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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: October 10, 1986 I am pleased to submit herewith the second interim report of the National Research Council Panel for the Technical Evaluation of the Redesign of the Space Shuttle Solid Rocket Booster. The purpose of this letter is to provide an independent technical evaluation of the progress of the redesign program thus far. Background During August and September, the Panel formally met twice with personnel of NASA Headquarters, Marshall Space Flight Center, and Morton Thiokol, Inc. The Panel also conducted a visit to the Kennedy Space Flight Center, where we inspected recovered parts of Mission 51-L, were briefed on techniques for recovering spent solid rocket boosters (SRB) from the ocean, and witnessed a demonstration of the preparation of booster segments for mating. Members of the Panel visited the Rocket Propulsion Laboratory at Edwards Air Force Base to inspect the dormant vertical test stands there, and some attended the meeting of the Program Requirements Review Board. Members also revisited Morton Thiokol for more thorough exchanges with the company's engineering personnel. Perspective on the Redesign Program The redesign effort has progressed greatly in these two months, principally refining in detail the earlier redesign ideas for the case field joint, igniter, and nozzle joints, as well as for changes of the nozzle linings. The focus of the effort is still on designs that will permit the use of booster rocket case forgings ordered previously. If this approach is successful, i.e., if the test program succeeds and the level of safety is judged acceptable, the Shuttle flight program can resume at the earliest time. The Panel concludes that the chances of success of this approach are sufficiently good that it should be pursued. 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 The New Baseline Design —2— The baseline design of the new case field joint incorpo- rates a capture feature that engages the inside leg of the clevis to restrict relative motion between the tang and clevis. We concur that the design will significantly con- strain the motion. There are however a number of practical aspects of the design that are uncertain and of concern. The increased rigidity of the joint will cause additional bending stresses to be carried by the case membrane on either side of the joint; the additional stresses need to be fully understood. The capture feature may make mating and demating the segments more difficult than in the past. The multiplicity of addi- tional seals in the new design (an adhesive bond in the insulation, the O-ring on the capture feature, and the meta1-to-metal interference fit of the capture feature) will make it difficult to qualify and verify by test the primary and secondary pressure seals, which by definition are the two O-rings between the clevis leg and tang. While the bond between facing insulators is not intended to qualify as a pressure seal, it will normally prevent exposure and evalua- tion of the metal joint during qualification tests (as the putty did in previous tests and flights). The new design imposes more stringent dimensional tolerances and the effects of reuse in meeting the tolerances remain to be determined. These and other uncertainties must be resolved through a vigorous test and engineering program. Regarding design of the insulation in the gap between case segments, the Panel notes that the baseline bonded insulator designs, while advantageous for protecting the metal joint and insulator-to-case bond, may also pose probe ems with assembly and demating. Alternatively, an unvented joint may be obtained without using an adhesive by employing unbended insu- Jating materials to fill the gap between facing inhibitors. The inserts would have to be compressed during assembly and be sufficiently resilient to accommodate the expansion of the gap that occurs at ignition. The program has also considered and will test a vented design employing a labyrinthine gap between ablative surfaces that causes combustion gases to travel a longer distance before reaching the joint. This design as presented may pre- vent hot combustion gases from impinging on the pressure seal components. Alternative approaches to a vented design may also be practical, including those intended to assure that any gases reaching the joint have cooled to acceptable tempera- tures. Such designs, which have not been considered in the program, could, for example, employ porous ablative materials 8
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Letter to the Honorable James C. Fletcher —3— in the gaps between segments to act as thermal barriers. The structure could incorporate a volume or reservoir of rela- tively cool, stagnant combustion gases between the mated insulating segments upstream of and in series with the first seal along a potential leak path. This volume could be fi1 leaf with a porous ablative material that would physically inhibit gas exchange and absorb the sensible heat that enters the reservoir. This approach could also, of course, be used with an unvented insulator design adding further to its overall reliability. In either case , such a reservoir should be designed to dampen acoustic oscil~ ations in the gap. Pending the results of full scale tests, the Panel regards both vented and unvented insulator designs as potentially satisfactory. Therefore, the Panel recommends that both vented and unvented designs be pursued in parallel in the development effort. This recommendation is based on serious concern about the validity and scope of the testing of the baseline (bonded) design. The Panel urges that a final decision on the design of the insulation between segments be delayed until: a) the performance of both unvented and vented designs has been evaluated in full diameter, full duration tests; and b) the feasibility of assembling and disassembling the unvented design is evaluated. Tests of potential O-ring materials performed to date have resulted in the identification of three elastomers that hold promise for use in field joints. Unfortunately, because end item specifications are not generally effective for control- ~ing elastomer formulations, uncertainties may arise regarding the exact composition of the tested materials. To avoid future problems, we strongly recommend that any candidate materials be thoroughly defined, in cooperation with their suppliers, in formal advance specifications. As described in my letter of August 1st, we believe that the best approach would include specifications, even for test materials, by product, process, and supplier at all levels and to "freeze" these specifications. These should supplement performance specifications for O-ring materials. The O-ring materials being considered retain the capacity to follow the gap opening at low temperatures. In light of reduced gap openings and improved cold-temperature resilience, the use of supplementary heaters should not be necessary. It seems prudent, however, to continue to develop the heaters as a precaution in the event that these materials cannot be used for some reason. 9
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Letter to the Honorable James C. Fletcher —4— The Panel also notes that the new O-ring materials are intended to be used in the case factory joints, which will not otherwise be changed. The factory joint is fundamentally different from the field joint. We do not understand the rationale for changing the O-ring material in the factory joint. It is also not clear to us why the program continues to use spliced O-rings. While an acceptable O-ring can be made either by splicing or by one-piece precision molding, good industrial practice avoids splices. The Shuttle solid rocket motor (SRM) has frequently experienced "minor" debonding between the case and insula- tion. Reliability of case-to-insulation bonds can be improved if procedures for removing grease from the metal can be enhanced or substitutes found for the grease that is used as protection against rust. We recommend first that recent improvements in the technology for testing bonds nondestruc- tive~y be exploited fully and second that any detectable debonds be appropriately repaired before flight. The Panel still has under consideration design issues in the case-to-nozzle joint, joints within the nozzle, and the igniter. These elements of the SRM are as important for the success of the redesign as the case field joint. The baseline design of the nozzle introduces many more bolt holes, leak check ports, and pressure seals. Reliability could be reduced by some of these changes. The Test Program Much progress has been made in laying out the program of tests, which has begun. Significant results, for example from the Joint Environment Simulation test, have already been obtained, but there is still much to do. The success of the redesign effort rests on the test program. The Marshall and Johnson teams and their contractors have briefed us on the potential role of a ful1-scale vertical test firing, including structural dynamics, in the test program. They have made the point that a variety of horizontal firings and other tests can, in fact, verify the essential features of the new design. It is further argued that a vertical firing on a test stand will not reproduce the precise conditions experienced during launch and flight. The design team has recommended against incorporating a vertical firing in the program. The Panel notes that the desirability of vertical testing has been evaluated by NASA and concurs that horizontal testing can be appropriate for this situation. We conclude however that to be comprehensive, the totality of the test program must include simulation of the dynamics of launch and flight. 10
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Letter to the Honorable James C. F1 etcher —5— Design of the test program to simulate these effects should take into consideration how loads are actually trans- ferred from the solid rocket booster (SRB) to the External Tank through both fore and aft struts, the dynamic conditions known as "twang", and vectoring of the nozzle under flight conditions. Some members of the Panel believe that this is best accomplished by constructing a new full scale horizontal test stand both to be used as an alternate to the current facility and to provide a facility suitable for conducting dynamic tests of the structure, nozzle, seals, etc. The new test stand should be constructed so that loads can be taken out through realistic struts, a displacement to start the twang oscillation can be imposed, and a complete simulation of nozzle vectoring can be obtained. We recommend the creation of an additional horizontal test stand to accommodate and test full-scale motors both to pro- vide the capability of performing more tests and to reduce the risk to the program should a catastrophic failure occur during a test. We also believe that a second Joint Environment Simulator test stand and associated hardware is warranted and should be established. Regarding other aspects of the test program, the Panel offers the following comments and recommendations: a) The current test schedule envisions only one full scale, full duration test of the new nozzle that incorporates all the changes of the redesign. We recommend additional full scale, full duration testing to assure that the various elements of the design, including ablative material and additional pressure seals and leak check ports, operate as expected over the range of conditions imposed by the requirements. In our opinion, a single full scale, full duration static test of the redesigned nozzle (prior to the first flight) is simply not sufficient. The number of tests required depends on the results of the tests and professional judgment. b) We endorse whole-heartedly the concept of testing articles that have incorporated inflicted flaws. c) We believe the testing program should be designed to distinguish between alternative designs regarding performance. For example, both vented and unvented joint insulation designs should be tested in regimes in which the alternatives could be expected to behave differently. 11
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Letter to the Honorable James C. Fletcher —6— d) In fight of the specification that Shuttle components must be acceptable after five years' storage before flight, a test and surveillance program should be established that will ensure satisfactory performance of components, such as O-rings, prior to use. Repre- sentative samples of O-ring materials applicable to each assembled SRB should be stored in a compressed state in an appropriate fixture to simulate the environment of the assembled O-rings. These mate- rials should be tested for dynamic resilience as a final verification before launch. e) The tests should be fully instrumented so that incipient failure modes can be identified. We plan to conduct a continuing review of the test pro- gram, including instrumentation needs and the simulation of dynamic conditions of launch and flight, and the results over the next several months. Our next interim report will address the test plan in detail. We also will continue to review aspects of the design including assembly and disassembly of field joints and reuse. Alternative Design Considerations The alternative designs being considered by NASA meet the constraint of using the case forgings that have already been ordered. The alternatives employ various designs for the insulation in the gap between segments, such as the ~abyrin- thine vented joint or the cooler-gas vented joint. At least one such alternative will be included in the test program; we hope that several of the more promising candidates can be pursued through testing. Alternatives to the baseline designs of the metal parts of both the case joint and the case-to-nozzle joint also exist which have not been thoroughly explored on the grounds that they will not be compatible with the forgings which have already been ordered. We recognize the importance to the nation of returning the Space Shuttle to service without unnecessary delays. If, however, the design process were not constrained by the forgings, we believe that more basic alter- natives to the baseline design would probably be preferred once thoroughly analyzed. For example, it is possible to design both case and case-to-nozzle joints, without capture features, that tend to rotate closed upon pressurization or not to rotate at all. Similarly, modifications of the simple tang-and-clevis joint, 12
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Letter to the Honorable James C. Fletcher such as by changing the dimensions of the parts, might reduce the rotation substantially. The concept of a joint that rotates closed or does not rotate at all might be implemented for the case joint either with bolted flanges, as suggested by the group at NASA/Langley Research Center, or with a pinned tang-c~evis joint. With the pin concept, the tang-clevis joint would have to be drawn into a smaller radius than the case cylinder. This approach might be accommodated by the current bi1 less. Using either technique might result in the reduction or elimination of joint rotation by design rather than mechanically by using a capture feature. We strongly recommend that NASA maintain a program to explore and develop original, possibly quite different designs for the next generation of SRB in parallel with the current redesign effort and for the contingency that the baseline design may not offer sufficiently good performance and margin of safety. In particular, an alternative SRB should take advantage of designs that are insensitive to or will benefit from the structural deformations that occur after ignition. Requirements, Specifications, and Institutional Relationships There does not appear to be a very clear separation of organizational responsibility for defining requirements or specifications and designing the hardware. The Project Office at Marshall not only develops requirements and reviews and approves designs, but also is deeply involved in the detailed design process. Thus it can happen that the design team can also approve waivers to the basic requirements. We conclude that a clear separation of the responsibilities for specifying requirements and implementing a design to meet those require- ments would allow designers greater flexibility to consider alternative solutions and assure a more independent evaluation of the design in light of requirements. Several issues were raised by Panel members regarding specific requirements for the redesign during the Program Requirements Review Board meeting on August 26-27. Of these, we include the following points here: a) We understand that the baseline design may not satisfy all the requirements. Some requirements, in fact, are actually design specifications. It appears to us that NASA may be setting itself up for new waivers as a consequence. We recommend that NASA carefully reconsider the design requirements to assure that unnecessary specifications are not incorporated that will necessarily result in waivers. 13
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Letter to the Honorable James C. Fletcher —8— by A number o f dif ferent types of safety factors are quantitative y imposed by the requirements . However, they are only loosely defined in terms of what specifics y shout ~ be measured, the method of measurement, and the details of the method of calculating safety factors. c) There seem to be different interpretations of terms such as f actor o f sa f ety, redundant, verif table, etc. We believe that it is important that concise, quantif iable def initions be prepared and used . Summary In summary, we have conch uded that the chances for success with the current approach to case f ield j oint redesign are sufficiently good that it should be pursued. This choice is the consequence of the understandable desire to use existing hardware to the greatest extent possible, including new case forgings previously ordered. The baseline case field joint will be difficult to evaluate by test. Implementation of alternative designs for metal joints, which may prove to have significant advantages, would take considerably longer but should be pursued in parallel with the current redesign program. The success of the redesign effort depends on the test program. We believe that the planned test program requires significant augmentation with additional facilities and tests. It need not, however, include a full scale, full duration vertical test provided the static and dynamic loads of launch and flight, separately and in combination, are appropriately simulated elsewhere in the program. Sincerely, H. Guyford Stever Chairman cc: Adm. Richard H. Truly Panel Members 14
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