The Report of the Presidential Commission on the Space Shuttle Challenger Accident - The Tragedy of Mission 51-L in 1986 - Volume Two, Appendix L, M: NASA Accident Analysis, Morton Thiokol Comments
National Aeronautics and Space Administration (NASA), World Spaceflight News, Presidential Commission on the Space Shuttle Challenger Accident, Rogers Commission
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Report of the Presidential Commission on the Space Shuttle Challenger Accident
June 6th, 1986
Washington, D.C.
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IN MEMORIAM
"The future is not free: the story of all human progress is one of a struggle against all odds. We learned again that this America, which Abraham Lincoln called the last, best hope of man on Earth, was built on heroism and noble sacrifice. It was built by men and women like our seven star voyagers, who answered a call beyond duty, who gave more than was expected or required and who gave it little thought of worldly reward."
President Ronald Reagan * January 31, 1986
Francis R. (Dick) Scobee - Commander
Michael John Smith - Pilot
Ellison S. Onizuka - Mission Specialist One
Judith Arlene Resnik - Mission Specialist Two
Ronald Erwin McNair - Mission Specialist Three
S. Christa McAuliffe - Payload Specialist One
Gregory Bruce Jarvis - Payload Specialist Two
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Report of the Presidential Commission on the Space Shuttle Challenger Accident - Volume 2
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Appendix L: NASA Accident Analysis Team Report
Appendix M: Comments by Morton Thiokol on NASA Report
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Appendix L - NASA Accident Analysis Team Report. [Part 1]
STS 51-L Data & Design Analysis Task Force Accident Analysis Team
April 1986
SUBMITTED BY: JOHN W. THOMAS, DEPUTY ACCIDENT ANALYSIS TEAM
1.0. Introduction
The Accident Analysis Team, NASA 51-L Data and Design Analysis Task Force, was chartered as delineated in Appendix A, to perform, as a priority task, the efforts necessary to respond to the Presidential Commission Accident Analysis Panel needs and interests. The Panel Commission Members were General Donald Kutyna, Mr. Neil Armstrong. Dr. Eugene Cover, Dr. Richard Feynman, and General Charles Yeager.
The Accident Analysis Team was organized as depicted in Figure 1.0.1 to develop possible causes of the 51-L accident. The team reviewed and analyzed all anomalous aspects of the STS 51-L preparation, launch, and flight and related those anomalies to the accident to the extent possible. The team developed comprehensive failure scenarios that postulated both the causes and sequence of events relating to the accident. The team considered all STS elements and developed and maintained a data base related to failure analysis.
The Accident Analysis Team responsibilities were executed and the conclusions and findings are documented in this and the 10 supporting element reports listed below.
1. Space Shuttle Main Engine Working Group
2. External Tank Working Group
*3. Solid Rocket Motor Working Group
4. Solid Rocket Booster Working Group
5. Space Shuttle Systems Working Group
6. IUS/TDRS Systems Working Group
7. Orbiter and GFE
8. Payload/Orbiter Interface Team
9. Review and Assessment of Spartan Halley Performance
10. Review and Assessment of TDRS-B Performance
* Reproduced with this volume. Begins on Page L-50.
2.0. Accident Timeline
The events surrounding the Space Shuttle Mission STS 51-L accident were principally derived from data retrieved electronically from the flight vehicle, ground based records, and photographic and video imagery. Additionally, other information was obtained from mathematical model reconstruction of the system response to certain external phenomena such as upper atmospheric winds acting upon the vehicle during its launch and ascent flight phase. These events, when chronologically ordered, constitute the "Timeline" of events used by all analysts throughout the Accident Analysis Team effort. The time reference selected is the elapsed time from issuance of the command to ignite the SRBs and is referred to as Mission Elapsed Time or MET.
The STS 51-L mission launch on January 28, 1986, at approximately 11:38 a.m. EST, proceeded normally through SSME startup and SRB ignition. The first anomaly noted was a small puff of smoke observed between the right SRB and the ET in a region near the SRM aft field joint. The smoke appeared to persist for a period between 0.678 seconds and 3.375 seconds. From this point onward to approximately 59 seconds, all systems appeared to perform within their design boundaries; however. there were significant vehicle attitude excursions and high thrust vector control (steering) system activity observed beginning at approximately 37 seconds. This high activity created by upper atmospheric wind gusts and planned maneuvers persisted through the time (59 seconds) when the vehicle was heavily loaded by dynamic pressure. At 58.788 seconds, a flame reappeared. emanating from the general region where the puff of smoke was observed near lift-off. This event occurred almost 5 seconds after the SRMs had experienced their expected chamber pressure reduction at approximately 54 seconds and the SRMs pressure was again rising. Within less than 2 seconds, at 60.004 seconds. the right SRM internal pressure began to diverge from that of the left SRM and did not rise as rapidly as normal. This correlated with a right SRM combustion gas leak. At 61 seconds. just over 2 seconds after the leak was observed, the well-defined plume was observed to be deflected, indicating that the hot gas had contacted the ET. Photographic analysis indicated that the plume breached the tank and produced an LH2 leak at 64.660 seconds. the LH2 leak was confirmed 1.2 seconds later, at 66.8 seconds. when the tank pressurization system could no longer maintain its normal repressurization rate; and at 72.6 seconds. the tank pressure could no longer be maintained indicating that the leak path had significantly increased and was growing rapidly. At 72.2 seconds, the guidance system showed that right SRB motion was not the same as the Orbiter and left SRB indicating that the lower ET-to-SRB attachment strut was severed or was pulled loose from the ET. Additionally, during this timeframe. large steering commands and systems responses were observed and at approximately 73 seconds, both LH2 and LOX pressures to the SSME showed a significant drop. This was followed at 73.124 seconds by the appearance of a circumferential white pattern around the ET aft bulkhead suggesting tank LH2 structural failure: and 0.013 seconds later, at 713.137 seconds, vapor was observed at the inter-tank indicative of the LOX tank failing. LOX tank failure can be attributed to abnormal loads induced by either or both the right SRB action at the forward attach point or the propulsive forces created by LH2 tank aft bulkhead structural failure. LOX was then observed streaming along the ET. At 73.191 seconds, a flash was observed between the ET and Orbiter that was immediately followed by total vehicle structural breakup explosion at 73.213 seconds. Intermittent telemetry data were obtained until 73.6 seconds at which time the SSME had responded normally as expected to reduced propellant pressures. Both SRBs exited the vehicle breakup propulsively and continued to fly erratically until destroyed by Range Safety Command at 110. 25 seconds. Details regarding post structural breakup are contained in the Search, Recovery, and Reconstruction Team Report.
3.0. Anomalies/Significant Observations
The anomalies/significant observations relating to the STS 51-L accident were identified throughout the investigation from careful review of manufacturing and launch processing records, engineering analyses, telemetry flight data, photographic and video imagery, and personal observation. The anomalies/significant observations described below are listed in the order they occur in the mission preparation and flight sequence.
3.1. Assembly
Launch site records review showed that the right SRM aft field joint was mated per approved procedures. However, significant out-of-round (ovality) conditions existed at mating as illustrated in Figure 3.1.1. In order to mate the aft center segment to the aft segment, it was necessary to install an infrequently used tool to adjust the aft center segment diametrical shape. Even with use of the tool. the relative diameters of the two segments were such that the joint tang would interfere with the joint clevis inner leg and also with the sealing O-rings. The circumferential area of maximum interference was located at the 1200-3000 position of the joint as noted in Figure 3.1.2. The significance of this observation is detailed in subsequent sections of this report.
3.2. Cold Weather
The ambient temperature at the pad ground level in the 24 hours before the STS 51-L launch reached a low of 24°F at 7 a. m. EST. The ambient temperature at launch time, 11:38 a. m., was 36°F. This translated to 28°F minimum SRM temperature at launch. The flight with the next coldest SRM temperature, 51°F, was STS 51-C. These low overnight temperatures necessitated that KSC implement facility and equipment protection procedures which called for, among other measures, small continuous water flow rates in the pad Firex system. This free water froze and resulted in various ice accumulations on the pad structure and to a lesser degree on the left SRB.
Figure 3.8.1. First Evidence of Fire.
Figure 3.9.1. Continuous Plume.
Figure 3.10.1. Breached External Tank Hydrogen Tank.
3.3. Water in SRM joint
Following the STS 51-L accident, launch site personnel reported that water was observed in SRM joints on one SRM from an earlier mission, STS-9. The presence of water, previously unrecorded, in the STS-9 joints was discovered while destacking one of the SRMs to exchange a motor nozzle. Water overcoming the grease barrier and entering the joint is not immediately detrimental unless its presence exists in combination with cold weather at launch. This combination could produce ice in the SRM joints that if positioned at or near the secondary O-ring as depicted in Figure 3.3.1 could interfere with proper seal operation. The potential for water in the joints was present on STS 51-L since over 7 inches of rain fell while it was on the launch pad and all joints had some or all their circumferences below freezing.
3.4. Right SRB Smoke On Ignition
At 0.678 seconds, smoke was observed between the right SRB and the ET. Due to malfunction of the cameras with the best view of this area, the exact point of origin of the smoke was not visible. From other cameras, the smoke was first observed above the aft SRB field joint and moved in an upward direction. The smoke quantity appeared to increase incrementally at a rate of 3-4 puffs per second until 2.499 seconds. With increasing upward motion of the vehicle, the smoke was observed trailing off behind the vehicle. Smoke was last seen above the attach ring at 2.733 seconds and was last seen dissipating below the vehicle at 3.375 seconds. Through film analysis, considering ground winds and vehicle liftoff motion, the smoke was estimated to have originated in the circumferential sector between 270° and 310° on the SRB.
3.5. Turbulence
At approximately 37 seconds into the flight, STS 51-L encountered the first of several turbulent wind conditions which lasted until about 62 seconds MET. The effect of this turbulence produced by wind gusts was relatively large fluctuations in forces applied to the vehicle. This causes rapid attitude changes that were immediately sensed and corrected by the GN&C system. These corrections were not only to maintain the proper flight path but also position the vehicle to minimize certain loading conditions. These changes and corrections are of little significance unless they induce unusual loading conditions or cause the control system to exceed its design limit. The best portrayal of the turbulence experienced by STS 51-L is a graphical display of the product of aerodynamic pressure (Q) and vehicle side slip, B (yaw) [Greek letter beta] and angle of attack, a (pitch) [Greek letter alpha] versus speed and time. The early wind gusts were from the side in the yaw plane, Figure 3.5.1, whereas the later the Figure 3.5.2. were from the direction of flight in the pitch plane. The early yaw plane gusts were significant and at one time exceeded prior flight experience in the subsonic region. The pitch plane gusts were large and produced Qa [Greek letter alpha] values as large as - 4.200 PSF degrees. At two times in the flight, around 55 seconds and 68 seconds, Qa [Greek letter alpha] exceeded prior flight experience. Even though STS 51-L exceeded prior flight experience in both pitch and yaw planes, the maximum encountered were well within design limits.
3.6. Thrust Vector Control (TVC) Duty Cycle
The SRB TVC (steering) system responded properly to all routine commands and turbulence effects throughout the truncated mission; however, the turbulence effects caused the TVC system to be more active than was experienced on any prior flight. A graphical display of this high activity, Figure 3.6.1, is the angular movement of the SRB nozzle after about 35 seconds into the flight: the earlier movement was for the planned roll maneuver combined with any low altitude wind effects. Angular movements of this magnitude were experienced before, but, the character of high activity steering on STS 51-L differs from prior flights in that the number and rate or the excursions on STS 51-L were greater. This high activity is well within system capability and therefore would not normally produce loading conditions detrimental in any way to the vehicle structure. However, this activity could potentially have a deleterious effect if coupled with an anomalous condition that had previously weakened the structure. Specifically, if the smoke observed at lift-off was indicative of damage to the SRM joint, this large amount of steering could potentially have been associated with the flame observed originating on the right SRB at 58 seconds.
3.7. "Flashes"
Several "flashes" were noted in the SSME plumes and were initially included in the events timeline. The flashes or briefly visible streaks of light are quite common and were visible on other flights. Since the flashes are not peculiar to STS 51-L, they were removed from the event timeline.
3.8. Right SRB Flames
The first indication of the right SRB hot gas leak is listed on the timeline in section 2.0 as "first evidence of flame." This occurred at 58.788 seconds as a light visible between the right SRB and the ET, Figure 3.8.1. This light was observed to flicker briefly and then to grow in intensity and size until 59.262 seconds. The light was then masked by an increasingly large combustion product plume emanating from the side of the SRM. The flame location was determined using enhanced photography and computerized graphics to be very near the SRB aft field joint at approximately the 300° position.
3.9. Plume
The plume emanating from the side of the SRM is characterized as continuous and well defined at 59.262 as shown in Figure 3.9.1. The plume continued to grow in size and at 60.238 seconds was evidently impinging on the ET due to the observed plume deflection. The plume continued to grow after that time until 64.660 seconds when there was an abrupt change in the character of the plume.
3.10. LH2 Tank Breach
The sudden change in the plume at 64.660 seconds was the first indication that the ET hydrogen tank had been breached, Figure 3.10.1. Cryogenic liquid hydrogen exiting the tank changed the flowfield and cooled the region where the plume was impinging on the ET. The plume was thus substantially changed which in some camera views appears as a sudden shrinkage. The hydrogen leakage was confirmed several seconds later through telemetered measurements of changes in the hydrogen tank pressure.
3.11. Right Aft SRB Strut Release
At 72.201 seconds, the lower attaching strut between the right SRB and the ET, Figure 3.11.1, released. Loss of this structural attachment, most likely caused by the strut pulling away from the hot gas-weakened hydrogen tank permitted the right SRB to rotate counterclockwise around the aft upper strut and the forward attachment at the inter-tank. This rotation is graphically displayed in Figure 3.11.2, indicated by divergent yaw and pitch rates between the left and right SRBs.
3.12. Drop in LH2 Tank Pressure
At 72.546 seconds, the ET hydrogen pressurization system could no longer compensate for the amount of hydrogen spilling from the aft tank area. Even with two flow control valves open to increase the amount of ullage pressurization gas, the pressure in the hydrogen tank began to decrease as shown in Figure 3.12.1.
Figure 3.13.1. "Vapors" at External Tank Aft Dome.
Figure 3.13.2. "Vapors" at External Tank Intertank Region.
At 73.124 seconds. a circumferential white pattern was observed, Figure 3.13.1, on the + Z side of the ET aft dome. This was the beginning of hydrogen tank structural failure which released massive amounts of hydrogen from the aft tank area. A sudden large forward thrust resulted from this hydrogen expulsion which concentrated large compressive loads on the intertank where it joins the hydrogen tank. Either this abnormal loading condition or the effects of the right SRB rotation, possibly into the intertank, caused the intertank to fail. This in turn caused the LOX feedline and tank to structurally fail at 73.137 seconds as evidenced by the vapors appearing in the intertank region, Figure 3.13.2. This led directly to total vehicle structural breakup.
3.14. Recovered SRM Hardware
Both left and right SRM hardware fragments of varying sizes and locations were recovered during salvage operations. Two large pieces were retrieved that contained burned areas, Figures 5.4.1 and 5.4.2. They are confirmed by part and serial numbers and configuration features to be from the right SRM aft center and aft segments. The burned area spanned circumferential positions 291° to 318° at the right SRM field joint and measured approximately 33 inches circumferentially and 35 inches longitudinal]y. Additionally, the aft segment fragment contained a small hole, 1.5 inches by 4 inches located between the ET attach rings. Initial indication is that this small hole is a secondary effect from the large burned hole. The position of the large burned hole generally coincides with estimates of the smoke at lift-off and the observed flames on the right SRM at approximately 39 seconds.
3.15. SRM Field Joint/Seal Anomaly History
The anomaly records for all previous flights were researched to identify any past field joint/seal anomalies. The records showed nine joints on seven flights with O-ring anomalies tabulated in Figure 3.15. 1. The anomalies included combustion gas passing by the primary O-ring called blowby, primary O-ring erosion, and heat indications on both O-rings; but in no case did the joint leak.
4.0. Areas of Consideration
Immediately following the STS 51-L accident, working groups were activated to begin implementing predefined contingency procedures. Using immediately available flight data, visual observations, and photographic and video imagery, the working groups either identified possible faults that could originate in their respective flight element system and potentially led to the vehicle breakup of STS 51-L and/or they systematically examined flight data in search of any anomalous conditions that could cause the accident. The final disposition of flight elements is shown in Figure 4.0.1. The Orbiter. SSME, and Cargo were eliminated early in the investigation. Major areas of investigation for the External Tank and SRB are depicted in Figure 4.0.2. The Accident Analysis Team reviewed the assessment process and results for each element/fault to arrive at the conclusions and findings set forth below. The investigation concluded that the accident was initiated by a failure of the right SRM aft field joint which resulted in a hot gas leak.
4.1. External Tank (ET)
The ET is comprised of three primary structures: an LO2 tank, an intertank, and an LH2 tank. The basic ET configuration is shown in Figure 4.1.1. In addition to the structures system, there are the propulsion system, electrical system, thermal protection system and interface hardware -all of which were considered initially in identifying possible faults or failures potentially contributing to the STS 51-L accident. The ET tank faults or failures that could have possibly contributed to the STS 51-L accident were:
Premature Detonation of ET Range Safety System
Structural Flaw
Overheating
Damage at Lift-off
Load Exceedance
Each fault or failure was assessed to determine its validity and possible implication in the events leading to the accident. A summary of each fault or failure and the Accident Analysis Team conclusions and findings are presented below.
a. ET Range Safety System
The possibility of the ET Range Safety System (RSS) prematurely detonating and destroying the ET and causing the accident was addressed; various failure modes were postulated such as aerodynamic overheating, electrical malfunction or premature arm and fire command. Assessment of all these modes found no credible failure mechanism associated with premature detonation of the ET Linear Shaped Charge (LSC). Additionally, large portions of LSCs from both the L02 and LH2 tanks were recovered intact, eliminating premature detonation as a possible cause. It is concluded that the ET RSS did not initiate the STS 51-L accident.
b. Structural Flaw
The possibility of a structural imperfection existing in either the pressurized or nonpressurized ET hardware elements, that could grow to a sufficient size to cause structure failure was examined in detail. All build paper, structural qualification test data, proof test inspection records, and x-rays, were re-reviewed and only one previously undetected imperfection was found: a 0.400-inch imperfection in the ET Station 2058 ring frame weld located on the + Z axis of the tank. This particular imperfection was found in recovered hardware with no propagation indicated. Other data from the Ice/Frost Team inspections, on-pad film and video coverage, pressurization records, and flight data revealed no leakage evidence. It is concluded that no imperfections existed that could have grown to a size to create a leak or cause catastrophic failure of the ET.