Sierra-class submarine
The Sierra class, Soviet designations Project 945 "Barrakuda and Project 945A "Kondor, (NATO reporting names Sierra I and Sierra II respectively), are series of nuclear-powered attack submarines intended for the Soviet Navy and currently in service with the Russian Navy. The class is unique due to its light and strong titanium pressure hull which enables the submarines of the class to dive to greater depths, reduce the level of radiated noise and increase resistance to torpedo attacks. It is powered by a single OK-650 pressurized water reactor. The upgraded version, the Sierra II class was specifically  developed for search and destroy missions against US nuclear submarines. It has a smaller turning circle than any other modern submarine, with speeds and diving depth greater than its American counterparts. It has also improved quieting and sonar. Versions. Project 945 "Barrakuda" (Sierra I). The first submarine of the Project 945, "Carp", was laid down in July 1979 at the Gorky shipyard and was launched in August 1983 before being transferred to Severodvinsk for fitting out. It was laid up in 1997. The next hull to be built was "Kostroma", which was launched in July 1986 and was commissioned in  September 1987. "K-276 Kostroma" was put into a drydock after its 11 February 1992 collision with the US submarine in the Barents Sea, off Kildin Island. The submarine was repaired on 3 June 1992 and was renamed "Krab" on 6 April 1993, but in 1996 its original name, "Kostroma" was restored and it is in active service with the Russian Northern Fleet. The Sierra I class was also fitted with a releasable escape pod for the crew. The pod is covered by a V-shaped casing on the port side of the sail. Project 945A "Kondor" (Sierra II). The Project  945A has a considerably larger sail which is longer than the Sierra I class. The sail also has a curious flat, square leading edge. The masts are offset on the starboard side to make way for two escape pods in the sail. The starboard side also has a 10-point environment sensor fitted at right angles to the front end of the sail. Also, the Sierra II class has a much larger pod on its after fin. The pod houses the Skat 3 passive very low frequency towed sonar array. In October 2012, a submarine of the Sierra II class  was deployed to the North Atlantic and carried out a patrol off the US East coast. Out of the two existing submarines of this class, the "Pskov" was in overhaul between 2011-2015 according to the Russian website Deep storm, thus it's likely that the submarine present in the North Atlantic in 2012 was the other one ("Nizhniy Novgorod"). The aforementioned website acknowledges activity of "Nizhniy Novgorod" that period under the command of Captain 1st Rank Alexey Ananko (submarine was reportedly active in 2008 and 2013) Both submarines took part in large exercise in October 2019. Project 945AB (Sierra  III). The sole possible submarine of the Project 945AB was laid down in March 1990 but was scrapped in November 1993 before completion. Modernization. In January 2013 a contract for refit and recommission of the submarines "Carp" and "Kostroma" of Project 945 was signed with the Zvezdochka Shipyard, Severodvinsk. The necessary upgrade work was expected to take three years. It was expected that the submarines would be transferred to Zvezdochka before end of April 2013 and the overhaul of the submarines would begin in summer 2013. Zvezdochka would carry out the refit, repair mechanical parts, and replace nuclear fuel and  all electrical equipment of the submarines. The submarines were also expected to receive a new sonar, combat information management system (BIUS), the GLONASS navigation system and new armament consisting of the Kalibr cruise missiles. In March 2015 it was reported that the final decision on the modernization of submarines "Carp" and "Kostroma" was yet to be made due to cost issues.