Return to KSC Home PageThe ascent profile of a mission determines if one or two OMS thrusting periods are used and the interactions of the RCS. After main engine cutoff, the RCS thrusters in the forward and aft RCS pods are used to provide attitude hold until external tank separation. At ET separation, the RCS provides a minus (negative) Z translation maneuver of about minus 4 feet per second to maneuver the orbiter away from the ET. Upon completion of the translation, the RCS provides orbiter attitude hold until time to maneuver to the OMS-1 thrusting attitude. The targeting data for the OMS-1 thrusting period is selected before launch; however, the target data in the onboard general-purpose computers can be modified by the flight crew via the cathode ray tube keyboard, if necessary, before the OMS thrusting period.
During the first OMS thrusting period, both OMS engines are used to raise the orbiter to a predetermined elliptical orbit. During the thrusting period, vehicle attitude is maintained by gimbaling (swiveling) the OMS engines. The RCS will not normally come into operation during an OMS thrusting period. If, during an OMS thrusting period, the OMS gimbal rate or gimbal limits are exceeded, RCS attitude control is required. If only one OMS engine is used during an OMS thrusting period, RCS roll control is 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 through the starboard (right) side T-0 umbilical overboard fill and drain. This velocity was precomputed in conjunction with the OMS-1 thrusting period.
Upon completion of the OMS-1 thrusting period, the RCS is used to null any residual velocities, if required. The flight crew uses the rotational hand controller and/or translational hand controller to command the applicable RCS thrusters to null the residual velocities. The RCS then provides attitude hold until time to maneuver to the OMS-2 thrusting attitude.
The second OMS thrusting period using both OMS engines occurs near the apogee of the orbit established by the OMS-1 thrusting period and is used 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 GPCs can be modified by the flight crew via the CRT keyboard, if necessary, before the OMS thrusting period.
Upon completion of the OMS-2 thrusting period, the RCS is used to null any residual velocities, if required, in the same manner as during OMS-1. The RCS is then used to provide 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 on orbit. Normally, the vernier RCS thrusters are selected for on-orbit attitude hold.
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 main propulsion system dump. The RCS provides attitude hold after the translation maneuver. The OMS-2 thrusting period is then used to achieve orbit insertion. The direct-insertion ascent profile allows the MPS to provide more energy to orbit insertion and permits easier use of onboard software.
Additional OMS thrusting periods using both or one OMS engine are performed on orbit according to the mission's requirements to modify the orbit for rendezvous, payload deployment or transfer to another orbit.
The two OMS engines are used to deorbit. Target data for the deorbit maneuver is computed by the ground and loaded in the onboard GPCs via uplink. This data is also voiced to the flight crew for verification of loaded values. After verification of the deorbit data, the flight crew initiates an OMS gimbal test on the CRT keyboard unit.
Before the deorbit thrusting period, the flight crew maneuvers the spacecraft to the desired deorbit thrusting attitude using the rotational hand controller and RCS thrusters. Upon completion of the OMS thrusting period, the RCS is used to null any residual velocities, if required. The spacecraft is then maneuvered 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 thrusters before the entry interface if it is necessary to control the orbiter's center of gravity.
The aft RCS plus X jets can be used to complete any planned OMS thrusting period in the event of an OMS engine failure. In this case, the OMS-to-aft-RCS interconnect would feed OMS propellants to the aft RCS.
From entry interface at 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.
The OMS in each pod consists of a high-pressure gaseous helium storage tank, helium isolation valves, dual pressure regulation systems, vapor isolation valves for only the oxidizer regulated helium pressure path, quad check valves, a fuel tank, an oxidizer tank, a propellant distribution system consisting of tank isolation valves, crossfeed valves, and an OMS engine. Each OMS engine also has a gaseous nitrogen storage tank, gaseous nitrogen pressure isolation valve, gaseous nitrogen accumulator, bipropellant solenoid control valves and actuators that control bipropellant ball valves, and purge valves.
In each of the OMS pods, gaseous helium pressure is supplied to helium isolation valves and dual pressure regulators, which supply regulated helium pressure to the fuel and oxidizer tanks. The fuel is monomethyl hydrazine and the oxidizer is nitrogen tetroxide. The propellants are Earth-storable liquids at normal temperatures. They are pressure-fed to the propellant distribution system through tank isolation valves to the OMS engines. The OMS engine propellant ball valves are positioned by the gaseous nitrogen system and control the flow of propellants into the engine. The fuel is directed first through the engine combustion chamber walls and provides regenerative cooling of the chamber walls; it then flows into the engine injector. The oxidizer goes directly to the engine injector. The propellants are sprayed into the combustion chamber, where they atomize and ignite upon contact with each other (hypergolic), producing a hot gas and, thus, thrust.
The gaseous nitrogen system is also used after the OMS engines are shut down to purge residual fuel from the injector and combustion chamber, permitting safe restarting of the engines. The nozzle extension of each OMS engine is radiation-cooled and is constructed of columbium alloy.
Each OMS engine produces 6,000 pounds of thrust. The oxidizer-to-fuel ratio is 1.65-to-1. The expansion ratio of the nozzle exit to the throat is 55-to-1. The chamber pressure of the engine is 125 psia. The dry weight of each engine is 260 pounds.
Each OMS engine can be reused for 100 missions and is capable of 1,000 starts and 15 hours of cumulative firing. The minimum duration of an OMS engine firing is two seconds. The OMS may be utilized to provide thrust above 70,000 feet. For vehicle velocity changes of between 3 and 6 feet per second, normally only one OMS engine is used.
Each engine has two electromechanical gimbal actuators, which control the OMS engine thrust direction in pitch and yaw (thrust vector control). The OMS engines can be used singularly by directing the thrust vector through the orbiter center of gravity or together by directing the thrust vector of each engine parallel to the other. During a two-OMS-engine thrusting period, the RCS will come into operation only if the OMS gimbal rate or gimbal limits are exceeded and should not normally come into operation during the OMS thrust period. However, during a one-OMS-engine thrusting period, roll RCS control is required. The pitch and yaw actuators are identical except for the stroke length and contain redundant electrical channels (active and standby), which couple to a common mechanical drive assembly.
The OMS/RCS pods are designed to be reused for up to 100 missions with only minor repair, refurbishment and maintenance. The pods are removable to facilitate orbiter turnaround, if required.
The helium storage tank in each pod has a titanium liner with a fiberglass structural overwrap. This increases safety and decreases the weight of the tank 32 percent over that of conventional tanks. The helium tank is 40.2 inches in diameter and has a volume of 17.03 cubic feet minimum. Its dry weight is 272 pounds. The helium tank's operating pressure range is 4,800 to 460 psia with a maximum operating limit of 4,875 psia at 200 F.
A pressure sensor downstream of each helium tank in each pod monitors the helium source pressure and transmits it to the N 2 , He , kit He switch on panel F7. When the switch is in the He position, the helium pressure of the left and right OMS is displayed on the OMS press left, right meters. This pressure also is transmitted to the CRT and displayed.
The two helium pressure isolation valves in each pod permit helium source pressure to the propellant tanks or isolate the helium from the propellant tanks. The parallel paths in each pod assure helium flow to the propellant tanks of that pod. The helium valves are continuous-duty, solenoid-operated. They are energized open and spring loaded closed. The OMS He press/vapor isol switches on panel O8 permit automatic or manual control of the valves. With the switches in the GPC position, the valves are automatically controlled by the general-purpose computer during an engine thrusting sequence. The valves are controlled manually by placing the switches to open or close.
The pressure regulators reduce the helium source pressure to the desired working pressure. Pressure is regulated by assemblies downstream of each helium pressure isolation valve. Each assembly contains primary and secondary regulators in series and a flow limiter. Normally, the primary regulator is the controlling regulator. The secondary regulator is normally open during a dynamic flow condition. It will not become the controlling regulator until the primary regulator allows a higher pressure than normal. All regulator assemblies are in reference to a bellows assembly that is vented to ambient. The primary regulator outlet pressure at normal flow is 252 to 262 psig and 247 psig minimum at high abort flow, with lockup at 266 psig maximum. The secondary regulator outlet pressure at normal flow is 259 to 269 psig and 254 psig minimum at high abort flow, with lockup at 273 psig maximum. The flow limiter restricts the flow to a maximum of 1,040 stan dard cubic feet per minute and to a minimum of 304 standard cubic feet per minute.
The vapor isolation valves in the oxidizer pressurization line to the oxidizer tank prevent oxidizer vapor from migrating upstream and over into the fuel system. These are low-pressure, two-position, two-way, solenoid-operated valves that are energized open and spring loaded closed. They can be commanded manually or automatically by the positioning of the He press/vapor isol switches on panel O8. When either of the A or B switches is in the open position, both vapor isolation valves are energized open; and when both switches are in the close position, both vapor isolation valves are closed. When the switches are in the GPC position, the GPC opens and closes the valves automatically.
The check valve assembly in each parallel path contains four independent check valves connected in a series-parallel configuration to provide a positive checking action against a reverse flow of propellant liquid or vapor, and the parallel path permits redundant paths of helium to be directed to the propellant tanks. Filters are incorporated into the inlet of each check valve assembly.
Two pressure sensors in the helium pressurization line upstream of the fuel and oxidizer tanks monitor the regulated tank pressure and transmit it to the RCS/OMS press rotary switch on panel O3. When the switch is in the OMS prplnt position, the left and right fuel and oxidizer pressure is displayed. If the tank pressure is lower than 234 psia or above 284 psia, the left or right OMS red caution and warning light on panel F7 will be illuminated. These pressures also are transmitted to the CRT and displayed.
The relief valves in each pressurization path limit excessive pressure in the propellant tanks. Each pressure relief valve also contains a burst diaphragm and filter. If excessive pressure is caused by helium or propellant vapor, the diaphragm will rupture and the relief valve will open and vent the excessive pressure overboard. The filter prevents particulates from the non-fragmentation-type diaphragm from entering the relief valve seat. The relief valve will close and reset after the pressure has returned to the operating level. The burst diaphragm is used to provide a more positive seal of helium and propellant vapors than the relief valve. The diaphragm ruptures between 303 and 313 psig. The relief valve opens at a minimum of 286 psig and a maximum of 313 psig. The relief valve's minimum reseat pressure is 280 psig. The maximum flow capacity of the relief valve at 60 F and 313 psig is 520 cubic feet per minute.
The OMS propellant tanks of both pods enable the orbiter to reach a 1,000-foot- per-second velocity change with a 65,000-pound payload in the payload bay. An OMS pod crossfeed line allows the propellants in the pods to be used to operate either OMS engine.
The propellant is contained in domed cylindrical titanium tanks within each pod. Each propellant tank is 96.38 inches long with a diameter of 49.1 inches and a volume of 89.89 cubic feet unpressurized. The dry weight of each tank is 250 pounds. The propellant tanks are pressurized by the helium system.
Each tank contains a propellant acquisition and retention assembly in the aft end and is divided into forward and aft compartments. The propellant acquisition and retention assembly is located in the aft compartment and consists of an intermediate bulkhead with communication screen and an acquisition system. The propellant in the tank is directed from the forward compartment through the intermediate bulkhead through the communication screen into the aft compartment during OMS velocity maneuvers. The communication screen retains propellant in the aft compartment during zero-gravity conditions.
The acquisition assembly consists of four stub galleries and a collector manifold. The stub galleries acquire wall-bound propellant at OMS start and during RCS velocity maneuvers to prevent gas ingestion. The stub galleries have screens that allow propellant flow and prevent gas ingestion. The collector manifold is connected to the stub galleries and also contains a gas arrestor screen to further prevent gas ingestion, which permits OMS engine ignition without the need of a propellant-settling maneuver employing RCS thrusters. The propellant tank's nominal operating pressure is 250 psi, with a maximum operating pressure limit of 313 psia.
A capacitance gauging system in each OMS propellant tank measures the propellant in the tank. The system consists of a forward and aft probe and a totalizer. The forward and aft fuel probes use fuel (which is a conductor) as one plate of the capacitor and a glass tube that is metallized on the inside as the other. The forward and aft oxidizer probes use two concentric nickel tubes as the capacitor plates and oxidizer as the dielectric. (Helium is also a dielectric, but has a different dielectric constant than the oxidizer.) The aft probes in each tank contain a resistive temperature-sensing element to correct variations in fluid density. The fluid in the area of the communication screens cannot be measured.
The totalizer receives OMS valve operation information and inputs from the forward and aft probes in each tank and outputs total and aft quantities and a low level quantity. The inputs from the OMS valves allow control logic in the totalizer to determine when an OMS engine is thrusting and which tanks are being used. The totalizer begins an engine flow rate/time integration process at the start of the OMS thrusting period, which reduces the indicated amount of propellants by a preset estimated rate for the first 14.8 seconds. After 14.8 seconds of OMS thrusting, which settles the propellant surface, the probe capacitance gauging system outputs are enabled, which permits the quantity of propellant remaining to be displayed. The totalizer outputs are displayed on the OMS/RCS prplnt qty meters on panel O3 when the rotary switch is positioned to the OMS fuel or oxid positions.
When the wet or dry analog comparator indicates the forward probe is dry, the ungaugeable propellant in the region of the intermediate bulkhead is added to the aft probe output quantity, decreasing the total quantity at a preset rate for 98.15 seconds, and updates from the aft probes are inhibited. After 98.15 seconds of thrusting, the aft probe output inhibit is removed, and the aft probe updates the total quantity. When the quantity decreases to 5 percent, the low-level signal is output.
Parallel tank isolation valves in each pod located between the propellant tanks and the OMS engine and the OMS crossfeed valves permit propellant to be supplied to the OMS engine and OMS crossfeed valves or isolate the propellant. The left or right OMS tank isolation A switch on panel O8 controls the A fuel and A oxidizer valve in that pod, and the B switch controls the B fuel and B oxidizer valve in that pod. When the left or right tank isolation switches in a pod are positioned to GPC , pairs of valves are automatically opened or closed upon command from the orbiter computer. When a pair of valves is opened, fuel and oxidizer from the corresponding propellant tanks are allowed to flow to that OMS engine and OMS crossfeed valves; and when that pair of valves is closed, fuel and oxidizer are isolated from the OMS engine and OMS crossfeed valves. The switch positions open, GPC and close are permanent-position switches. Electrical power is provided to an electrical motor controller assembly, which supplies power to the ac-motor-operated valve actuators. Once the valve is in the commanded position, logic in the motor controller assembly removes power from the ac-motor-operated valve actuator. A talkback indicator above each tank isolation switch on panel O8 indicates the status of the fuel valve and oxidizer valve. The talkback indicator is controlled by microswitches in each pair of valves. The talkback indicator indicates op when that pair of valves is open, barberpole when the valves are in transit or one valve is open or closed, and cl when that pair of valves is closed. The open and close positions of each left or right tank isolation A, B switch permits manual control of the corresponding pair of valves (one for fuel and one for oxidizer).
In each pod, parallel left or right OMS crossfeed valves are controlled by the left, right crossfeed A, B switches on panel O8. The A switch controls the A fuel and A oxidizer ac-motor-operated valve actuators in the pod selected, and the B switch controls the B fuel and B oxidizer valve in the pod selected. When the A or B switch in a pod is positioned to GPC , the A or B pair of fuel and oxidizer valves is automatically opened or closed upon command from the orbiter computer. For example, when the A or B pair of crossfeed valves in the left pod is opened, fuel and oxidizer from the left pod are routed to the OMS crossfeed valves of the right pod; thus, a pair of A or B crossfeed valves in the right pod must be opened to permit the left pod fuel and oxidizer to be directed to the right OMS pod engine. A talkback indicator above the pod crossfeed switches on panel O8 indicates the status of the selected pair's fuel and oxidizer valves. The talkback indicator indicates op when both valves are open, barberpole when the valves are in transit or one valve is open and one closed, and cl when both valves are closed. The left, right crossfeed A, B open/close switches on panel O8 permit manual control of the corresponding pair of fuel and oxidizer valves.
The left and right OMS crossfeed A, B switches also provide the capability to supply OMS propellants to the left and right aft RCS engines. The left and right aft RCS will not be used to supply propellants to the OMS due to differences in pressures between the OMS and RCS.
The OMS crossfeed fuel and oxidizer line pressures are monitored on telemetry and are transmitted to the flight deck CRT.
There are 64 ac -motor-operated valve actuators in the OMS/RCS nitrogen tetroxide and monomethyl hydrazine propellant systems. Each valve actuator was modified to incorporate a 0.25-inch-diameter stainless steel sniff line from the actuator to the mold line of the orbiter. The sniff line permits the monitoring of nitrogen tetroxide or monomethyl hydrazine in the electrical portion of each valve actuator during ground operations.
There are sniff lines in the 12 ac -motor-operated valve actuators in the forward RCS and in the 44 actua tors in the aft left and aft right RCS. The remaining 0.25-inch-diameter sniff lines are in the eight OMS tank isolation and crossfeed ac-motor-operated valve actuators in the left and right orbital maneuvering systems. The 44 aft left and right RCS sniff lines and the eight OMS left and right sniff lines are routed to the respective left and right OMS/RCS pod Y web access servicing panels.
During ground operations, an interscan can be connected to the sniff ports to check for the presence of nitrogen tetroxide or monomethyl hydrazine in the electrical portion of the ac-motor-operated valve actuators.
An electrical microswitch in each of the ac-motor-operated valve actuators signals the respective valves' position (open or closed) to the onboard flight crew displays and controls as well as telemetry. An extensive improvement program was implemented to reduce the probability of floating particulates in the electrical microswitch portion of each ac-motor-operated valve actuator. Particulates could affect the operation of the microswitch in each valve and, thus, the position indication of the valves to the onboard displays and controls and telemetry.
A gaseous nitrogen spherical storage tank is mounted next to the combustion chamber to supply pressure to its engine pressure isolation valve. The tank contains enough nitrogen to operate the ball valves and purge the engine 10 times. Nominal tank capacity is 60 cubic inches. The maximum tank operating pressure is 3,000 psi, with a proof pressure of 6,000 psig.
Each tank's pressure is monitored by two pressure sensors. One sensor transmits the tank pressure to the N 2 , He, kit He switch on panel F7. When the switch is positioned to N 2 , tank pressure is displayed on the OMS press N 2 tank left, right meters on panel F7. The other sensor transmits pressure to telemetry.
A dual-coil, solenoid-operated engine pressure isolation valve is located in each gaseous nitrogen system. The valve is energized open and spring-loaded closed. The engine pressure isolation valve permits gaseous nitrogen flow from the tank to the regulator, accumulator, the bipropellant ball valve control valves and purge valves 1 and 2 when energized open and isolates the nitrogen tank from the gaseous nitrogen supply system when closed. The engine pressure isolation valves in each system are controlled by the OMS eng left, right switches on panel C3. When the OMS eng left switch is placed in the arm press position, the left OMS engine pod's pressure isolation valve is energized open. When the OMS eng right switch is placed in the arm press position, the right OMS engine pod's pressure isolation valve is energized open. The gaseous nitrogen engine pressure isolation valve, when energized open, allows gaseous nitrogen supply pressure to be directed into a regulator, through a check valve, an in-line accumulator and to a pair of engine bipropellant control valves. The engine bipropellant control valves are controlled by the OMS thrust on/off commands from the GPCs.
A single-stage regulator is installed in each gaseous nitrogen pneumatic control system between the gaseous nitrogen engine pressure isolation valve and the engine bipropellant control valves. The regulator reduces the gaseous nitrogen service pressure to a desired working pressure of 315 to 360 psig.
A pressure relief valve downstream of the gaseous nitrogen regulator limits the pressure to the engine bipropellant control valves and actuators if a gaseous nitrogen regulator malfunctions. The relief valve relieves between 450 and 500 psig and resets at 400 psig minimum.
A pressure sensor downstream of the regulator monitors the regulated pressure and transmits it to the CRT display and to telemetry.
The check valve located downstream of the gaseous nitrogen regulator will close if gaseous nitrogen pressure is lost on the upstream side of the check valve and will isolate the remaining gaseous nitrogen pressure on the downstream side of the check valve.
The 19-cubic- inch gaseous nitrogen accumulator downstream of the check valve and upstream of the bipropellant control valves provides enough pressure to operate the engine bipropellant control valves one time with the engine pressure isolation valve closed or in the event of loss of pressure on the upstream side of the check valve.
Two solenoid-operated, three-way, two-position bipropellant control valves on each OMS engine control the bipropellant control valve actuators and bipropellant ball valves. Control valve 1 controls the No. 1 actuator and the fuel and oxidizer ball valves. Control valve 2 controls the No. 2 actuator and two ball valves, one fuel and oxidizer ball valve in series to the No. 1 system. Each control valve contains two solenoid coils, either of which, when energized, opens the control valve.
The right OMS engine gaseous nitrogen solenoid control valves 1 and 2 are energized open by computer commands if the right OMS eng switch on panel C3 is in the arm or arm/press position and the right OMS eng vlv switch on panel O16 is on; the valves are de-energized normally when thrust off is commanded or if the right OMS eng switch is positioned to off . The left OMS engine gaseous nitrogen solenoid control valves 1 and 2 are controlled in the same manner, but through the left OMS eng switch on panel C3 and the left OMS eng vlv switch on panel O14.
When the gaseous nitrogen solenoid control valves are energized open, pressure is directed into the two actuators in each engine. The nitrogen acts against the piston in each actuator, overcoming the spring force on the opposite side of the actuators. Each actuator has a rack-and-pinion gear; and the linear motion of the actuator connecting arm is converted into rotary motion, which drives two ball valves, one fuel and one oxidizer, to the open position. Each pair of ball valves opens simultaneously. Fuel and oxidizer are then directed to the combustion chamber of the engine, where the propellants atomize and ignite upon contact. The hypergolic propellants produce a hot gas, thus thrust.
The chamber pressure of each engine is monitored by a pressure sensor and is transmitted to the OMS press left and right Pc (chamber pressure) meter on panel F7.
When the computer commands thrust off or an engine's OMS eng switch on panel C3 or eng vlv switch on panel O14/O16 is positioned off, the solenoid control valves are de-energized, removing gaseous nitrogen pressure from the actuators; and the gaseous nitrogen pressure in the actuators is vented overboard through the solenoid control valve. The spring in the actuator forces the actuator's piston to move in the opposite direction, and the actuator drives the fuel and oxidizer ball valves closed simultaneously. The series-redundant arrangement of ball valves ensures engine thrusting is terminated.
Each actuator incorporates a linear position transducer, which supplies ball valve position to a CRT.
Check valves are installed in the vent port outlet of each gaseous nitrogen solenoid control valve on the spring pressure side of each actuator to protect the seal of these components from atmospheric contamination.
Each engine has two gaseous nitrogen purge valves in series. These valves are solenoid-operated open and spring-loaded closed. They are normally energized open after each thrusting period by the GPCs unless inhibited by a crew entry on the maneuver CRT display. The two purge valves of an engine are energized open 0.36 second after OMS engine thrust off has been commanded and permit gaseous nitrogen to flow through the valves and check valve into the fuel line downstream of the ball valves and out through the combustion chamber and engine injector to space for two seconds. This purges the residual fuel from the combustion chamber and injector of the engine, permitting safe engine restart. The purge valves are then de-energized and spring-loaded closed. When the purge is completed, the gaseous nitrogen tank pressure isolation valve is closed by placing the respective OMS eng switch (panel C3) to off. The check valve downstream of the purge valves prevents fuel from flowing to the engine purge valves during engine thrusting.
The contoured nozzle extension is bolted to the aft flange of the combustion chamber. The nozzle extension is made of a columbium alloy and is radiantly cooled.
The nominal flow rate of oxidizer and fuel to each engine is 11.93 pounds per second and 7.23 pounds per second, respectively, producing 6,000 pounds of thrust at a vacuum specific impulse of 313 seconds.
The sequence determines which engines are selected and then provides the necessary computer commands to open the appropriate helium vapor isolation valves and the engine gaseous nitrogen solenoid control valves and sets an engine-on indicator. The sequence will monitor the OMS engine fail flags and, if one or both engines have failed, issue the appropriate OMS cutoff commands as soon as the crew has confirmed the failure by placing the OMS eng switch in the off position. This will then terminate the appropriate engine's control valve commands.
In a normal OMS thrusting period, when the OMS cutoff flag is true, the sequence terminates commands to the helium pressurization, helium vapor isolation valves and two gaseous nitrogen engine control valves. If the engine purge sequence is not inhibited, the sequence will check for the left and right engine arm press signals and after 0.36 second open the engine gaseous nitrogen purge valves for two seconds for the engines that have the arm press signals present.
The gimbal ring assembly contains two mounting pads to attach the engine to the gimbal ring and two pads to attach the gimbal ring to the orbiter. The ring transmits engine thrust to the pod and orbiter.
The pitch and yaw gimbal actuator assembly for each OMS engine provides the force to gimbal the engines. Each actuator contains a primary and secondary motor and drive gears. The primary and secondary drive systems are isolated and are not operated concurrently. Each actuator consists of two redundant brushless dc motors and gear trains, a single jackscrew and nut-tube assembly and redundant linear position feedback transducers. A GPC position command signal from the primary electronic controller energizes the primary dc motor, which is coupled with a reduction gear and a no-back device. The output from the primary power train drives the jackscrew of the drive assembly, causing the nut-tube to translate (with the secondary power train at idle), which causes angular engine movement. If the primary power train is inoperative, a GPC position command from the secondary electronic controller energizes the secondary dc motor, providing linear travel by applying torque to the nut-tube through the spline that extends along the nut-tube for the stroke length of the unit. Rotation of the nut-tube about the stationary jackscrew causes the nut-tube to move along the screw. A no-back device in each drive system prevents backdriving of the standby system.
The electrical interface, power and electronic control elements for active and standby control channels are assembled in separate enclosures designated the active actuator controller and standby actuator controller. These are mounted on the OMS/RCS pod structure. The active and standby actuator controllers are electrically and mechanically interchangeable.
The gimbal assembly provides control angles of plus or minus 6 degrees in pitch and plus or minus 7 degrees in yaw with clearance provided for an additional 1 degree for snubbing and tolerances. The engine null position is with the engine nozzles up 15 degrees 49 seconds (as projected in the orbiter XZ plane) and outboard 6 degrees 30 seconds (measured in the 15-degree 49-second plane).
The thrust vector control command subsystem operating program processes and outputs pitch and yaw OMS engine actuator commands and the actuator power selection discretes. The OMS TVC command SOP is active during operational sequences, orbit insertion (OMS-1 and OMS-2), orbit coast, deorbit, deorbit coast and return-to-launch-site abort.
The flight crew can select either the primary or the secondary motors of the pitch and yaw actuators by item entry on the maneuver display or can select actuators off. The actuator command outputs are selected by the TVC command SOP depending on the flag that is present, i.e., major modes, deorbit maneuver, orbit coast, and RTLS abort, center-of-gravity trim and gimbal check. The deorbit maneuver coast flag causes the TVC command SOP to output I-loaded values to command the engines to the entry stowed position. The presence of the RTLS abort and center-of-gravity trim flags causes the engines to be commanded to a predefined position with the thrust vector through the center of gravity. The major mode RTLS flag by itself will cause the engines to be commanded to a stowed position for return-to-launch-site entry. The gimbal check flag causes the engines to be commanded to plus 7 degrees yaw and 6 degrees pitch, then to minus 7 degrees yaw and 6 degrees pitch, and back to zero degrees yaw and pitch. In the absence of these flags, the TVC command SOP will output the digital autopilot gimbal actuator commands to the engine actuators. The backup flight control system allows only manual TVC during a thrusting period, but it is otherwise similar.
The OMS TVC feedback SOP monitors the primary and secondary actuator selection discretes from the maneuver display and performs compensation on the selected pitch and yaw actuator feedback data. This data is output to the OMS actuator fault detection and identification and to the maneuver display. The OMS TVC feedback SOP is active during orbit insertion (OMS-1 and OMS-2), orbit coast, deorbit maneuver and deorbit maneuver coast. The present OMS gimbal positions can be monitored on the maneuver CRT display when this SOP is active and the primary or secondary actuator motors are selected.
Each OMS/RCS pod is divided into eight heater areas. Each of the heater areas in the pods contains an A and B element, and each element has a thermostat that controls the temperature from 55 to 75 F. These heater elements are controlled by the left pod and right pod switches on panel A14. Sensors located throughout the pods supply temperature information to the propellant thermal CRT display and telemetry.
The crossfeed line thermal control in the aft fuselage is divided into 11 heater areas. Each area is heated in parallel by heater systems A and B, and each area has a control thermostat to maintain temperature at 55 F minimum to 75 F maximum. Each circuit also has an overtemperature thermostat to protect against a failed-on heater switch. These heater elements are controlled by the respective crsfd lines switch on panel A14. Temperature sensors near the control thermostats on the crossfeed and bleed lines supply temperature information on the propellant thermal CRT display and telemetry.
The orbital interconnect sequence is available during orbit operations and on-orbit checkout.
The flight crew must first configure the following switches (using a feed from the left OMS as an example): (1) posi tion the aft left RCS tank isolation 1/2, 3/4/5A and 3/4/5B and aft right RCS tank isolation 1/2, 3/4/5A and 3/4/5B switches on panel O7 to close; (2) check that the talkback indicator above these switches indicates cl, and position the aft left RCS crossfeed 1/2, 3/4/5 and aft right RCS crossfeed 1/2, 3/4/5 switches to open; (3) check that the indicators show op and open the left OMS tank isolation A and B valves (panel O8) and verify the talkback indicators show op ; (4) open the left OMS crossfeed A and B valves and verify the indicators show op ; (5) close the right OMS crossfeed A and B valves and verify the indicators show cl; and (6) position the left OMS He press/vapor isol valve A switch in the GPC position. The left OMS-to-aft-RCS interconnect sequence can then be initiated by item entry on the RCS SPEC display.
The left OMS helium pressure vapor isolation valve A will be commanded open when the left OMS tank (ullage) pressure decays to 236 psig, and the open commands will be terminated 30 seconds later. If the left OMS tank (ullage) pressure remains below 236 psia, the sequence will set an OMS/RCS valve miscompare flag and will set a Class 3 alarm and a CRT fault message. The sequence also will enable the OMS-to-RCS gauging sequence at the same time.
The flight crew can terminate the sequence and inhibit the OMS-to-RCS gauging sequence by use of the OMS press ena-off item entry on the RCS SPEC display. The valves can then be reconfigured to their normal position on panels O7 and O8. The OMS-to-aft-RCS interconnect sequence is not available in the backup flight control system.
The gauging sequence maintains a cumulative total of left and right OMS propellant used during OMS-to-aft-RCS interconnects and displays the cumulative totals as percentage of left and right OMS propellant on the RCS SPEC display. The flight crew will be alerted by a Class 3 alarm and a fault message when the total quantity used from either OMS pod exceeds 1,000 pounds or 8.37 percent.
Premission-determined parameters are provided for the OMS and aft RCS thrusting periods during aborts since the propellant loading and orbiter center of gravity vary with each mission.
The premission-determined parameters for the abort-to-orbit thrusting period are modified during flight, based on the vehicle velocity at abort initiation. The premission-determined parameters for abort once around are grouped with different values for early or late AOA. The return-to-launch-site parameters are contained in a single table.
The abort control sequence is available in OPS 1 and 6 and is initiated at SRB separation if selected before then or at the time of selection if after SRB separation.
ATO and AOA Aborts. The OMS and aft RCS begin thrusting as soon as an ATO or AOA is initiated with one main engine out.
For some aborts, an OMS-to-aft-RCS interconnect is not desired. A parallel aft RCS plus X thrusting period using aft RCS propellant and the four aft RCS plus X thrusters will be performed during the OMS-1 thrusting period to achieve the desired orbit. If a plus X aft RCS thrusting period is required before main engine cutoff, the abort control sequence will command the four aft plus X RCS jets on if vehicle acceleration is greater than 0.8 g and will monitor the RCS cutoff time to terminate the thrusting period. If an RCS propellant dump (burn) is required before MECO and vehicle acceleration is greater than 1.8 g, the abort control sequence will command an eight-aft-RCS-jet null thrust and monitor the RCS cutoff time to terminate the thrusting period.
In other abort cases, an OMS-to-aft-RCS interconnect is desired. This thrusting is performed with the OMS and four aft RCS plus X thrusters to consume OMS propellant for orbiter center-of-gravity control. More aft RCS jets can be commanded if needed to increase OMS propellant usage. For example, for an OMS propellant dump (burn), 14 aft RCS null jets can be commanded to thrust to improve orbiter center-of-gravity location.
If the amount of OMS propellant used before MECO leaves less than 28 percent of OMS propellants, a 15-second aft RCS ullage thrust is performed after MECO to provide a positive OMS propellant feed to start the OMS-1 thrusting period.
The OMS-to-aft-RCS interconnect sequence provides for an automatic interconnect of the OMS propellant to the aft RCS when required and reconfigures the propellant feed from the OMS and aft RCS tanks to their normal state after the thrusting periods have ended. The interconnect sequence is initiated by the abort control sequence.
In order to establish a known configuration of the valves, the interconnect sequence terminates the GPC commands to the following valves if they have not been terminated before honoring a request from the abort control sequence: left and right OMS crossfeed A and B valves, aft RCS crossfeed valves and aft RCS tank isolation valves.
A request from the abort control sequence for an OMS-to-aft-RCS interconnect will sequentially configure the OMS/RCS valves as follows: close the left and right aft RCS propellant tank isolation valves, open the left and right OMS crossfeed A and B valves, and open the left and right aft RCS crossfeed valves. The OMS-to-aft-RCS interconnect complete flag is then set to true.
When the abort control sequence requests a return to normal configuration, all affected OMS/RCS propellant valve commands are removed to establish a known condition; and the interconnect sequence will then sequentially configure the valves as follows: close aft RCS crossfeed valves, close left and right OMS crossfeed valves and open aft RCS propellant tank isolation valves. The OMS-to-aft-RCS reconfiguration complete flag is then set to false, and the sequence is terminated.
Return-to-Launch-Site Abort. An RTLS abort requires the dumping of OMS propellant by burning the OMS propellant through both OMS engines and through the 24 aft RCS thrusters to improve abort performance and to achieve an acceptable entry orbiter vehicle weight and center-of-gravity location. The thrusting period is premission-determined and depends on the OMS propellant load.
The OMS engines start the thrusting sequence; and after the OMS-to-aft-RCS interconnect is complete, the aft RCS thrusters are commanded on. The OMS engines and RCS thrusters then continue their burn for a predetermined period. The interconnect sequence is the same for ATO and AOA aborts. The OMS and aft RCS will begin thrusting at SRB staging if the abort is initiated during the first stage of flight or immediately upon abort initiation during second stage.
Contingency Abort. A contingency abort is selected automatically at the loss of a second main engine or manually by the flight crew using an item entry on the RTLS TRAJ or RTLS TRANS CRT displays. For the contingency aborts, the OMS-to-aft-RCS interconnect is performed in a modified manner to allow continuous flow of propellants to the aft RCS jets for vehicle control and to allow contingency rapid dump (burning) of OMS and RCS propellants. The abort control sequence tracks the total time the OMS and aft RCS are on to determine the amount of propellants used.
The request for an interconnect will cause the interconnect sequence to configure the valves sequentially as follows: open the aft RCS crossfeed valves, open the left OMS crossfeed valves A, open the right OMS crossfeed valves B, close the left and right aft RCS tank isolation valves, open the left OMS crossfeed valves B and open the right OMS crossfeed valves A. The OMS-to-aft-RCS interconnect complete flag will then be set to true.
If the rapid dump is selected before MECO, the OMS-to-aft-RCS interconnect occurs, and both OMS engines and the 24 aft RCS jets are commanded to thrust until the desired amount of propellant has been consumed. The rapid dump will be interrupted during external tank separation if the thrusting period is not completed before MECO; otherwise, the thrusting period terminates when thrusting time equals zero or if the normal acceleration exceeds a threshold value.
Upon completion of the thrusting period, the OMS-to-aft-RCS configuration flag will be set to false, and the sequence will be terminated. A return-to-normal-configuration request by the abort control sequence will cause the interconnect sequence to configure the valves sequentially as follows: open aft RCS propellant tank isolation valves, close the aft RCS crossfeed valves, and close the left and right OMS crossfeed A and B valves. The OMS-to-aft-RCS interconnect complete flag will be set to false, and the sequence will be terminated.
Redundancy management software performs OMS engine FDI. It is assumed that the flight crew arms only the OMS engine to be used; the OMS engine not armed cannot be used for thrusting. FDI will be initialized at SRB ignition and terminated after the OMS-1 thrusting period or, in the case of an RTLS abort, at the transition from RTLS entry to the RTLS landing sequence program. The FDI also will be initiated before each OMS burn and will be terminated after the OMS thrusting period is complete.
The OMS engine FDI uses both a velocity comparison and a chamber pressure comparison method to determine a failed-on or failed-off engine. The velocity comparison is used only after MECO since the OMS thrust is small compared to main propulsion thrust before MECO.
The measured velocity increment is compared to a predetermined one-engine and two-engine acceleration threshold value by the redundancy management software to determine the number of engines actually firing. This information, along with the assumption that an armed engine is to be used, allows the software to determine if the engine has low thrust or has shut down prematurely.
The chamber pressure comparison test compares a predetermined threshold chamber pressure level to the measured chamber pressure to determine a failed engine (on, off or low thrust).
The engine-on command and the chamber pressure are used before MECO to determine a failed engine. The velocity indication and the chamber pressure indication are used after MECO to determine a failed engine. If the engine fails the chamber pressure test but passes the velocity test after MECO, the engine will be considered failed. Such a failure would illuminate the red right OMS or left OMS caution and warning light on panel F7 and the master alarm and produce a fault message. In addition, if an engine fails the chamber pressure and velocity tests, a down arrow is displayed on the maneuver CRT next to the failed engine.
When the flight crew disarms a failed engine by turning the arm/press switch on panel C3 to off , a signal is sent to the OMS thrusting sequence to shut down the engine and to signal guidance to reconfigure. Guidance reconfigures and downmodes from two OMS engines, to one OMS engine, to four plus X RCS jets.
The OMS gimbal actuator FDI is divided into two processes. The first determines if the actuators should move from their present position. If the actuators must move, the second part determines how much they should move and whether the desired movement has occurred.
The first part checks the actuators' gimbal deflection error (which is the difference between the commanded new position and the actuators' last known position) and determines whether the actuators should extend or retract or if they are being driven against a stop. If the actuators are in the desired position or being driven against a stop, the first part of the process will be repeated. If the first part determines that the actuator should move, the second part of the actuator FDI process is performed.
The second part of the actuator FDI process checks the present position of each actuator against its last known position to determine whether the actuators have moved more than a threshold amount. If the actuators have not moved more than this amount, an actuator failure is incremented by one. Each time an actuator fails this test, the failure is again incremented by one. When the actuator failure counter reaches an I-loaded value of four, the actuator is declared failed and a fault message is output. The actuator failure counter is reset to zero any time the actuator passes the threshold test.
The first and second parts of the actuator FDI process continue to perform in this manner. The actuator FDI process can detect full-off gimbal failures and full-on failures indirectly. The full-on failure determines that the gimbal has extended or retracted too far and commands reverse motion. If no motion occurs, the actuator will be declared failed. The flight crew's response to a failed actuator is to select the secondary actuator electronics by item entry on the maneuver CRT display.
The contractors are McDonnell Douglas Astronautics Co., St. Louis, Mo. (OMS/RCS pod assembly and integration); Aerojet Tech Systems Co., Sacramento, Calif. (OMS engine); Aerojet Manufacturing Co., Fullerton, Calif. (OMS propellant tanks); Aircraft Contours, Los Angeles, Calif. (OMS pod edge member); Brunswick-Wintec, El Segundo, Calif. (OMS propellant tank acquisition screen assembly); Consolidated Controls, El Segundo, Calif. (high- and low-pressure solenoid valves and OMS regulators); Fairchild Stratos, Manhattan Beach, Calif. (hypergolic servicing couplings); Metal Bellows Co., Chatsworth, Calif. (alignment bellows); Simmonds Precision Products Inc., Vergennes, Vt. (OMS propellant gauging system); SSP Products, Burbank, Calif. (gimbal bellows assembly); Tayco Engineering, Long Beach, Calif. (electrical heaters); AiResearch Manufacturing Co., Torrance, Calif. (gimbal actuators and controllers); Futurecraft Corp., City of Industry, Calif. (OMS engine valve components); L.A. Gauge, Sun Valley, Calif. (ball valves); PSM Division of Fansteel, Los Angeles, Calif. (OMS nozzle extension); Rexnord Inc., Downers Grove, Ill. (OMS engine bearings); Sterer Engineering and Manufacturing, Pasadena, Calif. (OMS engine pressure regulator/relief valve assembly); Parker-Hannifin, Irvine, Calif. (OMS propellant tank isolation valves, relief valves, manifold interconnect valves); Rockwell International, Rocketdyne Division, Canoga Park, Calif. (OMS check valves); Brunswick, Lincoln, Neb. (OMS helium tanks); Sundstrand, Rockford, Ill. (heater thermostats).
Click Here for REACTION CONTROL SYSTEM
Click Here for Shuttle Mission Info
Return to KSC Home Page 
Table of Contents