Usually located in the uninhabited area of the spacecraft, the service module serves a storehouse of critical subsystems and supplies for the mission such as electrical systems, environmental control, and propellant tanks.
In addition to the film cameras, similar to those used on the Lunar Orbiter spacecraft and requiring the Command Module Pilot to perform a deep-space EVA during the return trip, two of the SIM bays, on Apollos 15 and 16, also launched a lunar "subsatellite" before the astronauts performed the Trans-Earth Injection burn with the onboard service propulsion system.
[citation needed] A unique inhabitable variation of the service module concept is the Functional Cargo Block developed for the Soviet TKS Transport Supply Spacecraft.
In addition to full functionality of a service module, it featured a sizeable pressurized cargo bay, and a docking port – as opposed to its conventional location on the front of the re-entry capsule, which in case of the TKS instead possessed its own downscaled service module with de-orbiting thrusters – allowing the FGB to remain docked as an extension of the space station.
It provides the water and oxygen needed for a habitable environment, generates and stores electrical power, and maintains the temperature of the vehicle's systems and components.