The S5.4 (AKA TDU-1, GRAU Index 8D66), was a Russian liquid rocket engine burning TG-02 and AK20F in the gas generator cycle. It was originally used as the braking (deorbit) engine of the Vostok, Voskhod, and Zenit spacecraft, which later switched to solid engines.^{[citation needed]}

The engine produced 15.83 kilonewtons (3,560 lbf) of thrust with a specific impulse of 266 seconds in vacuum, and burned for 45 seconds, enough for the deorbit. It had a main fixed combustion chamber and four small verniers to supply vector control. It was housed in the service module and had two toroidal tanks for pressurization.^[4][5][6]

It was designed by OKB-2, the Design Bureau led by Aleksei Isaev, for the Vostok program. The braking engine for the first manned spacecraft was a difficult task that no design bureau wanted to take.^{[citation needed]} It was considered critical, as a failure would have left a cosmonaut stranded in space. A solid engine was considered, but the ballistic experts predicted a 500-kilometer (270-nautical-mile) landing error, versus a tenth of that for a liquid engine. It took the coordinated efforts of Boris Chertok and Sergei Korolev to convince Isaev to accept the task.^[7]