The Orbital Maneuvering System, or OMS, is a system of rocket engines used on the Space Shuttle for orbital injection and modifying its orbit. It consists of two "packs" at the back of the Shuttle, the large lumps on either side of the vertical stabilizer. Each pack contains a single 6,000 pounds thrust engine burning hydrazine, which can be reused for 100 missions and is capable of 1,000 starts and 15 hours of firing. The OMS pods also contain the rear set of RCS engines as well, which are referred to as the OMS/RCS.
Orbital maneuvering system can be used to describe any system for moving about in orbit, so the term is found in non-Shuttle related topics as well.
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