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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 The Honorable James C. Fletcher Administrator National Aeronautics & Space Administration 400 Maryland Avenue, S.W., Room 7137 Washington, DC 20546 Dear Jim: November 20, 1987 I am pleased to submit herewith the fifth interim report of the National Research Counci1's Panel for the Technical Evaluation of the Redesign of the Space Shuttle Solid Rocket Booster. Since our last report, the Panel has conducted two formal meetings and members have met with the redesign team during three test readiness reviews, three technical interchanges, and the DM-8 hot firing. In contrast to our previous getters, which have dealt with the SRB program as a whole, this report addresses only the test program in preparation for first flight. The test pro- gram is progressing well and has had some notable successes in recent months. A consequence has been heightened confidence in the baseline design. As additional tests are successfully carried out, the need to continue even the current minimal effort on contingent designs for first flight diminishes. The overall testing program, including subscale tests, short-duration simulations, and full-duration firings, repre- sents a commendably extensive effort--certainly large compared with that which preceded the first Shuttle flight and very de- manding of resources. When completed, it is reasonable to anticipate that the design and performance of the joints and insulation of the Shuttle SRM will be better understood than those of any other large solid rocket motor. The timing of the program, working toward the next launch in June 1988, is "tight" and the Panel approaches with great reluctance sugges- tions which could impose additional items or time constraints on the effort. 33 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. F1 etcher 2 Nonetheless, our review of the test program has leg us to conclude that it couth be further improved in a few signifi- cant aspects, which we commend to your attention: o Despite a formal designation of the primary and secondary O-rings as the redundant seals, the designs of both the case-to-case field and case-to-nozzle joints use bonded insulation upstream of the primary O-ring to contain combustion gases within the motor. This characteristic of the designs emphasizes the importance of understanding how this insulation works as a j oint seal. In addition to the work already planned, we recommend an evaluation of the seal ing characteristics of the designs, inch uding the respective flaps and adhesives, as functions of temperature ~ def ormation rates, exposure to varied environments, and aging. O Because both the case-to-case j oint and the case-to- nozzle joint feature unvented designs, attention is re qu ired to a s sure that the pert ormance o f the pr i - mary and secondary O-rings is evaluated in the test program. We concur with the decision for a timely fur 1-scale, full-duration motor firing in which pressurization of the primary O-ring in both j oints is ensured. With no similar tests of the secondary O-rings planned on full-duration motors, we believe that the number of such tests planned for the short-duration simulation test articles shout ~ be increased. Furthermore, measures taken to ensure that the O-rings experience pressure in the respec- tive tests also prevent the pretest application of pressure in the usual way to seat the O-rings in the proper direction. Special care will therefore be required in these tests to be sure that the primary and secondary O-rings are properly seated before firing. o Because O-ring leakage can occur and not leave discernible traces in fired hardware and since the timing of leakage cannot be evaluated by post-test inspection, we recommend that the pressure both immediately upstream of the primary and between the primary and secondary O-rings in both the case-to- case field and case-to-nozzle joints be routinely measured in all ground tests, including full-scale firings. 34

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Letter to the Honorable James C. Fletcher o The repetitive tests planned to demonstrate how to make an effective insulation bondline in the case-to- nozzle joint by assembly and disassembly are very important. In addition, we believe that there should be at least one test each in which the case-to-nozzle and the case-to-case field joints are assembler, leak-tested, disassembled, cleaned-up, reassembled, and fired before first flight to simulate in advance an operation that may well be requ i red a fter flight articles are delivered to the launch site ~ This latter type of test could be accomplished in short- duration simulation tests in the Joint Environment Simulator (JES), Nozzle Joint Environment Simulator (NJES), and/or the Transient Pressure Test Article (TPTA). While most of these recommendations relate to the manner in which presently planned tests are to be carried out or to measurements to be made, some of the recommendations require additional testing on TPTA, JES, or NJES hardware. Recog- nizing the severe time constraints already imposed on these test units, the Panel recommends that if the schedule of test activities before the next flight is not sufficiently flexible to accommodate our recommendations for additional tests, con- sideration be given to providing the necessary room in the test program by deferring some of the testing now planned for so-called "manufacturing defects." In closing, I am again pleased to congratulate NASA for the high degree of profess tonal ism and dedication of the redesign team. We are grateful for the informative technical presentations and discussions which have f acid itated our task. S incerely, H. Guyford Stever Chairman cc: Adm. Richard H. Truly Panel Members 35

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