Orbital Maneuvering System and Reaction Control System.

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 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



• Thrust direction control employs electromechanical thrust vector gimbal control

• OMS functions are controlled by the Digital Autopilot (DAP) or by manual operation


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 engine schematic


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



• 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



• 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


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


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

• 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


Mounted assembly minus nozzle


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 mechanical assembly


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



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

• 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

• 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

• 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 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 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 Functions

1. Orbit entry2. Deorbit3. Orbit altitude/periapse change ΔV4. Orbit plane change ΔV5. NC, TI, MCC and NH rendezvous burns


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

• 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


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


3. Orbit altitude/periapse change ΔV

• ΔV orbit altitude change uses approximately 2 ft/s for a 1 nm orbit change


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


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


• 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



• 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



• 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



• 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

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)


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

• 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


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


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


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


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


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


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

Orbital Maneuvering System and Reaction Control System.

Documents

control gncthe oms

oms chamber

oms engine bipropellant

oms pair

low thrust

v thrust

rcs thrustersomsrcs

nto propellants