 Sikorsky proposed a new version, originally designated "CH-53X"; in April 2006, the USMC signed a contract for 156 aircraft as the "CH-53K" valued at $18.8 billion with deliveries to be completed by 2021.The USMC planned to start retiring CH-53Es in 2009, thus needing replacements quickly as some rotorcraft reached their structural life limits in 2011–12.Flight testing of the CH-53K was expected to begin in 2011.