The S-99 (Russian: С-99 ) experimental submarine was the only ship of the Soviet Project 617 submarine class (NATO reporting name: Whale class) that the Soviet Union built during the early Cold War and the only Soviet submarine which had a Walter engine fuelled by high test peroxide (HTP).
The Soviet Union used a Walter engine fuelled by high test peroxide (HTP) only once - for the experimental submarine S-99 (Russian: С-99).