1. Orbital Maneuvering System and Reaction Control System

2. 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

3. 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)

4. OMS/RCS System

5. Orbital Maneuvering System

6. 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

7. 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

8. OMS/RCS System

9. 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

10. OMS specs OMS/RCS System - OMS Thrust 6,000 ± 200 lb each
Weight 260 lb each
Size 77" x 46" Isp 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

11. OMS/RCS System - OMS
OMS engine schematic

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. OMS/RCS System - OMS
Mounted assembly minus nozzle

19. 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

20. OMS/RCS System - OMS
OMS mechanical assembly

21. 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

22. OMS/RCS System - OMS

23. 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

24. 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

25. 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

26. He propellant tank pressurization
OMS/RCS System - OMS
He propellant tank pressurization
He pressure tanks
Two in each OMS pod
Internal volume is ft3 
Initial pressure 4,600-4,800 psia
Distribution pressure regulated at psi
Check valves prevent backflow of fuel and oxidizer

27. OMS/RCS System - OMS

28. 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 lb/s flow rate

29. 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 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. Reaction Control System

41. 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

42. 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

43. 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

44. 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

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

46. 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

47. 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

48. OMS/RCS System - RCS
RCS primary thruster cutaway

49. OMS/RCS System - RCS
RCS vernier thruster cutaway

50. 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

51. 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

52. 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

53. 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

54. 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

55. 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

56. 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

57. 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