S1.5400

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The S1.5400 was a Soviet rocket engine designed by V. M. Melnikov for the Molniya fourth-stage Block-L. It was the world’s first engine to use an oxidizer-rich staged combustion cycle, burning liquid oxygen and kerosene. It could start and restart in vacuum and had the highest specific impulse (efficiency) at the time it was used. Development ran from 1958 to 1960, with production beginning in May 1960. The first flight failed before the engine fired, but a later Venera mission in 1961 succeeded.

From 1961 to 1964, it was improved to the S1.5400A1 (11D33M). Thrust rose to 66.69 kN and specific impulse to 340 s, while the engine mass stayed the same. The combustion chamber used titanium alloy to withstand up to 700 °C. Turbopump spin-up was pyrotechnic, and the engine was mounted on a Cardan suspension that allowed about 3° of gimbal movement in two axes.

Used on the Molniya upper stage (Block-L), the S1.5400 is now retired. Its dry mass was about 153 kg, and it delivered up to 207 seconds of burn time. The last flight date was 2010-09-30.