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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. F1 etcher Administrator National Aeronautics & Space Administration 400 Maryland Avenue, S.W., Room 7137 Washington, DC 20546 Dear Jim: August 1, 1986 Based on an initial appraisal of the objectives and focus of the program to redesign the SRB, I am pleased to provide this interim report containing observations, conclusions, and recommendations of the NRC Panel for the Technical Evaluation of the Redesign of the Space Shuttle Solid Rocket Booster. Background Beginning with its formation in early June 1986, the Panel has had introductory briefings by personnel of NASA Head- quarters, Marshall Space Flight Center (MSFC), and Morton Thiokol, Inc., on the current design and manufacture of the solid rocket motor (SRM), the mission 51-L accident, and preliminary plans for the redesign. We have had more detailed briefings from and discussions with personnel of Rockwell International, Thioko1, and NASA/MSFC on the structural loads and dynamics of the total assembler shuttle and the resultant loads on the SRM, as well as briefings from personnel of United Technologies Chemical Systems Division, Aerojet Stra- tegic Propulsion Company, and Hercules Aerospace Company on alternative solid rocket designs and alternative joint and seal designs. We have toured the Thiokol manufacturing and test facilities and engaged in many detailed technical dis- cussions with Thiokol personnel on design and testing of proposed future SRB designs. We have alto been informed by Air Force personnel about the results of their investigation of the recent accident involving the Titan solid rocket motor. 1 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 Focus of the SRB Redesign Program —2— In addressing its charge to provide a technical evaluation of the redesign effort, the Panel is aware of four interde- pendent factors that influence the program. They are safety, time, costs, and performance. Above all, safety is the prime consideration. The critical national need for the launch capability of the Shuttle makes time a close second. Conse- quently, we recognize that NASA is initially concentrating on those redesign approaches that will provide the required margins of safety with maximum use of existing facilities, hardware, and components. In addition to engineering design, the Panel is also reviewing the program for testing and veri- fication of safe performance of new designs, selection and specification of materials, and quality assurance or control. In parallel with this initial approach, the Panel intends to address alternative designs for the SRM to raise the levee of not only safety and economy, but alto performance. Design Specifications and Requirements We understand that the specifications and requirements for redesign of the SRM are being completely reexamined. We have noted differences between NASA/MSFC and Thiokol personnel on this subject, particularly on the application of past specifi- cations and requirements for the redesign. Great care and prompt action should be taken to eliminate these differences and misunderstandings. We conclude that the redesign process cannot proceed successfully unless and until all parties agree to and understand the meaning of specifications and require- ments placed on the performance of the new SRB. Joints between Motor Segments While the Panel agrees with the Presidential Commission that faulty design of the pressure seal, compounded by the cold temperature at launch and dynamic joint rotation, is the most probable cause of a blow-by of the two O-ring seals in the aft field joint of one SRB, the ensuing events which resulted ultimately in the loss of the shuttle (almost 60 seconds later) are much less clear. It has been hypothesized that sealing was reestablished by a plug of material which was dislodged later, possibly during conditions of high dynamic pressure including wind shear, leading to melting of the case metal in and adj acent to the j oint by hot gas . The details, however, remain uncertain and this uncertainty is very important f or the redes ign . 2
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Letter to the Honorable James C. Fletcher —3— All failure modes must be examined, and all potential weaknesses in the design of the motor should be eliminated or reduced to the extent possible. For example, the probability of debonding of insulation from the metal case should be minimized as should the probability of the introduction of moisture into the case where corrosion of metal parts or degradation of performance of the elastomeric parts can occur. We recommend that NASA consider using existing SRM cases to perform tests on the joint to investigate the mode of failure of mission 51-L to assure that our understanding of the weaknesses in the original design is correct. The tests should be designed to investigate alternative modes of failure and to duplicate the failure of the 51-L joint. Based on its work to date, the Panel recommends that each of the following be addressed in the new design: joint rota- tion from all sources; O-ring materials; O-ring and groove design; verification of O-ring seating; insulation at the joint, considering the potential for gas flow in the gaps; asymmetries in gas circulation in the bore, gaps between seg- ments, and nozzle from various causes; the bond between the metal case and insulation, and the role and properties of putty or alternative means for protecting the O-rings from exposure to hot gases. The previous design employing two pressure-activated O-rings was adopted in response to the requirement that the seal be both redundant and verifiable. The redesign program focuses on joint designs that employ a double clevis arrangement to reduce rotation and either two or three O-ring seals, among other things. It is not cd ear to the Panel that two pressure-activated O-rings always provide redundancy. In these early stages of the redesign program, it is important to maintain consideration of a diversity of designs, including both modifications of the proposed double-clevis joint, and simpler alternatives such as reinforcing the case walls to reduce rotation. An array of alternatives should be investigated and carried through the redesign process as far as is practical. Additional discussions of joint redesign are scheduled for an early August meeting of the Panel. The Nozzle The nozzle of the SRM has experienced serious problems in the nozzle-to-case joint and the ablating material in the nozzl e itself. Work to correct these probe ems has been underway for several years but much more needs to be done. 3
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Letter to the Honorable James C. Fletcher —4— Alternative designs in which joint rotation tends to close gaps between parts rather than to open the gaps (as in previous shuttle SRM designs) appear to be inherently safer. We recommend that NASA include such designs in the redesign program. Materials Materials have always posed serious challenges in solid propulsion rocket design and new problems are emerging. Clearly the putty used heretofore in the SRB is one such material. The expected and actual performance of the putty seem quite unclear. . . . require Its use. Designs are being considered that do not Results of the investigation of the Titan accident suggests a new materials problem, a potentially erosive interaction between molten alumina and the steel case, that accentuates the need to avoid exposing steel casing to prope1- lant gases. We recommend that this phenomenon be researched vigorously because of its potential importance for the safe design of future solid rocket motors. We also recommend that NASA review its requirements and specifications for assurance of the adequacy of bonds in light of this recent development. There is also a great dependence on bonded materials without techniques for assured inspection of bonds. Progress is currently being made in non-destructive inspection tech- niques, but they sti11 leave great room for improvement. We recommend that NASA carefully monitor development of tech- niques for non-destructive testing of bonds being carried out by the Air Force and apply the techniques, if appropriate. It may also be anticipated that asbestos, currently incorporated into the inhibitor material for the SRM, may not be avail able in the future. We recommend that NASA consider developing and qualifying asbestos-free insulating material s for the SRM within the current redesign effort to avoid a subsequent requal i f ication when asbestos is no longer available . Quality control procedures for materials suppl fed by vendors appear to have depended largely on the ability of product specifications to define adequately and compl etely the material to be supplied, backed up by the qualif ication of vendors, at least for critical items. This procedure is not adequate for many materials, polymers in particular being notorious for their inability to be completely described by product speci f ication. The system should be strengthened for 4
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Letter to the Honorable James C. Fletcher —5— most components by: qualifying vendors' vendors (and in a few cases, such as the O-rings, vendors' vendors' vendors) and imposing a process "freeze" on the part of vendors (and vendors' vendors, etc.) with requalification required for any process change. We recommend that NASA review the experience of the military with control and surveillance of materials for its solid rocket motors and where applicable adapt that experience to the SRM program. Testing The fact that a solid fuel rocket can never truly be tested except in flight places unusual emphasis on adequate subsca~e and full scale testing with appropriate simulations and statistical analysis. It is our first impression that the proposed test program at best only meets a minimal require- ment; it is clearly success oriented. The Panel needs more information on the detailed tests to be conducted before we can reach an opinion about the feasibility of meeting the current test schedule, but it appears clear that the planned sample size may not be adequate for testing all specified requirements. An opportunity exists, however, to reduce some limit testing through design choices, for example using heaters at the joints would obviate the need to test joints at cold temperatures. The special issue of vertical vs. horizontal testing is sti11 open and will be further discussed when the Panel meets in early August. Dynamic performance of the shuttle system and its effects on the SRB need both testing and analysis. The critical issue is the extent to which the actual dynamic conditions of launch and flight can be simulated in a vertical test stand. A corollary issue is whether vertical testing provides enough additional data--or indeed as much useful data--as to be worth the additional cost. For equivalent expenditures and time, a larger number of horizontal tests can be performed. Sincere] my , H. Guyford Stever Chairman cc: Adm. Richard H. Truly Panel Members 5
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Representative terms from entire chapter: