Service Module - Design

Design

Depending upon the spacecraft architecture and system design, a typical service module will usually contain the following:

• Fuel cells, solar panels, or batteries to provide electrical power to the spacecraft (batteries are also used in the crew capsule)
• Liquid hydrogen (LH₂) and liquid oxygen (LOX) for fuel cell operation and water production, with LOX also being used to provide breathing oxygen for the crew.
• Pressurized helium or nitrogen to force consumables and fuel from source tanks to their destinations.
• Guidance computer systems and related sensors
• Fuel and oxidizer for reaction control and propulsion systems.
• Thermal control systems for proper heating and cooling of above systems.

While this would be used for a "baseline" service module, a service module may also be modified for additional functions. An example would be the equipment module on Gemini IX-A, when it was modified to carry the U.S. Air Force-developed Astronaut Maneuvering Unit that would have been tested by astronaut Eugene Cernan, but was cancelled when his spacesuit overheated, causing his visor to fog up. But the best example would be the final three Apollo missions, in which the J-series service modules included scientific instrument module (SIM) bays that took pictures and other readouts in lunar orbit. 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.