I. Introduction.

II. Directive Appointing Team/Charter.

III. Organizational Structure Technique.

A. Organization.

B. Methods of Investigation.

IV. Definition of Terms and Acronyms.

A. Acronyms.

B. Definition of Terms.

V. Report Summaries.

A. Surface Search.

B. Metric Data.

C. Sonar Mapping.

D. Contact Classification.

E. Recovery.

F. Structural Reconstruction and Evaluation.

1. Reconstruction Technique.

2. Probable Failure Mode (Orbiter with Payload).

3. Probable Failure Mode (External Tank).

4. Probable Failure Mode (Solid Rocket Boosters).

VI. Findings and Conclusions.

A. Physical Evidence vs Fault Tree Analyses Findings.

B. Findings.

C. Conclusions.

 

Appendices

A. List of Recovered Parts.

B. List of Search Photographs.

C. List of Recovery Photographs.

D. List of Search Video Tapes.

E. List of Recovery Video Tapes.

F. 51-L Launch Readiness Review (DOD) dated January 14, 1986.

 

 

LIST OF VOLUMES (separately bound)

 

VOLUME 1	THIS VOLUME- REPORT SUMMARIES
VOLUME 2	Enclosure 1. MISSION 51-L SHUTTLE MISHAP-SURFACE SEARCH Enclosure 2. RADAR DATA ANALYSIS AND IMPACT ESTIMATION FOR STS 51-L DEBRIS . Enclosure 3. STS 51-L RIGHT SRB REPORT NO. 82-SR-86-07, Enclosure 4. SEARCH, CLASSIFICATION AND RECOVERY
VOLUME 3	Enclosure 5. SRB INCREMENTAL RECOVERY PLANNING
VOLUME 4	Enclosure 6. RECOVERED ET DEBRIS HARDWARE ASSESSMENT, Enclosure 7. OCEANOGRAPHY Enclosure 8. SPACE TRANSPORTATION SYSTEM, MISSION 51-L STRUCTURAL RECONSTRUCTION AND EVALUATION REPORT (Standard Accident Report) Enclosure 9. CONTACT #131 AND #712 RECOVERED RH SRB AFT FIELD JOINT EVALUATION
NOTES: Volume 2 of the SRR report dated April 1986 is now broken into three volumes (Vol. 2, 3, and 4). Enclosure 9 is added as a part of Volume 4.

 

LIST OF CHARTS

 

Chart	Title
1.	STS 51-L Data & Design Analysis Task Force.
2.	Search, Recovery and Reconstruction Team.

 

 

LIST OF FIGURES

 

Figure	Title
0.	Shuttle Coordinate Systems and Dimensions.
0-1.	Orbiter Vehicle Dimensions.
1.	Right SRB and FAA Radar Impacts.
2.	Impacts From Radar Data.
3.	Current Search Area.
4.	Orbiter and Reference Axes.
5.	External Tank & Reference Axes.
6.	SRB Coordinate System Looking Aft.
7.	Physical Evidence vs 51-L Fault Tree.
8.	Physical Evidence vs 51-L Fault Tree.
9.	Physical Evidence vs 51-L Fault Tree.

 

LIST OF PHOTOGRAPHS

 

Photograph	Title
1.	Layout of Orbiter Debris.
2.	Layout of External Tank Debris.
3.	SRB Fracture Surface.
4.	SSME as Recovered.
5.	Undetonated Linear Shape Charge.
6.	Undetonated Linear Shape Charge.
7.	Unburned or Unscorched Debris from Payload Bay.
8.	Bent IUS Beam.
9.	Aft Center Segment with Burn Thru Hole Contact #131.
10.	Aft Center Segment with Burn Thru Hole Contact #131.
11.	Mockup of Hole on Right-Hand SRB Aft Field Joint.

 

 

LIST OF TABLES

 

Table	Title
1.	Search and Salvage Vessel Assets and Capabilities.
2.	Summary of Underwater STS 51-L Contacts Recovered.
3.	Orbiter Components Returned for Analysis.

 

[O4] I. Introduction

At 11:38:00.010 Eastern Standard Time (EST) on January 28 1986, Mission 51-L Shuttle was launched from Launch Complex (LC) 39B at the John F. Kennedy Space Center, Florida, with seven astronauts aboard. As observed and recorded on video tape at 74.130 seconds after liftoff, the Shuttle was engulfed in flames. It began a catastrophic breakup. Both Solid Rocket Boosters (SRB's) separated from the External Tank (ET) and continued powered flight on erratic courses until the Range Command Destruct signal was sent by the Range Safety Officer (RSO) at 11:39:50.2. Numerous large pieces of the Orbiter, "Challenger," and the ET were observed to descend from the point of initial breakup and fall into the Atlantic Ocean east-northeast of the Kennedy Space Center (KSC). After Command Destruct action, the SRB's started to break into many pieces and fall into the ocean east of the Orbiter and ET impact area.

The Air Force Eastern Space and Missile Center (ESMC) was tasked by the Program Requirement Document (PRD) to provide both radar and optical tracking for Mission 51-L launch. This tasking is provided for all Space Transportation System (STS) launches in order to provide data for the performance of the Range Safety function and also to provide engineering and trajectory data to the National Aeronautics and Space Administration (NASA). The status of these systems was briefed at the Commander's Mission Briefing (see DDATF Pre-Launch Activities Team Report, Appendix J, Appendix 6) and the NASA Launch Readiness Review on January 14, 1986 (see Appendix F). The radar and optical data was used by the RSO to verify the vehicle had experienced a catastrophic breakup and both SRB's were still propulsive following the initial breakup at T+ 74 seconds. This data also provides the basis for water impact predictions used in the initial stages of surface recovery. The lack of a multi-object tracking system made the prediction of radar and optical data extremely difficult and, to a large measure, accounted for the lengthy search and recovery operations.

The Department of Defense Manager's STS Contingency Support Operations Center initiated a search and rescue operation on behalf of NASA. The surface search was directed by the Launch Recovery Director (LRD), in accordance with the STS Contingency Support Operations Plan dated 1 December 1985 (see Appendix G), and the Department of Defense Manager for Space Shuttle Contingency Support (DDMS). The surface search was coordinated by the United States Coast Guard (USCG) with assistance from ESMC. The surface search was terminated on February 7, 1986. The Department of the Navy was also requested to initiate preparation for an underwater salvage operation.

NASA established three areas for storage and reconstruction. The Logistics Building and an adjacent area on KSC were used for Orbiter and ET debris, respectively, and on Cape Canaveral Air Force Station (CCAFS), Hangar "O" and the EOD Range were used for SRB debris.

On January 28, 1986, Dr. William Graham, NASA Acting Administrator, appointed Mr. Jesse Moore to act as the interim 51-L Mishap Investigation Board chairman. Mr. Moore requested that Col. Edward O'Connor direct the activity of the NASA LRD and coordinate the development of the necessary search and recovery organizations required to support the recovery of the 51-L flight crew and those flight components that would provide the basis for the accident investigation. Col. O'Connor also directed the development of the recovered components preservation plan and requested Mr. Terry Armentrout of the National Transportation Safety Board (NTSB) to establish a Structural Reconstruction and Evaluation Group composed of engineering representatives from the STS element contractors, NASA, and the NTSB. Col. O'Connor provided continuing direction to these activities until formally chartered as Team Lead for the Search, Recovery, and Reconstruction Team of the Task Force on March 20, 1986.

Initial priorities for the underwater salvage operations were as follows:

1. Right-Hand Solid Rocket Booster (SRB)

2. Left-Hand SRB

3. Orbiter Crew Compartment

4. Payload

The recovery of Mission 51-L Shuttle components for support of the accident investigation activities of the Task Force was terminated on May 1, 1986. The deep water SRB recovery was terminated after the recovery of Contacts #131 and #712 which contain approximately 90% of the burn-through area of the right-hand aft field joint. This is believed to be the origin of the initial failure. Shallow water recovery was deemed to have provided sufficient material to confirm that the right-hand SRB failure was the initiating failure mode and that all other flight components failed subsequent to this failure.

Some shallow water recovery will continue in order to provide material for NASA Design Centers' Material Properties Studies.

The following pages provide a narrative of the activities leading to the recovery of debris as well as the structural reconfiguration and evaluation. The detailed source data for this report are in Enclosures 1 through 9.

Remarks:

This report utilizes the manufacturer's coordinate system. This system was selected because the reconstruction process required use of the manufacturer's drawings. Reference Figures 0 and 0-1 for the axes and coordinates.

The Photo and TV Support Team utilized the Flight Dynamics coordinate system which is illustrated in Volume 1 of their report.

III. Organizational Structure Technique.

A. Organization

The following two charts show the top levels of the organization as charted by Rear Admiral Richard Truly and does not represent the multitude of people that participated in the Search, Recovery, and Reconstruction Task.

B. Methods of Investigation

Immediately following the mishap, the NASA LRD requested that the NASA SRB retrieval vessels proceed to the water impact location and also requested that DDMS dispatch all immediately available surface vessels and aircraft to the area of the mishap. The ESMC RSO held all aircraft and surface vessels outside of the falling debris area until entry was deemed to be safe from falling materials that could cause a hazard to the crews entering the area. Approximately one hour after the mishap, a surface search was initiated in the immediate area surrounding the water impact location of the Orbiter and ET. A large amount of Orbiter and ET debris was found floating and was recovered by surface ships. Due to the currents of the Gulf Stream, the surface search was expanded to the north and, over the next several days, extended to an area 450 nautical miles north of the initial search area.

Radar tracking data from the ESMC was utilized to determine the area for the underwater search and sonar mapping of this area commenced. The search area was subsequently expanded based on further data reduction efforts that identified a wider dispersion of Right SRB components following the Command Destruct.

Identification of individual sonar contacts was accomplished by remotely-operated vehicles (ROV's) and manned submersibles in both shallow water and deep water areas as well as divers in shallow (less than 200 ft. (61 meters)) water. This identification established the debris to be recovered as well as the priorities of recovery. Plans were developed for the recovery technique, safing, and preservation of the components.

II. Directive Appointing Team/Charter.

[O5] Figure 0. Shuttle Coordinate Systems and Dimensions.

[06] Figure 0-1. Orbiter Vehicle Dimensions.

[O7-O8] Letter. [To: M/Chairman, STS 51-L Data and Design Analysis Task Force; From: A/AD/Acting Administrator; Subject: Establishment of the STS 51-L Data and Design Analysis Task Force]

[O9] 51-L DATA AND DESIGN ANALYSIS TASK FORCE [To: E. O'Connor; From: Task Force Chairman; Subject: Appointment of Lead; Search, Recovery and Reconstruction Team]

[O10] GENERAL ASSIGNMENT FOR SEARCH, RECOVERY AND RECONSTRUCTION TEAM.

[O11] CHART 1. STS 51-L DATA & DESIGN ANALYSIS TASK FORCE. CHART 2. SEARCH RECOVERY AND RECONSTRUCTION TEAM.

[O12] The recovery was accomplished by the divers and submersibles. The recovered parts were delivered to impound areas for possible reconstruction.

The Orbiter, ET, SRB's, and cargo (payloads) were placed in separate areas for reconstruction. The reconstruction placed the components in as close a proximity to the original configuration as possible. The debris was photographed and examined for physical evidence and samples were then taken for laboratory examination.

The analyses of the recovered components conducted by the National Transportation Safety Board (NTSB) were used to verify the conclusions of the Accident Analysis Team in determining the specific failure modes/scenarios dispositions (probable, improbable, and possible).

At the time of writing this report, the identification, recovery and reconstruction tasks are still in progress.

IV. Definition of Terms and Acronyms

A. Acronyms

B. Definition of Terms

V. Report Summaries

A. Surface Search

During the first few minutes following the Mission 51-L Shuttle Mishap, the DOD Manager's STS Contingency Support Operations Center (SOC) alerted appropriate search and rescue operations and established communications with the National Military Command Center (NMCC) as well as the Joint Chiefs of Staff WCS). The JCS formed a Shuttle Response Cell which was briefed on the situation.

During the first fifteen (15) minutes following the mishap, search and rescue aircraft and ships were dispatched to holding points short of the safety zone established for falling debris. During this period, one aircraft reported sightings of falling debris impacting the ocean surface. The two SRB retrieval vessels, Liberty Star and Freedom Star, were directed from launch support positions to the impact area.

At 1737Z (1237 EST), the Range Safety Officer gave a clearance for aircraft to enter the safety area.

By 2130Z (1630 EST), a major Search and Rescue (SAR) effort was requested. By 2400Z (1900 EST), twelve (12) aircraft and eight (8) ships were participating in the area of debris impact.

On February 5, 1986, the Coast Guard sent a notice to mariners declaring the search area a safety area, which suspended all fishing operations in the area. This was done because numerous scallop boats reported finding Shuttle debris in their nets.

The Surface Search and Rescue effort continued until February 7, 1986. Debris recovered by the ships assigned to the effort was brought into Port Canaveral and off-loaded. Debris found off the coast of Georgia and South Carolina was returned to the Cape Canaveral Air Force Station (CCAFS).

During the period of January 28, 1986 through February 7, 1986, the Search and Rescue Operation was simultaneously supported with as many as 14 ships and 11 aircraft. The Coast Guard stated, "The operation was the largest surface search in which they had participated."

B. Metric Data (Radar and Optics)

The source materials for this section were two Eastern Space and Missile Center (ESMC) reports, "Radar Data Analysis and Impact Estimation for STS 51-L Debris," Report No. 82-SR-86-06, dated 7 March 1986, Enclosure 2, and "STS 51-L Right SRB," Report 82-SR-86-07, dated 4 April 1986, Enclosure 3.

The prime purpose of evaluation of the metric data (radar/optics) was to provide impact locations for debris recovery and, when possible, identify the tracked object.

Following the breakup of the External Tank (ET) at T+74 seconds, the right Solid Rocket Booster (SRB) continued under thrust for approximately 37 seconds. It was tracked continuously by Radar 1.17 and the Ponce de Leon Metric Integrated Ground Optical Recorder (PDL MIGOR). Radar 0.14 and the Universal Camera Site (WCS) #15 Intermediate Focal Length Optical Tracker (IFLOT) tracked briefly.

Figure 1 shows the geographical locations of the radar and optical stations as well as the track of the right SRB Aft skirt with a short section of SRB segment and impact points of other SRB debris.

Analysis of the PDL MIGOR video tape shows the right-hand SRB, following the ET breakup, to be rolling clockwise viewed from aft approximately once every 10 seconds. Possibly this period decreased by about 1.5 seconds during the 3i seconds of powered flight indicating an angular acceleration about the longitudinal axis.

From T+93.6 to T+95.8 seconds, Radar 0.14 detected a number of small objects separating from the main target. None of these objects was visible to the optics sites or on the radar boresight video tape. Enough data could be extracted to permit a trajectory estimate to be made for one of these objects, designated RD on Figure 1. Impact was at approximately T+ 398 seconds, and the separation speed relative to the main SRB body was on the order of 60 meters per second (m/s).

[O14] At T+110.3 seconds, Range Safety issued a command destruct to the SRB's. The shock wave from the destruct explosion of the right SRB was visible on the WCS-15 E-705 16mm camera, permitting a computed estimate of the explosion at time T+110.275 seconds. The propagation speed of the shock wave was on the order of 1,000 m/s during the first few hundredths of a second with the expansion slowing to essentially zero within 0.1 second.

Following the command destruct, the SRB separated into a number of fragments. Optical resolution was not sufficient to permit the identification of these fragments. Radar 1.17 and PDL MIGOR continued to track a large piece later identified as part of the aft segment and skirt. A number of smoke trails, possibly two dozen small (pinpoint) glowing objects and four bright glowing objects, could be seen on the WCS-15 IFLOT 70mm film. The four bright glowing objects and possibly three smoke trails were also seen on the PDL MIGOR video tape. The radar boresight video tape showed no visible object after the command destruct.

Because the smoke trails are at the limit of the PDL MIGOR video resolution, first priority was given to the three bright glowing objects not tracked by Radar 1.17. Triangulation on these objects (RA, RB, and RC) was possible for periods of four (4) to six (6) seconds, and impact trajectories have been generated. Figure 1 is a map showing the ground traces of the tracked (solid) and projected (dotted) trajectories. Separation speeds relative to the booster aft segment ranged from 45 m/s to 120 m/s.

From T+115 to T+141 seconds, an object flashing twice per second was in the PDL MIGOR field of view. Although it appears near the tracked segment, it cannot be positively linked to the right SRB.

The Federal Aviation Administration (FAA) radar observations cannot be positively associated with the right or the left SRB. The six impact estimates may be from either source, but Objects 2, 5, 6, and 7 appeared more likely to be from the right SRB.

Estimated impact locations of nine objects were determined using the ESMC radar data only. The impact locations are shown on Figure 2.

1. Object A's impact location was of prime importance as it represented debris from the right SRB.

2. Two objects (F and G) were parachutes believed to have been picked up during the surface search. Note: One drogue chute was recovered during the surface search. The right-hand main chutes were recovered with the right-hand SRB forward skirt.

3. Two objects (B and D) were believed to be sizeable (2 to 5 meters, 6 to 16 feet) based on radar boresight recordings and were believed to be dense based on their early impact times.

4. One object (K) was relatively dense but the cross-section was less than 0.1 square meter (1.1 square feet).

5. Two objects (C and I) had moderately slow final descent rates of about 134 feet per second (41 meters per second) with Object C having a relatively large cross-section.

6. Object H was of little interest for recovery operations since it appeared to be a small, light object which may very well have floated after impact.

Both of the reports estimated impact locations of Shuttle-related debris thought to be from the SRB's. Data from both reports support the impact location of the right SRB aft skirt with partial segment attached, reference Figure 2, Object "A" and Figure 1, Point "R".

C. Sonar Mapping

1. The Supervisor of Salvage, Charles A. Bartholomew, Captain, U.S. Navy, with the assistance of the SRB Incremental Recovery Group lead by Alexander A. McCool, Director, Structures and Propulsion Laboratory, MSFC, directed the Search and Recovery Operation. This section and the next two sections describe the methods utilized during this operation. A more detailed description is contained in Enclosure 4, Search, Classification, and Recovery, and Enclosure 5, SRB Incremental Recovery Planning.

The planning for an underwater salvage operation is dependent upon a changing set of contact data, priorities, and environmental conditions. By utilizing the data available, the proper assets were acquired and effectively utilized (see Table 1 for assets used). The search, contact classification, and recovery phases were accomplished concurrently.

Table 1. Search and Salvage Vessel Assets and Capabilities.

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¹ Primary Employment

² Vessel Employment Phases

³ SUPSALV Supplied Assets

The Supervisor of Salvage held daily meetings with NASA, DDMS, ESMC, and Coast Guard personnel to determine priorities and deployment of assets. All phases of the operation were discussed with alternatives planned if conditions changed.

Radar data (both ESMC and FAA) were used to establish the basic search area, a parallelogram 10 miles by 25 miles, 5 miles on each side of the major radar tracks. Water depth was just under 100 feet to approximately 1200 feet. The area was completely within the Gulf Stream. The area was later expanded to include the entire right and left hand SRB debris fields based on refined analyses of trajectory data (reference Figure 1). The expanded search area was 480 square nautical miles (307,200 acres) in size. Figure 3 is a map of the search area.

2. The Global Positioning System (GPS) was selected to be used as the primary navigation system. LORAN-C and LORAC-A were also used but corrected using GPS as a standard to adjust for inaccuracies between systems.

The search area was mapped for targets by side scan sonar. This is a device towed by a ship 15 meters (49 feet) above the ocean bottom at a constant speed (between 2 to 4 knots). In order to maintain a constant speed while moving within the Gulf Stream, most mapping runs were made north to south against the current. This extended the operation because no data was gathered on the return trip (to the north).

Search lines 5 seconds longitude apart (135 meters, 148 yards) were used to provide sufficient overlap of bottom surface coverage. Sonar scan width was 300 meters (328 yards). This provides for 118 % overlap. This is done to prevent any voids based on navigation systems inaccuracies, sea conditions, and winds. Through May 1, 1986, a total of 691 contacts have been identified using side scan sonar.

D. Contact Classification

Contact classification was accomplished by documentation and small debris provided by divers (many), remotely-operated submersibles (2), and manned submersibles (2). This documentation consisted of still photographs, video tapes, audio descriptions from manned submersibles, and diver descriptions. Small pieces from debris fields were recovered in an effort to determine a possible general location of adjacent components. Each contact was classified as Shuttle-related, non-Shuttle-related, or unconfirmed. The Shuttle-related were put into categories of each element: Orbiter, ET, Right SRB, and Left SRB.

The majority of the contacts were classified as non-Shuttle-related. These items were Geological, such as shells, coral ridge lines, fish, debris of previous unsuccessful launches from CCAFS, and discarded equipment from vessels using the shipping lane, such as refrigerator, 55-gallon drums, coils of wire.

Through May 1, 1986, 490 contacts have been classified; 408 contacts have been non-Shuttle, while 82 contacts have been Shuttle-related (5 to 1 ratio).

E. Recovery

During the contact phase, photos, video tapes, and divers' logs were examined to determine what techniques were to be used for recovery. Special tools were designed and fabricated. The variety of tools ranged from large baskets to contain small pieces in dense debris fields to attach fittings used to lift pieces of SRB weighing up to 15 tons.

Special individual plans were prepared to recover pieces of SRB. This was done to prevent or minimize any handling damage and provide instructions for preservation to prevent additional corrosion. These plans are listed in Enclosures 4 and 5 with an example included in Enclosure 5.

Again, the daily meetings held by SUPSALV were used to establish the priorities and dispatch assets for contact recovery. This was invaluable as it provided a forum to redirect assets as priorities changed.

Divers were used extensively for recovery operations in shallow water. Only surface-supplied air and scuba diving techniques were utilized. Surface-supplied mixed gas (helium/oxygen) and saturation mixed gas (helium/oxygen) diving techniques were not used in the initial search effort. However, this technique is planned to be used later in depths of approximately 300 feet.

The submersibles were used in the deeper water areas.

Table 2 lists the major debris recovered. This operation utilized the largest number of assets ever assembled for marine salvage.

Table 2. Summary of Underwater STS 51-L Contacts Recovered.

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[O18]

Figure 1. RIGHT SRB AND FAA RADAR IMPACTS.

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---

[O19]

Figure 2. IMPACTS FROM RADAR DATA.

Figure 3. Search Area.

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Several pieces from the Orbiter and ET debris contained pyrotechnic devices. The majority of the SRB debris contained pyrotechnics or unburned propellant. The pyrotechnics were either safed or destroyed by Explosive Ordnance Disposal (EOD) personnel. The fragments of the cases were used as physical evidence in the reconstruction process; therefore, a burn plan was developed to remove (burn) the unused propellant without damaging the case material. The unburned propellant was burned off the case without destroying any evidence. Video tapes were made to document the "before" and "after" state of the debris.

F. Structural Reconstruction and Evaluation

The National Transportation Safety Board (NTSB) evaluated the recovered debris in an effort to determine the probable failure mode of the Orbiter including the Payload, the External Tank (ET), and each Solid Rocket Booster (SRB). The following is a brief description of the techniques for control and reconstruction of the debris with a probable failure mode for each element. A more detailed explanation is found in Enclosure No. 8, Space Transportation System, Mission 51-L Structural Evaluation Report.

1. Reconstruction Technique

Recovered Shuttle debris was returned by ship to the docks at Port Canaveral and screened for material. The debris was loaded onto pallets and trucks for transport to KSC or the Cape Canaveral Air Force Station (CCAFS) where impoundment areas had been established. Access to the impoundment areas was controlled by KSC and Air Force security forces on a 24-hour, 7-day per week basis. The three major impoundment areas were large enough to accommodate almost the full dimensions of the STS subsystems-Orbiter, ET, and SRB's. A separate area was established on the EOD Range for hazardous material. The impoundment area for the SRB's was located in a remote hangar (Hangar "O") near the Eastern Space and Missile Center (ESMC) EOD Range because the pieces of SRB contained unburned and potentially hazardous propellant.

As the debris arrived at the impoundment area, it was unloaded from the trucks with forklifts and transferred to the identification area. In the latter area, the pieces were video taped, photographed, identified by Quality Assurance personnel, and appropriately tagged. Each piece of debris was assigned an identification number based on its time of arrival at the impoundment area, and a chronological record was prepared which included the identification number; description; time and date of arrival; related photograph numbers; date; time; and location (latitude and longitude) of recovery (if known); and the name of the ship that delivered the debris to the docks.

The impoundment areas for the Orbiter and ET were divided into a grid with yellow tape. The pieces of debris were placed in the appropriate squares which corresponded to their original configuration. As the quantity of debris increased, platforms and stanchions were constructed upon which the pieces of debris were placed or secured vertically. Various internal storage tanks from the Orbiter were identified and collected in one location for further examination-see Photographs 1 and 2 for layouts of the Orbiter and ET debris, respectively.

Table 3 lists recovered electronic components such as General Purpose Computers (GPC's) and tape recorders. These components were washed with fresh water and sent to other facilities for evaluation. This report will not include the analysis of those components.

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[O21]

Photograph 1. Layout of Orbiter Debris.

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[O22]

Photograph 2. Layout of External Tank Debris.

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[O23] Pieces of debris that were removed from the impoundment areas for metallurgical or chemical analysis were accounted for by hand receipt. Video tapes, pertinent photographs, and records of the photographs taken were kept in safes in the impoundment areas.

After transport to the EOD area (Hangar "O"), the pieces of SRB debris were initially evaluated and identified. The pieces containing unburned propellant were then taken to the EOD range where the propellant was burned in accordance with a propellant disposal plan that was developed and tested. The plan provided for safe disposal of the propellant and complete protection of the evidence in the process. Records including video tapes of the propellant burning processes were maintained. Following propellant disposal, the pieces of the SRB's were returned to Hangar "O" for storage and evaluation.

2. Probable Failure Mode (Orbiter With Payload)

Insufficient structure was recovered to definitely establish the failure mode of the Orbiter, Challenger. However, the fact that all material failures occurred from overload with no evidence of internal burn damage or exposure to explosive forces indicates that destruction of the Orbiter occurred predominantly from aerodynamic, acceleration, and inertial forces that exceeded design limits.

There is evidence that the right SRB contacted the bottom surface of the outboard section of the right wing which may have contributed to the separation of the right wing from the Orbiter. The evidence includes crush damage on the bottom surface of the right wing and outboard elevon, positive ( + z) bending and fractures in sections of the right elevon and inboard structure of the right wing, and high inertial loading on the right main landing gear and on the IUS's right spreader beam. The Orbiter axes are shown in Figure 4. Further, the proliferation of alpha aluminum oxide residue from molten SRB propellant on various surfaces on the right side of the Orbiter in conjunction with the approximate geometric relationship of the right wing to the inboard circumference of the right SRB during normal flight indicates that as the SRB moved upward ( + z) into contact with the right wing, and as the wing separated from the Orbiter, the right side of the Orbiter was sprayed by hot gases exhausting from the hole in the inboard circumference of the SRB. It is possible that this contact between the right SRB and the Orbiter assisted in separating the Orbiter from the ET in a rapid counter-clockwise rolling movement that exposed the Orbiter to destructive aerodynamic and inertial forces.

The structural evaluation established clearly that the crew module, including most of its outer shell, remained essentially intact until impact with the water and that the module was fragmented extensively from extreme overload and inertial forces associated with water impact. The structural deformations and fragmentations indicate that the module struck the water in a slight nose down and steep left bank attitude.

Evaluation of the SSME's indicated extensive internal thermal damage as a consequence of oxygen-rich operation that resulted from depletion of the hydrogen fuel supply. The positions of various engine valves indicate the engine control system for No. 1 engine was correcting for the loss of the hydrogen fuel supply; the No. 2 engine was in the shutdown phase; and the No. 3 engine was in mainstage operation when hydraulic and pneumatic power sources were lost. There was no evidence to indicate that SSME or MPS malfunctions or failures contributed to the destruction of the Orbiter.

There was no evidence to indicate that the IUS contributed to premature structural failure of the Orbiter.

3. Probable Failure Mode (External Tank)

Insufficient structure was recovered to positively establish the failure mode of the ET. However, the structure recovered indicates that the LH2 tank probably failed from external forces and thermal damage from the fire that erupted from the inboard circumference of the right SRB about 58 seconds after liftoff. The fire probably impinged on the ET near the lower attachment fittings for the right SRB and separated one or more of the three lower attachment fittings. The separation of the fitting(s) allowed the right SRB to twist about its single upper ET attachment fitting to the extent that the frustum joint penetrated the intertank near the 70-degree and XT1000 location. The penetration of the intertank by the right SRB frustum probably forced the intertank structure into the LO₂ tank which permitted escape and vaporization of the LO₂. Figure 5 shows the External Tank and reference axes.

The separation of the lower attachment fittings on the right SRB indicates that breach of the LH₂ tank preceded compromise of the LO₂ tank. The LH₂ tank probably failed from thermal damage near the welded seam that attaches the aft ellipsoidal dome to the tank cylinder. The thermally-weakened seam probably failed circumferentially under the weight of the LH₂ as magnified by longitudinal acceleration forces.

4. Probable Failure Mode (Solid Rocket Booster)

As of May 1, 1986, 42 pieces of the two SRB's had been recovered, including 2 pieces of the right SRB (sidescan sonar Contacts Nos. 131 and 712) that identified the approximate dimensions of the hole burned through the side of the SRB near the lower field joint. Since these two pieces of the right SRB contained the physical evidence pertinent to the failure of the right SRB that initiated the accident sequence, further search and recovery efforts for pieces of SRB were discontinued as of the above date. The evaluation of Contact Nos. 131 and 712 is contained in Volume 4, Enclosure 9, of this report.

The right frustum sustained more nose damage than the left frustum. There was evident and unique damage to the base of the frustum at the 240- to 255-degree circumferential location (reference Figure 6). Chemical analyses of deposits in the damaged area established the presence of polyurethane of the type used for outer insulation of the ET intertank. Also, indentations in the damaged area were spaced about 5 inches apart which corresponds to the spacing between the vertical stringers in the ET intertank.

None of the 19 pieces of the right SRM or the pieces of the left SRM evaluated in this report provided indication of any structural failures that may have occurred before the SRM's were destroyed by detonation of the LSC's attached to the casings along the system tunnel of each SRM.

VI. Findings and Conclusions

A. Physical Evidence Versus Fault Tree Analyses Findings

The Search, Recovery, and Reconstruction Team compared the physical evidence obtained from the Mission 51-L Shuttle recovery debris with the conclusion of the Accident Analysis Team's Fault Tree. The Fault Tree was annotated with symbols indicating those areas where the physical evidence supports or contradicts the analytical findings. These are shown on Figures 7, 8, and 9 with a brief explanation following.

1. No physical evidence exists or the physical evidence available is insufficient to support or contradict the following nodes on the fault tree analyses.

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[O24]

Figure 4. ORBITER AND REFERENCE AXES.

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Figure 5. External Tank & Reference Axes.

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Figure 6. SRB COORDINATE SYSTEM.

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[O25]

The events or forces preceding the recovery of the physical evidence, such as aerodynamic breakup, nine-mile free fall, water impact, winds, currents, all combine to obscure or shield the possible findings and conclusions.

2. Physical evidence (fractured surfaces) indicated structural overload. No signs of stress corrosion, fatigue, or hydrogen embrittlement was found. This physical evidence supports the following nodes on the fault tree analyses.

The types and quantities of deformation and fractures seen on the recovered debris are consistent with the forces involved in an aerodynamic breakup and water impact. Physical evidence from recovered structures does not indicate structural overload was a cause but rather a result of some other malfunction. This conclusion is based on approximately 20% recovery of the ET. (See Photograph 2, Layout of ET Debris.)

3. Physical evidence (fractured surfaces) indicated structural overload. No signs of stress corrosion, fatigue, or hydrogen embrittlement were found. This physical evidence supports the following nodes on the fault tree analyses.

The ET intertank failed due to structural overload caused by contact with the right hand SRB. This in turn caused the LO₂ tank to fail.

The ET LO₂ tank failed due to structural overload caused by internal overpressurization. The ET LH₂ tank failed due to structural overload caused by external overloads rather than from an internal overpressurization.

The SRB's failed due to structural overload. All fractured surfaces exhibited either the characteristic herringbone or chevron markings of rapid tensile overload, a complete bending failure due to overload, or a jet-separation fracture due to the detonation of the linear-shape charges (LSC's).

The above findings are based on physical evidence, however, observations, video tape, and film show the tank structurally failing after other abnormal RH SRB events, and the SRB's continuing in erratic powered flight until the Command Destruct signal was sent by the Range Safety Officer. (Reference Photograph 3 for SRB Fracture Surface.)

4. The physical evidence indicated the SSME's (main engines) were operating until Orbiter breakup. This supports the Fault Tree analysis.

Valve positions on the three main engines were examined to determine what phase of operation each engine was experiencing. Several valves were damaged by impact such that their position was invalid. Two engines were found to be in main stage (full operation) and one in a shutdown phase when hydrogen depletion occurred. Damage to the engines was consistent with oxygen-rich operation and loads associated with water impact. (Reference Photograph 4, SSME as Recovered.)

5. The physical evidence indicated the external tank (ET) LSC's did not have a premature detonation. This supports the Fault Tree analyses.

The linear-shaped charges from both the LO₂ and LH₂ tanks were recovered; the charges had not detonated. (Reference Photographs 5 and 6, Undetonated Linear-Shaped Charges.)

6. The physical evidence of the Inertial Upper Stage (IUS) recovered components shows no evidence of fire, explosion, or any premature element separation. This supports the Fault Tree.

7. The physical evidence of the recovered IUS/TDRS/ASE components indicates a structural breakup not caused by premature ignition, explosion, or fire. This supports the Fault Tree analyses.

The debris from the payload was found to be shattered into many small fragments. There was no evidence of burning or fire on the recovered pieces. Unburned propellant fragments were among the pieces found. (Reference Photograph 7, Unburned or Unscorched Debris from Payload Bay.)

8. The physical evidence of the recovered IUS ASK indicates damage consistent with the IUS remaining attached to the Orbiter mounting until Orbiter structural breakup. This supports the Fault Tree analyses.

The two aft trunnion spreader beams which support the IUS in the Orbiter payload bay were recovered. Both beams contained a permanent deformation indicative of a large + z acceleration while the IUS was still attached. This is considered evidence the structural breakup of the Orbiter began while the IUS was still being supported by the ASK. (Reference Photograph 8, Bent IUS Beam.)

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[O26]

Figure 7. PHYSICAL EVIDENCE Vs 51-L FAULT TREE.

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Figure 8. PHYSICAL EVIDENCE Vs 51-L FAULT TREE.

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[O27]

Figure 9. PHYSICAL EVIDENCE Vs 51-L FAULT TREE.

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[O28]

Photograph 3. SRB Fracture Surface.

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[O29]

Photograph 4. SSME as Recovered.

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[O30]

Photograph 5. Undetonated Linear Shape Charge.

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[O31]

Photograph 6. Undetonated Linear Shape Charge.

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[O32]

Photograph 7. Unburned or Unscorched Debris from Payload Bay.

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[O33]

Photograph 8. Bent IUS Beam.

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[O34] 9. The physical evidence of portions of both sides of the righthand SRB aft field joint indicate a burn through in the 270° to 360° quadrant. This supports the Fault Tree Analysis.

The portions of the RH SRB aft field joint indicate a hole caused by a violation of pressure integrity of the joint. The portions, when aligned, indicate the hole to be consistent with area where smoke was observed during the lift off sequence by two cameras. Photographs 9 and 10 show the physical evidence of the aft center segment with burn through hole. Photograph 11 shows the hole as mocked up on another RH SRB aft field joint. These two portions of RH SRB were recovered Contacts #131 and #712

NOTE: Sections B and C are exact duplicates of the findings and conclusions found in Enclosure 8, National Transportation Safety Board's Structural, Reconstruction and Evaluation Report.

B. Findings.

1. About 30 percent of the Orbiter's total structure was recovered including about 75 percent of the crew module and surrounding forward fuselage shell structure.

2. About 20 percent of the ET was recovered; the structure was predominantly from the intertank and LH₂ tank of the ET.

3. All fractures examined on the STS structure had failed from overload forces; there was no evidence of fatigue, stress corrosion cracking, or manufacturing defect.

4. The lower structure of the LH₂ tank portion of the ET contained a minor weld imperfection; the imperfection did not contribute to the failure of the LH₂ tank.

5. There was no evidence of internal fire or explosion in the Orbiter preceding its disintegration.

6. The Orbiter was destroyed predominantly by high acceleration loads, high inertial load, and adverse aerodynamic forces.

7. Following separation of one or more of its lower attachment fittings to the ET, the right SRB struck the lower outboard portion of the Orbiter's right wing contributing to the separation of the wing from the Orbiter.

8. The aft right side of the Orbiter was burned by hot propellant gases exhausted probably from the hole in the inboard circumference of the right SRB.

9. The SSME's were functioning properly until the LH₂ supply was terminated by the failure of the LH₂ tank and/or tank connections to the Orbiter.

10. There was no evidence of malfunction or failure of the ASE/IUS/TDRS package in the Orbiter's payload bay before the right SRB forcefully contacted the Orbiter's right wing.

11. The crew module separated from the Orbiter at frame 582; the aft pressure bulkhead (576) remained with the crew module.

12. The crew module and surrounding forward fuselage shell structure forward to bulkhead 378 descended to the surface of the Atlantic Ocean essentially intact; these structures probably struck the surface in a nose down and steep left bank attitude.

13. The crew module and forward fuselage structure disintegrated from high deceleration and inertial forces associated with water impact.

14. The base of the right SRB frustum penetrated the intertank structure of the ET following release of one or more of the SRB's lower attachment fittings to the ET.

15. The LO₂ tank in the ET probably was breached by intertank structure which permitted LO₂ to escape and vaporize in the relatively warm atmosphere.

16. Fragmented LO₂ tank structure may have been burned by vaporized LO₂ following failure of the LO₂ tank.

17. The LH₂ tank in the ET probably failed from external forces and thermal damage near the aft ellipsoidal dome.

18. The pieces of SRB evaluated in this report were unremarkable; all fractures were from overload or from detonation of LSC's.

C. Conclusions

1. There is no evidence that structural failures in either the Orbiter or its payload package preceded the destruction of STS 51-L.

2. The Orbiter's MPS and SSME's functioned properly and did not contribute to the loss of STS 51-L.

3. The Orbiter's crew module was essentially intact until it struck the surface of the Atlantic Ocean; the crew module was disintegrated by water impact forces.

4. The ET probably failed from thermal and structural damage near the base of its LH₂ tank and from overpressurization of its LO₂ tank following partial separation of the right SRB from the ET.

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[O35]

Photograph 9. Aft Center Segment with Burn Through Hole. Contact #131.

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[O36]

Photograph 10. Aft Center Segment with Burn Through Hole. Contact #131.

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[O37]

Photograph 11. Mock-up of Hole on Right Hand SRB Aft Field Joint.

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