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In a normal airplane, hydraulics or even cable-pulled systems are used to power the control surfaces. In a missile, that seems infeasible. What device is used to move the control surfaces, and where does it get its power?

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Battery powered servo actuators.

From www.acc.af.mil, for the AIM-120 Scorpion:

The control section consists of control electronics, actuator batteries, and four independently controlled servo-actuators.

Further on the actual AIM-120 control mechanism: (Courtesy of the Turkish AF http://web.deu.edu.tr/atiksu/ana44/air3.html)

The Weapons Control Unit (WCU)-11/B consists of four independently controlled electro-mechanical servo actuators, four lithium-aluminum batteries connected in parallel, and a steel fuselage section that is bolted to the propulsion section aft skirt. Each actuator consists of a brushless DC motor ballscrew, an infinite resolution potentiometer directly coupled to the output shaft, and pulse width modulated control electronics. The output shaft is engaged directly to a squib actuated lock so that it does not interfere with the fin (control surface) installation and removal.

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    $\begingroup$ Thanks but can you explain what exactly is a servo-actuator? E.g., a DC motor powering a worm gear? $\endgroup$ – DrZ214 Jan 21 at 5:50
  • $\begingroup$ @DrZ214 - Info added. $\endgroup$ – WPNSGuy Jan 21 at 11:23
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    $\begingroup$ What is a squib actuated lock, please? Sounds like an explosive bolt sort of arrangement that... locks when fired, or unlocks when fired? Does the lock keep the shaft (or fin) from moving until fired, or connect the actuator to the fin when fired? $\endgroup$ – Technophile Jan 21 at 16:16
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    $\begingroup$ @Technophile - I believe in this case it is a spring loaded catch mechanism. AMRAAM are often loaded on the jet with only some (or none) of the fins installed. They are stupidly sharp, and right at head height. Bumping into one can easily draw a little bit of skull blood. To install, you push the fin on until the catch grabs. $\endgroup$ – WPNSGuy Jan 21 at 16:55
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    $\begingroup$ @WPNSGuy thanks! That covers fin installation but not the "squib actuated lock". Found material at globalsecurity.org/military/systems/munitions/…. Guessing the lock is released by firing the squib, and holds the fins lined up straight until it's time for the servos to take control. $\endgroup$ – Technophile Jan 21 at 23:31

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