3.8 IMPACT ANALYSIS AND TESTING

¶The importance of understanding this potential impact damage and the need to prove or disprove the impression that foam could not break an RCC panel prompted the investigation to develop computer models for foam impacts and undertake an impact-testing program of shooting pieces of foam at a mockup of the wing leading edge to re-create, to the extent practical, the actual STS-107 debris impact event.

¶Based on imagery analysis conducted during the mission and early in the investigation, the test plan included impacts on the lower wing tile, the left main landing gear door, the wing leading edge, and the carrier panels.

¶A main landing gear door assembly was the first unit ready for testing. By the time that testing occurred, however, analysis was pointing to an impact site in RCC panels 6 through 9. After the main landing gear door tests, the analysis and testing effort shifted to the wing leading edge RCC panel assemblies. The main landing gear door testing provided valuable data on test processes, equipment, and instrumentation. Insignificant tile damage was observed at the low impact angles of less than 20 degrees (the impact angle if the foam had struck the main landing gear door would have been roughly five degrees). The apparent damage threshold was consistent with previous testing with much smaller projectiles in 1999, and with independent modeling by Southwest Research Institute. (See Appendix D.12.)

¶Impact Test – Wing Leading Edge Panel Assemblies

¶The test concept was to impact flightworthy wing leading edge RCC panel assemblies with a foam projectile fired by a compressed-gas gun. Target panel assemblies with a flight history similar to Columbiaʼs would be mounted on a support that was structurally equivalent to Columbiaʼs wing. The attaching hardware and fittings would be either flight certified or built to Columbia drawings. Several considerations influenced the overall RCC test design:

RCC panel assemblies were limited, particularly those with a flight history similar to Columbiaʼs.
The basic material properties of new RCC were known to be highly variable and were not characterized for high strain rate loadings typical of an impact.
The influence of aging was uncertain.
The RCCʼs brittleness allowed only one test impact on each panel to avoid the possibility that hidden damage would influence the results of later impacts.
The structural system response of RCC components, their support hardware, and the wing structure was complex.
The foam projectile had to be precisely targeted, because the predicted structural response depended on the impact point.

¶Because of these concerns, engineering tests with fiberglass panel assemblies from the first Orbiter, Enterprise,12 were used to obtain an understanding of overall system response to various impact angles, locations, and foam orientations. The fiberglass panel impact tests were used to confirm instrumentation design and placement and the adequacy of the overall test setup.

¶Test projectiles were made from the same type of foam as the bipod ramp on STS-107ʼs External Tank. The projectileʼs mass and velocity were determined by the previously described "best fit" image and transport analyses. Because the precise impact point was estimated, the aiming point for any individual test panel was based on structural analyses to maximize the loads in the area being assessed without producing a spray of foam over the top of the wing. The angle of impact relative to the test panel was determined from

¶T-Seal

¶High speed cameras

¶Compressed

¶the transport analysis of the panel being tested. The foamʼs rotational velocity was accounted for with a three-degree increase in the impact angle.

¶Computer Modeling of Impact Tests

¶The investigation used sophisticated computer models to analyze the foam impact and to help develop an impact test program. Because an exhaustive test matrix to cover all feasible impact scenarios was not practical, these models were especially important to the investigation.

¶The investigation impact modeling team included members from Boeing, Glenn Research Center, Johnson Space Center, Langley Research Center, Marshall Space Flight Center, Sandia National Laboratory, and Stellingwerf Consulting. The Board also contracted with Southwest Research Institute to perform independent computer analyses because of the instituteʼs extensive test and analysis experience with ballistic impacts, including work on the Orbiterʼs Thermal Protection System. (Appendix D.12 provides a complete description of Southwestʼs impact modeling methods and results.)

¶The objectives of the modeling effort included (1) evaluation of test instrumentation requirements to provide test data with which to calibrate the computer models, (2) prediction of stress, damage, and instrumentation response prior to the Test Readiness Reviews, and (3) determination of the flight conditions/loads (vibrations, aerodynamic, inertial, acoustic, and thermal) to include in the tests. In addition, the impact modeling team provided information about foam impact locations, orientation at impact, and impact angle adjustments that accounted for the foamʼs rotational velocity.

¶Flight Environment

¶A comprehensive consideration of the Shuttleʼs flight environment, including temperature, pressure, and vibration, was required to establish the experimental protocol.

¶Lights

Wing Leading Edge Sub-Sytem

¶gas tank Figure 3.8-1. Nitrogen-powered gun at the Southwest Research Institute used for the test series.

¶Based on the results of Glenn Research Center sub-scale impact tests of how various foam temperatures and pressures influence the impact force, the Board found that full-scale impact tests with foam at room temperature and pressure could adequately simulate the conditions during the foam strike on STS-107.13

¶The structure of the foam complicated the testing process. The bipod ramp foam is hand-sprayed in layers, which creates "knit lines," the boundaries between each layer, and the foam compression characteristics depend on the knit linesʼ orientation. The projectiles used in the full-scale impact tests had knit lines consistent with those in the bipod ramp foam.

¶A primary concern of investigators was that external loads present in the flight environment might add substantial extra force to the left wing. However, analysis demonstrated that the only significant external loads on the wing leading edge structural subsystem at about 82 seconds into flight are due to random vibration and the pressure differences inside and outside the leading edge. The Board concluded that the flight environment stresses in the RCC panels and the attachment fittings could be accounted for in post-impact analyses if necessary. However, the dramatic damage produced by the impact tests demonstrated that the foam strike could breach the wing leading edge structure subsystem independent of any stresses associated with the flight environment. (Appendix D.12 contains more detail.)

¶Test Assembly

¶The impact tests were conducted at a Southwest Research Institute facility. Figure 3.8-1 shows the nitrogen gas gun that had evaluated bird strikes on aircraft fuselages. The gun was modified to accept a 35-foot-long rectangular barrel, and the target site was equipped with sensors and high-speed cameras that photographed 2,000 to 7,000 frames per second, with intense light provided by theater spotlights and the sun.

¶Test Impact Target

¶The leading edge structural subsystem test target was designed to accommodate the Boardʼs evolving determination of the

Support structure

Panel 6 slot

Panel 9 Panel 8 Panel 7 T-Seal Panel 5

¶Figure 3.8-2. Test assembly that provided a structural mounting for RCC panel assemblies 5 to 10 and would accommodate some

¶most likely point of impact. Initially, analysis pointed to the main landing gear door. As the imaging and transport teams refined their assessments, the likely strike zone narrowed to RCC panels 6 through 9. Because of the long lead time to develop and produce the large complex test assemblies, investigators developed an adaptable test assembly (Figure 3.8-2) that would provide a structurally similar mounting for RCC panel assemblies 5 to 10 and would accommodate some 200 sensors, including high-speed cameras, strain and deflection gauges, accelerometers, and load cells.14

¶Test Panels

¶RCC panels 6 and 9, which bracketed the likely impact region, were the first identified for testing. They would also permit a comparison of the structural response of panels with and without the additional thickness at certain locations.

¶Panel 6 tests demonstrated the complex system response to impacts. While the initial focus of the investigation had been on single panel response, early results from the tests with fiberglass panels hinted at "boundary condition" effects. Instruments measured high stresses through panels 6, 7, and 8. With this in mind, as well as forensic and sensor evidence that panel 8 was the likeliest location of the foam strike, the Board decided that the second RCC test should target panel 8, which was placed in an assembly that included RCC panels 9 and 10 to provide high fidelity boundary conditions. The decision to impact test RCC panel 8 was complicated by the lack of spare RCC components.

¶The specific RCC panel assemblies selected for testing had flight histories similar to that of STS-107, which was Columbiaʼs 28th flight. Panel 6 had flown 30 missions on Discovery, and Panel 8 had flown 26 missions on Atlantis.

¶Test Projectile

¶The preparation of BX-250 foam test projectiles used the same material and preparation processes that produced the foam bipod ramp. Foam was selected as the projectile material because foam was the most likely debris, and materials other than foam would represent a greater threat.

¶Figure 3.8-3. A typical foam projectile, which has marks for determining position and velocity as well as blackened outlines for

¶200 sensors and other test equipment. indicating the impact footprint.

¶The testing required a projectile (see Figure 3.8-3) made from standard stock, so investigators selected a rectangular cross-section of 11.5 by 5.5 inches, which was within 15 percent of the footprint of the mean debris size initially estimated by image analysis. To account for the foamʼs density, the projectile length was cut to weigh 1.67 pounds, a figure determined by image and transport analysis to best represent the STS-107 projectile. For foam with a density of 2.4 pounds per cubic foot,15 the projectile dimensions were 19 inches by 11.5 inches by 5.5 inches.

¶Impact Angles

¶The precise impact location of the foam determined the impact angle because the debris was moving almost parallel to the Orbiterʼs fuselage at impact. Tile areas would have been hit at very small angles (approximately five degrees), but the curvature of the leading edge created angles closer to 20 degrees (see Figure 3.4-4).

¶The foam that struck Columbia on January 16, 2003, had both a translational speed and a rotational speed relative to the Orbiter. The translational velocity was easily replicated by adjusting the gas pressure in the gun. The rotational energy could be calculated, but the impact force depends on the material composition and properties of the impacting body and how the rotating body struck the wing. Because the details of the foam contact were not available from any visual evidence, analysis estimated the increase in impact energy that would be imparted by the rotation. These analyses resulted in a three-degree increase in the angle at which the foam test projectile would hit the test panel.16

¶The "clocking angle" was an additional consideration. As shown in Figure 3.8-4, the gun barrel could be rotated to change the impact point of the foam projectile on the leading edge. Investigators conducted experiments to determine if the corner of the foam block or the full edge would impart a

¶Figure 3.8-4. The barrel on the nitrogen gun could be rotated to adjust the impact point of the foam projectile.

¶greater force. During the fiberglass tests, it was found that a clocking angle of 30 degrees allowed the 11.5-inch-edge to fully contact the panel at impact, resulting in a greater local force than a zero degree angle, which was achieved with the barrel aligned vertically. A zero-degree angle was used for the test on RCC panel 6, and a 30-degree angle was used for RCC panel 8.

¶Test Results from Fiberglass Panel Tests 1-5

¶Five engineering tests on fiberglass panels (see Figure 3.8-5) established the test parameters of the impact tests on RCC panels. Details of the fiberglass tests are in Appendix D.12.

¶Figure 3.8-5. A typical foam strike leaves impact streaks, and the foam projectile breaks into shards and larger pieces. Here the foam is striking Panel 6 on a fiberglass test article.

¶Test Results from Reinforced Carbon-Carbon Panel 6 (From Discovery)

¶RCC panel 6 was tested first to begin to establish RCC impact response, although by the time of the test, other data had indicated that RCC panel 8-left was the most likely site of the breach. RCC panel 6 was impacted using the same parameters as the test on fiberglass panel 6 and developed a 5.5-inch crack on the outboard end of the panel that extended through the rib (see Figure 3.8-6). There was also a crack through the "web" of the T-seal between panels 6 and 7 (see Figure 3.8-7). As in the fiberglass test, the foam block deflected, or moved, the face of the RCC panel, creating a slit between the panel and the adjacent T-seal, which ripped the projectile and stuffed pieces of foam into the slit (see Figure 3.8-8). The panel rib failed at lower stresses than predicted, and the T-seal failed closer to predictions, but overall, the stress pattern was similar to what was predicted, demonstrating the need to incorporate more complete RCC failure criteria in the computational models.

¶Without further crack growth, the specific structural damage this test produced would probably not have allowed enough superheated air to penetrate the wing during re-entry to cause serious damage. However, the test did demonstrate that a foam impact representative of the debris strike at 81.9 seconds after launch could damage an RCC panel. Note that

¶Figure 3.8-6. A 5.5-inch crack on the outboard portion of RCC Panel 6 during testing. Figure 3.8-8. Two views of foam lodged into the slit during tests.

¶Figure 3.8-9. The large impact hole in Panel 8 from the final test.

¶Figure 3.8-7. Two views of the crack in the T-seal between RCC Panels 6 and 7.

¶the RCC panel 6-left test used fiberglass panels and T-seals in panel 7, 8, 9, and 10 locations. As seen later in the RCC panel 8-left test, this test configuration may not have adequately reproduced the flight configuration. Testing of a full RCC panel 6, 7, and 8 configuration might have resulted in more severe damage.

¶Test Results from Reinforced Carbon-Carbon Panel 8 (From Atlantis)

¶The second impact test of RCC material used panel 8 from Atlantis, which had flown 26 missions. Based on forensic evidence, sensor data, and aerothermal studies, panel 8 was considered the most likely point of the foam debris impact on Columbia.

¶Based on the system response of the leading edge in the fiberglass and RCC panel 6 impact tests, the adjacent RCC panel assemblies (9 and 10) were also flown hardware. The reference 1.67-pound foam test projectile impacted panel 8 Figure 3.8-10. Numerous cracks were also noted in RCC Panel 8.

¶at 777 feet per second with a clocking angle of 30 degrees and an angle of incidence of 25.1 degrees.

¶F3.8-3 The impact created a hole roughly 16 inches by 17 inches, which was within the range consistent with all the findings of the investigation (see Figure 3.8-9). Additionally, cracks in the panel ranged up to 11 inches in length (Figure 3.8-10). The T-seal between panels 8 and 9 also failed at the lower outboard mounting lug. F3.8-4

¶Three large pieces of the broken panel face sheet (see Figure 3.8-11) were retained within the wing. The two largest F3.8-5 pieces had surface areas of 86 and 75 square inches. While this test cannot exactly duplicate the damage Columbia in- F3.8-6 curred, pieces such as these could have remained in the wing cavity for some time, and could then have floated out of the damaged wing while the Orbiter was maneuvering in space. This scenario is consistent with the event observed on Flight Day 2 (see Section 3.5). F3.8-7

¶The test clearly demonstrated that a foam impact of the type Columbia sustained could seriously breach the Wing Leading Edge Structural Subsystem.

¶R3.8-1 Conclusion

¶At the beginning of this chapter, the Board stated that the physical cause of the accident was a breach in the Thermal Protection System on the leading edge of the left wing. The breach was initiated by a piece of foam that separated from the left bipod ramp of the External Tank and struck the wing R3.8-2 in the vicinity of the lower half of the Reinforced Carbon- Carbon (RCC) panel 8.

¶The conclusion that foam separated from the External Tank bipod ramp and struck the wing in the vicinity of panel 8 is documented by photographic evidence (Section 3.4). Sensor data and the aerodynamic and thermodynamic analyses (Section 3.6) based on that data led to the determination that the breach was in the vicinity of panel 8 and also accounted for the subsequent melting of the supporting structure, the spar, and the wiring behind the spar that occurred behind panel 8. The detailed examination of the debris (Section 3.7) also pointed to panel 8 as the breach site. The impact tests (Section 3.8) established that foam can breach the RCC, and also counteracted the lingering denial or discounting of the analytic evidence. Based on this evidence, the Board concluded that panel 8 was the site of the foam strike to Columbia during the liftoff of STS-107 on January 23, 2003.

¶Findings:

¶F3.8-1 The impact test program demonstrated that foam

can cause a wide range of impact damage, from cracks to a 16- by 17-inch hole.

¶F3.8-2 The wing leading edge Reinforced Carbon-Car-

bon composite material and associated support hardware are remarkably tough and have impact capabilities that far exceed the minimal impact resistance specified in their original design requirements. Nevertheless, these tests demonstrate that this inherent toughness can be exceeded by

¶impacts representative of those that occurred during Columbiaʼs ascent. The response of the wing leading edge to impacts is complex and can vary greatly, depending on the location of the impact, projectile mass, orientation, composition, and the material properties of the panel assembly, making analytic predictions of damage to RCC assemblies a challenge.17 Testing indicates the RCC panels and T-seals have much higher impact resistance than the design specifications call for. NASA has an inadequate number of spare Reinforced Carbon-Carbon panel assemblies. NASAʼs current tools, including the Crater model, are inadequate to evaluate Orbiter Thermal Protection System damage from debris impacts during pre-launch, on-orbit, and post-launch activity. The bipod ramp foam debris critically damaged the leading edge of Columbiaʼs left wing.

¶ions:

¶Obtain sufficent spare Reinforced Carbon-Carbon panel assemblies and associated support components to ensure that decisions related to Reinforced Carbon-Carbon maintenance are made on the basis of component specifications, free of external pressures relating to schedules, costs, or other considerations. Develop, validate, and maintain physics-based computer models to evaluate Thermal Protection System damage from debris impacts. These tools should provide realistic and timely estimates of any impact damage from possible debris from any source that may ultimately impact the Orbiter. Establish impact damage thresholds that trigger responsive corrective action, such as on-orbit inspection and repair, when indicated.

¶11.5" 12.25"

¶75 in2 86 in2

6.5" 7"

Three large pieces of debris from the panel face lodged within the hollow area behind the RCC panel.

¶The citations that contain a reference to "CAIB document" with CAB or

¶CTF followed by seven to eleven digits, such as CAB001-0010, refer to a document in the Columbia Accident Investigation Board database maintained by the Department of Justice and archived at the National Archives.

¶See Dennis R. Jenkins, Space Shuttle: The History of the National Space

¶Transportation System – The First 100 Missions (Cape Canaveral, FL,

¶Specialty Press, 2001), pp. 421-424 for a complete description of the

¶External Tank.²² before the 1986 Challenger accident NASA adopted an alternate Space For detailed discussions of the origins of the Space Shuttle, see Dennis R. Shuttle mission numbering scheme, this report uses the original STS flight Jenkins, Space Shuttle: The History of the National Space Transportation designations. System – The First 100 Missions (Cape Canaveral, FL: Specialty Press, 12 2001); T. A. Heppenheimer, The Space Shuttle Decision: NASAʼs Search President Reaganʼs quote is contained in President Ronald Reagan, for a Reusable Space Vehicle, NASA SP-4221 (Washington: Government "Remarks on the Completion of the Fourth Mission of the Space Shuttle Printing Office, 1999; also published by the Smithsonian Institution Press, Columbia," July 4, 1982, p. 870, in Public Papers of the Presidents of the 2002); and T. A. Heppenheimer, Development of the Space Shuttle, United States: Ronald Reagan (Washington: Government Printing Office, 1972-1981 (Washington: Smithsonian Institution Press, 2002). Much of 1982-1991). The emphasis noted is the Boardʼs. the discussion in this section is based on these studies. 13 "Pricing Options for the Space Shuttle," Congressional Budget Office National Commission on Space, Pioneering the Space Frontier: An Exciting Vision of Our Next Fifty Years in Space, Report of the National Commission on Space (Bantam Books, 1986), p. 2. Guidelines per NASA Policy Guideline 8621.Show full note

¶Scotty Sparks and Lee Foster, "ET Cryoinsulation," CAIB Public Hearing,

¶April 7, 2003. CAIB document CAB017-03140371.³³ See John M. Logsdon, "The Space Shuttle Program: A Policy Failure?" Report, 1985. Science, May 30, 1986 (Vol. 232), pp. 1099-1105 for an account of this 14 The quote is from page 2 of the We Deliver brochure, reproduced in decision process. Most of the information and quotes in this section are Exploring the Unknown Volume IV, p. 423. taken from this article. 15 NASA Johnson Space Center, "Technology Influences on the Space President George H. W. Bush, "Remarks on the 20th Anniversary of the Apollo 11 Moon Landing," Washington, D.C., July 20, 1989.Show full note

¶Scotty Sparks and Steve Holmes, Presentation to the CAIB, March 27,

¶2003, CAIB document CTF036-02000200.

¶See the CAIB/NAIT Joint Working Scenario in Appendix D.7 of Volume

¶II of this report.⁵⁵ Heppenheimer, The Space Shuttle Decision, pp. 278-289, and Roger in 1985 when adjusted for inflation becomes $52.9 million in 1971 A. Pielke, Jr., "The Space Shuttle Program: ʻPerformance vs. Promise,ʼ" dollars or nearly seven times the 1971 estimate. "Pricing Options for the Center for Space and Geosciences Policy, University of Colorado, August Space Shuttle." 31, 1991; Logsdon, "The Space Shuttle Program: A Policy Failure?" pp. 17 See Diane Vaughan, The Challenger Launch Decision: Risky Technology, 1099-1105. Culture, and Deviance at NASA (Chicago: The University of Chicago U.S.C. App § §1 et seq. (1972). Harold W. Gehman to Sean OʼKeefe, February 25, 2003. 238 Report Volume I August 2003Show full note

¶Boeing Specification MJ070-0001-1E, "Orbiter End Item Specification for the Space Shuttle Systems, Part 1, Performance and Design Requirements,

¶November 7, 2002.⁶⁶ Quoted in Jenkins, Space Shuttle, p. 171. Press, 1996). 7 18 Memorandum from J. Fletcher to J. Rose, Special Assistant to the See John M. Logsdon, "Return to Flight: Richard H. Truly and the President, November 22, 1971; Logsdon, John, "The Space Shuttle Recovery from the Challenger Accident," in Pamela E. Mack, editor, Program: A Policy Failure?" Science, May 30, 1986, Volume 232, pp. From Engineering to Big Science: The NACA and NASA Collier Trophy 1099-1105. Research Project Winners, NASA SP-4219 (Washington: GovernmentShow full note

¶Ibid., Paragraph 3.3.1.8.16.

¶NSTS-08171, "Operations and Maintenance Requirements and

¶Specifications Document (OMRSD)" File II, Volume 3. CAIB document

¶CAB033-12821997.⁸⁸ The only actual flight tests conducted of the Orbiter were a series of Printing Office, 1998) for an account of the aftermath of the accident. Approach and Landing Tests where Enterprise (OV-101) was dropped Much of the account in this section is drawn from this source. from its Boeing 747 Shuttle Carrier Aircraft while flying at 25,000 feet. 19 Logsdon, "Return to Flight," p. 348. These tests – with crews aboard – demonstrated the low-speed handling 20 Presidential Commission on the Space Shuttle Challenger Accident capabilities of the Orbiter and allowed an evaluation of the vehicleʼs (Washington: Government Printing Office, June 6, 1986). landing characteristics. See Jenkins, Space Shuttle, pp. 205-212 for more information. 26 Report Volume I August 2003Show full note

¶Dr. Gregory J. Byrne and Dr. Cynthia A. Evans, "STS-107 Image Analysis

¶Team Final Report in Support of the Columbia Accident Investigation,"

¶NSTS-37384, June 2003. CAIB document CTF076-15511657. See

This section based on information from the following reports: MIT Lincoln Laboratory "Report on Flight Day 2 Object Analysis;" Dr. Brian M. Kent, Dr. Kueichien C. Hill, and Captain John Gulick, "An Assessment of Potential Material Candidates for the ʻFlight Day 2ʼ Radar Object Observed During the NASA Mission STS-107 (Columbia)", Air Force Research Laboratory Final Summary Report AFRL-SNS-2003-001, July 20, 2003 (see Appendix E.2); Multiple briefings to the CAIB from Dr. Brian M. Kent, AFRL/SN (CAIB document CTF076-19782017); Briefing to the CAIB from HQ AFSPC/XPY, April 18, 2003 (CAIB document CAB066-13771388).

The water tanks from below the mid-deck floor, along with both Forward Reaction Control System propellant tanks were recovered in good condition.

Enterprise was used for the initial Approach and Landing Tests and ground tests of the Orbiter, but was never used for orbital tests. The

vehicle is now held by the National Air and Space Museum. See Jenkins, Space Shuttle, pp. 205-223, for more information on Enterprise.

Philip Kopfinger and Wanda Sigur, "Impact Test Results of BX-250 In Support of the Columbia Accident Investigation," ETTP-MS-03-021, July 17, 2003.

¶Details of the test instrumentation are in Appendix D.12.

Evaluations of the adjustments in the angle of incidence to account for rotation are in Appendix D.12.

The potential damage estimates had great uncertainty because the database of bending, tension, crushing, and other measures of failure were incomplete, particularly for RCC material.

¶Appendix E.2 for a copy of the report.

¶Other Factors Considered

¶During its investigation, the Board evaluated every known factor that could have caused or contributed to the Columbia accident, such as the effects of space weather on the Orbiter during re-entry and the specters of sabotage and terrorism. In addition to the analysis/scenario investigations, the Board oversaw a NASA "fault tree" investigation, which accounts for every chain of events that could possibly cause a system to fail. Most of these factors were conclusively eliminated as having nothing to do with the accident; however, several factors have yet to be ruled out. Although deemed by the Board as unlikely to have contributed to the accident, these are still open and are being investigated further by NASA. In a few other cases, there is insufficient evidence to completely eliminate a factor, though most evidence indicates that it did not play a role in the accident. In the course of investigating these factors, the Board identified several serious problems that were not part of the accidentʼs causal chain but nonetheless have major implications for future missions.

¶In this chapter, a discussion of these potential causal and contributing factors is divided into two sections. The first introduces the primary tool used to assess potential causes of the breakup: the fault tree. The second addresses fault tree items and particularly notable factors that raised concerns for this investigation and, more broadly, for the future operation of the Space Shuttle.