The S-99 (Russian: С-99) experimental submarine was the only ship of the Project 617 class (codenamed Whale) that the Soviet Union built during the early Cold War.

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.

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 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.

[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 963 °C (1,765 °F).

S-99's turbine could generate 7,250 metric horsepower (5,330 kW) in shallow water, but only 6,050 PS (4,450 kW) at depths of 30–40 meters (98–131 ft) because the pressure of the deeper water created more back pressure which reduced the turbine's efficiency.

[5] The submarine's maximum speed on the surface was 11 knots (20 km/h; 13 mph) and she had a range of 120 nautical miles (220 km; 140 mi) at maximum speed on the Walter turbine and 198 nmi (367 km; 228 mi) at 14.2 knots (26.3 km/h; 16.3 mph).

Using just the electric motor underwater, the submarine had a range of 13.9 nmi (25.7 km; 16.0 mi) at 9.3 knots (17.2 km/h; 10.7 mph).

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).

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 resulting decomposition blew a 80-millimetre (3.1 in) hole in the pressure hull which caused the two rear compartments to partially flood.