp: 0.2
$Max Velocity: 0.0, 0.0, 0.0
$Rotation time: 20.0, 20.0, 20.0
$Rear Velocity: 0.0
$Forward accel: 0.0
$Forward decel: 0.0
$Slide accel: 0.0
$Slide decel: 0.0
$Expl inner rad: 0.0
$Expl outer rad: 0.0
$Expl damage: 0.0
$Expl blast: 0.0
$Expl Propagates: NO ;; If set to Yes, then when the ship dies, the explosion propagates through it.
$Shockwave Speed: 0.0 ;; speed shockwave expands at, 0 means no shockwave
$Default PBanks: ()
$Default SBanks: ()
$SBank Capacity: ()
$Shields: 300 ***
$Power Output: 0.0
$Max Oclk Speed: 0.0
$Max Weapon Eng: 0.0
$Hitpoints: 250
$Flags: ("sentrygun" "in tech database")
$AI Class: Captain
$Afterburner: NO
$Countermeasures: 0
$Scan time: 2000
$EngineSnd: -1 ;; Engine sound of ship
$Closeup_pos: 0.0, 0.5, -14
$Closeup_zoom: 0.85
$Score: 9
$Subsystem: turret01a, 20, 1.0
$Default PBanks: ( "Subach HL-7" )
$Subsystem: turret02a, 20, 1.0
$Default PBanks: ( "Subach HL-7" )
