The S1.5400 was a Soviet single-nozzle liquid-propellant rocket engine burning liquid oxygen and kerosene in an oxidizer-rich staged combustion cycle, the first rocket engine to use this cycle in the world. It was designed by V. M. Melnikov, an alumnus of Isaev, within Korolev's Bureau, for the Molniya fourth stage, the Block-L. It was the first Soviet engine designed for start and restart in vacuum, and had the highest Isp at the time of its deployment.