The Orbital Maneuvering System (OMS, pronounced like ohms) is a system of reaction jets on board the Space Shuttle orbiters. It is designed to provide thrust for orbit insertion, orbit transfer, orbit circularization, rendezvous and de-orbit burns. It consists of two fuel storage pods (the characteristic 'bumps' beneath the tail of the orbiter) and two main engines just outboard of the top SSME.

The OMS uses hypergolics for operation. The brew of choice is monomethyl hydrazine fuel and nitrogen tetroxide as an oxidizer. These two substances react in engine thrust reaction chambers before being ejected through the main engine bells and thrust vectoring nozzles at the rear of the orbiter. The total fuel capacity of the system is enough to produce a total delta-v of approximately 1,000 feet per second with a 65,000 lb. payload in the orbiter's cargo bay, at a specific impulse (measured in vacuum) of approximately 313 seconds. Operating thrust is 6,000 lbs. per engine.

There are pressurized helium tanks in the system as well to provide fuel and oxidizer tank pressures of around 250 p.s.i. during flight, as well as pressurized nitrogen to purge the OMS engines after burn. The OMS shares fuel types with the Reaction Control System (RCS), and there are several cross-feed valves through which fuel and oxidizer can be exchanged between systems. The system is plumbed to allow either tank set (one per pod) to fuel both engines.

Fuel is first fed through 102 cooling channels in the walls of the engine reaction chamber and bell before flowing to its injector. This allows the engine to regeneratively cool itself, as well as increases the fuel temperature and pressure during operation. The engine nozzle itself is constructed of Columbium (a.k.a. Niobium) alloy and is bolted to the aft end of the reaction chamber. The oxidizer flows directly to its injector in the engine. During operation, the engine consumes 11.93 pounds per second of oxidizer and 7.23 pounds per second of fuel.

The engines are operated (one or both) through the maneuvering control systems of the orbiter; the crew has the option of using either or both, and of inhibiting the post-burn nitrogen purge (if, for example, they foresee immediate re-use of the system, as in a rendezvous manuever).

During intact aborts or contingency aborts, the OMS will operate in various modes, at times in conjunction with the RCS. During an intact Abort Once Around or Abort To Orbit maneuver where one main engine has failed, the OMS and all 24 aft Reaction Control System jets will fire both to provide added thrust as well as to burn OMS/RCS propellant and lighten the Shuttle. If the Shuttle enters a Return to Launch Site or Trans-Atlantic Abort Landing mode, the abort sequencing software in the OMS controllers will begin to burn the OMS engines as well as produce a null-thrust RCS burn using eight of the aft jets. It does this by firing oppositely placed RCS jets on the rear of the orbiter. These will both serve to dump OMS fuel and oxidizer, both of which are extremely hazardous materials and would interfere with safety and crew retrieval operations.

Sources: NASA websites, The Space Shuttle Operator's Manual, Discovery Channel documentaries

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