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
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.