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]}

S5.4

Vostok spacecraft replica at the Technik Museum Speyer, Germany. The lower conical section was the service module with the S5.4/TDU-1 engine.
Country of origin	USSR
Date	1959-1961
First flight	1959
Designer	OKB-2, A.M. Isaev
Application	Spacecraft breaking engine
Successor	S5.35
Status	Retired
Liquid-fuel engine
Propellant	AK20F / TG-02
Mixture ratio	3.07
Cycle	Gas Generator
Configuration
Chamber	1 main + 4 vernier
Performance
Thrust, vacuum	15.83 kilonewtons (3,560 lbf)
Chamber pressure	5.6 megapascals (810 psi)
Specific impulse, vacuum	266 seconds
Burn time	45 seconds
Propellant capacity	250 kilograms (550 lb)
Dimensions
Length	1.13 metres (44 in)
Diameter	0.95 metres (37 in)
Dry mass	98 kilograms (216 lb)
Used in
Vostok, Voskhod and Zenit
References
References	[1][2][3]

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 crewed 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]