The orbiter's reaction control system comprises the forward and
aft RCS. The forward RCS is located in the forward fuselage nose
area. The aft (right and left) RCS is located with the orbital
maneuvering system in the OMS/RCS pods.
Each RCS consists of high-pressure gaseous helium storage tanks,
pressure regulation and relief systems, a fuel and oxidizer tank,
a system that distributes propellant to its engines, and thermal
control systems (electrical heaters).
The forward and aft RCS units provide the thrust for attitude
(rotational) maneuvers (pitch, yaw and roll) and for small velocity
changes along the orbiter axis (translation maneuvers).
The ascent profile of a mission determines the interaction of
the RCS units, which depends on the number (one or two) of OMS
thrusting periods. After main engine cutoff, the forward and aft
thrusters are used to maintain attitude hold until external tank
separation. Then the reaction control system provides a minus
(negative) Z translation maneuver of about 4 feet per second to
move the orbiter away from the external tank. Upon completion
of the maneuver, the RCS holds the orbiter attitude until it is
time to maneuver to the OMS-1 thrusting attitude. Although the
targeting data for the OMS-1 thrusting period is selected before
launch, the target data in the onboard general-purpose computers
can be modified by the flight crew via the CRT and keyboard, if
necessary, before the OMS thrusting period.
The first thrusting period of the orbital maneuvering system
(OMS-1) uses both OMS engines to raise the orbiter to a predetermined
elliptical orbit. During the OMS-1 thrusting period, vehicle attitude
is maintained by gimbaling (swiveling) the OMS engines. The reaction
control system normally does not operate during an OMS thrusting
period. If, during an OMS thrusting period, the gimbal rate or
gimbal limits are exceeded, RCS roll control would be required;
or if only one OMS engine is used during a thrusting period, RCS
roll control would be 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
out through the right-side T-0 umbilical overboard fill and drain
system. This velocity is precomputed in conjunction with the OMS-1
Upon completion of the OMS-1 thrusting period, the reaction control
system can be used to null any residual velocities, if required.
The flight crew uses the rotational hand controller or translational
hand controller to command the applicable RCS thrusters to null
the residual velocities. The reaction control system then provides
attitude hold until it is time to maneuver to the OMS-2 thrusting
The second thrusting period of the orbital maneuvering system
(OMS-2) uses both OMS engines near the apogee of the orbit established
by the OMS-1 thrusting period 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 computers can be modified by the flight crew on the
computer keyboard, if necessary, before the OMS thrusting period.
Upon completion of the OMS-2 thrusting period, the reaction control
system can be used to null any residual velocities, if required.
It is then used for 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 in orbit.
Normally, the vernier thrusters are selected for on-orbit attitude
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 MPS dump. The RCS is used for attitude
hold after the 5-feet-per-second translation maneuver. The OMS-2
thrusting period is then used to achieve orbit insertion. This
profile allows the MPS to provide more energy for orbit insertion
and permits easier use of onboard software.
Additional OMS thrusting periods using one or both OMS engines
are performed on orbit as needed for rendezvous, for payload deployment
or for transfer to another orbit.
For the deorbit thrusting maneuver, the two OMS engines are used.
Target data for the deorbit maneuver is computed on the ground,
loaded in the onboard general-purpose computers by uplink and
voiced to the flight crew for verification of loaded values. The
flight crew then initiates an OMS gimbal test by item entry in
the CRT keyboard unit.
Before the deorbit thrusting period, the flight crew moves the
spacecraft to the desired attitude using the thrusters. After
the OMS thrusting period, the RCS is used to null any residual
velocities, if required. The spacecraft is then moved 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 yaw thrusters before entry interface if orbiter
center-of-gravity control is necessary.
The aft RCS plus X jets can be used to complete any OMS deorbit
thrusting period if an OMS engine fails. In this case, the OMS-to-aft-RCS
interconnect can be used to feed OMS propellant to the aft RCS.
From an entry interface of 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.
Two helium tanks supply gaseous helium pressure to the oxidizer
and fuel tanks. The oxidizer and fuel are then supplied under
gaseous helium pressure to the RCS engines. Nitrogen tetroxide
is the oxidizer, and monomethyl hydrazine is the fuel. The propellants
are Earth-storable and hypergolic (they ignite upon contact with
each other). The propellants are supplied to the engines, where
they atomize, ignite and produce a hot gas and thrust.
The forward RCS has 14 primary and two vernier engines. The aft
RCS has 12 primary and two vernier engines in each pod. The primary
RCS engines provide 870 pounds of vacuum thrust each, and the
vernier RCS engines provide 24 pounds of vacuum thrust each. The
oxidizer-to-fuel ratio for each engine is 1.6-to-1. The nominal
chamber pressure of the primary engines is 152 psia. For each
vernier engine, it is 110 psia.
The primary engines are reusable for a minimum of 100 missions
and are capable of sustaining 20,000 starts and 12,800 seconds
of cumulative firing. The primary engines are operable in a maximum
steady-state thrusting mode of one to 150 seconds, with a maximum
single-mission contingency of 800 seconds for the aft RCS plus
X engines and 300 seconds maximum for the forward RCS minus X
engines as well as in a pulse mode with a minimum impulse thrusting
time of 0.08 second above 125,000 feet. The expansion ratio (exit
area to throat area) of the primary engines ranges from 22-to-1
to 30-to-1. The multiple primary thrusters provide redundancy.
The vernier engines' reusability depends on chamber life. They
are capable of sustaining 330,000 starts and 125,000 seconds of
cumulative firings. The vernier engines are operable in a steady-state
thrusting mode of one to 125 seconds maximum as well as in a pulse
mode with a minimum impulse time of 0.08 second. The vernier engines
are used for finite maneuvers and stationkeeping (long-time attitude
hold) and have an expansion ratio that ranges from 20-to-1 to
50-to-1. The vernier thrusters are not redundant.