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

Both OMS and RCS thrusters use the same propellants 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

Orbital Maneuvering System - Major subsystems and components 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

Orbital Maneuvering System 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

Orbital Maneuvering 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 specs OMS/RCS System - OMS 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 engine operation OMS/RCS System - OMS 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

Combustion chamber cooling is an active process 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

Nozzle OMS/RCS System - OMS 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

Bipropellant valve 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

Nitrogen gas propellant flow control 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

Nitrogen gas propellant flow control 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

He propellant tank pressurization 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

He propellant tank pressurization 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 propellant system OMS/RCS System - OMS 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 propellant system Tanks 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

3. Orbit altitude/periapse change ΔV 4. Orbit plane change ΔV OMS/RCS System - OMS OMS Functions 1. Orbit entry 2. Deorbit 3. Orbit altitude/periapse change ΔV 4. Orbit plane change ΔV 5. NC, TI, MCC and NH rendezvous burns

Two orbit entry burns with the OMS engines were used initially 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 Functions 1. Orbit entry OMS/RCS System - OMS 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

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

Reaction Control System 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

Reaction Control System 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

Reaction Control System 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

RCS specs OMS/RCS System – RCS Primary thruster 38 total   14 in forward section   12 in each aft pod     Thrust 870 lb       Isp 280 s Chamber Pressure 152 psia Nominal lifetime 100 missions, 20,000 starts, 12,800 s accumulated time Operation 1-125 s continuous, 0.08 s minimum pulse Vernier thruster 6 total    2 fore    2 per aft pod     Thrust 24 lb Isp 265 s Chamber pressure 110 psia Nominal Lifetime 330,000 starts, 125,000 s accumulated time Operation 1 to 125 sec continuous, 0.08 sec minimum pulse Propellants 928 lb Fuel Monomethyl hydrazine (MMH) Oxidizer Nitrogen tetroxide (NTO)

Two thruster types are used on the Orbiter's RCS system 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

2. Vernier thrusters OMS/RCS System – RCS 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