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Orbital Maneuvering System and Reaction
Control System
OMS/RCS System
Flight control of the Orbiter beyond the atmosphere is provided by the OMS and RCS thrusters
OMS/RCS functions are under the control of the operational software used for Guidance Navigation and Control (GN&C)
The OMS and RCS thrusters are combined to furnish both high and low thrust for the Orbiter’s two flight functions on orbit– Orbit change - OMS– Attitude control - RCS
OMS/RCS System
Propellants• Both OMS and RCS thrusters use the same propellants
– Fuel - monomethyl hydrazine (MMH)– Oxidizer - Nitrogen tetroxide (NTO)
Thruster placement• OMS – Only aft thrusters• RCS – Both fore and aft thrusters
Thrust• OMS
– 6,000 lb (2 thrusters)
• RCS– 870 lb (38 thrusters)– 24 lb (6 thrusters)
OMS/RCS System
Orbital Maneuvering System
OMS/RCS System - OMS
Orbital Maneuvering System - Major subsystems and components
– Two aft-mounted OMS engines
– Nitrogen tetroxide and monomethyl hydrazine propellants and propellant tanks
– Propellant pressurization subsystem
– Pressurized nitrogen on-off valve subsystem
– Associated plumbing and control components
– Thrust vector control subsystem
OMS/RCS System - OMS
Orbital Maneuvering System
• OMS engines can be used simultaneously, or individually for smaller thrusts
• Engine thrust operations can be either pulse mode or continual thrust – 2 sec minimum pulse
• The two engines generate approximately 2 ft/s2 (0.06g) acceleration in continual thrust
• Single-engine operation of the OMS pair is possible– Generally used when required ΔV thrust is less than 6 ft/s to
conserve engine life
OMS/RCS System
OMS/RCS System - OMS
Orbital Maneuvering System
• Thrust direction control employs electromechanical thrust vector gimbal control
• OMS functions are controlled by the Digital Autopilot (DAP) or by manual operation
OMS/RCS System - OMS
OMS specs
• Thrust 6,000 ± 200 lb each• Weight 260 lb each• Size 77" x 46"
Isp 313 sec• ΔVmax 1,000 fps with a 65,000 lb payload• Propellant 23,867 lb total
Fuel Monomethyl hydrazine (MMH = CN2H6) 9,010 lb
Oxidizer Nitrogen tetroxide (NTO = N2O4)14,866 lb
• Gimbaling ± 6o pitch ±7o yaw• Nominal lifetime 100 missions, 1000 starts, 15 hr total
operation • Chamber pressure 125 psia (psi absolute)• Expansion ratio 55:1• Minimum burn time 2 sec
OMS/RCS System - OMS
OMS engine schematic
OMS/RCS System - OMS
OMS engine operation
• Combustion of the liquid MMH and NTO propellants in the OMS chamber is hypergolic and does not require an ignition system
• Fuel and oxidizer are injected onto an internal mixing plate (injector plate) that helps catalyze the combustion reaction
• Heat energy released in the combustion chamber is removed by using regenerative cooling– Circulated fuel cools combustion chamber and heats the
fuel for more efficient combustion– Nozzle cooling is accomplished with simple radiative
cooling
OMS/RCS System - OMS
OMS engine operation
• The OMS burn sequence includes the on and off commands from the Digital Autopilot (DAP)– Off command is followed by the engine purge function
• Purge function sends pressurized nitrogen through the feed lines after the engine cutoff sequence
• Sufficient nitrogen is carried in the supply tanks to operate the OMS engine bipropellant valves and purges for a minimum of 10 times
• Pressurized helium is used to force propellants from the storage tanks into the feed line
OMS/RCS System - OMS
OMS engine operation
• Pressurized nitrogen is used for control and regulation functions of the engine by driving the dual injector on-off valves
• Cross feed is possible for the OMS engines, making it possible to feed propellant to the right engine from the left pod and vise versa– Crossfeed lines also link the aft RCS thruster
propellant tanks with the OMS tanks
OMS/RCS System - OMS
Injector plate
• Combustion takes place spontaneously as propellants are injected onto the surface of the combustion chamber injector plate
• Injection onto the hot injection plate vaporizes and mixes the hypergolics
OMS/RCS System - OMS
Injector plate
• Oxidizer flows directly from the valve assembly to the injector plate– Fuel is first circulated through a cooling jacket
that surrounds the thrust chamber for regenerative cooling
• Normal operating temperature of the combustion chamber is 218oF– Safe operation limit is set at 260oF – Monitored at the injector inlet
OMS/RCS System - OMS
Combustion chamber
• OMS combustion/thrust chamber drives the hot gas through the exhaust nozzle– Operates at a nominal pressure of 130 psia– Measured with internal transducers
• Combustion chamber cooling is an active process– Results from the circulation of the monomethyl
hydrazine fuel through the surrounding jacket before injection into the combustion chamber
OMS/RCS System - OMS
Mounted assembly minus nozzle
OMS/RCS System - OMS
Nozzle
• OMS nozzle is a light-weight columbium alloy structure
• Cooled by radiation– This technique requires it be placed outside the OMS pod
and not covered with insulation tiles
• Thrust vectoring of the nozzle exhaust is driven by electromechanical actuators– Actuators are attached to the thruster body which is attached
directly to the nozzle
OMS/RCS System - OMS
OMS mechanical assembly
OMS/RCS System - OMS
Bipropellant valve assembly
• OMS engine combustion is regulated by pressure-fed propellants passing through dual fuel and oxidizer valves
• Bipropellant valve assembly consists of two fuel valves in series and two oxidizer valves– These are driven by pressurized nitrogen– Provides redundant protection against leakage– Also requires both valves to be open to allow propellant flow
• Each assembly fuel valve is mechanically linked to an oxidizer valve so that they open and close together
OMS/RCS System - OMS
OMS/RCS System - OMS
Nitrogen gas propellant flow control
• Nitrogen pressurization for the OMS and RCS system is used to drive the propellant valves– Also used to purge propellants after each
operation
• Regulated nitrogen gas pressurization lines are turned on and off themselves by valves actuated by the control circuitry– Circuitry managed by the GN&C software
OMS/RCS System - OMS
Nitrogen gas propellant flow control
• N2 pressurization system includes control and regulation valves and distribution lines
• N2 storage tanks– Two in each OMS pod – Internal volume of 60 in3 – Initial pressure is 3,000 psi
OMS/RCS System - OMS
He propellant tank pressurization
• Helium pressurization is used for fluid flow from the OMS and RCS propellant tanks– He is an inert gas and does not react with either
propellant
• He pressurization system includes– Tanks– Pressure sensors– Pressure regulators– Isolation and distribution valves– Distribution lines
OMS/RCS System - OMS
He propellant tank pressurization
• He pressure tanks– Two in each OMS pod – Internal volume is 17.03 ft3 – Initial pressure 4,600-4,800 psia – Distribution pressure regulated at 252-273
psi – Check valves prevent backflow of fuel and
oxidizer
OMS/RCS System - OMS
OMS/RCS System - OMS
OMS propellant system
• OMS and RCS thrusters use liquid bipropellants that are stable under low and high pressures, are hypergolic, and produce a relatively high specific impulse– Propellants require no cryogenic storage and are stable over
long periods (good) but are extremely toxic (bad)
• Fuel– Monomethyl hydrazine (CN2H6) – Circulated around nozzle for engine cooling then fed into
injector – 7.23 lb/s flow rate
• Oxidizer – Nitrogen tetroxide (N2O4) – Direct injection into combustion chamber
11.93 lb/s flow rate
OMS/RCS System - OMS
OMS propellant system
• Tanks – One for fuel and one for oxidizer (each pod)– Titanium structure – Helium pressurized – Internal volume is 89.89 ft3
– Propellant acquisition and retention assembly maintains positive flow in zero gravity
– Screen is used as a wick assembly to retain part of the fuel/oxidizer at the feed end of the tank
OMS/RCS System - OMS
OMS Functions
1. Orbit entry2. Deorbit3. Orbit altitude/periapse change ΔV4. Orbit plane change ΔV5. NC, TI, MCC and NH rendezvous burns
OMS/RCS System - OMS
OMS Functions
1. Orbit entry
• Orbit entry consists of a single burn of the OMS engines to furnish the remaining ΔV after Main Engine Cutoff
• Two orbit entry burns with the OMS engines were used initially– First to establish apogee– Second to circularize the orbit at a point 180o from the first
burn
OMS/RCS System - OMS
OMS Functions
1. Orbit entry
• Missions with small payloads at modest altitudes can be placed in orbit by the SSMEs at MECO– Orbit trim provided by the OMS engines if necessary
• OMS-1 – Not normally used
• OMS-2 – Boost to apogee and circularize orbit simultaneously– Both OMS engines used– RCS used to null residual velocities above computed values at
the end of the burn
OMS/RCS System – OMS Functions
2. Deorbit
• Digital Autopilot rotates the vehicle 180o
• Places the OMS engines forward in preparation for the deorbit burn
• Rotates the Orbiter 180o again after the burn is completed
OMS/RCS System – OMS Functions
2. Deorbit
• Both OMS engines used for the 250 ft/s ΔV
• One hour later the deorbit burn places the Orbiter in the upper atmosphere roughly 100o from the burn point
• Once the Orbiter reaches the top of the sensible atmosphere the drag begins to reduce its velocity irreversibly
OMS/RCS System – OMS Functions
3. Orbit altitude/periapse change ΔV
• ΔV orbit altitude change uses approximately 2 ft/s for a 1 nm orbit change
OMS/RCS System – OMS Functions
4. Orbit plane change ΔV
• After being established in an orbit, the Orbiter can change orbit planes but within a very limited range
• Plane change known as inclination angle, or wedge angle, is limited to approximately 5o because of the severe penalty on propellants needed for the maneuver– Plane change is also limited to the intersecting
nodes of the new and old orbit planes
OMS/RCS System – OMS Functions
5. NC, TI, MCC and NH rendezvous burns
Phase adjustment, and orbit correction and trim burns used for precise positioning for satellite deployment and target rendezvous
OMS/RCS System – OMS
Thrust Vector Control
• OMS engines have directional capability, or thrust vector control (TVC)– TVC employs electromechanical actuators– TVC is driven by Digital Autopilot commands
• Two servoactuators on each OMS engine are anchored to the OMS/RCS pod thrust structure and mount to the OMS near the nozzle
OMS/RCS System – OMS
Thrust Vector Control
• Rotating gimbal is located on the top of the engine similar to the SSME engines
• Two OMS thrust vector actuators on each engine drive the nozzles in 2 dimensions– ± 6o in pitch and ±7o in yaw
• Calculations made for OMS single-engine or dual operation are calculated by the GN&C software and commanded by the automated functions, or from manual controls for some orbital functions
Reaction Control System
OMS/RCS System – RCS
Reaction Control System
• The Orbiter's RCS system provides 3-axis attitude control from the 16 small thrusters in the forward RCS section and 28 in the aft OMS/RCS pods
• Two sizes of the RCS thrusters furnish precise attitude control under both manual and automatic control of the Digital Autopilot
OMS/RCS System – RCS
Reaction Control System
• RCS thrusters are also used for correcting the OMS burns since the OMS engines' thrust line is offset from the Orbiter's centerline.
• RCS thrusters are also used for aerodynamic functions that include:– Augmenting STS guidance during ascent– RTLS abort control– Augmenting aerodynamic flight during most of the
reentry descent
OMS/RCS System – RCS
Reaction Control System
• RCS thrusters are also used for small rotational and translational maneuvers to close in on, and for separating from rendezvous and docking targets
• RCS thrusts are also used for small changes to the orbital parameters and even trimming the OMS-2 orbit entry burn
• Orbiter flight modes such as Local Vertical-Local Horizontal and Inertial modes are maintained by the RCS thrusters commanded by the Digital Autopilot
OMS/RCS System – RCS
Reaction Control System
• Thrusters on the forward RCS section and on the OMS/RCS pods provide precise rotational (roll, pitch and yaw) and translational motion with two types of thrusters– Larger primary thrusters– Smaller vernier thrusters
• Fuel and oxidizer for the RCS thrusters are the same nitrogen tetroxide and monomethyl hydrazine used in the OMS system
OMS/RCS System – RCS
RCS specs
Primary thruster 38 total 14 in forward section 12 in each aft pod
Thrust 870 lb Isp 280 sChamber Pressure 152 psiaNominal lifetime 100 missions, 20,000 starts, 12,800 s accumulated
timeOperation 1-125 s continuous, 0.08 s minimum pulse
Vernier thruster 6 total 2 fore 2 per aft pod
Thrust 24 lbIsp 265 sChamber pressure 110 psiaNominal Lifetime 330,000 starts, 125,000 s accumulated timeOperation 1 to 125 sec continuous, 0.08 sec minimum pulsePropellants 928 lb
Fuel Monomethyl hydrazine (MMH) Oxidizer Nitrogen tetroxide (NTO)
OMS/RCS System – RCS
Two thruster types are used on the Orbiter's RCS system
1. Primary thrusters
• 870 lb thrust
• Activated by electrical signals generated by the GPC software and commanded by the reaction jet driver
• Thruster propellant valve is operated by both electrical solenoids and propellant hydraulic pressure
• Cooling of the combustion chamber is augmented by fuel flow in outer injector holes into the thrust/combustion chamber
OMS/RCS System – RCS
2. Vernier thrusters
• 24 lb thrust
• Used for fine adjustment in attitude, and for low-Z docking approaches
• Activated solely by electrical signals generated by the GPC software and commanded by the reaction jet driver
OMS/RCS System - RCS
RCS primary thruster cutaway
OMS/RCS System - RCS
RCS vernier thruster cutaway
OMS/RCS System – RCS
RCS propellant tanks
• Propellants used for the RCS system is stored in separate tanks from the OMS propellants– OMS and RCS propellants are identical, and can be
cross fed between OMS and RCS tanks
• RCS tanks are unique because of their difference between the forward supply in the forward RCS section and the aft RCS supply in the OMS pods– To allow positive feed during reentry, the forward
tanks have a fluid collector in the upper compartment
OMS/RCS System – RCS
RCS propellant tanks
• The same set of feed lines and compartments are used for powered flight, for low-g operations on orbit, and for higher reentry accelerations– The feed mechanisms interact differently
during the different orientations
OMS/RCS System – RCS Propellant Tanks
RCS forward tanks on the left have positive feed
for both low-g and powered flight shown on the left
Reentry configuration shown on the right
OMS/RCS System – RCS
Thermal control
• Maximum temperature extremes at the thrusters are encountered during space exposure (-250oF) and during RCS combustion
• Temperatures within the OMS chamber/nozzle are 1,260-1,740oF in the aft flange– Maximum rating is 2,400oF
OMS/RCS System – RCS
Thermal control
• Insulation is positioned in both the forward RCS section and in the OMS pods to prevent thruster & feed line heat loss– Helps maintain operating temperatures– Also helps keep propellants from freezing
• Heaters are placed in the RCS thruster blocks to maintain proper operating temperature range– Heaters also prevent injected fuel from freezing
OMS/RCS System – RCS
RCS operations
• Fore and aft RCS thrusters provide pitch, yaw, and roll control on orbit– Controlled by GN&C/DAP software
• Fore and aft RCS thrusters provide low thrust along the X-axis for minor translational control (±ΔV) on-orbit
• Used for attitude control functions during abort operations
• Used for Orbiter separation from ET (-Z burn during separation, then attitude hold before OMS-2 burn alignment
OMS/RCS System – RCS
RCS operations
• Used for low-Z separation and approach to/from space station or spacecraft
• RCS is used during ascent to trim SSME and OMS engine vectored thrust
• RCS is used during deorbit, descent and approach phases of the mission– Completely deactivated when its last function which is
yaw augmentation is complete as the Orbiter speed decreases below Mach 1
References
– National Space Transportation System Press Kit, NASA, 1988
– Reaction Control System Training Manual, NASA, 1995
– Orbital Maneuvering System - Orbiter Systems Training Manual, NASA, April, 1995
– Shuttle Crew Operations Manual - OMS, NASA
– Shuttle Crew Operations Manual - RCS, NASA