SUBMARINE (Project 615-Quebec class)

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The Soviet navy's small patrol submarine class boasted a distinctive propulsion system: the REDO engine. Originally, the USSR had been developing a Kreislauf-type system since the mid-1940s, but this accelerated after acquiring German technology in 1945. The REDO engine was not strictly air-independent; it required external air for starting. Subsequent CO2 removal from exhaust and recycling residual oxygen back into the engine were its remarkable features. However, Soviet sailors humorously called these submarines "the cigarette lighters" due to the hazards of handling volatile liquid oxygen around hot exhaust pipes and live electrical cables.