At the end of the Second World War, the Soviets captured components for the HTP-fuelled Type XXVI submarine as well as engineers and technicians who had worked on the Walter turbine that used HTP. To make use of this new technology, they established a new submarine design bureau in Germany headed by Engineer-Captain Aleksei A. Antipin, formerly head of Central Design Bureau No. 18 (TsKB-18), which was tasked with the collection of plans and documents relating to the Type XXVI and its components. TsKB-18 began designing a copy of the German submarine as Project 616 in 1946, intending to order components from East Germany, but this was rejected by the Soviet Navy as it did not meet Soviet standards for reserve buoyancy and other issues. Together with the Central Research Shipbuilding Institute No. 45, the bureau immediately then began an original design utilizing the Walter HTP turbine. Preliminary design work on Project 617 was finished at the end of 1947.[1]
A new design bureau for high-speed submarines (SKB-143) was established on 30 March 1948 and the Antipin Bureau was folded into the new organization with Antipin in charge. Knowing that the Walter HTP turbine technology was unproven, he decided to construct the submarine's propulsion plant and its hull compartment in a building at the Sudomekh Shipyard in Leningrad. The turbine used original German-built components as much as possible and was tested through the beginning of 1951. It was carefully disassembled in May and repaired as necessary in preparation for installation in S-99 which had begun construction a few months prior.[2]
S-99 was a double-hulled submarine that displaced 950t on the surface and 1500t submerged. The boat had an overall length of 62.2m (204.1feet), a beam of 6.08m (19.95feet) and a draft of 5.08m (16.67feet). S-99s crew numbered 51 officers and men. Her hull was divided into six compartments; the sail was very small and did not have a compartment. The hull was fitted with anechoic tiles captured from the Germans. The boat had a reserve buoyancy of 28%, enough to remain afloat with any single compartment flooded, a figure more than two and a half times greater than that of the Type XXVI. S-99 had a test depth of 170m (560feet) and a design depth of 200m (700feet). In service, she proved to be maneuverable and was very noisy at full speed.[3]
The submarine was primarily powered by a Walter HTP turbine that drove a single propeller shaft using steam generated when highly concentrated hydrogen peroxide was sprayed onto a layer of activated charcoal to produce high-pressure steam and oxygen at a temperature of . This passed into a combustion chamber into which kerosene was injected. The resulting combustion converted the oxygen into carbon dioxide and carbon monoxide and further increased the mixture's temperature and pressure. Water was then added to the mixture to reduce the temperature down to about, double the volume of the steam and convert the supersaturated steam into saturated steam better suited for powering a geared steam turbine. The steam/gas mixture was passed through a condenser to recover the water while the combustion gases were exhausted into the ocean. S-99s turbine could generate 7250PS in shallow water, but only at depths of 30– because the pressure of the deeper water created more back pressure which reduced the turbine's efficiency. The turbine gave the boat a submerged speed in excess of 20kn.[4]
S-99 was also provided with a diesel-electric system that consisted of a 8Ch-23/30 diesel engine and a PG-100 electric motor for cruising. An additional electric motor was intended for slow speeds underwater and could be powered by either the 112 Type 26-SU battery cells or a DG-17 diesel generator. The boat was fitted with a folding snorkel to allow the diesel engines to operate while underwater.[5]
The submarine's maximum speed on the surface was and she had a range of 120nmi at maximum speed on the Walter turbine and at . Using her diesel-electric system on the surface gave S-99 a range of at ; using her snorkel reduced it to at . Using just the electric motor underwater, the submarine had a range of at . She had internal fuel tanks for of diesel fuel and of kerosene. Plastic bladders between the pressure hull and the outer hull held of very expensive hydrogen peroxide. S-99 carried enough supplies for 45 days of operation.[6] [7]
The boat's armament consisted of six 533sp=us0sp=us 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[5] (NATO reporting name: Snoop Plate).[8]
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,[6] but Friedman says she was commissioned on 6 June 1952[9] 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.[10]
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.[6] [10]
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 80mm 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.[6] [11] [12]