OMS

The Orbital Maneuvering System is a pair of rocket engines mounted in pods on the aft fuselage of the Space Shuttle orbiter. Each engine produces approximately 6,000 pounds of thrust and burns monomethylhydrazine and nitrogen tetroxide propellant stored in dedicated tanks. The OMS is distinct from the main engines, which fire only during launch; the OMS performs all significant orbital maneuvers after the orbiter reaches space.

An OMS burn typically lasts between 30 seconds and several minutes, depending on the velocity change required. The system can execute orbital circularization at apogee, plane changes, rendezvous with target spacecraft, and deorbit burns to initiate reentry. Because the orbiter carries a fixed propellant load, mission planners must calculate all maneuvers in advance and verify that fuel reserves meet minimum safety margins.

Each OMS pod contains one engine, with both engines piped to a common avionics control unit. The engines can fire together or independently, allowing attitude control during longer burns. Unlike the main engines, the OMS engines are hypergolic, igniting on contact between fuel and oxidizer with no ignition system required. This makes them highly reliable for repeated firings, which the main engines cannot support.

Maintenance crews inspect OMS plumbing, injectors, and combustion chambers for erosion and corrosion after each flight. Propellant tanks require periodic inspection for internal degradation. The system includes isolation valves and check valves that must function precisely to prevent backflow or crossfeed between tanks. Any sign of leakage in the hypergolic system triggers mandatory repair before the next launch.

The OMS is heritage technology from the Shuttle development era in the 1970s, and the design matured through decades of flight data. It remains operational on remaining orbiter frames, though no new orbiters have been built since 1991. The terminology reflects NASA's distinction between launch propulsion, main engines, and in-orbit maneuvering, a classification that influenced the design of subsequent spacecraft.