Design. The faulty Solid Rocket Motor joint and seal must be changed. This could be a new design eliminating the joint or a redesign of the current joint and seal. No design options should be prematurely precluded because of schedule, cost or reliance on existing hardware. All Solid Rocket Motor joints should satisfy the following requirements:
Independent Oversight. The Administrator of NASA should request the National Research Council to form an independent Solid Rocket Motor design oversight committee to implement the Commission's design recommendations and oversee the design effort. This committee should:
SOLID ROCKET MOTOR DESIGN
NASA has reviewed the Commission findings and recommendations and developed a plan to provide a solid rocket motor (SRM) design that satisfies all program design requirements and addresses the Commission recommendations. The primary objective of the redesign effort is to provide a solid rocket motor that is safe to fly. A secondary objective is to minimize the impact on the schedule by using existing hardware if it can be done without compromising safety.
An SRM Redesign Project Plan was developed to formalize the methodology for redesign and requalification of the solid rocket motor, including evaluation and implementation of the recommendations of the Commission. The plan provides an overview of the organizational responsibilities and relationships; the design objectives, criteria, and process; the verification approach and process; and a master schedule. The companion Development and Verification Plan defines the test program and the analyses required for verification of the redesigned and the unchanged components of the SRM. The solid rocket booster configuration is shown in Figure 1.
All aspects of the existing solid rocket motor have been assessed, and design changes required in the field joint, nozzle-to-case joint, nozzle, igniter, factory joint insulation, and ground support equipment have been identified. Design criteria have been established for each component to ensure a safe design with an adequate margin of safety. These criteria focus on loads, environments, performance, redundancy, margins of safety, and verification philosophy.
The criteria were converted into specific design requirements during preliminary requirement reviews held in July and August 1986. The design developed from these requirements was assessed at the preliminary design review in September 1986 and baselined in October 1986. The final design will be approved at the critical design review (CDR) to be held in October 1987. Manufacturing of the flight hardware will begin after the CDR and will occur in parallel with the hardware certification process.
The field joint metal parts, insulation, and seals have been redesigned to provide improved structural capability, seal redundancy, and thermal protection. A comparison of the original and new design for the field joint is shown in Figures 2 and 3. A capture latch provides a positive metal-to-metal interference around the circumference of the tang and clevis ends of the mating segments. This interference limits the amount of movement (deflection) between the tang and clevis sealing surfaces due to motor pressure and structural loads. The O-ring seals are designed to not leak under structural deflection of twice the expected values. In the STS 51-L-type design, the application of actuating pressure to the upstream face of the O-ring was essential for proper joint-sealing performance. This was necessary because large sealing gaps were created by pressureinduced deflections, O-ring groove dimensions, the O-ring diameter, and temperature.
The new design, with the tang capture....
....latch and the use of custom-fit shims between the outer surface of the tang and inner surface of the outer clevis leg, controls the sealing gap dimension and ensures positive sealing under operating conditions. The sealing gap and the O-ring seals are being designed so that there is always a positive compression (squeeze) on the O-rings. The minimum and maximum squeeze requirements provide for the effects of temperature, O-ring resiliency and compression set, and pressure. The clevis O-ring groove dimension has been increased so that the O-ring, when fully compressed, never fills more than 90 percent of the groove, and will accommodate, but does not require, pressure actuation.
The new design includes an additional leak check port to verify that there is no leakage after assembly and that the primary and secondary O-rings are positioned in the proper sealing location.
The internal case insulation has been modified to be sealed with a deflection relief flap rather than the putty used in the original configuration. A third O-ring is used to seat the primary O-ring in the proper direction and serve as a thermal barrier should the sealed insulation be breached. Longer casemating pins, with reconfigured retainer bands, will be used to improve safety margin.
External heaters with integral weather seals will maintain the field joint seal temperatures at or above 75°F and will ensure that a safe seal is maintained within specified operating environments. The weather seal will prevent water from entering the joint.
Analyses and tests identified Viton as the O-ring seal material that best meets the specified requirements to seal under all operating environments with safety margin. The joint seal safety margin will be verified in tests that expose the seal to a combination of ambient temperature limits, storage compression, grease, assembly stresses, and other environments.
The nozzle-to-case joint (Figure 4), which [16] experienced several instances of O-ring damage in flight, has been redesigned to meet the same requirements as the field joint. Sealed radial bolts have been added to minimize joint gap opening, and the insulation has been modified with additional adhesive and an interference fit. Joint closure is enhanced through use of a stress relief flap with a flow baffle and with a wiper O-ring in front of the primary O-ring. The material and size of the primary O-ring have been changed.
The nozzle metal parts, ablative components, and seals have been redesigned. The seals are redundant and verifiable. Improved bonding techniques will be used for nozzle inlet, cowl/boot, and aft exit assemblies. Nozzle inlet assembly distortion is being minimized by increasing the thickness of the aluminum housing and improving fabrication processes. The angle of the carbon cloth phenolic tape wrap (ply), for areas of the throat inlet assembly and the nozzle inlet assembly, has been changed to improve ablative insulation erosion tolerance. The cowl/ outer boot ring has additional structural support. These changes will increase the overall margin of safety in the nozzle.
The igniter and the motor case factory joints are being modified. The igniter case chamber, which houses the igniter nozzle insert, is being increased in thickness to improve the margin of safety. The factory joint (Figure 5) is being modified to provide increased margin. Additional internal insulation has been added to the factory joint. The O-ring size and groove and the pin, retainer band, and weather seal of the factory joint
....have the same, or similar, modifications as those being incorporated into the field joint. Since the factory joints have sufficient insulation and a continuous internal seal, they do not require joint heaters.
The ground support equipment has been redesigned to minimize case distortion during handling and launch site storage; improve the case measurement system; improve methods for case rounding; and improve leak testing capability. These improvements will increase the accuracy of the measurement of case diameters to facilitate stacking, lessen the risk of O-ring damage during assembly, and permit verification of the integrity of the igniter, segment, and nozzle joints after assembly.
Two ground support equipment assembly aids, a guide and round-out rings, shown schematically in Figure 6, will be used in the field joint assembly process. The guide unit clamps to the clevis joint and forces the tang to conform to the same shape as the clevis, while guiding the tang into place. The roundout rings circularize the tang and clevis to assist in joint engagement. Other modifications include changes to the transportation pallet, shaping tools, and the lifting beam. These changes will resolve transportation, handling, and assembly problems that occurred in the past.
Analyses related to structural strengths, loads, stress, dynamics, fracture mechanics, gas and thermal dynamics, and materials characterization and behavior have been conducted to increase the understanding of the joint behavior and to support the design modifications. Continuing analyses will ensure that the design integrity and system compatibility are in agreement with the requirements. These analyses will be verified through correlation of test results and pretest predictions.
The strength of the improved joint design is expected to approach that of the case walls. The selected joint redesign approaches will minimize the sensitivity to manufacturing tolerances, handling, assembly and test procedures, flight operating characteristics, water impact, recovery, and reuse. The solid rocket design process is summarized in Figure 7.
Verification
The SRM Development and Verification Plan describes the test program necessary to demonstrate that the SRM meets all design and performance requirements and that failure [18] modes and hazards have been eliminated or controlled. The verification program includes the development, qualification, acceptance, preflight checkout, flight, and postflight phases.
Final hardware certification will be based, in part, on the results of the subscale tests, development and qualification motor firings, and data analyses. Whereas the development tests are principally engineering-oriented, the qualification tests will be formal demonstrations to verify that flight hardware meets the specified performance and design requirements. The Development and Verification Plan defines a test program that follows a rigorous sequence in which successive tests build on the results of previous tests, leading to formal certification.
Test activities include laboratory and component tests, subscale tests, full-scale simulation, and full-scale motor static firings. Laboratory and component tests are used to determine component properties and characteristics; subscale motor firings are used to simulate gas dynamics and thermal conditions for component and subsystem design.
Ten small-scale motor (70-pound) tests to evaluate both bonded (sealed) and unbonded insulation joint configurations have been completed. The test results were as expected, with no evidence of damage to the primary or secondary O-rings. Four circumferential flow tests have been completed with 400-pound motors; and the results were as predicted.
Fourteen full-scale vertical mate/demate tests have been performed using the joint assembly device as a test article. These tests used the redesigned capture feature hardware and included eight interspersed hydrostatic pressure tests to simulate the flight hardware case growth that results from the initial pressurization cycles. The mate/demate test results were as expected and confirmed the predictions of joint loads.
SRM case growth was identified as a potential problem contributing to improper joint performance during the accident investigation. The growth was suspected to have occurred during hydrostatic proof testing of the motor cases. In order to confirm this, two new cases were selected for measurement before and after several proof-testing cycles. Results confirmed that case growth had occurred during proof-testing cycles but that it became diminishingly smaller after three cycles.
The cause of SRM case growth will be fully understood by NASA prior to the first flight, and any necessary corrective action
Test Plan
Objectives
Configuration
....will be taken to ensure that both new and refurbished segments will meet all safety and reliability requirements. All case segments will be dimensionally stabilized by multiple proof cycles prior to flight use. Measurements will be made before stacking to confirm that all cases conform to engineering requirements.
The structural test article (STA)-Figure 8-provides the capability to test a flighttype forward segment, aft segment, and aft skirt. Tests utilizing the STA will demonstrate the structural integrity of the redesigned hardware under prelaunch and flight loads and will permit assessment of joint deflections under loaded conditions.
Verification of the new design includes component testing of the actual launch configuration over the full range of operating conditions. Full-scale, short-duration component tests of the field and nozzle joints that include joint flaws and flight loads will be used to verify analytical models and to determine hardware assembly characteristics, deflection characteristics, and overall performance. The results of these tests and analyses will be used to determine redesigned hardware structural characteristics.
Test programs that utilize full-scale flight design hardware include the nozzle joint environment simulator, the joint environment simulator (JES), and the transient pressure test article. These tests subject the SRM design features to the maximum expected operating pressure, maximum pressure rise rate, and temperature extremes during ignition tests. The transient pressure test article will be subjected to prelaunch loads during firing. Figures 9 and 10 depict test configurations...
Test Plan
Objectives
Configuration
Performance.
Test Plan
Objectives
Configuration
Performance
..... and describe the objectives for two of these full-scale hardware simulators. Figure 11 is a sketch of the complete transient pressure test article facility.
Three nozzle joint simulator tests have been successfully conducted. The STS 51-L configuration test confirmed predicted nozzle-to-case joint deflection. The other two tests using the new configurations with the radial bolts confirmed predictions of nozzle closure at maximum motor pressure.
Four joint environment simulator tests have been conducted. The JES-1 test series of two tests used the STS 51-L hardware configuration with and without a prefabricated defect in the putty and with joint temperatures of 20°F. These tests established a structural and performance data base for the STS 51-L configuration.
The JES-2 tests were conducted using STS 51-L motor case hardware with the new bonded seal insulation-application technique. Tests were conducted with and without flaws built into the seals in the joints, and neither test showed any evidence of O-ring erosion or blow-by.
Full-scale motor static firings will be conducted to confirm the integrated SRM performance. Six full-scale motor, full-duration static firings are planned. These firings [21] include the engineering test motor (Figure 12), which was successfully fired on May 27, 1987, and will provide a data base for the 51-L-type field, case-to-nozzle, and factory joints. The engineering test motor evaluated changes in the nozzle and the effectiveness of graphite composite stiffener rings to reduce joint deflections. Early analysis of the data indicates that the test met its objectives.
Two development motor firing tests (DM8 and DM9) and three qualification motor firing tests (QM6, QM7, and QM8) are planned for completion prior to the first flight. At least three successful qualification motor firings are required for final configuration and performance certification. Two of the qualification motors (QM7 and QM8) will be subjected to flight dynamic loads and a predetermined thermal environment during firings.
The static firing test attitude required to completely verify the design changes was assessed. The assessment included the establishment of test objectives, definition and quantification of the attitude-sensitive parameters, and evaluation of the attitude options. The horizontal and vertical (nozzle up and down) test attitudes were assessed.
In all options, consideration was given to testing with and without externally applied loads. The horizontal attitude was determined to represent a more demanding test configuration for the conditions influencing the joint and insulation behavior and has been retained.
A second horizontal test stand is being constructed and will support testing for first flight. The new stand, designated as the T-97 Large Motor Static Test Facility (Figure 13), will simulate environmental stresses, loads, and temperatures experienced during an actual Space Shuttle launch and ascent.
Nondestructive Evaluation (NDE)
NASA and several contractors are addressing both near-term and far-term nondestructive inspection testing at various stages of the SRM manufacturing process. X-ray of the propellant for all segments is being reinstated for the near-term flights. This X
Test Plan
Objectives
Configuration
....ray effort will be performed at the manufacturing facility. For the long term, continuation of full X-ray inspection versus a sampling approach will be assessed. This assessment will be based upon technology advancements, other inspections performed, and the accumulated experience base.
NASA has consulted with the Department of Defense in detail on the nondestructive testing plans of the Titan recovery program. The Associate Administrator for SRM&QA formed an agency-level group on nondestructive testing of the SRM that is addressing both near- and far-term methodology. This group, chaired by Dr. Joe Heyman of the Langley Research Center, draws upon the expertise of several appropriate contractors. The full spectrum of possible techniques is being assessed, including infrared thermography, various ultrasonic methods, and computer tomography.
Contingency Planning
To provide additional program confidence, both near- and long-term contingency planning has been implemented. Alternate designs, which might be incorporated into the flight program at discrete decision points, [23] include field joint graphite composite overwrap bands and alternate seals for the field joint and nozzle-to-case joint. Alternate designs for the nozzle include a different composite layup technique and a steel nose inlet housing.
Alternate designs with long lead time implications are also being developed. These designs focus on the field joint and nozzle-to-case joint. Since fabrication of the large steel components dictates the schedule, long lead procurement of maximum-size steel ingots has been initiated. This will allow machining of case joints to either the new baseline or to an alternate design configuration. Ingot processing will continue through forging and heat treating. At that time, the final design will be selected. A principal consideration in this configuration decision is the result of verification testing on the baseline configuration.
INDEPENDENT OVERSIGHT
The National Research Council (NRC) established an Independent Oversight Panel to review the SRM redesign. This panel, chaired by Dr. H. Guyford Stever, reports directly to the NASA Administrator. The panel was briefed on the Shuttle system requirements, the original design and manufacturing of the SRM, the mission 51-L accident analyses, and preliminary plans for the redesign.
Panel members have met with SRM manufacturers and vendors and have visited some of their facilities. They have reviewed the SRM design criteria, engineering analyses and design, certification program planning, verification testing, material specification selection, and quality assurance and control. They will continue to review the design as it progresses through manufacturing,....
[24] ....assembly of the first flight SRM, and design certification. Panel members participated in the preliminary requirements review and the preliminary design review, and will participate in future reviews.
The panel has held a number of full meetings and numerous subpanel and individual member meetings, and has submitted three written status reports to the NASA Administrator. Although NASA has not yet formally responded to these status reports, actions have been taken to implement most of the committee recommendations. NASA has held several meetings with the committee to discuss and review the status of the response to the recommendations. The NRC membership and a summary of the panel responsibilities are provided in Appendix A.
In addition to the NRC panel, an advisory group of 12 experienced senior engineers from NASA and the aerospace industry are supporting the redesign team. They review the design activities and provide recommendations for major program decisions. The membership and a summary of the group's responsibilities are provided in Appendix B.
NASA requested four other major solid rocket motor companies-Aerojet Strategic Propulsion Company, Atlantic Research Corporation, Hercules Incorporated, and United Technologies Corporation's Chemical Systems Division-to participate in the redesign efforts. Each company was given a short study contract and requested to critique the present redesign approach and to submit concepts for alternate designs. Their critiques were used in finalizing the design criteria and ensuring that industry standards are implemented into the final design selection. Hercules, Atlantic Research, and United Technologies are continuing to support the redesign by conducting special design and test activities.
As a result of the studies by these companies and others by NASA, a study to define a new advanced solid rocket motor has been initiated.
Further changes to the SRB are discussed in Part 2 of this report.
