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NATIONAL RESEARCH COUNCIL COMMISSION ON ENGINEERING AND TECHNICAL SYSTEMS 2101 Constitution Avenue Washington, D. C. 20418 COMMITTEE ON NASA SCIENTIFIC AND TECHNOLOGICAL PROGRAM REVIEWS Panel on Redesign of Space Shuttle Solid Rocket Booster March 17, 1988 The Honorable James C. Fletcher Administrator National Aeronautics and Space Administration 400 Maryland Avenue, S.W., Room 7137 Washington, D.C. 20546 Dear Jim: I am pleased to submit herewith the sixth interim report of the National Research Counci1's Panel for the Technical Evaluation of NASA's Redesign of the Space Shuttle Solid Rocket Booster. Since our last report, the Panel has conducted three meetings and some of our members have attended the DM-9 Test Readiness Review, one run of the Assembly Test Article, three meetings of the Outer Boot Ring Anomaly investigation team, and the SRM Critical Design Review Level ITI preboard and board meetings. I also presented a testimony on the progress of the redesign effort to the Senate Subcommittee on Science, Technology, and Space on February 16th. The Test Program The test program has entered the critical certification phase; with test articles and the components of the first set of flight hardware already manufactured and no slack time in the program, the remaining tests will have to proceed essen- tia1ly perfectly if the target date for the return to flight is to be met. We concur with the decision to conduct three additional fulI-sca~e, full-duration motor firings before the return to flight, i.e., Qualification Motors #6 and #7 (QM-6, QM-7) and the so-called Production Verification Motor (P~M). In par- ticular, the PVM will provide the only test in the program to verify the performance of the primary pressure seals in the case field joint and case-to~nozzle joint throughout the motor 37 The National Research Council is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering to serve government and other organizations

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Letter to the Honorable James C. Fletcher 2 burn; we believe that this test is essential to the certifi- cati on o f the design . The results of the test, including assessments of the performance of the j oints that are inten- tionally modif fed to assure pressurization of the primary O-rings, must be available for review before the return to fl ight. We also agree that the fu~i-scale, ful1-duration low ambient temperature qualification motor test (QM-8) can occur af ter the return to flight provided that the test i s conducted before the f irst launch at low ambient temperature. The Nozzle Outer Boot Ring. A1 though the motor used in the DM-9 test might have resulted in a successful flight, the structural performance of its outer boot ring was unsatis- factory. Post-test examination revealed that the margins of safety for both the structural and thermal properties of the DM-9 outer boot ring (involute design) were intolerably small. It is our understanding that the exact causes of the unsatisfactory performance are still to be determined. The needs now are to (a) understand the reasons for the poor thermal and structural behavior of the involute design and (b) determine whether these reasons are applicable to the alterna- tive, structural support design that is now baselined for flight. Any anomalous behavior of nozzle parts should be understood and corrective action taken if necessary before committing to flight. The structural support design will be subjected to test in the three additional static motor firings before the return to flight. It should be noted, however, that as a consequence of the detailed investigation of the DM-9 result, fewer data are currently available on the structural support design than on the involute. Almost all of the confidence in the structural support boot ring arises from the DM-8 test, which was less demanding than the DM-9 test, and from the experience with the design flown in previous Shuttle flights, from which the con- cept of the structural support outer boot ring has evolved. The DM-9 result illustrates that coupon data are insuf- ficient for predicting the failure modes and characteristics of composite structures for design purposes. Even though the carbon cloth pheno~ic materials used in the two outer boot ring designs are ostensibly the same based on coupon tests and chemistry, the performance of the parts depends strongly on their internal geometrical configurations and fabrication procedures. The results of future full-scale tests of the outer boot ring should be assessed from a sound and adequate base of data from laboratory tests and in-process inspections. 38

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Letter to the Honorable James C. Fletcher 3 The criteria for successful testing of the outer boot ring should be established before QM-6 is fired. We believe that test criteria should derive from design requirements. It is our understanding that the outer boot ring/flexible boot assembly is designed to prevent thermal damage to the flexible bearing. If that is the case, then suitable criteria would be expressed in terms of permissible damage (delamination, fracture, erosion, etc.) or the conditions needed to preclude damage to the bearing that would disqualify it for subsequent use. Thrust Vector Control Duty Cycles. The duty cycle of the thrust vector control (TVC) system in the DM-9 test has been suggested as a potential contributing factor to failure of the outer boot ring in that test. We believe that the TIC duty cycles used in static motor tests should be realistic but should be significantly more severe than those experienced in flight to demonstrate the margins in the design. Since the largest mismatch between the thrusts of flight boosters is likely to occur during the last 10 seconds of motor operation, the largest angular deflections in nozzle tests should be imposed early during that period. Hydroburst Tests. A case hydroburst test was recently deleted from the program. We recommend that the test be reinstated to demonstrate the performance not only of the baseline design but also of recently manufactured case material qualified to current processes. The test can and should be completed before the return to flight without introducing delays. In addition, a second hydroburst test should be conducted after flights resume in which flight-qualified hardware is subjected to repeated cycles of assembly, pressurization, and disassem- bly to simulate 20 uses before being burst. In the latter test, the case segments should be examined between cycles to monitor performance. Requirements The interpretations of several important design require- ments are as yet unresolved. The manner in which they are resolved can have significant effects on the program, particu- larly in light of the limited amount of testing that can be accomplished before the next flight. Seal Systems. The program requirements specify that each pressure vessel leak path shall have redundant, verifiable seals and that each seal shall provide independent sealing 39

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Letter to the Honorable James C. Fletcher -4- capabi~ity throughout the motor burn without evidence of blow-by or erosion. Questions have been raised about the interpretation of the requirement in the development of criteria for success of tests in which the design has been intentionally flawed. Three types of tests are envisioned: of flight design motors, of joints with intentional flaws to simulate defects that might occur in manufacturing or assem- bly, and of joints that have been modified to study the performance of specific seals. First, the success criterion for tests of joint systems without any intentionally introduced defects must be the re- quirement itself, no erosion or blow-by of seals, since these joint systems are of the flight design. These joints would only be tested if they passed post-assemb~y leak checks. We conclude that the same criterion should also apply in tests where joints are flawed in such a way that the flaws are not detectable by leak checks (the so-called manufacturing flaws), since flight articles must be able to accommodate this type of defect. This type of defect will be introduced into joints in QM-6 as well as on several short-duration test articles. Third, tests that incorporate modifications of the hard- ware to assure that motor gas pressure reaches either the primary or secondary seal (the so-called pressure-assuring flaws) are intended to verify that the pressure seals will work if and when called upon. In our view, these tests, in which the seals will not pass leak checks, are essential for certifying the design. We agree with the viewpoint, expressed by some within the program, that the success criteria for these tests, including the PVM, need not be the same as those for tests of the flight design since the test hardware must be significantly modified from the flight design in the pressure- assuring tests. We conclude that an appropriate success criterion in the later tests might allow some erosion of the seal under test but should prohibit leakage past it. Thus, in tests where modifications are introduced to assure pressure to the primary O-ring, there should be no leakage past the pri- mary O-ring and no erosion of or leakage past the secondary O-ring. Reuse of Motor Cases. The current requirement specifies that , case segments shall be designed for 20 uses. Currently, data are not available from which to judge whether the require- ment will be met. Dimensional control in the area of the field joint interference fit may prove to be very difficult to achieve after a number of reuse cycles. In light of the uncertainties about reuse and because the lead time to obtain 40

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Letter to the Honorable James C. Fletcher additional new cases is long, it is essential for both safety and maintenance of the schedule of future flights that a practical program to determine the realistic reuse potential of case segments be completed as soon as possible. The pro- gram should include the second of the hydroburst tests recommended earlier. Grease. The requirements for reuse, surface finish, and other considerations argue for the liberal use of the corrosion- preventive grease. Liberal applications of grease, however, make it more difficult to satisfy the requirement for veri- fication of pressure seals after assembly, since the grease may mask small leaks. The masking effect may, in fact, be beneficial; other rocket motor systems intentionally rely on liberal applications of grease to assist in the sealing func- tion. In view of the clear benefit of corrosion protection, potential benefit of assisting in the sealing function, and uncertain arguments concerning the detection of small leaks, we conclude that in resolving the conflicting implications of the requirements for the design, priority should be given to assuring that liberal amounts of grease are applied to the O-rings. Storage/Ag~ng. The requirements also specify that motor segments must be usable after storage for five years. The motor contains a large number of polymeric/composite com- ponents; examples are the J-seal insulation and the case-to- nozz~e joint insulation protecting the O-rings, the flexible boot and bearing, and the propellant. Materials such as these are subject to changes in properties due to aging during storage, changes that can be irreversible. Except for the propellant, the current program appears to be inadequate for understanding the aging of these materials and how aging characteristics under practical storage conditions influence the dimensional or other requirements on the polymeric parts. Control of Materials and Processes As we have stated previously, next to the design itself, the most important determinant of success, not only for the return to flight but throughout the flight program, will be assurance of the quality of materials, manufacturing, and assembly of motor components. Defining and maintaining the essential controls on materials and processes is critical to the reliability of the booster especially because the flight article itself cannot be test fired before use. Installation of environmental controls in the Vehicle Assembly Building at 41

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Letter to the Honorable James C. Fletcher 6 Kennedy Space Center represents an important improvement in controlling cleanliness for bonding, repair of debonds, and protection of components. We note below three additional areas where further attention is needed. First, when lack of bonding occurs between the insulation and case, it is usually attributed to contamination of the surface with grease. For large debonds, a procedure has been specified for repair. We suggest that NASA qualify the proce- dure using realistic laboratory specimens to be sure that the technique for repairing defects does not inadvertently weaken the bonds. A second area is the assembly of the case-to-nozzle joint while avoiding or controlling the occurrence of flaws in the polysulfide adhesive caused by trapped gas bubbles. A series of tests is being conducted to characterize the formation of these flaws and if possible to develop means for preventing them. The tests should include evaluation of the charac- teristics of the adhesive as well as procedures for its application that minimize or eliminate the occurrence of voids. The objective should be to establish appropriate controls on the materials, processes, and environment for assembly of this joint. The third area concerns the insulation barriers in the case field joint and case-to-nozzle joint. Both employ an interference fit at the mating surfaces of rubber parts as well as open slots elsewhere so that the motor pressure will act to press the parts together. The dimensional tolerances and materials properties of the rubber parts are important for assuring proper joining. The case field joint insulation cur- rently is accepted as a "product of the mold," of which there are six. The dimensions of molded parts can vary signifi- cantly, however. Therefore, we recommend that these parts be accepted based on "inspection to dimensions," the tolerances for which are determined to assure appropriate interference fits and adequately open slots. The desirable material properties of the components of the joint should also be defined and controlled. Continuing Development and Product Improvement The objective of the redesign effort is to return a safer and more reliable Shuttle to service at the earliest possible date. Safety and reliability, however, are not absolutes. The desire to provide the safest and most reliable design can conflict with the desire to return to flight as soon as possible. NASA has to make judgments about whether additional 42

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Letter to the Honorable James C. F1 etcher 7 improvements are worth the delays in the return to service that they imply. F1ying the Shuttle or any other space transportation vehicle will always entail accepting some risks. Thus far, we find no evidence that the redesign team has taken or Will be taking undue risks in favor of an earlier return to flight. However, in this as in other large aero- space systems, schedule and budgetary constraints have affected the program so that the risks associated with the first flight of the redesigned SRB, while acceptable, will be higher than if the program were not so constrained. It should not be necessary to continue to incur this additional level of risk throughout the service of the redesigned SRB, however, if there were a program of continuing development and product improvement beyond flight testing. Maintaining such a program through the operational phases of aeronautical and space systems, in fact of any complex technological system, is simply prudent practice. The purpose is to improve not only reliability and hence safety, but also performance and cost on the basis of ground test data, analy- sis, manufacturing and operational experience. Such a program permits tracking and improving design features, materials, and processes that are identified as potential problems. This kind of program is particularly important for the SRB because of the complexity of the system, limitations on the testing program, and the national importance of reliable operation. The nature of the SRB is such that only flight subjects all of its components to realistic environments. Thus, the first few flights, which will be subjected to a more intensive scrutiny than later flights, should provide much data useful for defining and reducing risks. We believe that it is essential that NASA have in place an appropriately staffed and funded program for planning and implementing a continuing development program, including provision for the requisite ground tests and determination of flight performance; the means to analyze results, upgrade and evaluate designs; and provision to introduce risk-reducing improvements into flight hardware and operations. An impor- tant result of a continuing development and product improve- ment program wit ~ be an increased understanding of the capabilities of the SRB, leading in turn perhaps to lower operational costs. We recommended such a program in previous reports. Have ng such a program requires advanced planning, which shout ~ begin now if it is to be effective. The program should have continuity with the technical efforts of the redesign 43

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Letter to the Honorable James C. Fletcher 8 program and should include dedicated personnel, continued development of analytical and computational capabilities, continued use of appropriate subscale and short-duration test facilities, full utilization of all currently planned ground firings, and provision for use of selected flight instrumen- tation after the sixth flight of the redesigned SRM. In those instances in which the element under consideration affects safety, it is essential that the program incorporate a plan for certification and adoption of modified designs, materials, or processes on a timely basis. Critical Design Review As we understand it, the Critical Design Review (CDR) is conducted to assure that the design meets the requirements. In the current instance, the timing of the CDR was unusual because the first flight set had already been built. Thus, it seems unlikely that this CDR would result in a change in the design, except in the unexpected event that an egregious error were discovered. The plan for verifying that the design meets the requirements was also considered in the CDR, and its review was more timely. The CDR process is designed to assure that no member of the review team is inhibited from raising issues that he or she believes to be important. While this objective is laudable, it also plays a role in the major weakness of the process: There appears to be no means for assuring that the CDR board deals in sufficient depth with the most important questions. The board considers only the recommendations of reviewers that have not been resolved in the several review teams or by the preboard. Some of these may have been important, but a number of issues brought to the board appeared to be of ques- tionable overall importance. Almost all requests for addi- tional studies, tests, or analyses were approved; which ones must be completed before return to flight remains to be determined. Whether many of the others will ever be done 8 e remains uncertain. It appears to us that issues of critical importance to the program may often be agreed and resolved at a lower level in the review thus escaping the proper attention of the board. Lower level reviews should concentrate on the less critical items with higher level reviews concentrating on the more important issues, whether or not the issues are contro- versial. We recommend that NASA reconsider future review board procedures to assure that the board formally considers and concentrates on the issues of highest priority. 44

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Letter to the Honorable James C. Fletcher ;.. _ 9 _ In our second interim report, we commented on the lack of a clear separation of organizational responsibility for defining requirements, making design choices, and conducting design reviews. Based on the observations of members of the Panel who attended the recent Level III SRM CDR preboard and board meetings, we could not distinguish the differences between the roles of NASA Level III and those of its contrac- tor in the preboard meeting nor was it clear that the par- ticipation of NASA Level II in either the preboard or board meetings thus far has assured an independent review. The current observations confirm our earlier comment. we c onc lude . however . that the CDR has provided an essential forum For 1uentltylng and airing issues that may be considered unresolved at this time. The atmosphere at both meetings encouraged free and open discussion. There was obviously considerable effort to be impartial, treating all comments fairly. In closing, ~ wish again to the cooperation of the redesign in conducting our activities. cc: Adm. Richard H. Truly Panel Members 45 _ The atmosphere at express our appreciation for team and other NASA personnel Sincerely, H. Guyford Stever Chairman

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