Find a Security Clearance Job!

Space



OMS GIMBAL ACTUATOR FDI...

REACTION CONTROL SYSTEM

REACTION CONTROL SYSTEM

The orbiter's reaction control system comprises the forward and aft RCS. The forward RCS is located in the forward fuselage nose area. The aft (right and left) RCS is located with the orbital maneuvering system in the OMS/RCS pods.

Each RCS consists of high-pressure gaseous helium storage tanks, pressure regulation and relief systems, a fuel and oxidizer tank, a system that distributes propellant to its engines, and thermal control systems (electrical heaters).

The forward and aft RCS units provide the thrust for attitude (rotational) maneuvers (pitch, yaw and roll) and for small velocity changes along the orbiter axis (translation maneuvers).

The ascent profile of a mission determines the interaction of the RCS units, which depends on the number (one or two) of OMS thrusting periods. After main engine cutoff, the forward and aft thrusters are used to maintain attitude hold until external tank separation. Then the reaction control system provides a minus (negative) Z translation maneuver of about 4 feet per second to move the orbiter away from the external tank. Upon completion of the maneuver, the RCS holds the orbiter attitude until it is time to maneuver to the OMS-1 thrusting attitude. Although the targeting data for the OMS-1 thrusting period is selected before launch, the target data in the onboard general-purpose computers can be modified by the flight crew via the CRT and keyboard, if necessary, before the OMS thrusting period.

The first thrusting period of the orbital maneuvering system (OMS-1) uses both OMS engines to raise the orbiter to a predetermined elliptical orbit. During the OMS-1 thrusting period, vehicle attitude is maintained by gimbaling (swiveling) the OMS engines. The reaction control system normally does not operate during an OMS thrusting period. If, during an OMS thrusting period, the gimbal rate or gimbal limits are exceeded, RCS roll control would be required; or if only one OMS engine is used during a thrusting period, RCS roll control would be 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 out through the right-side T-0 umbilical overboard fill and drain system. This velocity is precomputed in conjunction with the OMS-1 thrusting period.

Upon completion of the OMS-1 thrusting period, the reaction control system can be used to null any residual velocities, if required. The flight crew uses the rotational hand controller or translational hand controller to command the applicable RCS thrusters to null the residual velocities. The reaction control system then provides attitude hold until it is time to maneuver to the OMS-2 thrusting attitude.

The second thrusting period of the orbital maneuvering system (OMS-2) uses both OMS engines near the apogee of the orbit established by the OMS-1 thrusting period 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 computers can be modified by the flight crew on the computer keyboard, if necessary, before the OMS thrusting period.

Upon completion of the OMS-2 thrusting period, the reaction control system can be used to null any residual velocities, if required. It is then used for 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 in orbit. Normally, the vernier 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 MPS dump. The RCS is used for attitude hold after the 5-feet-per-second translation maneuver. The OMS-2 thrusting period is then used to achieve orbit insertion. This profile allows the MPS to provide more energy for orbit insertion and permits easier use of onboard software.

Additional OMS thrusting periods using one or both OMS engines are performed on orbit as needed for rendezvous, for payload deployment or for transfer to another orbit.

For the deorbit thrusting maneuver, the two OMS engines are used. Target data for the deorbit maneuver is computed on the ground, loaded in the onboard general-purpose computers by uplink and voiced to the flight crew for verification of loaded values. The flight crew then initiates an OMS gimbal test by item entry in the CRT keyboard unit.

Before the deorbit thrusting period, the flight crew moves the spacecraft to the desired attitude using the thrusters. After the OMS thrusting period, the RCS is used to null any residual velocities, if required. The spacecraft is then moved 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 yaw thrusters before entry interface if orbiter center-of-gravity control is necessary.

The aft RCS plus X jets can be used to complete any OMS deorbit thrusting period if an OMS engine fails. In this case, the OMS-to-aft-RCS interconnect can be used to feed OMS propellant to the aft RCS.

From an entry interface of 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.

Two helium tanks supply gaseous helium pressure to the oxidizer and fuel tanks. The oxidizer and fuel are then supplied under gaseous helium pressure to the RCS engines. Nitrogen tetroxide is the oxidizer, and monomethyl hydrazine is the fuel. The propellants are Earth-storable and hypergolic (they ignite upon contact with each other). The propellants are supplied to the engines, where they atomize, ignite and produce a hot gas and thrust.

The forward RCS has 14 primary and two vernier engines. The aft RCS has 12 primary and two vernier engines in each pod. The primary RCS engines provide 870 pounds of vacuum thrust each, and the vernier RCS engines provide 24 pounds of vacuum thrust each. The oxidizer-to-fuel ratio for each engine is 1.6-to-1. The nominal chamber pressure of the primary engines is 152 psia. For each vernier engine, it is 110 psia.

The primary engines are reusable for a minimum of 100 missions and are capable of sustaining 20,000 starts and 12,800 seconds of cumulative firing. The primary engines are operable in a maximum steady-state thrusting mode of one to 150 seconds, with a maximum single-mission contingency of 800 seconds for the aft RCS plus X engines and 300 seconds maximum for the forward RCS minus X engines as well as in a pulse mode with a minimum impulse thrusting time of 0.08 second above 125,000 feet. The expansion ratio (exit area to throat area) of the primary engines ranges from 22-to-1 to 30-to-1. The multiple primary thrusters provide redundancy.

The vernier engines' reusability depends on chamber life. They are capable of sustaining 330,000 starts and 125,000 seconds of cumulative firings. The vernier engines are operable in a steady-state thrusting mode of one to 125 seconds maximum as well as in a pulse mode with a minimum impulse time of 0.08 second. The vernier engines are used for finite maneuvers and stationkeeping (long-time attitude hold) and have an expansion ratio that ranges from 20-to-1 to 50-to-1. The vernier thrusters are not redundant.

PRESSURIZATION SYSTEM

    Each RCS has two helium storage tanks, four helium isolation valves, four pressure regulators, two relief valves, two check valves, two manually operated valves and servicing connections for draining and filling.

    The helium storage tanks are composite spheres and consist of a titanium liner with a Kevlar structural overwrap that increases safety and decreases the tank weight over conventional titanium tanks. Each helium tank is 18.71 inches in diameter with a volume of 3,043 cubic inches and a dry weight of 24 pounds. Each helium tank is serviced to 3,600 psia.

    The two helium tanks in each RCS supply gaseous helium individually, one to the fuel tank and one to the oxidizer tank.

    There are two parallel helium isolation valves between the helium tanks and the pressure regulators in each RCS. When open, the helium isolation valves permit the helium source pressure to flow to the propellant tank. The helium isolation valves are controlled by the fwd RCS He press A/B switches on panel O8 and the aft left RCS He press A/B and aft right RCS He press A/B switches on panel O7. Each switch controls two helium isolation valves, one in the oxidizer helium line and one in the fuel helium line. The switch positions are open, GPC and close. When positioned to GPC , the pair of valves is automatically opened or closed upon command from the orbiter computer. The open/close position permits manual control of that pair of valves.

    Electrical power is momentarily applied through logic in an electrical load controller assembly to energize the two helium isolation solenoid valves open and to magnetically latch the valves open. To close the two helium isolation valves, electrical power is momentarily applied through the load controller to energize a solenoid surrounding the magnetic latch of the two helium isolation valves, which allows spring and helium pressure to force the valve closed.

    A position microswitch in each valve indicates valve position to an electrical controller assembly and controls a position indicator (talkback) above each switch on panels O7 and O8. When both valves (helium fuel and helium oxidizer) are open, the talkback indicates op ; and when both valves are closed, the talkback indicates cl . If one valve is open and the other is closed, the talkback indicates barberpole.

    The RCS helium supply pressure is monitored on panel O3. The rotary switch on panel O3 positioned to RCS He X10 allows the forward and aft RCS helium pressures to be displayed on the RCS/OMS press fuel and oxid meters on panel O3.

    Helium pressure is regulated by two regulator assemblies, connected in parallel, downstream of the helium isolation valves. Each assembly contains two stages, a primary and a secondary, connected in series. If the primary stage fails open, the secondary stage regulates the pressure. The primary regulates the pressure at 242 to 248 psig, the secondary at 253 to 259 psig.

    The check valve assembly, which consists of four poppets in a series-parallel arrangement, is located between the pressure regulator assemblies and the propellant tank. The series arrangement limits the backflow of propellant vapor and maintains propellant tank pressure integrity in the event of an upstream helium leak. The parallel arrangement ensures the flow of helium pressure to the propellant tank if a series check valve fails in the closed position.

    A helium pressure relief valve assembly is located between the check valve assemblies and propellant tank and will vent excessive pressure overboard before it reaches the propellant tank. Each valve consists of a burst diaphragm, filter and relief valve. The non-fragmentation diaphragm provides a positive seal against helium leakage and will rupture between 324 to 340 psig. The filter prevents any particles of the burst diaphragm from reaching the relief valve seat. The relief valve relieves at 315 psig minimum. The relief valve is sized to handle, without damaging the propellant tank, helium pressure flow volume if a regulator malfunctions to a full-open position.

PROPELLANT SYSTEM

    The system that distributes the propellants to the RCS thrusters consists of fuel and oxidizer tanks, tank isolation valves, manifold isolation valves, crossfeed valves, distribution lines and filling and draining service connections.

    Each RCS contains two spherical propellant tanks, one for fuel and one for oxidizer, constructed of titanium and 39 inches in diameter.

    The nominal full load of the forward and aft RCS tanks in each pod is 1,464 pounds in the oxidizer tanks and 923 pounds in the fuel tanks. The dry weight of the forward tanks is 70.4 pounds. The dry weight of the aft tanks is 77 pounds.

    Each tank is pressurized with helium, which expels the propellant into an internally mounted, surface-tension, propellant acquisition device that acquires and delivers the propellant to the RCS thrusters on demand. The propellant acquisition device is required because of the orbiter's orientation during boost, on orbit, and during entry and because of the omnidirectional acceleration spectrum, which ranges from very high during boost, entry or abort to very low during orbital operation. The forward RCS propellant tanks have propellant acquisition devices designed to operate primarily in a low-gravity environment, whereas the aft RCS propellant tanks are designed to operate in both high and low gravity, ensuring propellant and pressurant separation during tank operation.

    A compartmental tank with individual screen devices in both the upper and lower compartments supplies propellant independent of tank load or orientation. The devices are constructed of stainless steel and are mounted in the titanium tank shells. A titanium barrier separates the upper and lower compartments in each tank.

    At orbiter and external tank separation and for orbital operations, propellant flows from the upper compartment bulk region, into the channel network, to the upper compartment transfer tube and into the lower compartment bulk region. Flow continues from the upper compartment until gas is ingested into the upper compartment device and transferred to the lower compartment.

    The lower compartment of the forward RCS propellant tanks will expel propellant to depletion, as in the case of the upper compartment; however, orbital operations are terminated with the forward RCS at an expulsion efficiency of 91 percent to preclude gas ingestion to the forward RCS engines.

    The aft RCS propellant tanks' lower compartment is not used on orbit, but is required for entry. The aft RCS tank propellants are positioned approximately 100 degrees away from the tank outlet because of the influence of up to 2.5-g acceleration. As the acceleration builds up, the channel screen in the ullage area of both devices breaks down and ingests gas. As entry expulsion continues, propellant is withdrawn from the lower compartment until a 96.5-percent expulsion efficiency is achieved.

    The aft RCS propellant tanks incorporate an entry collector, sumps and gas traps to ensure proper operation during abort and entry mission phases. Because of these components, the aft RCS propellant tanks are approximately 7 pounds heavier than the forward RCS propellant tanks.

    The left, forward and right RCS fuel and oxidizer tank ullage pressures can be monitored on panel O3. When the rotary switch on panel O3 is positioned to RCS prplnt , the pressures are displayed on the RCS/OMS press fuel, oxid meters on panel O3. The pressures will illuminate the left RCS, fwd RCS or right RCS red caution and warning light on panel F7, respectively, if that module's tank ullage pressure is below 200 psia or above 312 psia.

RCS QUANTITY MONITO

    The RCS quantity monitor sequence uses the general-purpose computer to calculate the usable percent of fuel and oxidizer in each RCS module. The RCS quantities are computed based on the pressure, volume and temperature method, which requires that pressure and temperature measurements be combined with a unique set of constants to calculate the percent remaining in each of the six propellant tanks. Correction factors are included for residual tank propellant at depletion, gauging inaccuracy and trapped line propellant. The computed quantity represents the usable (rather than total) quantity for each module and makes it possible to determine if the difference between each pair of tanks exceeds a preset tolerance (leakage detection).

    The calculations include effects of helium gas compressibility, helium pressure vessel expansion at high pressure, oxidizer vapor pressure as a function of temperature, and oxidizer and fuel density as a function of temperature and pressure. The sequence assumes that helium flows to the propellant tanks to replace propellant leaving. As a result, the computed quantity remaining in a propellant tank will be decreased by normal usage, propellant leaks or helium leaks.

    The left, right and forward RCS quantities are displayed to the flight crew on panel O3. When the rotary switch on panel O3 is positioned to the RCS fuel or oxid position, the RCS/OMS qty meters on panel O3 will indicate, in percent, the amount of fuel or oxidizer. If the switch is positioned to RCS lowest, the gauging system selects whichever is lower (fuel or oxidizer) for display on the RCS/OMS prplt qty, left, fwd and right meter.

    The left, right and forward RCS quantities also are sent to the cathode ray tube, and in the event of failures, substitution of alternate measurements and the corresponding quantity will be displayed on the CRT. If no substitute is available, the quantity calculation for that tank is suspended with a fault message.

    The sequence also provides automatic closure of the high-pressure helium isolation valves on orbit when the propellant tank ullage pressure is above 312 psia. The caution and warning red light on panel F7 is illuminated for the respective forward, left or right RCS, and a fault message is sent to the CRT. When the tank ullage pressure returns below this limit, the close command is removed.

    Exceeding a preset absolute difference of 12.6 percent between the fuel and oxidizer propellant masses will illuminate the respective left RCS, right RCS or fwd RCS red caution and warning light on panel F7; activate the backup caution and warning light; and cause a fault message to be sent to the CRT. A bias of 12.6 percent is added when a leak is detected so that subsequent leaks in that same module may be detected.

ENGINE PROPELLANT FEED

    The propellant tank isolation valves are located between the propellant tanks and the manifold isolation valves and are used to isolate the propellant tanks from the remainder of the propellant distribution system. The isolation valves are ac-motor-operated and consist of a lift-off ball flow control device and an actuator assembly that contains a motor, gear train and sector gear. One pair of valves (one fuel valve and one oxidizer valve) isolates the propellant tanks from the 1/2 manifold in the forward and aft left and right RCS. One pair of valves isolates the propellant tanks from the 3/4/5 manifold in the forward RCS; and two pairs of valves, in parallel, identified as A and B, isolate the propellant tanks from the 3/4/5 manifold in the aft left and right RCS.

    The forward RCS tank isolation valves are controlled by the fwd RCS tank isolation 1/2 and 3/4/5 switches on panel O8. The aft RCS tank isolation valves are controlled by the aft left RCS tank isolation 1/2 and 3/4/5 A and B and aft right RCS tank isolation 1/2 and 3/4/5 A and B switches on panel O7. These are permanent-position switches that have three settings: open, GPC and close.

    When the fwd RCS tank isolation 1/2 and 3/4/5 switches are positioned to GPC, that pair of valves is automatically opened or closed upon command from the orbiter computer. When the corresponding pair of valves is opened, fuel and oxidizer from the propellant tanks are allowed to flow to the corresponding manifold isolation valves. Electrical power is provided to an electrical motor controller assembly that 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 actuator.

    A talkback indicator above each tank's isolation switch on panel O8 shows the status of that pair of valves. The talkback indicator is controlled by microswitches in each pair of valves. The talkback indicator shows op or cl when that pair of valves is open or closed and barberpole when the valves are in transit or one valve is open and the other is closed. The open and close positions of the fwd RCS tank isolation 1/2 and 3/4/5 switches on panel O8 permit manual control of the corresponding pair of valves.

    The forward RCS manifold isolation valves are between the tank isolation valves and the forward RCS engines. The manifold isolation valves for manifolds 1, 2, 3 and 4 are the same type of ac-motor-operated valves as the propellant tank isolation valves and are controlled by the same type of motor-switching logic. The forward RCS manifold valve pairs are controlled by the fwd RCS manifold isolation 1, 2, 3, 4 and 5 switches on panel O8. When a switch is positioned to GPC , that pair of valves is automatically opened or closed upon command from the orbiter computer. A talkback indicator above the 1, 2, 3, 4 and 5 switch on panel O8 indicates the status of that pair of valves. The talkback indicator is controlled in the same manner as the tank isolation valve indication. The open and close positions of the manifold isolation 1, 2, 3, 4 or 5 switch on panel O8 permit manual control of the corresponding pair of valves. The fwd RCS manifold 1, 2, 3 and 4 switches control propellants for the forward primary RCS engine only.

    The fwd RCS manifold 5 switch controls the manifold 5 fuel and oxidizer valves, which control propellants for the forward vernier RCS engines only. The switch is normally in the GPC position, but it can be placed in either open or close for manual override capability. Electrical power is momentarily applied through logic in an electrical load controller assembly to energize the solenoid valves open and magnetically latch the valves. To close the valves, electrical power is momentarily applied to energize the solenoids surrounding the magnetic latches of the valves, which allows spring and propellant pressure to force the valves closed. A position microswitch in each valve indicates valve position to an electrical controller assembly and controls a position talkback indicator above the switch on panel O8. When both valves are open, the indicator shows op ; and when both valves are closed, it indicates cl . If one valve is open and the other is closed, the talkback indicator shows barberpole.

    The open, GPC and close positions of the aft left RCS tank isolation 1/2 and 3/4/5 A and B and aft right RCS tank isolation 1/2 and 3/4/5 A and B switches on panel O7 are the same type as those of the forward RCS tank isolation switches and are controlled electrically in the same manner. A talkback indicator above each switch indicates the position of the pair of valves as in the forward RCS. The 3/4/5 A and B switches control parallel fuel and oxidizer tank isolation valves to permit or isolate propellants to the respective aft left and aft right RCS manifold isolation valves 3, 4 and 5.

    The aft left and aft right manifold isolation valves are controlled by the aft left RCS manifold isolation 1, 2, 3, 4, 5 and aft right RCS manifold isolation switches on panel O7. The open, GPC and close positions of each switch are the same type as the forward RCS manifold isolation switch positions and are controlled electrically in the same manner. The aft left and aft right RCS manifold 1, 2, 3 and 4 switches provide corre sponding tank propellants to the applicable primary RCS engines or isolate the propellants from the engines. The aft left and aft right RCS manifold 5 switch provides corresponding tank propellants to the applicable vernier RCS engines or isolates the propellants from the engines.

    Each RCS engine is identified by the propellant manifold that supplies the engine and by the direction of the engine plume. The first identifier is a letter-F, L or R. These designate an engine as forward, left aft or right aft RCS. The second identifier is a number-1 through 5. These designate the propellant manifold. The third identifier is a letter- A (aft), F (forward), L (left), R (right), U (up), D (down). These designate the direction of the engine plume. For example, engines F2U, F3U and F1U are forward RCS engines receiving propellants from forward RCS manifolds 2, 3 and 1, respectively; the engine plume direction is up.

    If either aft RCS pod's propellant system must be isolated from its RCS jets, the other aft RCS propellant system can be configured to crossfeed propellant. The aft RCS crossfeed valves that tie the crossfeed manifold into the propellant distribution lines below the tank isolation valves can be configured so that one aft RCS propellant system can feed both left and right RCS engines. The aft RCS crossfeed valves are ac-motor-operated valve actuators and identical in design and operation to the propellant tank isolation valves. The aft RCS crossfeed valves are controlled by the aft left and aft right RCS crossfeed 1/2 and 3/4/5 switches on panel O7. The positions of the four switches are open, GPC and close. The GPC position allows the orbiter computer to automatically control the crossfeed valves, and the open and close positions enable manual control. The open position of the aft left RCS crossfeed 1/2 and 3/4/5 switches permits the aft left RCS to supply propellants to the aft right RCS crossfeed valves, which must be opened by placing the aft right RCS crossfeed 1/2 and 3/4/5 switches to the open position for propellant flow to the aft right RCS engines. (Note that the aft right RCS tank isolation 1/2 and 3/4/5 A and B valves must be closed.) The close position of the aft left and aft right RCS crossfeed 1/2 and 3/4/5 switches isolates the crossfeed capability. The crossfeed of the aft right RCS to the left RCS would be accomplished by positioning the aft right and left RCS crossfeed switches to open and positioning the aft left RCS tank isolation 1/2 and 3/4/5 A, B switches to close . (Note that the aft left RCS tank isolation 1/2 and 3/4/5 A and B valves must be closed.)

    There are 64 ac-motor-operated valves in the OMS/RCS nitrogen tetroxide and monomethyl hydrazine propellant systems. Each of these valves was modified to incorporate a 0.25-inch-diameter stainless steel sniff line from each valve actuator to the mold line of the orbiter. The sniff line from each valve actuator permits the monitoring of nitrogen tetroxide or monomethyl hydrazine in the electrical portion of each valve actuator during ground operations.

    The sniff lines from each of the 12 forward RCS valve actuators are routed to the respective forward RCS nitrogen tetroxide or monomethyl hydrazine servicing panels (six to the nitrogen tetroxide servicing panel and six to the monomethyl hydrazine servicing panel). The remaining 52 sniff lines are in the left and right OMS/RCS pods. During ground operations, an interscan checks for the presence of nitrogen tetroxide or monomethyl hydrazine in the electrical portion of the valve actuators.

    An electrical microswitch located in each of the ac-motor-operated valve actuators provides an electrical signal (open or closed) to the onboard flight crew displays and controls and to telemetry. An extensive program was implemented to reduce the probability of floating particulates in the electrical microswitch portion of each ac-motor-operated valve actuator, which could affect the operation of the microswitch in each valve.

RCS ENGINES

    Each RCS engine contains a fuel and oxidizer valve, injector head assembly, combustion chamber, nozzle and electrical junction box.

    Each primary RCS engine has one fuel and one oxidizer solenoid-operated pilot poppet valve that is energized open by an electrical thrust-on command, permitting the propellant hydraulic pressure to open the main valve poppet and allow the respective propellant to flow through the injector into the combustion chamber. When the thrust-on command is terminated, the valves are de-energized and closed by spring and pressure loads.

    Each vernier RCS engine has one fuel and one oxidizer solenoid-operated poppet valve. The valves are energized open by an electrical thrust-on command. When the thrust-on command is terminated, the valves are de-energized and closed by spring and pressure loads.

    The primary RCS engine injector head assembly has injector holes arranged in two concentric rings; the outer ring is fuel and the inner ring is oxidizer. They are canted toward each other to cause impingement of the fuel and oxidizer streams for combustion within the combustion chamber. Separate outer fuel injector holes provide film cooling of the combustion chamber walls.

    Each of the six vernier RCS engines has a single pair of fuel and oxidizer injector holes canted to cause impingement of the fuel and oxidizer streams for combustion.

    The combustion chamber of each RCS engine is constructed of columbium with a columbium disilicide coating to prevent oxidation. The nozzle of each RCS engine is tailored to match the external contour of the forward RCS module or the left and right aft RCS pods. The nozzle is radiation-cooled, and insulation around the combustion chamber and nozzle prevents the excessive heat of 2,000 to 2,400 F from radiating into the orbiter structure.

    The electrical junction box for each RCS engine has electrical connections for an electrical heater, a chamber pressure transducer, a leak detection device for each valve, and the propellant valves.

    Because of the possibility of random but infrequent combustion instability of the primary RCS thrusters, which could cause a burnthrough in the combustion chamber wall of a RCS primary thruster in a very few seconds, an instability protection system is incorporated into each of the 38 primary RCS thrusters. The electrical power wire of each primary RCS thruster fuel and oxidizer valve is wrapped around the outside of each primary RCS thruster combustion chamber wall. If instability occurs within a primary RCS thruster, the burnthrough would cut the electrical power wire to that primary RCS thruster's valves, remove electrical power to the valves, close the valves and render the thruster inoperative for the remainder of the mission.

HEATERS

    Electrical heaters are provided in the forward RCS module and the OMS/RCS pods to maintain the propellants in the module and pods at safe operating temperatures and to maintain safe operating temperatures for the injector of each primary and vernier RCS thruster.

    Each primary RCS thruster has a 20-watt heater, except the four aft-firing thrusters, which have 30-watt heaters. Each vernier RCS thruster has a 10-watt heater.

    The forward RCS has six heaters mounted on radiation panels in six locations. Each OMS/RCS pod is divided into nine heater zones. Each zone is controlled in parallel by an A and B heater system. The aft RCS thruster housing contains heaters for the yaw, pitch up, pitch down and vernier thrusters in addition to the aft OMS/RCS drain and purge panels. The OMS/RCS heater switches are located on panel A14.

    The forward RCS panel heaters are controlled by the fwd RCS auto A, B, off switch on panel A14. When the fwd RCS switch is positioned to auto A or B, thermostats on the forward left-side panel and right-side panel automatically control the respective forward RCS heaters. When the respective forward RCS panel temperature reaches a minimum of approximately 55 F, the respective panel heaters are turned on. When the temperature reaches a maximum of approximately 75 F, the heaters are turned off. The off position removes all electrical power from the forward RCS heaters.

    The aft RCS heaters are controlled by the left pod auto A and auto B and right pod auto A and auto B switches on panel A14. When the switches are positioned to either auto A or auto B, thermostats automatically control the nine individual heater zones in each pod. Each heater zone is different, but generally the thermostats control the temperature between approximately 55 F minimum to approximately 75 F maximum. The off position of the respective switch removes all electrical power from that pod heater system.

    The forward and aft RCS primary and vernier thruster heaters are controlled by the fwd and aft RCS jet 1, 2, 3, 4 and 5 switches on panel A14. When the switches are positioned to auto , individual thermostats on each thruster automatically control the individual heaters on each thruster. The primary RCS thruster heaters turn on between approximately 66 to 76 F and turn off between approximately 94 to 109 F. The vernier RCS thruster heaters turn on between approximately 140 to 150 F and off between approximately 184 to 194 F. The off position of the switches removes all electrical power from the thruster heaters. The 1, 2, 3, 4 and 5 designations refer to propellant manifolds. There are two to four thrusters per manifold.

RCS JET SELECTION

    The RCS sends pressure, temperature and valve position data to the data processing system through the flight-critical multiplexers/demultiplexers for processing by the orbiter computers. The computers use the data to monitor and display the configuration and status of the RCS. The DPS provides valve configuration and jet on/off commands to the RCS by way of the aft and forward reaction jet drivers. Data from the RCS through the MDMs also are sent to the pulse code modulation master unit for incorporation into the downlink to ground telemetry and to the orbiter's onboard recorders.

    The RJDs AND fire commands A and B for an RCS jet. If both are true, they send a voltage to open the RCS fuel and oxidizer solenoid valves. This voltage is used to generate the RJD discrete. Fire command B also is sent and used by the RCS redundancy management. The RJD driver and logic power for the aft and forward RJDs are controlled by the RJDA-1A L2/R2, RJDA-2A L4/R4 and RJDF-1B F1 manf logic and driver on and off switches on panel O14; RJDA-1B L1/L5/R1 and RJDF-1A F2 manf logic and driver switches on panel O15; and RJDA-2B L3/R3/R5, RJDF-2A F3 and RJDF-2A F4/F5 manf logic and driver switches on panel O16.

    The RCS redundancy management monitors the RCS jets' chamber pressure discretes, fuel and oxidizer injector temperatures, RJD on/off output discretes, jet fire commands and manifold valve status.

    The DPS software provides status information on any RCS errors to the RCS redundancy management software. The errors are referred to as communications faults. When an RCS error is detected by any orbiter computer for two consecutive cycles, the data on the entire chain are flagged as invalid for the applications software. Communications faults in the RCS redundancy management help to prevent the redundant orbiter computers from moding to dissimilar software, to optimize the number of RCS jets available for use, and to prevent the RCS redundancy management from generating additional alerts to the flight control operational software. The RCS redundancy management will reconfigure for communications faults regardless of whether the communications faults are permanent, transient or subsequently removed. On subsequent transactions, if the problem is isolated, only the faulty element is flagged as invalid.

    The RCS-jet-failed-on monitor uses the jet fire command B discretes, the RJD on/off output, the jet deselect inhibit discretes and the jet communications fault discretes as inputs from each of the 44 jets. The RCS-jet-failed-on logic checks for the presence of an RJD-on discrete when no jet fire command B exists. It outputs that the RCS jet has failed on if this calculation is true for three consecutive cycles during any flight phase. Note that the consecutive cycles are not affected by communications faults or by cycles in which there are fire commands for the affected RCS jet. However, the three-consecutive-cycle logic will be reset if the non-commanded jet has its RJD output discrete reset to indicate the jet is not firing. A jet-failed-on determination sets the jet-failed-on discrete (even for a minimum jet fire command pulse of 80 milliseconds on and off) and outputs the jet-failed-on indication to the backup caution and warning light, the yellow caution and warning RCS jet light on panel F7, a fault message on the CRT and an audible alarm. These discretes will be reset when the associated RCS jet redundancy management inhibit discrete is reset by the flight crew. A jet failed on will not be automatically deselected by the RCS redundancy management, and the orbiter digital autopilot will not reconfigure the jet selections.

    The RCS-jet-failed-off monitor uses the RCS jet fire command B discretes, the jet chamber pressure discretes, the RCS jet-deselect inhibit discretes and the jet communications fault discrete as inputs from each of the 44 jets. The RCS-jet-failed-off logic checks for the absence of the jet chamber pressure discretes when a jet fire command B discrete exists. It outputs that the RCS jet has failed off if true for three consecutive cycles. The consecutive cycles are not affected by the communications faults or by cycles in which there are no fire commands for the affected RCS jet. However, the three-consecutive-cycle logic leading to a failed-off indication must begin anew if, before the third consecutive cycle is reached, the fire command and its associated chamber pressure indicate that the RCS jet has fired. A jet-failed-off determination sets the jet-failed-off discrete (even for a minimum jet fire command pulse of 80 milliseconds on and off) and outputs the jet-failed-off indication to the backup caution and warning light, the yellow RCS jet light on panel F7, a fault message on the CRT and an audible alarm. The RCS-jet-failed-off monitor will be inhibited for the jet failed off until the flight crew resets the redundancy management inhibit discrete. The RCS redundancy management will automatically deselect a jet that has failed off, and the DAP will reconfigure the jet selection accordingly. The RCS redundancy management will announce a failed-off jet, but will not deselect the jet if the jet's redundancy management inhibit discrete has been set in advance.

    The RCS-jet-failed leak monitor uses the RCS jet fuel and oxidizer injector temperatures for each of the 44 jets with the specified temperatures of 30 F for oxidizer and 20 F for fuel for the primary and 130 F for the vernier jets (in OPS 2 and 8). It declares a jet-failed leak if any of the temperatures are less than the specified limit for three consecutive cycles. An RCS-jet-failed leak monitor outputs the RCS-jet-failed leak to the backup caution and warning light, the yellow RCS jet caution and warning light on panel F7, a fault message on the CRT and an audible alarm. The RCS-jet-failed leak monitor will be inhibited until the flight crew resets the RCS redundancy management inhibit discrete. The RCS redundancy management will automatically deselect a jet declared leaking, and the DAP will reconfigure the jet selection accordingly. The RCS redundancy management will announce a failed leak jet, but it will not deselect the jet if the jet's redundancy management inhibit discrete has been set in advance.

    The RCS jet fault limit module limits the number of jets that can be automatically deselected in response to failures detected by RCS redundancy management. The limits are modifiable by the flight crew input on the RCS SPEC display (RCS forward, left, right jet fail limit). This module also reconfigures a jet's availability status. Automatic deselection of a jet occurs if all the following are satisfied: jet detected failed off or leak (jet-on failures do not result in automatic deselection), jet-select/deselect status is select, jet's manifold status is open, redundancy management is not inhibited for this jet, jet failure has not been overridden, and the number of automatic deselections of primary jets on that aft RCS pod is less than the associated jet fail limit (no limit on vernier jets). A jet's status can be changed from deselect to select only by item entry on the RCS SPEC page. Automatic deselection of a jet can be prevented by use of the inhibit item entries on the RCS SPEC page.

    The manifold status monitor uses the open and close discretes of the oxidizer and fuel manifold isolation valves to determine their open/close status independently of status changes made by the flight crew. The flight crew can override the status of all manifolds on an individual basis by item entries on the RCS SPEC. The use of the manifold status override feature will not inhibit or modify any of the other functions of the manifold status monitor.

    The available jet status table module provides a list of jets available for use to the flight control system. The available jet status table uses the manifold open/close discretes from the manifold status monitor and the jet-deselect output discretes from the jet fault limit module as inputs. This table outputs the jet available discretes and the jet status discrete. The available jet status module shows a jet as available to the flight control system if the jet-deselect output discretes and the manifold open/close discretes indicate select and open, respectively. The available jet status table will be computed each time the jet status change discrete is true.

    The digital autopilot jet-select module contains default logic in certain instances. When the orbiter is mated to the ET, roll rate default logic inhibits roll rotation, and yaw commands are normally in the direction of favorable yaw-roll coupling. During insertion, a limit of seven aft RCS jets per tank set applies for ET separation and for return-to-launch-site aborts. If negative Z and plus X translation commands are commanded simultaneously, both will be degraded. A limit of four aft RCS jets per tank set normally applies. Plus X is degraded when simultaneous negative Z and plus X and Y translation and yaw rotation commands exceed a demand of five aft RCS jets. If plus X and negative Z translations are commanded simultaneously, plus X is given priority.

    The DAP jet-select module determines which aft RCS jets (right, left or both) must be turned on in response to the pitch, roll and yaw jet commands from the entry flight control system. The forward RCS jets are not used during entry. After entry interface, only the four Y-axis and six Z-axis RCS jets on each aft RCS pod are used. No X-axis or vernier jets are used. The DAP sends the discretes that designate which aft RCS jets are available for firing (a maximum of four RCS jets per pod may be fired) and, during reconfiguration or when the RCS crossfeed valves are open, the maximum combined total number of yaw jets available during certain pitch and roll maneuvers.

    During ascent or entry, the DAP jet-select logic module in the flight control system receives both RCS rotation and translation commands. By using a table lookup technique, the module outputs 38 jet on/off commands to the RCS command subsystem operating program, which then generates dual fire commands A and B to the individual RCS reaction jet drivers to turn each of the 38 primary RCS jets on or off. The fire commands A and B for each of the 38 primary RCS jets are set equal to the digital autopilot RCS commands. Commands are issued to the six RCS vernier jets similarly on orbit.

    The transition digital autopilot becomes active immediately after main engine cutoff and maintains attitude hold in preparation for ET separation. The transition DAP controls the spacecraft in response to control stick steering or automatic commands during orbit insertion OMS thrusting periods, orbit coast, on-orbit checkout, deorbit maneuver and deorbit maneuver coast. These commands are converted to OMS engine deflections (thrust vector control) during OMS insertion thrusting periods and RCS jet firing during the insertion phase. RCS commands are issued to support OMS rotations (roll control) when only one OMS engine is used or for rotation, attitude hold or translation when the OMS engines are not thrusting. The transition DAP uses attitude feedback and velocity increments from the inertial measurement units through the attitude processor. This feedback information allows the transition DAP to operate as a closed-loop system for pointing and rotation, but not for translation.

    The on-orbit DAP and RCS command orbit subsystem operating program generate the dual fire commands to the individual RCS jets in response to commands from the flight control system during orbit operations and on-orbit checkout. The fire A and fire B commands for each jet are set equal to the on-orbit DAP RCS commands. The fire B commands are also sent to redundancy management. There are automatic or control stick steering rotation mode, manual translation and primary or vernier RCS capabilities on orbit.

    The automated or guided rotation commands are supplied by the universal pointing processor, and control stick steering rotation or translation commands are supplied by the rotational hand controller or translational hand controller. Crew commands from the flight deck forward or aft station are accepted. Three selectable control stick steering rotation modes and two selectable translation modes (for X, Y and Z translations) are provided. The capability to select nose (forward RCS) or tail (aft RCS) only for pitch and/or yaw control is provided by the primary jets. Primary jet roll control is provided only by the aft RCS jets.

    The vernier jets are used for tight attitude dead bands and fuel conservation. The loss of one down-firing vernier jet results in the loss of the entire vernier mode.

    The on-orbit DAP has two sets of initialized dead bands - DAP A and DAP B. DAP A is used for maneuvers that do not require accurate pointing. DAP B has a narrow dead band and is used for maneuvers that require accurate pointing, such as IMU alignment.

    The entry and landing RCS command subsystem operating program generates the dual fire commands to the individual RCS thrusters in response to commands from the flight control system during entry guidance, terminal area energy management, and approach and landing. This program sets the fire A and fire B commands equal to the aerojet DAP commands or the return-to-launch-site abort DAP commands, depending on the one selected by the flight control system. These commands are sent to the 20 aft RCS Y and Z jets. The fire B commands are also sent to redundancy management.

    The aerojet DAP is a set of general equations used to develop effector commands that will control and stabilize the orbiter during its descent to landing. The aerojet DAP resides in the entry OPS but is used only during entry, terminal area energy management, and approach and landing.

    This is accomplished by using either control stick steering commands or automatic commands as inputs to the equations. The solution of these equations results in fire commands to the available RCS jets and/or appropriate orbiter aerosurfaces.

    The on-orbit and transition digital autopilots also are rate command control systems. Sensed body rate feedback is employed for stability augmentation in all three axes. This basic rate system is retained in a complex network of equations whose principal terms are constantly changing to provide the necessary vehicle stability while ensuring sufficient maneuvering capability to follow the planned trajectory.

    For exoatmospheric flight or flight during the trajectory in which certain control surfaces are rendered ineffective by adverse aerodynamics, a combination of aft RCS jet commands and aerosurface commands is issued. For conventional vehicle flight in the atmosphere, the solution of equations results in deflection commands to the elevons, rudder, speed brake and body flap. Inputs from entry guidance can consist of automatic attitude, angle of attack, surface position and acceleration commands and control stick steering roll, pitch and yaw rate commands from the flight-crew-operated controllers or a combination of the two, since the software channels may be moded independently.

    Roll, pitch and yaw indicator lights on panel F6 indicate the presence of an RCS command during entry, terminal area energy management, and approach and landing. The indicators are L and R for roll and yaw left or right and U and D for pitch up and down. Their primary function is to indicate when more than two yaw jets are commanded and when the elevon drive rate is saturated.

    From entry interface until the dynamic pressure is greater than 10 pounds per square foot, the roll l and roll r lights indicate that left or right roll commands have been issued by the DAP. The minimum light-on duration is extended to allow the light to be seen even for a minimum impulse firing. When a dynamic pressure of 10 pounds per square foot has been sensed, neither roll light will be illuminated until 50 pounds per square foot has been sensed and two RCS yaw jets are commanded on.

    The pitch lights indicate up and down pitch jet commands until a dynamic pressure of 20 pounds per square foot is sensed, after which the pitch jets are no longer used. When 50 pounds per square foot is sensed, the pitch lights assume a new function. Both pitch lights will be illuminated whenever the elevon surface drive rate exceeds 20 degrees per second (10 degrees per second if only one hydraulic system is remaining).

    The yaw lights function as yaw jet command indicators throughout entry until the yaw jets are disabled at approximately 45,000 feet. The yaw lights have no other function.

    The forward RCS module and OMS/RCS pods can be removed to facilitate orbiter turnaround, if required, and are reusable for a minimum of 100 missions.

    The contractors are McDonnell Douglas Astronautics Co., St. Louis, Mo. (OMS/RCS pod assembly and integration); CCI Corp., Marquardt Co., Van Nuys, Calif. (primary and vernier thrusters); Brunswick, Lincoln, Neb. (RCS helium tanks); Consolidated Controls, El Segundo, Calif. (dc solenoid RCS high-pressure helium isolation valves and low-pressure vernier engine manifold isolation valves); Cox and Co., New York, N.Y. (RCS electrical heaters); Fairchild Stratos, Manhattan Beach, Calif. (RCS helium pressure regulators, propellant couplings, nitrogen tetroxide/monomethyl hydrazine and helium fill disconnects); Honeywell Inc., Clearwater, Fla. (RCS reaction jet drivers); Martin Marietta, Denver, Colo. (RCS propellant tanks); Metal Bellows Co., Chatsworth, Calif. (RCS flexible line assembly); Parker Hannifin, Irvine, Calif. (ac-motor-operated tank and manifold isolation valves, OMS/RCS crossfeed interconnect valves and manually operated isolation OMS/RCS valves); Rockwell International, Rocketdyne Division, Canoga Park, Calif. (RCS check valves); Brunswick-Wintec, Los Angeles, Calif. (filters).

    Click Here for ELECTRICAL POWER SYSTEM

Table of Contents


Information content from the NSTS Shuttle Reference Manual (1988)
Last Hypertexed Wednesday October 11 17:44:07 EDT 1995
Jim Dumoulin (dumoulin@titan.ksc.nasa.gov)



NEWSLETTER
Join the GlobalSecurity.org mailing list