10. Mercury E-6

Joined the Service in 1989
Producing company: Boeing
Price: $141.7 million
The E-6 Mercury, which entered service in 1989, continues to be a cornerstone of American airborne command and control. This Boeing-built aircraft serves as a crucial conduit for communication between the leadership of the United States and its strategic nuclear forces, ensuring the availability of U.S. armed troops.

Mercury ©E-6 Greg L. Davis and Wikipedia
The Mercury, a converted commercial 707 aircraft, is primarily used to transmit commands to the submarine fleet in order to maintain an efficient nuclear deterrent. It has advanced communication technology and is built to withstand electromagnetic pulses, ensuring that command will continue even in the case of a nuclear accident. The E-6 Mercury is still a vital tool for efforts related to national security today.