seven AH-1Z/UH-1Y attack/utility helicopters, and two Navy MH-60S Knighthawks for air-sea rescue. The exact make-up of the ship's aircraft complements may vary according to their missions. They can carry about 20 AV-8Bs or F-35Bs, and 2 MH-60Ss, to serve as a small aircraft carrier as demonstrated by landing helicopter dock (LHD) operations in Operation Iraqi Freedom. The U.S. Marine Corps is now more concerned about anti-ship missile attacks from fast attack craft and long-range precision fires from land. To counter such attacks the Marine Corps wants to keep amphibious ships farther offshore. For this Marines will be transported ashore in larger and longer range MV-22 V/STOL aircraft. To accommodate these requirements, America has twice the displacement of the retired s. The America-class amphibious assault ships are engineered with a (CODLOG) hybrid-electric propulsion system derived from the one used on Makin Island. The ships can use gas-turbines for high speeds and the diesel-electric engines when required. Setting the beam of America at was dictated by the need for these ships to pass through the Panama Canal. The Congressional Budget Office found that if before 2040 the price of oil reached and remained above $140 per barrel then the use of nuclear propulsion for LHX-class ships would be more cost-effective. The LHX or LH(X) was a warship that was proposed in the late 1990s to replace the s, but with a dry deck for hovercraft rather than a floodable "well deck". After 2000, the LHX, the "Amphibious Assault Ship Future Replacement", was put forward to replace all of the LHDs. The new LHX could be a Flight 2 design of the America class built with a well deck and a smaller island superstructure, which would give it 20 percent more capacity on the flight deck. This would remove the current restriction on MV-22s to land on spots 5 and 6, and also giving room for four MV-22B, three F-35B Lightning IIs, or three CH-53Ks to use the flight deck. In 2008, the procurement of Flight 2 ships was tentatively planned for 2024, but that might not be practical or affordable by then. A modified version of the design of America, designated the MPF(F), LHA(R), or T-LHA(R), was proposed for two ships of the Maritime Prepositioning Force (Future). The MPF(F) is the Navy's concept for a "sea base" to support operations ashore starting in about 2025. These two ships would hypothetically be manned