of operation. The boat's armament consisted of six torpedo tubes mounted in the bow, each with one reload. Alternatively, 20 AMD-1000 naval mines could be carried. S-99 was fitted with a Tamir-5LS sonar, Mars-24-KEG hydrophones and a Nakat surface-search radar (NATO reporting name: Snoop Plate). Construction and career S-99 was laid down on 5 February 1951 at the Shipyard No. 196 and launched on 5 February 1952. Naval historians vary on when the boat was commissioned or completed: Pavlov says she was completed in December 1955, but Friedman says she was commissioned on 6 June 1952 while Polmar and Moore give 26 March 1956. The boat's sea trials began on 16 June 1952 and lasted for years. Less than a year later, SKB-143 was pulled from Project 617 in March 1953 and tasked with developing a nuclear-powered submarine; TsKB-18 assumed responsibility for the project. Despite the testing of the Walter turbine ashore, the sea trials revealed that S-99 still had problems when HTP would decompose on contact with dirt or oil, causing fires or explosions. During this time, TsKB-18 worked on Project 617M, an enlarged design with additional HTP and fuel as well as improved weapons and sensors. The bureau also considered two variants that utilized closed-cycle diesel engines, Project 635, a (surface displacement), twin-turbine design and an even larger boat, Project 643. Work on all of these was canceled in 1960 when all of the closed-cycle projects were terminated. Upon commissioning, S-99 was assigned to a training brigade of the Baltic Fleet. Between 1956 and 1959, she went to sea 98 times, during which the boat cruised more than on the surface and about submerged. About of the latter used the Walter turbine. On 17 May 1959 S-99 was cruising at a depth of when the turbine was started. This caused an explosion in the turbine compartment when HTP encountered mud in the hull valve of the HTP supply pipe. The resulting decomposition blew a hole in the pressure hull which caused the two rear compartments to partially flood. The submarine was able to surface and reached base on battery power. S-99 was not repaired because it was unfeasible to replace all of the damaged German parts. The submarine was not fully repaired and was reclassified as an experimental submarine on 31 August 1961. She was decommissioned on 28 February 1964 and later scrapped. See also German Type XVII