Orbital
Maneuvering System
The Orbital
Maneuvering System (OMS) is basically an hypergolic propellant powered engines
that are used by the shuttle during three phases: The ascent, the orbit and the
deorbit.
During the
initial launch, the engines sometimes may be used after main engine cutoff in
order to power up or boost the shuttle to the predetermined elliptical orbit. This
is called the OMS-1 burn. But it may not be required based on the payload and
mission.
Now, the
Orbital Maneuvering System-2 burn is used to circularize the elliptical orbit
that the shuttle first enters after launch. These engines may be used to change
the shuttle’s orbital characteristics during the mission.
The Orbital
Maneuvering System engines are used to deorbit the shuttle so it may reenter
Earth’s atmosphere.
The OMS provides enough power for orbit
insertion, orbit circularization, orbit transfer, rendezvous, deorbit, abort to
orbit, and abort once around. It offers more than 1000 pounds of propellant to
the aft reaction control system.
Aft Reaction Control System
The Aft Reaction Control System is a
spacecraft which uses thrusters to provide altitude control or even
translation. It uses diverted engine thrust to provide stable altitude control
of a short/vertical takeoff and landing aircraft.
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Spacecraft reaction control systems are often used for altitude control while reentering the Earth’s surface, station keeping in orbit and control orientation.
The OMS is
stored in 2 independent pods located on each side of the orbiter’s aft
fuselage. The pods also house the aft RCS (Reaction Control System) and are
referred to as the OMS/RCS pods.
Each one of these
pods contains one OMS engine and the hardware that is needed to pressurize,
store and distribute the propellants to perform the velocity maneuvers. The
vehicle velocity required for orbital adjustments is approximately 2ft per
second for each nautical mile of altitude change.
During the first OMS thrusting phase, both
engines are used to raise the orbiter to a predetermined elliptical orbit.
Next, during the thrusting phase, vehicle attitude is maintained by supporting
the OMS engines. The Reaction Control System usually does not come into
operation with the OMS thrusting period. However, if during an OMS thrusting
period the OMS supporting (or gimbal) rate or gimbal period exceeds their own
limits, RCS attitude is required. In case that just one OMS engine is used
during the thrusting period, but RCS control is also required.
Attitude Control:
It is the exercise of control over the
orientation of an object respecting the inertial frame of reference or any
other entity such as certain fields, nearby objects, etc.
During the OMS-1
thrusting period, the liquid oxygen and liquid hydrogen that are 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 side, which is the right side. This
velocity was pre-computed in conjunction with the OMS-1 thrusting period.
Once the
OMS-1 thrusting period is completed, the RCS is now used to null any residual
velocities (just in case it is required). The flight crew operates by using a
rotational hand controller or the translational hand controller to command the
applicable RCS thrusters to null the residual velocities.
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.
Bibliography:
“HSF-The Shuttle.” HSF-The
Shuttle. N.p., n.d. Web, 6 Feb. 2014
“Orbital Maneuvering
System.” Orbital Maneuvering System. N.p.,
n.d. Web, 6 Feb. 2014
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