Hale wayne

Date post:22-May-2015
View:13,631 times
Download:5 times
Share this document with a friend
  • 1. PROJECT MANAGEMENT CHALLENGE 2007SHARED VOYAGES: LESSONS LEARNED FROM THE EXTERNAL TANKHow I Found Out That We Are Not Nearly As Smart As We Thought We WereN. Wayne Hale1Manager, Space Shuttle Program

2. We dont know a millionth of one percent about anything.Thomas A. EdisonProgress comes from the intelligent use of experience.Elbert HubbardExperience teaches the teachable.Aldous HuxleyIt aint what ya dont know that will get ya, its what you thinkya know that aint so.Yogi BerraThe ET is just a big, dumb drop tank.anonymous Shuttle Commander in the early years 2 3. Quotes from Chapter 6: Engineering Culture the messy interior of engineering practice, which after the accident investigation looks like an accident waiting to happen is nothing more than normal technology. Normal technologyis unruly.experts are operating with far greater levels of ambiguity, needing to makeuncertain judgments in less than clearly structured situations.Practices do not follow rules, rather, rules follow evolving practices.In the implementation and operation of complex technological systems, new rules andrelationships are continually being invented and negotiated.Information generated by anomaly, by discrepancy between expected and actualoutcomes becomes the means by which fallible rule sets are corrected and movedtoward solution sets. This general tendency is profoundly realized in engineeringwork. Learning proceeds through iteration. 3 4. Quotes from Chapter 6: Engineering Culture Absolute certainty can never be attained for many reasons, one of them being that even without limits on time and other resources, engineers can never be sure they have foreseen all possible contingencies, asked and answered every question, played out every scenario.Many technologiescannot be tested in laboratory conditions. Tests are conductedon models, which can only approximate the complex systemic forces of nature andtechnical environment. This situation creates risk: the world outside the laboratorybecomes the setting for experiments.Judgments are always made under conditions of imperfect knowledge.The essence of engineering as a craft is to convert uncertainty to certainty, figuringprobabilities and predictions for technologies that seldom stay the samein theworkplace, engineers formulate the rules as they go along, attempting to capture theunruly technology with numbers, experienced based theories, and practical rules.Even in closure there is ambiguity.4 5. 5 6. History of the External Tank STS-1: December 29, 1980STS-2: Columbia is mated to its ET/SRB stack Repair operations to holescaused by woodpeckers on ETfor STS-70Only part of the Space Shuttle Vehicle not returned for reuse and evaluation!6 7. 7 8. ET Production History121 Units Delivered to DateDeliveredFlownThree Versions: Standard Weight Tank66Al 2219 (Al=Aluminum) (1981 83)Dry Wt. 77,099 lbs. (actual ET1) Lightweight Tank 8786Al 2219(1983 98, 2002, 2003)Dry Wt. 65,767 lbs(actual ET71) Super Lightweight Tank2821 Al 2195 (Al-Li = Aluminum Lithium)(1998 Present) Dry Wt. 58,319 lbs. (actual ET96)Substantially Completed Tanks 4 8 9. External Tank Foam pre-STS-107 Prior to STS-107,foam loss was regarded as a vehicle processing issue, not a safety of flight issue.9 10. ET CRYOINSULATION: General PropertiesFoam / Property (HCFC) NCFI 24-124(HCFC) NCFI 24-57 (HCFC) PDL 1034(HCFC) BX265(HCFC) SS 1171(CFC) CPR 488(CFC) NCFI 22-65(CFC) PDL 4034(CFC) BX 250(% of total foam)(77%) (7%)(1%)(14%)Application LH2,L02,I/T sidewall LH2 aft domeCloseouts and repairsLH2 forward dome, L02 aft dome, closeoutsProcessSpraySprayPour/Mold SprayDescriptionIsocyanurate Isocyanurate UrethaneUrethane RequirementsSpec TypFltSpecTypFlt Spec TypFltSpec Typ Flt ReqPropPredReq PropPred ReqPropPredRezProp PredDensity PCF 2.0-2.5 2.28Lighter6 2.6-3.12.97Heavier6 2.3-3.1**3.3** same61.8-2.6 2.42.1-2.6 2.42.6-3.12.902.3-3.12.6 1.8-2.6 2.4 same6 1.8-2.6 2.4Tensile RT (psi)30min441940min 6619 60113 19 35min 8035min5440min 71 6010435min 5319 35min 75Tensile -423 F (psi)N/A13419 N/A4919 N/A50 19 N/A 7419 41474962 53Tensile +300 F (psi)N/A 3219 N/A3619 N/A71219N/A5319 374553355 47 Compression (psi)25min332035min 4920 30 61 20 24min 432024min4035min 51 30 4224min 30 24min 42Recession Rate @ 7 BTU/ftN/A 0.0094lower6 N/A0.00997 same6N/A0.0303higher6 0.031 sq sec (in/sec) 0.01680.00997 0.0235 N/A0.0173lower6 0.024Thermal Cond @ R/T BTU/hr 0.0250.017 same6 0.02250.0180 higher60.0160.015higher60.0150.015ft F) 0.017 0.01580.01560.0160.012 0.0150.013 higher6 0.011Cryostrain (ksi) [email protected]@-423 [email protected] [email protected] passN/A [email protected] passN/A [email protected] [email protected] [email protected] [email protected] [email protected]/A- Not Applicable1 2+200F [email protected] 4 BTU/ft sq sec 4Max density 3.0 in dome area allowed [email protected] + 200F56Means new vs. old 7Radiant heating 82.4 2.8 PCF for thick/thin **Spec Req-cup pour; Typ Prop-dissectionReference: ET Project Design Values for Non-Metallic Materials, LM 809-9600 Rev C, May 2006. 11. ET CRYOINSULATION: Key Material Engineering AspectsLevels of Structure Polymeric StructureCellular StructureKnitline GeometrySubtrate Geometry11 12. Columbia (STS-107) In the early morning on Saturday, February 1, 2003, the Space Shuttle Columbia broke up during entry. All seven crew members were killed. 81 seconds after launch, foam insulation on the External Tank broke off and struck the Shuttles wing at Mach 2.46, creating a hole roughly the size of a pizza box. When Columbia reentered the atmosphere to land, highly heated plasma entered the breached wing, and burned or melted away the wings internal structure. The structural failure of the wing led to Crew of the Space Shuttle Columbia the loss of vehicle control and the vehicle broke apart as it descended toward Earth. CAIB: Foam Did It! but WHY did the foam do it? 12 13. ET Foam Certification Testing13 14. Loss of ET Thermal Protection System Acceptance Rationale14from the STS-121 FRR 15. How Air-Divots Are Formed15from the Physics Models for Foam Debris presentation on May 4, 2005 by Dr. Peter B. Pollock 16. Codes for Predicting Foam Debris16from the Physics Models for Foam Debris presentation on May 4, 2005 by Dr. Peter B. Pollock 17. Thermal Protection Verification (TPS) / Validation Issues17 18. 18 19. ET Major Design Changes19 20. Return to Flight (RTF) RTF concentrated on improved foam application processes to minimize defects (voids) Much tighter controls on workmanship More oversight and review Continuing practice Routine destructive evaluation of foam applied tonear flight fixtures 20 21. Return to Flight (RTF) RTF included redesign of Bipodsto eliminate the ramp andgreatly reduce foam in the area Serious review of PAL ramp,which is the largest manuallyapplied foam structure on the ET,showed no significant defects, noimprovement in safety byremoving and using newprocesses to reapply STS-107Bipod Ramp 21 STS-114 22. Return to Flight (RTF)Return to Flight was 2.5 years in the makingIt was noted in passingthat the defect/void divot debris generation theorycould not explain the STS-107 bipod loss.22 23. STS-114 RTF Results Major foam loss from Bipod wedge Major foam loss from Protuberance Air Load (PAL) ramp Significant foam losses around the Ice/Frost Ramps Near misses of Orbiter from all three areas could have beencatastrophic23 24. What Went Wrong?How Could That Happen? 24 25. Bipod wedge was lost because we introduceda new failure mechanism: Cryopumping Wires were not sealed which allowed air toliquefy and become the motive force to blow offsignificant foamClassic case of a new designhaving an undesirable side effect!25 26. What About The Other Losses? ST-120 underwent 2 tanking cycles at KSC and thenwas shipped back to the factory Evaluation of the tank delayed until MichoudAssembly Facility (MAF) operations resumed followingHurricane Katrina Immediate observation: cracks in and under the PALramp and in and under the Ice Frost Ramp (IFR)Not Seen in Previous Testing! 26 27. 12B is the second crack reported from visual inspection; A iscrack found during backscatter inspection; found in Zone 6inspectionOne of the first 2 cracks reported; crack that appears tobe closed at the surface; found in zone 5 inspection 35 This is the 4th crack detected during a backscatter inspection; this is the first crack detected in BX250; found in zone 84Second crack detected during inspection on 11/3/05;found in zone 14 Crack detected during inspection on 11/3/05; appears to run between plugs and under or into the larger plug; found in zone 13 27 28. Relearning the LessonTurns out that the full size (test) article shows there is significant differences inthermal expansion for foam on foam application, which leads to cracks, primarily onthe hydrogen tankThe Space Shuttle Program immediatelydirected the removal of the PAL rampThis caused a huge engineering recertification effort of the protuberances and their associated load capability! 28 29. But the time of loss during flight was not understood, soProbability Risk Assessment (PRA) analysis was based on theassumption that foam losses, as seen in ET separationphotography (end state), were evenly dispersed during theascent or assumed to all happen at the most critical times This lead to extremely high probabilities ofcatastrophic failure prior to the second return to flight, STS-12129 30. During STS-121, a very good image with new camera views showed adivot coming off the Ice Frost Ramp at a significant timeDetailed review of other video sources built up a record of when theCoefficient of Thermal Expansion (CTE) failure causes pieces to come offMost losses will occur after the aero region where there is not motive force to cause damage, ergo, no hazard 30 31. ET Debris Table Sensitivity(Foam on Tile Void DeltaP Risk Assessment) 31 32. Debris Overview32 33. Review of pre STS-107 imagery shows severalflights where bipod losses occurred also had lossesin foam adjacent to the bipod lossSeveral more flights showed just losses in theadjacent acreage, which tends to confirm the Bipodfoam loss of STS-107 was associated with CTEmismatch, not void defect divot33 34. MORAL(S) of the Story:1. You are never as smart as you think you are2. If the hypothesis does not explain reality, the hypothesis is not right3. Flight test is the only REAL test4. Continually question your fundamental assumptions5. Dont expect certainty34 35. The Universal AbstractWe have not succeeded in answering all ofour problems. Indeed, we have notcompletely answered any of them. Theanswers we have found have only served toraise a whole new set of questions. Insome ways, we feel as confused as ever,but we think we are confused on a muchhigher level about more important things. 35

Popular Tags:
of 35/35
1 N. Wayne Hale Manager, Space Shuttle Program PROJECT MANAGEMENT CHALLENGE 2007 SHARED VOYAGES: LESSONS LEARNED FROM THE EXTERNAL TANK How I Found Out That We Are Not Nearly As Smart As We Thought We Were
Embed Size (px)