Abstract: The fin lock system overcomes the major problems associated with the current deployment and locking mechanism by adding a center boss to the fin between the rear and front bosses and a sliding lock that engages between the housing and the center boss. The sliding lock, having a very low mass, engages with the center boss, and thus with the fin, very quickly and reliably when the fin reaches its fully deployed position, thereby arresting the motion of the fin and preventing it from rebounding. This, in turn, allows the aft-housing lug to lock the fin in its deployed position with one opening motion of the fin. The additional boss provides greater resistance to inertial and aerodynamic loads while the sliding lock completely eliminates both over-rotation of the fin and inconsistent engagement of the fin lock.