Standard Missile - 3 (SM-3) is being developed and produced as part of the U.S. Navy's sea-based ballistic missile defense system, providing theater-wide defense against medium and long range ballistic missiles. The Aegis BMD/SM-3 system is a keystone in the Phased Adaptive Approach (PAA) for missile defense in Europe. The SM-3 destroys incoming ballistic missile threats by colliding with them, a concept sometimes described as "hit-to-kill." The SM-3 missiles use the Aerojet Rocketdyne produced Mk 72 first stage propulsion system. The Mk 72 incorporates a moveable nozzle thrust vector control (TVC) system. The Aerojet Rocketdyne Mk 104 DTRM provides second stage propulsion for the SM-3 missile.