S5.4

{{Short description|Russian liquid rocket engine}}

{{Infobox rocket engine

|name = S5.4

|image =Vostok spacecraft replica.jpg

|image_size =220px

|caption =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_date =1959

|last_date =

|designer =OKB-2, A.M. Isaev

|manufacturer =

|purpose =Spacecraft braking engine

|associated =

|predecessor =

|successor =S5.35

|status =Retired

|type =liquid

|fuel =TG-02

|oxidiser =AK20F

|mixture_ratio =3.07

|cycle =Gas generator

|pumps =

|description =

|combustion_chamber=1 main + 4 vernier

|nozzle_ratio =

|thrust =

|thrust_at_altitude=

|thrust(Vac) ={{convert|15.83|kN|lbf}}

|thrust(SL) =

|thrust_to_weight=

|chamber_pressure={{convert|5.6|MPa|psi}}

|specific_impulse=

|specific_impulse_vacuum=266 seconds

|specific_impulse_sea_level=

|total_impulse =

|burn_time =45 seconds

|capacity ={{convert|250|kg|lb}}

|dimensions =

|length ={{convert|1.13|m|in}}

|diameter ={{convert|0.95|m|in}}

|dry_weight ={{convert|98|kg|lb}}

|used_in =Vostok, Voskhod and Zenit

|references =

|notes =

}}

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 and Voskhod crewed spacecraft, and Zenit satellites, which later switched to solid motors.{{Cite journal |last=Okninski |first=Adam |date=2022 |title=Solid rocket propulsion technology for de-orbiting spacecraft |url=https://linkinghub.elsevier.com/retrieve/pii/S1000936121003319 |journal=Chinese Journal of Aeronautics |volume=35 |issue=3 |pages=128–154 |doi=10.1016/j.cja.2021.07.038 |doi-access=free}}

The engine produced {{convert|15.83|kN|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.

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|date=May 2016}} It was considered critical, as a failure would have left a cosmonaut stranded in space. A solid motor was considered, but the ballistic experts predicted a {{convert|500|km|nmi|adj=on|sp=us|abbr=off}} 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.