The AH-1Z Viper is an upgraded twin-engine attack helicopter developed for the US Marine Corps based on the AH-1W SuperCobra. Key upgrades include a new bearingless four-blade composite main rotor system, more powerful engines, upgraded avionics, sensors, and weapons. South Korea had expressed interest in purchasing 36 AH-1Z helicopters for $2.6 billion but ultimately selected the AH-64E Apache instead. The AH-1Z can carry various rockets, missiles, and a 20mm cannon, and has capacity for over 6,600 pounds with a maximum take-off weight of 18,500 pounds.