There have been many unusual Soviet submarine modifications, and the modified Yankee class “Big Nose” Project 09780 Akson-2 was one of them!
K-403 began life as a Project 667A “boomer” SSBN Nuclear-powered Ballistic Missile Submarine, which received the NATO designation “Yankee class”. These were the first Russian missile boats with a conventional layout of missile tubes behind the sail – 16 SS-N-6 SLBMs. They were roughly contemporary to the USN “Forty-one for Freedom” classes, and, as Russian boats go, they looked downright normal – very similar to their Polaris-armed adversaries serving in the USN and Royal Navy.
K-403 Kazan was commissioned in 1971, and would be modified several times. The website RussianShips.Info gives a summary of these modifications. During the early 1980s, K-403 was modified to a Project 667AK Akson or NATO “Yankee Pod” configuration, with the missile compartment removed and a towed sonar housing atop the rudder, which looked like what wound up installed on Oscar II class SSGNs. A decade later came the Project 09780 Akson-2 “Big Nose” conversion – a distinctive swollen bow section to house the large Irtysh spherical sonar prototype. At some point, K-403 was fully disarmed, with the bow torpedo tubes also removed.
The Kazan then served as the test-bed for the Irtysh/Amfora sonar system. THe trials must have been successful, as the system is fitted to current Project 885M (NATO Yasen-M) Nuclear Cruise Missile/Attack boats. K-403 was reportedly decommissioned around 2004 and scrapped at Severodvinsk in 2010. K-403 and K-411 (another oddity-a heavily modified, stretched mothership) were the last of the Yankees known to exist.