The orbital maneuvering system provides the thrust for orbit insertion, orbit circularization, orbit transfer, rendezvous, deorbit, abort to orbit and abort once around and can provide up to 1,000 pounds of propellant to the aft reaction control system. The OMS is housed in two independent pods located on each side of the orbiter's aft fuselage. The pods also house the aft RCS and are referred to as the OMS/RCS pods. Each pod contains one OMS engine and the hardware needed to pressurize, store and distribute the propellants to perform the velocity maneuvers. The two pods provide redundancy for the OMS. The vehicle velocity required for orbital adjustments is approximately 2 feet per second for each nautical mile of altitude change.

The ascent profile of a mission determines if one or two OMS thrusting periods are used and the interactions of the RCS. After main engine cutoff, the RCS thrusters in the forward and aft RCS pods are used to provide attitude hold until external tank separation. At ET separation, the RCS provides a minus (negative) Z translation maneuver of about minus 4 feet per second to maneuver the orbiter away from the ET. Upon completion of the translation, the RCS provides orbiter attitude hold until time to maneuver to the OMS-1 thrusting attitude. The targeting data for the OMS-1 thrusting period is selected before launch; however, the target data in the onboard general-purpose computers can be modified by the flight crew via the cathode ray tube keyboard, if necessary, before the OMS thrusting period.

During the first OMS thrusting period, both OMS engines are used to raise the orbiter to a predetermined elliptical orbit. During the thrusting period, vehicle attitude is maintained by gimbaling (swiveling) the OMS engines. The RCS will not normally come into operation during an OMS thrusting period. If, during an OMS thrusting period, the OMS gimbal rate or gimbal limits are exceeded, RCS attitude control is required. If only one OMS engine is used during an OMS thrusting period, RCS roll control is required.

During the OMS-1 thrusting period, the liquid oxygen and liquid hydrogen trapped in the main propulsion system ducts are dumped. The liquid oxygen is dumped out through the space shuttle main engines' combustion chambers and the liquid hydrogen is dumped through the starboard (right) side T-0 umbilical overboard fill and drain. This velocity was precomputed in conjunction with the OMS-1 thrusting period.

Upon completion of the OMS-1 thrusting period, the RCS is used to null any residual velocities, if required. The flight crew uses the rotational hand controller and/or translational hand controller to command the applicable RCS thrusters to null the residual velocities. The RCS then provides attitude hold until time to maneuver to the OMS-2 thrusting attitude.

The second OMS thrusting period using both OMS engines occurs near the apogee of the orbit established by the OMS-1 thrusting period and is used to circularize the predetermined orbit for that mission. The targeting data for the OMS-2 thrusting period is selected before launch; however, the target data in the onboard GPCs can be modified by the flight crew via the CRT keyboard, if necessary, before the OMS thrusting period.

Upon completion of the OMS-2 thrusting period, the RCS is used to null any residual velocities, if required, in the same manner as during OMS-1. The RCS is then used to provide attitude hold and minor translation maneuvers as required for on-orbit operations. The flight crew can select primary or vernier RCS thrusters for attitude control on orbit. Normally, the vernier RCS thrusters are selected for on-orbit attitude hold.

If the ascent profile for a mission uses a single OMS thrusting maneuver, it is referred to as direct insertion. In a direct-insertion ascent profile, the OMS-1 thrusting period after main engine cutoff is eliminated and is replaced with a 5-feet- per-second RCS translation maneuver to facilitate the main propulsion system dump. The RCS provides attitude hold after the translation maneuver. The OMS-2 thrusting period is then used to achieve orbit insertion. The direct-insertion ascent profile allows the MPS to provide more energy to orbit insertion and permits easier use of onboard software.

Additional OMS thrusting periods using both or one OMS engine are performed on orbit according to the mission's requirements to modify the orbit for rendezvous, payload deployment or transfer to another orbit.

The two OMS engines are used to deorbit. Target data for the deorbit maneuver is computed by the ground and loaded in the onboard GPCs via uplink. This data is also voiced to the flight crew for verification of loaded values. After verification of the deorbit data, the flight crew initiates an OMS gimbal test on the CRT keyboard unit.

Before the deorbit thrusting period, the flight crew maneuvers the spacecraft to the desired deorbit thrusting attitude using the rotational hand controller and RCS thrusters. Upon completion of the OMS thrusting period, the RCS is used to null any residual velocities, if required. The spacecraft is then maneuvered to the proper entry interface attitude using the RCS. The remaining propellants aboard the forward RCS are dumped by burning the propellants through the forward RCS thrusters before the entry interface if it is necessary to control the orbiter's center of gravity.

The aft RCS plus X jets can be used to complete any planned OMS thrusting period in the event of an OMS engine failure. In this case, the OMS-to-aft-RCS interconnect would feed OMS propellants to the aft RCS.

From entry interface at 400,000 feet, the orbiter is controlled in roll, pitch and yaw with the aft RCS thrusters. The orbiter's ailerons become effective at a dynamic pressure of 10 pounds per square foot, and the aft RCS roll jets are deactivated. At a dynamic pressure of 20 pounds per square foot, the orbiter's elevons become effective, and the aft RCS pitch jets are deactivated. The rudder is activated at Mach 3.5, and the aft RCS yaw jets are deactivated at Mach 1 and approximately 45,000 feet.

The OMS in each pod consists of a high-pressure gaseous helium storage tank, helium isolation valves, dual pressure regulation systems, vapor isolation valves for only the oxidizer regulated helium pressure path, quad check valves, a fuel tank, an oxidizer tank, a propellant distribution system consisting of tank isolation valves, crossfeed valves, and an OMS engine. Each OMS engine also has a gaseous nitrogen storage tank, gaseous nitrogen pressure isolation valve, gaseous nitrogen accumulator, bipropellant solenoid control valves and actuators that control bipropellant ball valves, and purge valves.

In each of the OMS pods, gaseous helium pressure is supplied to helium isolation valves and dual pressure regulators, which supply regulated helium pressure to the fuel and oxidizer tanks. The fuel is monomethyl hydrazine and the oxidizer is nitrogen tetroxide. The propellants are Earth-storable liquids at normal temperatures. They are pressure-fed to the propellant distribution system through tank isolation valves to the OMS engines. The OMS engine propellant ball valves are positioned by the gaseous nitrogen system and control the flow of propellants into the engine. The fuel is directed first through the engine combustion chamber walls and provides regenerative cooling of the chamber walls; it then flows into the engine injector. The oxidizer goes directly to the engine injector. The propellants are sprayed into the combustion chamber, where they atomize and ignite upon contact with each other (hypergolic), producing a hot gas and, thus, thrust.

The gaseous nitrogen system is also used after the OMS engines are shut down to purge residual fuel from the injector and combustion chamber, permitting safe restarting of the engines. The nozzle extension of each OMS engine is radiation-cooled and is constructed of columbium alloy.

Each OMS engine produces 6,000 pounds of thrust. The oxidizer-to-fuel ratio is 1.65-to-1. The expansion ratio of the nozzle exit to the throat is 55-to-1. The chamber pressure of the engine is 125 psia. The dry weight of each engine is 260 pounds.

Each OMS engine can be reused for 100 missions and is capable of 1,000 starts and 15 hours of cumulative firing. The minimum duration of an OMS engine firing is two seconds. The OMS may be utilized to provide thrust above 70,000 feet. For vehicle velocity changes of between 3 and 6 feet per second, normally only one OMS engine is used.

Each engine has two electromechanical gimbal actuators, which control the OMS engine thrust direction in pitch and yaw (thrust vector control). The OMS engines can be used singularly by directing the thrust vector through the orbiter center of gravity or together by directing the thrust vector of each engine parallel to the other. During a two-OMS-engine thrusting period, the RCS will come into operation only if the OMS gimbal rate or gimbal limits are exceeded and should not normally come into operation during the OMS thrust period. However, during a one-OMS-engine thrusting period, roll RCS control is required. The pitch and yaw actuators are identical except for the stroke length and contain redundant electrical channels (active and standby), which couple to a common mechanical drive assembly.

The OMS/RCS pods are designed to be reused for up to 100 missions with only minor repair, refurbishment and maintenance. The pods are removable to facilitate orbiter turnaround, if required.