Service module

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A service module is a spacecraft compartment containing a variety of support systems used for spacecraft operations, but not any habitable area. Service modules are jettisoned when they are no longer needed, for example just before atmospheric re-entry.

The Russian phrase for service module for the Soyuz spacecraft is sometimes more directly translated "Instrument-Assembly Compartment." This comes from the design feature of having the guidance and other computer systems in a separate pressure chamber (the instruments) from the rocket engines, their propellant tanks, and the life support tanks (from the German Aggregat, which gets translated "assembly"). The Russians do not use the word "module" for their own spacecraft, though it is in the Russian language.

Service modules often contain:

Fuel cells or batteries to provide power to the spacecraft. (Batteries are also in the part of the spacecraft that re-enters, to provide power during re-entry.)
Solar cell arrays both to provide electric power directly and to recharge the batteries.
Liquid oxygen (LOX) for fuel cells and breathing.
Liquid hydrogen (LH2) for fuel cells.
Helium or Nitrogen to force liquids to their destinations.
Guidance sensors
Guidance computer systems
Propulsion engine and fuel
Reaction control system engines
Temperature-control system