14-July-2005, 12:17 PM
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Established Member
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Join Date: Apr 2004
Location: Arkansas, USA
Posts: 1,266
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STS Engine Cut Off Sensor Failure Mode Analysis, 11/29/2001
This analysis is dated Nov. 2001, but provides some good detail into the unit and some past technical issues:
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FUNCTION:
THIS SYSTEM PROVIDES A MEANS OF MONITORING THE PROPELLANT LEVEL IN THE LO2/LH2 EXTERNAL TANKS DURING LOADING AND ASCENT. POINT SENSOR TRANSDUCERS ARE LOCATED IN THE ET AND ON THE ORBITER (LO2 17-INCH FEEDLINE) WITH SIGNAL CONDITIONING ELECTRONICS ON THE ORBITER TO MONITOR AND CONTROL LOADING AND DRAINING OF PROPELLANTS. SENSORS ARE REDUNDANT AT ALL CRITICAL LEVELS.
FOUR SENSORS IN THE ORBITER LO2 FEEDLINE AND FOUR IN THE BOTTOM OF THE ET LH2 TANK PROVIDE A SAFE BACKUP SSME CUTOFF SIGNAL TO PRECLUDE ENGINE PROPELLANT STARVATION, IN CASE A GUIDED (VELOCITY) MECO IS NOT ATTAINED. THE ECO LOGIC IS ARMED WHEN THE CALCULATED TOTAL PROPELLANT MASS REMAINING REACHES 32,000 LBS OR UPON SECOND ENGINE FAILURE FOLLOWING SRB SEPARATION.
ANY TWO QUALIFIED LH2 OR LO2 DRY SENSORS WILL GENERATE A MECO COMMAND AFTER THE SYSTEM IS ARMED.
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Quote:
FUNCTIONAL CRITICALITY EFFECTS:
1R/3 3 SUCCESS PATHS. TIME FRAME - ASCENT.
1,2,3) 3 LO2 OR 3 LH2 ECO SENSORS FAIL WET.
PROPELLANT DEPLETION MECO COMMAND CAPABILITY WILL BE LOST. SYSTEM REQUIRES 2 LO2 OR 2 LH2 DRY SIGNALS AFTER ARM COMMAND BEFORE A PROPELLANT DEPLETION SSME SHUTDOWN COMMAND CAN BE GIVEN. PROPELLANT STARVATION RESULTS IN SSME PUMP CAVITATION AND UNCONTAINED ENGINE DAMAGE. POSSIBLE LOSS OF CREW/VEHICLE.
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Quote:
DESIGN:
THE POINT SENSOR TRANSDUCERS UTILIZE 0.0005 INCH DIAMETER PLATINUM WIRE SENSING ELEMENTS MOUNTED ON A CERAMIC FRAME. THE ORBITER TRANSDUCER (LO2 ECO) HAS TWO SENSING ELEMENT ASSEMBLIES CONTAINED WITHIN A FLANGE-MOUNTED HOUSING (304 CRES) AND CONTAINS ONE ELECTRONIC CONNECTOR. THE ET TRANSDUCERS HAVE ONE SENSING ELEMENT ASSEMBLY IN A HOUSING WHICH IS MOUNTED ON MASTS AND ON THE LH2 SIPHON ASSEMBLY (BAFFLE) WITHIN THE LH2/LO2 TANKS. THE ET SENSOR USES A 356-T6 ALUMINUM INVESTMENT CASTING HOUSING WHICH IS TEFLON COATED ON THE INTERIOR TO ASSIST SYSTEM PERFORMANCE.
[snip]
SIGNALS FOR GROUND CHECKOUT ARE DISTRIBUTED THROUGH OPTICAL ISOLATION CIRCUITS ON THE CHECKOUT COMMAND ISOLATION BOARD. GROUND CHECKOUTS ARE PERFORMED PRIOR TO AND DURING PROPELLANT LOADING OPERATIONS. SENSOR CABLE
AND CONNECTOR RESISTANCES ARE VERIFIED DURING OPF CHECKS.
VEHICLE CABLING/CONNECTORS ROUTING MAINTAINS CIRCUIT ISOLATION. SIX INTERFACE ELECTRICAL CONNECTORS ARE INSTALLED ON THE HOUSING. TWO ARE USED FOR ATP CHECKOUT AND THE REMAINING FOUR ARE USED FOR OPERATIONAL FUNCTIONS. THE
SYSTEM IS DESIGNED TO PROVIDE A WET SIGNAL FOR THE PREDOMINANT FAILURE MODES (PREVENT OVERFILLING OF ET TANKS AND PREMATURE MECO). EACH CRITICAL SENSING LOCATION HAS TRANSDUCERS LOCATED TO PROVIDE EITHER VERTICAL OR HORIZONTAL REDUNDANCY (BY LOCATION).
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Quote:
FAILURE HISTORY:
EARLY PRODUCTION VERSIONS OF POINT SENSOR TRANSDUCERS SUFFERED OPEN CIRCUITS DUE TO WELDING PROBLEMS.
FOUR FAILURES OCCURRED AT THE SENSING WIRE WELD JUNCTIONS (REFERENCE CAR A5872 AND A5838 AT THE SUPPLIER; CAR AB6152
AT MPTA AND CAR AC4210 AT MARTIN MARIETTA).
TWO OTHER FAILURES OCCURRED AT THE SUPPLIER ON THE LOX ECO SENSOR NICKEL RIBBON WELD ATTACHMENT (REFERENCE CAR AB7600 AND AB8336). CORRECTIVE ACTION TAKEN ON LATER SENSOR
CONFIGURATIONS FOR ALL VEHICLES INCLUDED IMPROVED WELDING TECHNIQUES, WELDEQUIPMENT, SCREENING AND WELD SAMPLING.
SIX TRANSDUCERS OPEN CIRCUITS OCCURRED DURING EARLY SUPPLIER ATP TESTING DUE TO THE OCCURRENCE OF ICE IN LN2 TEST FIXTURE (REFERENCE CAR A5138, A5160, A5708, A5142 AND A6182). CORRECTIVE ACTION CHANGED THE TEST PROCEDURE AND EQUIPMENT TO PREVENT ICE OCCURRENCE.
EARLY SUPPLIER PRODUCTION UNITS HAD FALSE WET SIGNALS DUE TO USE OF INCORRECTLY SIZED CURRENT CONTROLLER CIRCUIT RESISTORS FOR LOW TEMPERATURE (REFERENCE CAR A5498). THE CIRCUIT RESISTORS WERE MODIFIED ON ALL UNITS.
EARLY PRODUCTION SENSORS HAD ENCOUNTERED A SERIES OF BROKEN SENSING ELEMENTS DUE TO EXCESSIVE WIRE TENSION (REFERENCE CAR A9753 AND AB0195 DURING QUALIFICATION TESTS; A5179 AT SUPPLIER; AB7231, AB7230 MPTA; AND AB8051 AT MCC). ALL VEHICLES' SENSORS WERE CHANGED TO UTILIZE LOWER WIRE TENSION.
DURING QUALIFICATION TESTS, EXCESSIVE VOLTAGE WAS INADVERTENTLY INDUCED ON THE POINT SENSOR ELECTRONICS BOX.
ONE POWER SUPPLY WAS DAMAGED (REFERENCE CAR A8679). THE TEST SETUP WAS MODIFIED TO PROTECT THE BOX FOR FUTURE TESTS.
FOUR SENSORS WERE DAMAGED DUE TO OPERATOR ERROR. ONE SENSOR WAS DAMAGED DURING QUALIFICATION TESTING (REFERENCE CAR A8619) AND ONE DURING MPTA TESTING (REFERENCE CAR A1820)DUE TO EXCESSIVE VOLTAGE. OTHER UNITS WERE PHYSICALLY DAMAGED ON THE NSTL ENGINE TEST STAND (REFERENCE CAR AB8503) AND ON MPTA (REFERENCE CAR A7983). NO CORRECTIVE ACTION WAS TAKEN DUE TO THE UNKNOWN SOURCE OF THE PROBLEMS.
FALSE WET SIGNALS OCCURRED ON THE ELECTRONICS BOX DUE TO OPERATOR ERROR (REFERENCE CAR A9384 DURING QUALIFICATION TEST; AD1694, AB0705, A9627, AD1623 AT THE SUPPLIER). PRECAUTIONARY OPERATOR INSTRUCTIONS WERE ISSUED. ONE FALSE WET SIGNAL OCCURRED DURING THE SUPPLIER ATP VIBRATION TEST DUE TO A COLD SOLDER JOINT IN A POINT SENSOR ELECTRONICS BOX (REFERENCE CAR AC5955). THIS PROBLEM IS NOT ALWAYS DETECTABLE DURING VISUAL INSPECTION; HOWEVER, THE ATP SCREENING TEST IS USED TO IDENTIFY THIS OCCURRENCE. NO CORRECTIVE ACTION WAS TAKEN OTHER THAN PRECAUTIONARY OPERATOR NOTIFICATION.
DURING SUPPLIER ATP TESTING, LOW RESISTANCE BETWEEN THE SENSING CIRCUIT AND THE TRANSDUCER HOUSING OCCURRED DUE TO OPERATOR ERROR, MANUFACTURING DEFECTS AND EXCESSIVE MOISTURE IN THE TEST FIXTURE (REFERENCE CAR A6328, A7564, A6163, A8619 AND AC4846). CORRECTIVE ACTION INCORPORATED IMPROVED MANUFACTURING, SCREENING AND TEST PROCEDURES.
OPEN CIRCUITS OF THE LOX ECO POINT SENSOR TRANSDUCERS OCCURRED DUE TO USE OF MOLYBDENUM/RHENIUM SENSING WIRE WHICH OXIDIZED IN AN OXYGEN ATMOSPHERE (REFERENCE CAR AB6852 AND AB6853 AT MPTA AND AB5397, AB6613 AT KSC). CORRECTIVE ACTION FOR ALL VEHICLES CHANGED THE SENSING ELEMENTS TO PLATINUM.
FOUR OCCURRENCES OF OPEN CIRCUITS DUE TO VEHICLE WIRING PROBLEMS (REFERENCE CAR AB1742, AB5217 AT MPTS, CAR AC4828 AT KSC OV-099, CAT AC6930 AT KSC OV-102). NO CORRECTIVE ACTIONS WERE TAKEN, OTHER THAN TO REPAIR THE VEHICLE WIRING (ET AND ORB). DURING MPTA TESTING, LOW RESISTANCE IN THE SENSOR CIRCUIT CREATED FALSE WET SIGNALS DUE TO SUSPECTED EXCESSIVE CABLE/CONNECTOR MOISTURE (LEAKAGE TO GROUND). CORRECTIVE ACTIONS INCLUDED THE INSTALLATION OF DRAIN HOLES ON THE WET WIRING CONDUITS AND DRYING OF CABLE CONNECTIONS (REFERENCE CAR'S AC0940 AND AC1761).
TWO SENSORS ENCOUNTERED TEFLON DEBONDING ON THE HOUSINGS (REFERENCE CAR AB6854 AT MPTA AND AB7544 (ET) AT KSC). CORRECTIVE ACTION TAKEN FOR ALL VEHICLES INCLUDED REMOVAL OF THE TEFLON COATING ON THE ORBITER LOX ECO SENSORS AND USE OF A DIFFERENT PROCESS SUPPLIER FOR THE ET SENSOR TEFLON COATINGS PLUS IMPROVED QUALITY CONTROL.
DURING LAUNCH OPERATIONS OF OV-102 (STS-1, 2 AND 6), ONE SENSOR ON EACH OF THE ET'S (DIFFERENT LOCATIONS) FAILED TO PROVIDE THE PROPER DRY SIGNAL. A BROKEN SENSING ELEMENT IN THE TRANSDUCER IS SUSPECTED TO BE THE CAUSE OF THE FAILURE (REFERENCE CAR 06F036 AND AC0934). POST FLIGHT FAILURE ANALYSIS WAS NOT POSSIBLE DUE TO UNAVAILABILITY OF THE ET.
DURING CHECKOUT OF POINT SENSOR ELECTRONICS BOX ON OV-102 (STS-2) AT KSC, FALSE LOX ECO WET SIGNALS OCCURRED DUE TO USE OF RESISTORS SIZED FOR MOLYBDENUM/RHENIUM SENSING WIRE (REFERENCE CAR AB8685). ALL BOXES WERE MODIFIED TO UTILIZE RESISTORS SIZED FOR USE WITH PLATINUM SENSING WIRES.
ON OV-099 CHECKOUT AT KSC, THE POINT SENSOR ELECTRONICS BOX GENERATED FALSE WET SIGNALS DUE TO A STRUCTURAL FAILURE OF THE BOX HOUSING (REFERENCE CAR 13F006). ONE POWER SUPPLY PRINTED CIRCUIT BOARD FAILED DUE TO SUSPECTED BOX OVERPRESSURIZATION WHICH ALLOWED THE BOARD TO FALL OUT OF THE CARD GUIDE. THE DAMAGED HOUSING AND BOARD WERE REPLACED.
AT KSC ON OV-099, STS-8 CHECKOUT, A FALSE WET SIGNAL OCCURRED DUE TO A DEFECTIVE RESISTOR (REFERENCE CAR AC6567). THE FAULTY RESISTOR WAS REPLACED. THIS WAS CONSIDERED A RANDOM FAILURE.
VARIOUS FALSE DRY OR WET SIGNALS HAVE OCCURRED DUE TO DEFECTIVE HYBRID INTEGRATED CIRCUITS (REFERENCE CAR AB6709 AT KSC AND CAR'S A6390, A7100, A9315, AB0132 AT SUPPLIER). CORRECTIVE ACTION CONSISTED OF USE OF NEW HYBRID MANUFACTURER PLUS USE OF IMPROVED MANUFACTURING TECHNIQUES, TESTING AND SCREENING.
CURRENT DATA ON TEST FAILURE, FLIGHT FAILURE, UNEXPLAINED ANOMALIES, AND OTHER FAILURES EXPERIENCED DURING GROUND PROCESSING ACTIVITY CAN BE FOUND IN THE PRACA DATABASE.
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If anyone finds the report for the May 2005 failure, be sure to post it here. Sorry for the poor formatting in this post.
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In the progress of this discussion I shall endeavor to give a satisfactory answer to all the objections which shall have made their appearance, that may seem to have any claim to your attention.
Alexander Hamilton, Federalist No. 1
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