with a amphibious force. They are based upon the landing platform helicopter (LPH) ships, the USN's first class of dedicated helicopter carriers which began entering service in 1961. The basic design was modified to incorporate a bridge and operations superstructure located amidship along with a rear helicopter deck and communication and sensor masts located on the forward weather deck in place of the base vessels' island bridge and full-length flight deck. There are only two vessels in the class, USS Blue Ridge and USS Mount Whitney, and are the command vessels for the 7th and 6th Fleets respectively. While theses vessels have greater command and control capabilities than the class upon which they are based, they are limited by the fact that they lack a well dock and are thus deficient in terms of amphibious operations compared to other classes of AAVs in the USN. They are also limited in size of the rotorcraft they can operate, with the biggest being the CH-47 Chinook, and are not able to operate the CH-53 Sea Stallion nor the MV-22 Osprey. Therefore, away from shore-based facilities, their air- and sea-lift capabilities are limited to the helicopters that they can support They are the oldest commissioned vessels in the USN's fleet, with both vessels undergoing a major modernisation program in 2011 to extend their service lives for at least another 20 years, and Blue Ridge a US$60M propulsion plant overhaul in 2018. However, question marks exist surrounding their remaining scheduled operating lifespan, with the navy facing budget cutbacks and placing a greater emphasis on common capital infrastructure (i.e. weapons systems, power plants, etc.) to reduce cost, and with congressional pressure to reduce the size of its amphibious fleet, with just two class members and little commonality with other classes, they may be candidates for early retirement. San Antonio-class LPD The San Antonio-class LPD is the USN's most numerous AAV with 12 ships currently in service as of December 2022 and with three further vessels under construction and another on order. The class is marketed by its builders, Ingalls Shipbuilding, as a multi-role vessel, and was described by a former Chief of Naval Operations (CNO) as "modular trucks" in reference to its versatility and sealift capacity, who proposed the fitting of modular mission bays as utilised by the navy's littoral combat ships to increase its capabilities. Compared to earlier AAVs such as the Austin class