[O374] NUMERICAL TABLE OF CONTENTS
ALPHABETICAL TABLE OF CONTENTS
LIST OF FIGURES
LIST OF PHOTOGRAPHS
LIST OF TABLES
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 1 and 2 for the axes and coordinates.
The Photo and TV Support Team utilized the flight dynamics coordinate system which is illustrated in Volume I of their report.
B. STRUCTURAL EVALUATION GROUP.
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About 11:38 EST on January 28, 1986, Space Transportation System (STS) Mission 51-L departed Launch Pad 39B at the John F. Kennedy Space Center, Florida, with seven astronauts aboard. As observed and recorded on television and other film, about 72 seconds after liftoff, STS 51-L was nearly engulfed in a huge ball of fire, and it began to disintegrate. Both solid rocket boosters (SRB's) separated from the external tank (ET) and continued powered flight on erratic courses until linear-shaped charges (LSC's) on the SRB's were detonated by the Range Safety Officer (RSO); the charges destroyed both SRB's. Numerous large pieces of the Orbiter, "Challenger," and the ET were observed on the film to descend out of the fireball into the Atlantic Ocean east-northeast of the Kennedy Space Center (KSC). Also, the pieces of the SRB's fell into the ocean east-northeast of KSC.
During the several days following the accident, investigation teams were formed at the KSC to investigate the accident. A structural reconstruction and evaluation group was formed as a part of the Search, Recovery, and Reconstruction Team headed by Colonel Edward O'Connor, USAF, to evaluate all of the structure of STS 51-L that was recovered from the Atlantic Ocean. After cleaning with fresh water, the recovered parts of the structure were impounded in several buildings at the KSC where pieces [O377] were identified, cataloged, and arranged in two-dimensional mock-ups for detailed examination. A visual examination on the deformation and fractures of the pieces recovered was performed in order to assess their probable mode of failure and to establish a probable breakup sequence. Also, samples were taken of selected pieces which had either burn damage or deposits of a foreign matter. Laboratory analyses were then performed on these samples to determine the probable source of the heat or to identify the deposits of foreign material. The laboratory tests performed were X-ray Diffraction, Scanning Electron Microscopy/Energy Dispersive Spectroscopy, Infrared Spectroscopy, and Microprobe/Wavelength Dispersive Spectroscopy.
About 30 percent of the structure of the STS 51-L was recovered during the three months following the accident. Other pieces were identified by underwater television on the floor of the Atlantic Ocean but were not recovered because the pieces were deemed not significant to the determination of the probable cause of the accident. The structure deemed significant to the determination of probable cause consisted of certain portions of the right SRB from which fire was observed on film to have erupted about 58 seconds after lift-off. Also deemed significant were structural portions of the ET in the vicinity of the fire, including the lower fittings which attached the right SRB to the ET, and the crew module. The search for the structural component of STS 51-L consumed hundreds of thousands of hours of effort; however, much of the structure was never found. The suspect RH SRB aft field joint pieces were recovered and debris still being recovered should have no effect on the team findings.
The significant pieces of Orbiter structure recovered included all three Space Shuttle Main Engines (SSME's), the forward fuselage including the crew module, the right inboard and outboard elevons, a large portion of the right wing, a lower portion of the vertical stabilizer, three rudder speed brake panels, and portions of mid fuselage side walls from both the left and right sides. This represented about 30 percent of the Orbiter and did not provide sufficient evidence to conclusively establish the complete failure sequence of the entire Orbiter spacecraft. However, there was sufficient evidence to establish some of the probable structural failure modes that resulted in destruction of the Orbiter.
All fractures and material failures examined on the Orbiter with the exception of the SSME's, were the result of overload forces, and they exhibited no evidence of internal burn damage or exposure to explosive forces. This indicated that the destruction of the Orbiter occurred predominantly from aerodynamic, acceleration, and inertial forces that exceeded design limits. There was evidence that during the breakup sequence, the right SRB struck the outboard end of the Orbiter's right wing and right outboard elevon. Additionally, the right side of the Orbiter probably was sprayed by hot propellant gases exhausting from the hole in the inboard circumference of the right-hand (RH) SRB. Evaluation of the Orbiter main engines indicated extensive thermal damage to the engines as a consequence of oxygen-rich operation that results from a shut-off of the hydrogen fuel supply. The supply of hydrogen fuel to the main engines would have been abruptly shut off when the liquid hydrogen (LH2) tank in the ET disintegrated.
Evaluation of the limited amount of Orbiter structure recovered revealed no evidence to indicate that a failure or malfunction of structural components within the Orbiter, including the payload package, contributed to the cause of this accident.
The crew module wreckage was found submerged in about 90 feet of ocean water and was concentrated in an area of about 20 feet by 80 feet. Portions of forward fuselage outer shell structure were found among the pieces of crew module recovered. There was no evidence of an internal explosion, heat, or fire damage on the crew module pieces or on the forward fuselage pieces. The crew module was disintegrated with the heaviest fragmentation and crush damage on the left side. The fractures examined were typical of overload breaks and appeared to be the result of high forces generated by impact with the surface of the water. The sections of lower forward fuselage outer shell found floating on the ocean surface, which were recovered shortly after the accident, also contained crush damage of an impact on the left side. The consistency of damage to the left side of the outer fuselage shell and crew module indicates that these structures remained attached to each other until impact with the water. Supportive evidence which strongly indicates that the crew module remained an essentially intact structure until impact with the water was the concentration of its pieces in only one small area on the ocean bottom.
The majority of ET hardware recovered was found as floating debris and consisted mainly of pieces from the intertank and LH2 tank. The foam insulation on the recovered pieces of ET skin structure had various amounts of surface scorching; however there was no discernible pattern to the scorch damage except that some of the scorching occurred following disintegration of the ET. None of the metal structure beneath the insulation had melted or was burned through. Portions of LSC's from both the LH2 and LO2 tanks were recovered; they had not detonated. Examination of the fracture patterns on pieces from the LH2 tank indicated that it probably broke apart due to external overload forces rather than from an internal overpressure. Only a small portion of LO2 tank was recovered and examined The appearance of the fractures on the LO2 tank pieces was indicative of a tank failure caused by excessive internal pressure. Close examination of the fracture surfaces on the ET revealed several minor weld imperfections in the LH2 tank structure. Detailed evaluation of these imperfections indicated that they were not a factor to the STS Mission 51-L accident. All fractures examined on the recovered pieces of ET were the result of overload forces.
As of May 1, 1986, 42 pieces of SRB had been recovered including 2 pieces of the right SRB (sidescan sonar Contact Nos. 131 and 712) that identified the approximate dimensions of the hole burned through the side of the SRB near the lower field joint. The evaluation of these two pieces is contained in Volume 4, Enclosure 9 of this report. Since these two pieces of the right SRB contained the physical evidence pertinent to the failure of the SRB, further search and recovery efforts for pieces of SRB were discontinued as of the above date.
All casing fractures of the other 40 pieces of SRB were the results of overload forces or of case penetration by LSC explosives. The right SRB frustum showed clear evidence of forceful contact with ET intertank structure at the 245- to 255-degree position on the base of the frustum. This evidence indicates that failure of the liquid oxygen (LO2) tank in the ET may have been precipitated by the frustum's forceful contact with and disruption of intertank structure.
For the purpose of reconstructing and evaluating the recovered structure of STS Mission 51-L, the system (see Figure 1 for typical STS) was separated into three major subsystems: (1) The Orbiter, Challenger, including its main engines (SSME's), the Main Propulsion System (MPS), and the payload bay contents, (2) the External Tank (ET), and (3) the two Solid Rocket Boosters (SRB's). Brief descriptions of the above subsystems follow:
1. ORBITER (see Figures 2 and 3 for typical Orbiter)
The Orbiter structure is made primarily of conventional aluminum construction protected by reusable surface insulation. The structure is divided into six major sections, which are the forward fuselage, crew module, mid fuselage, wings, aft fuselage, and main engines. The forward fuselage structure is divided into upper and lower sections that fit clam-like around the pressurized crew module. The crew module, which is supported within the forward fuselage at four primary attachment points, is welded to create a pressure-tight vessel. The module has a side hatch for normal passage and a hatch from the airlock into the mid fuselage payload bay area. The mid fuselage connects to the rear of the forward fuselage at the 582 frame. The mid fuselage is a 60-foot....
[O381] ....section of primary load-carrying structure which contains the wing carry-through structure and the payload bay area. The left wing and right wings, which attach to the mid fuselage, are constructed of conventional aluminum alloy, and each wing has two elevon flight control surfaces attached to its trailing edge. The aft fuselage, which transfers the main engine thrust loads to the mid fuselage and ET, includes a truss-type internal structure of diffusionbonded elements. The main engines consist of three liquid propellant rocket engines with variable thrust. Additional data on the engines are provided in Figure 4.
2. EXTERNAL TANK (see Figure 5 for typical ET)
The ET has three major components which are the forward liquid oxygen (LO2) tank, an unpressurized intertank which contains most of the electrical components, and the aft liquid hydrogen (LH2) tank. The ET is 154.2 feet long and has a diameter of 27.5 feet and is the largest and heaviest (when loaded) subsystem of the STS. The LO2 tank is an aluminum monocoque (shell like) structure composed of a fusion-welded assembly of preformed chemmilled gores, panels, machined fittings, and ring frames. The intertank is a steel/aluminum semimonocoque cylindrical structure with flanges on each end for joining the LO2 and LH2 tanks. The intertank contains the forward SRB-to-ET thrust beam and fittings which distribute the SRB loads to the LO2 and LH2 tanks. The LH2 tank is an aluminum semi-monocoque structure of fusion-welded barrel sections, five ring frames, and forward and aft ellipsoidal domes. The LH2 tank provides, at its forward end, the ET-to-Orbiter forward attachment strut and, at its aft end, the two ET-to-Orbiter aft attachment ball fittings and the SRB-to-ET stabilizer attachment ball fittings.
3. SOLID ROCKET BOOSTERS (see Figure 6 for typical SRB)
The SRB's incorporate the largest solid propellant motors ever flown and the first designed for reuse. With the forward skirt, frustum, and nose fairing added, the complete SRB is just under 150 feet in length and weighs about 1,293,500 pounds. A solid propellant rocket motor consists of an insulated case, propellant, nozzle, and igniter. Because of its size and weight, the SRB is manufactured and shipped to KSC in four segments. At the launch site complex, the segments are assembled by tang and clevis joints machined into the case ends. Each joint is fastened with 177 headless, multiphase nickel chromium alloy pins installed in matching holes drilled in the tank and clevis. The pins are held in place by a retaining band around the entire circumference of the joint. The joint is sealed against pressure leaks with two synthetic rubber O-rings installed in the circumferential grooves machined in the clevis.
4. WEIGHT AND CENTER OF GRAVITY DATA
Dry weight data and wet weight data for STS Mission 51-L at SRB ignition on January 28, 1986, are shown in Table 1.
TABLE 1. DRY WEIGHT DATA AND WET WEIGHT DATA AT LIFT-OFF.
|
Component |
Weight in Pounds |
|
. | |
|
Orbiter | |
|
dry-with crew, w/o cargo |
177,372 |
|
wet-with crew, w/o cargo |
216,494* |
|
wet-with crew and cargo |
268,829 |
|
. | |
|
External Tank | |
|
dry |
66,933 |
|
wet |
1,663,966 |
|
. | |
|
Right Solid Rocket Booster | |
|
shell only |
180,583 |
|
full |
1,297,849* |
|
. | |
|
Left Solid Rocket Booster | |
|
shell only |
180,561 |
|
full |
1,297,828* |
|
. | |
|
Cargo |
52,335* |
|
. | |
|
Lift-Off weight (wet) |
4,528,472 |
|
Shuttle Center of Gravity |
Xs 1412.2, Ys 0.2, Zs 419.9 |
* Denotes Flight Element Weights.
E. STRUCTURAL IDENTIFICATION AND RECONSTRUCTION OF STS MISSION 51-L.
When recovered STS structure was returned by ship to the docks at Port Canaveral, it was washed with fresh water and loaded onto pallets and trucks for transport to KSC or the Cape Canaveral Air Force Station where impoundment areas had been established. Access to the impoundment areas was controlled by KSC security forces on a 24-hour, 7-day per week basis. The three major impoundment areas where large enough to respectively accommodate almost the full dimensions of the STS subsystems- Orbiter, ET, and SRB's. Separate areas were established for the crew module, the SSME's, and the MPS. The impoundment area for the SRB's was located in a remote hangar (Hangar "O") near the Eastern Space and Missile Center (ESMC) Explosive Ordnance Disposal (EOD) Range on the Cape Canaveral Air Force Station because the pieces of SRB contained unburned and potentially explosive propellant.
As the wreckage arrived at the impoundment area, it was unloaded from the trucks with forklifts and transferred to the identification area. The pieces were video-taped, photographed, identified by Quality Assurance personnel, and appropriately tagged. Each piece of wreckage 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 wreckage to the docks.
The impoundment areas for the Orbiter and ET were divided into 4-foot squares and marked with yellow tape. The pieces of wreckage were placed in the appropriate squares which corresponded to their full dimensional plan form location. As the quantity of wreckage increased, platforms and stanchions were constructed upon which the pieces of wreckage 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 wreckage, respectively.
Pieces of wreckage that were removed from the impoundment areas for metallurgical or chemical analysis were accounted for by hand receipt. Personal cameras were excluded from all impoundment areas, and all photographs were taken by KSC photographers. 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 wreckage 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 in coordination with KSC explosive safety personnel and ESMC Safety and EOD experts. 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- see Photograph 3 for SRB propellant burning setup.
F. STRUCTURAL EVALUATION OF STS MISSION 51-L.
1.0. STRUCTURAL EVALUATION OF ORBITER CHALLENGER.
1.1. General.
As of May 1, 1986, approximately 30 percent of the structure of the Orbiter, Challenger, was recovered from either the...
[O384] ....surface or the floor of the Atlantic Ocean. When recovered, most of the fracture surfaces of the metallic components were corroded to various degrees from exposure to salt water (see Figures 7 through 11 for illustrations of the portions of the wreckage recovered).
The recovered structure of the Orbiter was extensively torn, twisted, mangled, and fractured-see Photograph 4 for illustration of damage. Examination of the fracture surfaces of the metallic components was accomplished on a sampling basis because of the very large number of fracture surfaces. All of the surfaces examined exhibited characteristics of overload failure. No evidence was found of fatigue, stress corrosion cracking, or manufacturing defect.
Various components of the Orbiter were examined in detail in an attempt to determine the probable failure modes and the probable sources of various material transfers and deposits found on the components. These components include portions of the upper and lower rudder speed brake (RSB) and the vertical stabilizer, the right elevons, portions of the body flap, portions of the lower forward fuselage outer shell, major portions of the right wing, the crew module, and miscellaneous structure.
Chemical analysis of residues on the surfaces of the Orbiter were performed to help establish a breakup scenario. The interpretation of the analytical results was based on the type of residue and the expected source. This is best illustrated by the residue from the SRB exhaust.
The SRB propellant contains approximately 70% ammonium perchlorate, 16% aluminum powder, 14% binder and curing agent, and 0. 1% iron oxide. During combustion, the aluminum is converted to a specific crystalline state-alpha aluminum oxide-from the high temperature of the combustion process. Alpha aluminum oxide, known as corundum, produces a rough surface finish on otherwise smooth surfaces. Furthermore, aluminum oxide produced from oxidation of aluminum at atmospheric temperatures does not have the alpha structure. Therefore, the finding of alpha aluminum oxide on the Orbiter structure is strong evidence of impingement from SRB exhaust either from the hole in the inboard circumference of the right SRB or from the nozzle of the right SRB.
Splatters that appeared to be from molten metals were also analyzed to determine their composition and to establish the source. The pieces from which the samples were taken are shown in Figures 12 through 15. A summary of the analytical results is shown in Table 2.
During the week of April 7, 1986, large portions of the Orbiter's right wing, left mid fuselage side wall, and vertical stabilizer were recovered and transported to the impoundment area for examination. The portion of right wing included the entire length of the aft spar, the outboard end of the outboard elevon, and the elevon actuators. The recovered portion of left fuselage side wall included an intact length of left main longeron from its aft attachment end at bulkhead 1307 to about 30 feet forward. The vertical stabilizer included the structure from its base to a separation point approximately 15 feet above the base.
1.2. Orbiter Component Detail.
1.2.1. Rudder Speed Brake (RSB) and Vertical Stabilizer.
The portion of vertical stabilizer recovered consisted of the base structure up to the lower hinge area of the lower RSB (see Photograph 4A). Portions of aft fuselage structure were still mounted to the base of the vertical stabilizer. The attachment area at the aft lower right side of the vertical stabilizer was bent forward and inward, which indicated that the stabilizer may have separated from the fuselage toward the aft and right (+ y) direction.1 The surface structure of the stabilizer was extensively covered with sea life; however, of the fractured surfaces exposed for examination, their appearances were typical of overload breaks. There was some heat discoloration around the fracture area of the stabilizer at its upper right side, and the surface paint was blistered and burned. However, the aluminum structure in this area was not melted.
The Thermal Protection System (TPS) on the right side of the stabilizer was gouged, pitted, and partially eroded in some areas due to heat and mass flow over the surface. The TPS surface of the left side of the stabilizer was relatively undamaged. The direction of the erosion and penetration marks on the right side of the stabilizer are shown in Figure 15A. A small fragment of graphite epoxy skin structure from the Orbiter Maneuvering Subsystem (OMS) pod was found among the gouge marks embedded into the surface of the TPS. Other small fragments embedded into the TPS in the same area were pieces of gold insulation and a small green clamp. These items probably came from the inside area of the right OMS pod. The damage to the right side of the vertical stabilizer appeared to have been the result of SRB exhaust impingement and high energy contact with fragments of the right OMS pod. The OMS pod most likely broke apart and fragmented as it was subjected to the blast forces of the SRB exhaust.
The upper and lower RSB's were separated from the vertical stabilizer. The left speed brake panel was also missing from the lower RSB. The TPS on the right side of the upper and lower RSB exhibited extensive erosion due to heat and mass flow over the surface in a vertical direction (+ z) whereas the left side was relatively undamaged, indicating that the right side had been exposed to high temperatures-see Photographs 5 and 6. The two rotary actuator hinges in the lower right RSB failed in tension indicating the application of a large compressive load to the left (-y) on the outer surface of the RSB. The fracture surface on the top hinge was discolored from heat. Both hinges from the upper right RSB were missing, and severe crush damage to the upper aft corner indicates that following failure of the lower hinge, the RSB rotated about the upper hinge and struck the overhang of the vertical stabilizer tip. Both hinge fittings on the upper left RSB had failed in a manner indicating the application of a large lateral load to the left (-y) and an upward (+ z) load. The upper aft corner on the upper left RSB was damaged severely indicating contact with the vertical stabilizer tip.
The TPS on the right side of the vertical stabilizer tip had erosion damage similar to that on the right side of the RSB, whereas the left side of the tip had relatively little damage. The fracture surface of the interface between the tip and the rear spar of the vertical stabilizer occurred from the application of a compressive force to the left (-y).
The erosion pattern on the right RSB was vertical from bottom to top (+z) at a low angle of incidence and was one of the damaged areas in which a distinct impingement direction was discernible. The erosion of the tile coating exposed the underlying silica fibers and produced a grooved appearance in the fibers with a black molten appearance and blackened fibers at the end of the grooves. These features reflect the impingement direction which is shown in Photographs 5 and 6.
The coating on the trailing edge of the tiles on the outboard surfaces of the right RSB had a rough texture that is shown in Photograph 7. X-ray diffraction analysis of these deposits showed the presence of alpha aluminum oxide.
Gray deposits with a sandpaper-like texture were also found on the right outboard tile surfaces of the vertical stabilizer tip. These deposits were removed and analyzed by X-ray Diffraction. The analysis showed the presence of alpha aluminum oxide and aluminum silicate (Al6Si2O13) or mullite. The aluminum silicate and aluminum oxide were present in about equal amounts. The presence of aluminum silicate was probably due to a reaction between the molten aluminum oxide from SRB exhaust and the silica of the tile coating and/or fibers. The direction of exhaust impingement was also discernible on this component and was consistent with the bottom-to-top (+z) impingement on the RSB.
The RSB also showed various amounts of metallic material on the heavily-eroded surfaces. As shown in Table 2, iron, nickel,...
[O394] ....and chromium were found in all the samples analyzed from this area. In addition to these elements, smaller amounts of cobalt and titanium were identified. The metals were found predominantly in the outer surface of the deposits. The stabilizer tip residue contained the above elements, except for titanium, which was not present.
1.2.2. Right Wing and Elevons (See Figure 15A for typical wing structure).
A large section of right wing was recovered relatively intact and consisted of the entire aft spar at Station X01307 (Photograph 7A). The lower skin surface which was still attached to these spars was essentially intact from 1365 forward to about X01191. The upper skin surface was present from X01365 forward to X01307; however, the skin surface was collapsed downward into the wing's inner structure in several areas. An outboard portion of the outboard right elevon was still partially attached to the wing structure by one seal panel actuator tube. The upper skin surface of the attached elevon was also collapsed downward. There was no evidence of excessive heat damage on the wing structure, such as molten aluminum or heavy erosion of the TPS surfaces.
The fractures examined on the right wing structure were typical of breaks due to overload forces. The failure and bending patterns of the fractures indicated the right wing separated from the Orbiter due to an excessive positive (+ z) load on the wing. This evidence was indicated by compressive buckling at the inboard end of the wing's upper surface, upward (+ z) bending along the inboard end of the wing's lower surface, and tension failures along the lower surface of the wing. There were longitudinal indentations and crush damage in the TPS on the lower surface of the wing and elevon near the wing tips (Photograph 7B). The largest indentation mark was about 2 feet long by about 10 inches across, and was about 2 inches deep. The indentations and crush damage appeared to have been the result of an upward (+ z) contact by an external object. However, the damage did not appear substantial enough to have been caused by a force which would have created sufficient positive loads to break the wing off the Orbiter.
Examination of the elevons which had separated from the right wing of the Orbiter indicated that the outboard elevon had been subjected to a large upward force in the vertical (+ z) direction outboard of the elevon actuator. Similarly, the inboard elevon exhibited evidence of having been subjected to a substantial vertical (+ z) load. Four elevon flipper doors of titanium honeycomb showed evidence of burning which probably resulted from high-energy impact ignition of the titanium doors when the titanium clevises from the flipper door closure mechanism were forced into violent contact with the doors.
The inboard elevon forward closure panel was covered with a thin gray deposit as shown in Photograph 8. The deposit appeared almost like a paint film applied on top of the green super Koropon paint that is normally present. Removal of the deposit was accomplished by vigorous scraping of the sandpaper-like coating. X-ray diffraction analysis of the scrapings showed the presence of alpha aluminum oxide.
An additional sample of residue was taken from the elevon actuator tube. Gray deposits were present on the lower surface of this normally bare metal Inconel tube. One of the deposits was dark gray with a rough texture similar to the deposit on the forward closure panel ribs while the other deposit was a smooth lighter gray deposit. Diffraction analysis of these deposits showed both alpha aluminum oxide and the lower temperature gamma form of aluminum oxide. The significance of the gamma form is not known at this time. The right inboard elevon forward closure panel (Photograph 9) had many black splatters which appeared to originate from the top of the elevon surface. A sample removed by scotch tape showed titanium-rich spheres dispersed on the surface splatters. Other elements present in the black splatter were similar to that of the underlying super Koropon paint. Titanium metal splatters were also deposited on the elevon actuator tubes.
The tile surfaces of the inboard closure surface of the right outboard elevon also contained residue (Photograph 10). These tiles had a very rough feel and appeared to have rust and silver-colored deposits on the tile surface. Alpha aluminum oxide was found by X-ray Diffraction analysis. In addition, moderate to minor amounts of nickel, iron, titanium, aluminum, chromium, and cobalt were found.
1.2.3. Mid and Aft Fuselage Sidewalls.
Portions of fuselage sidewall from both the left and right sides of the Orbiter were recovered for examination (see Photographs 10A and 10B). The fracture surfaces on both the left and right sidewall pieces appeared to have been the result of overload forces. The outer surface of a section of aft fuselage right sidewall was heat damaged and eroded due to heat and mass flow over the surface. The direction of flow was forward and upward at about a 50-degree angle relative to the Orbiter's x axis (see Figure 15B). The heat and erosion damage was consistent with the same type damage on the right sides of the vertical stabilizer and RSB's, and was most likely the result of exhaust blast from the right SRB.
The sidewall pieces from the left side of the Orbiter fuselage were relatively large portions of structure, and there was no evidence of heat or fire damage on the pieces. The largest piece was about 30 feet long and 12 feet high, and it consisted of payload bay sidewall structure and the upper main longeron from the 1307 bulkhead attachment area to about 30 feet forward of the 1307 bulkhead. Also recovered was the adjacent piece of fuselage sidewall normally located aft of the 1307 bulkhead. This piece was about 15 feet high and 12 feet long. The outside surfaces of this aft fuselage piece and the larger piece of payload bay sidewall structure were gouged in several areas; however, there was no apparent pattern to these gouge marks.
Three bridge fittings for support of the Inertial Upper Stage (IUS) payload remained attached to the main longeron on the large portion of left payload bay sidewall. Two trunnion retention fittings remained attached to the bridge fittings. The retention fittings were intact and in the latched position. Portions of two saddle straps for the IUS payload were still attached to the bridge fittings. Both straps were fractured at a location approximately I foot below the attachment pins for the straps. The fracture surfaces on the saddle straps were typical of overload failures.
1.2.4. Crew Module (See Figure 16 for Structure).
The crew module was found about one month after the accident at a water depth of about 90 feet and was concentrated in an area of about 20 feet by 80 feet. Approximately 75 % of the crew module was recovered. Other pieces of structure also recovered from the same area included the 582 fuselage frame, portions of the forward fuselage outer shell, and all four primary attachment fittings which secure the crew module to the outer shell. The fact that much of the crew module structure was concentrated in such a small area underwater indicates that the module did not disintegrate in-flight but remained essentially intact until it struck the surface of the water. The fracture surfaces examined on pieces of the crew module were typical of breaks resulting from overload forces. There was no evidence found of heat or fire damage on the pieces of crew module recovered.
The largest fragment of crew module recovered consisted of the right half of the 576 aft pressure bulkhead (see Photograph 11). The piece was about 12 feet by 17 feet in size and consisted of the majority of the right side of the bulkhead structure and the airlock hatch. The airlock hatch was intact and secured to the bulkhead with a portion of the airlock tunnel on the forward side of the hatch. The tunnel was separated approximately 6 inches forward of the bulkhead. The remaining pieces from the left side of the 576 bulkhead were recovered and were notably more fragmented than the right side of the 576 bulkhead.
The internal crew module components recovered were crushed and distorted, but showed no evidence of heat or fire. A general consistency among the components was a shear deformation from the top of the components toward the +y direction from a force acting from the left. Components crushed or sheared in the above....
[O401] ....manner included avionics boxes from all three avionics bays, crew lockers, instrument panels, and the seat frames for the Commander and the Pilot. The more extensive and heavier crush damage appeared on components nearer the upper left side of the crew module. The magnitude and direction of the crush damage indicates that the module was in a nose down and steep left bank attitude when it hit the water.
The fact that pieces of forward fuselage upper shell were recovered with the crew module indicates that the upper shell remained attached to the crew module until water impact. Pieces of upper forward fuselage shell recovered or found with the crew module included cockpit window frames, the ingress/egress hatch, structure around the hatch frame, and pieces of the left and right sides. The window glass from all of the windows, including the hatch window, was fractured with only fragments of glass remaining in the frames.
The lower outer shell structure of the forward fuselage was recovered as floating debris on the ocean surface and was found within the first few days after the accident. Much of the outer shell (see Photograph 12) was recovered in just four major pieces. The lower outer shell separated at or just below the manufacturing interface with the upper shell. There was no evidence of burning or of significant heat damage on any of the lower shell structure. The lower shell appeared to have broken from frame 582 in one large section. The fractures along frame 582 were in tension overload with negative (- z) bending which indicates that the shell separated from the Orbiter mid fuselage in a nose down direction. The left side of the forward fuselage lower shell was crushed and deformed inward (+ y) with the greatest deformation just below the side hatch frame structure. Adjacent pieces of outer shell structure from around the hatch frame were recovered with the submerged crew module. These pieces had similar crush deformation damage toward the right (+ y) direction. The crush damage appeared typical of water impact damage. The apparent consistency of crush damage on the left side of the forward fuselage lower shell and the crew module would indicate that these two items were attached until impact with the water.
The forward ET attachment fitting plate in the Orbiter was fractured along bulkhead 378. One nut remained attached to the shaft of a bolt in the right side of the fitting plate; the bolt was fractured at a position near the underside of the fuselage skin. The aft bolt recess slot in the fitting plate was gouged in a manner which indicates that the bolt was twisted off in a clockwise direction as viewed from above toward the - z direction.
1.2.5 Miscellaneous Structures and Components
Most of the body flap was recovered except for the four rotary actuator support fittings. The failures which occurred in the hinge fittings indicated that the body flap had been subjected to a large vertical (+ z) load with the failures progressing from the right outboard hinge laterally (- y) to the left outboard hinge.
Some of the upper tile surfaces on the body flap had a sandpaper feel as well as metal impingement on them. In addition, a leading edge tile had a bubbled appearance on the coating surface. X-ray Diffraction identified alpha aluminum oxide on the upper tile surfaces. Metallic spheres with varying composition were under the bubbled area. Several were high in iron with moderate to minor amounts of chromium, nickel, aluminum, cobalt, and traces of niobium. X-ray Diffraction of the coating on this surface showed the presence of iron aluminum oxide (FeAl204) and quartz. The iron aluminum oxide appears to have been formed when the molten metal struck the tile surface. Other metallic splatters were primarily aluminum. Minor amounts of nickel, iron, chromium, cobalt, and trace amounts of magnesium were found. The results of this analysis are summarized in Table 2.
Portions of both main landing gear doors were recovered. The impressions of both tires on the inner surface of the right door indicates that the gear was inertially deflected downward in response to a high normal (+ z) acceleration load while the door was closed.
Numerous storage tanks from the Orbiter were recovered as floating debris from the ocean surface-See Figure 17. Five of the tanks had various degrees of heat damage. Two metallic LH2 tanks from the mid fuselage area were the most heavily damaged. The aluminum outer shields on each of the two tanks were partially burned away-see Photographs 13A and 13B. In their normal configuration, the two LH2 tanks would be about 5 feet apart with the burned area of each tank facing toward the other tank. The burn patterns on the LH2 tanks indicated that the tanks had been subjected to a sustained heat source while some LH2 remained in the tanks. The patterns of burning and melting on the outer shields of both tanks indicate that LH2 was present in the cooling tubes beneath the shields when the burning occurred. Consequently, the fuel for the sustained fire could have been LH2 if an oxidizer was present or the fuel could have been the aluminum shields themselves burning in an oxygen-rich environment as the result of LO2 leaks from the nearby LO2 tanks or lines. From the symmetry of the burn/melt patterns on the two tanks, they must have remained secured in their normal positions, probably until impact with the water. Of the pieces of Orbiter components and structure recovered, the two LH2 tanks were the only components/structure, except for the SSME's, that showed evidence of a sustained fire. However, there is no evidence to indicate that the fire began before the Orbiter broke apart.
The TPS tile surfaces on the base heat shield had a rough sandpaper feel with heavier deposits in the lower right quadrant near Main Engine No. 2. Analysis of the deposits in these areas confirmed the presence of alpha aluminum oxide. Blackened silica fibers where the coating on the tile was missing showed both alpha aluminum oxide and aluminum silicate (mullite). Analysis of residue taken from the tile between Engine No. 1 and No. 2 showed only traces of alpha aluminum oxide.
Analysis of molten splatters on the base heat shield showed nickel, iron, chromium, cobalt, copper, zinc, and manganese varying from major to trace depending on location of the sample. Only trace amounts of niobium and titanium were found. These findings are summarized in Table 2.
1.3 Main Propulsion System and Space Shuttle Main Engines (See Figure 18)
On February 23, 1986, components of the Main Propulsion System (MPS) and the three main engines (SSME's) were recovered from the floor of the Atlantic Ocean. The MPS components and the engines were in close proximity which indicates that all three SSME's probably were attached to the thrust structure (see Figure 19) upon impact with the water. The three engines were relatively intact. All metallic surfaces, except the titanium surfaces and those buried in the sand, were covered by marine life.
The gimbal bearings on all three engines apparently failed in overload, and the engine nozzles were sheared off just below the forward manifolds. All of the accessible fractures were examined and all appeared representative of ductile overload. The forward nozzle manifold, main combustion chamber, and the powerhead for each engine were attached. The six thrust vector control hydraulic actuators were attached to the engine quadropod and a segment of the thrust structure.
The powerheads on all three engines exhibited burn through damage on the two outboard fuel transfer tubes and a total loss of the line tubes on the fuel preburner sides. Also, there was extensive burn through damage on other components of each engine all of which is indicative of damage from oxygen-rich operation due to depletion of the hydrogen fuel supply. There was no evidence of heating or burning from sources other than the engines themselves.
The fuel metering valve positions on the three engines were examined to determine the phase of operation each engine may have been in when the hydrogen fuel supply was lost. For Engine 2023, Position No. 1, the fuel preburner oxidizer valve was the only valve sufficiently intact to assume that it provided a valid data position. This valve was full open, indicating that the control system for Engine 2023 was correcting for mixture ratio during....
[O405] ....hydrogen fuel depletion. For Engine 2020, Position No. 2, the main oxidizer valve was about 50 percent open, and the oxidizer preburner oxidizer valve was fully closed. The positions of these two valves indicate that Engine 2020 was about 1/2 seconds into a computer-controlled shutdown when the hydrogen fuel supply was lost. The remaining valves of Engine 2020 were too badly damaged to provide valid data positions. For Engine 2021, Position No. 3, the main oxidizer valve and the main fuel valve were full open, and the oxidizer preburner oxidizer valve was at the mainstage position. The positions of these valves indicate that Engine 2021 was still in mainstage when it lost both hydraulics and pneumatics.
The MPS had very little evidence of burning or heat damage, but was heavily corroded by sea water and broken apart by impact forces. The components of MPS from the right side of the Orbiter were generally in better condition than those from the left side. The positions of the LO2 and LH2 valves in the feed lines were examined and are provided in Table 3.
1.4. Inertial Upper Stage (IUS), Airborne Support Equipment (ASE), and Tracking and Data Relay Satellite (TDRS)
The payload bay of the Orbiter, Challenger, contained a Tracking and Data Relay Satellite (TDRS) attached to an Inertial Upper Stage (IUS) launch vehicle, and associated Airborne Support Equipment (ASK) (see Figures 20 and 21). The combined weight of these components was about 40,000 pounds. The IUS contained two solid rocket motors (SRM's): SRM-1 and SRM-2.
A significant amount of the ASE/IUS/TDRS package was recovered from the Atlantic Ocean. Components recovered included segments of the cases of both SRM's, the ignition safe/arm device for each SRM, the ignitor for SRM-2, fragments of unburned propellant from each SRM, five explosive separation bolts that secure the two SRM's together, the forward ASK trunnions, the aft ASK trunnions with spreader beams, and an undetonated section of Super*Zip fasteners.
There was no evidence of scorching, burning, or melting on any of the components and structure recovered, and all fractures were typical overload fractures. The safe/arm device for each SRM was in the safe position, the five explosive SRM-1/SRM-2 separation bolts were intact, and pieces of propellant were not burned indicating that the SRM's had not ignited. The two aft trunnion spreader beams were intact but were bent in the downward (- z) direction. The right spreader beam was deformed about 7.5 inches and was cracked (see Photograph 14), and the left spreader beam was deformed about 1.5 inches. These deformations indicate that the ASE/IUS/TDRS package was subjected to significant inertial loads in the downward (- z) direction, while the package was intact and secure in the payload bay, as the result of positive normal (+ z) acceleration of the Orbiter.
For a complete systems evaluation of the IUS/TDRS, see IUS/TDRS Systems Working Group Final Report dated March 14, 1986.
1.5. Probable Failure Mode (Orbiter).
Insufficient structure was recovered to definitely establish the complete failure sequence 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. Further, the proliferations 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 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 indicates that the engine control system for No. 1 engine was correcting for the loss of the hydrogen fuel supply; that 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 to the destruction of the Orbiter.
There was no evidence to indicate that the IUS contributed to premature structural failure of the Orbiter.
2.0. STRUCTURAL EVALUATION OF THE EXTERNAL TANK (ET)
2.1. General
Approximately 20% of the ET structure was recovered and the majority of these pieces were from the intertank and LH2, tank-Photographs 15 through 18 provide illustrations of the portions of ET structure recovered. Most of the pieces recovered still had the external foam insulation material attached. Portions of the ET destruct system hardware, which included LSC's from both the LO2 and LH2 tanks, were recovered; the charges had not detonated.
The external foam insulation on the piece of LH2 tank and intertank structure of the ET was scorched and discolored in various locations; however, there was no evidence of melting or burn through of the ET metallic portions of this structure. Burn patterns across the pieces of insulation on these pieces of the ET indicate that various areas were subjected to fire both before and after the ET broke up in-flight.
The fracture surfaces of the recovered pieces from the LH2 and LO2 tanks were examined to assess the possible modes of ET breakup. The actual failure mode of the LO2 tank could not be adequately assessed because of the limited amount of structure recovered. However, of the LO2 tank pieces examined, the fracture patterns were consistent with a failure due to internal overpressurization. The fracture patterns on the LH2 tank pieces indicated that the tank broke apart due to external loads rather than an internal overpressure. The propagation of the fracture patterns indicated that the area around station XT1871 of the LH2 tank failed relatively early in the breakup sequence. Fractures at the aft end of the intertank near the LH2 tank were crushed forward indicating a forward motion of the LH2 tank relative to the intertank during the latter's disintegration.
Close examination of the ET fracture surfaces and production close-out X-rays disclosed a 0.400-inch crack-like imperfection and an area of four very small (up to 0.500-inch diameter) pores which were present before and after the flight of STS Mission 51-L. These imperfections were located along a weld line within about 2 inches of each other on the LH2 tank near the 180 degree (- z0) and XT2060 location. This location is on the bottom side of the tank (opposite the Orbiter side) and toward the aft end. Even though the fractures propagated through the two [O406] imperfections, the directions of propagation indicated that the fractures did not originate at either of the two imperfections. Comparison of imperfection measurements as shown in X-rays before the flight with post accident imperfection measurements showed that the imperfections had not increased in size before the ET failed.
Pieces from the right circumference of the ET were examined to determine if the right SRB may have contacted the ET during the breakup. The majority of ET structure adjacent to the left and right SRB's was not recovered. However, gouge marks were noted in the integral stiffeners of the intertank thrust panel on the right side of the ET near the 70 degree and XT 1000 location (0 degrees is the + z position). A small section of thrust panel was removed from this area for chemical analysis. The analysis showed that a small amount of iron was embedded into the ET thrust panel which indicates contact with the right SRB. Examination of the right SRB frustum disclosed several gouge marks located along the 245- to 255-degree location (0 degrees is the -z position on the right SRB) on the outside circumference of the frustum base. Chemical analysis of materials extracted from the gouge marks indicated the presence of polyurethane material similar to the external foam insulation material on the ET. The distance between each of the several gouge marks on the frustum was approximately 5 inches, which corresponded to the spacing distance between the integral stiffeners of the thrust panel on the intertank. Further evidence of contact between the ET and the right SRB was exhibited by a small piece of SRB surface insulation material lodged into a fracture on the intertank skin panel near the 45-degree and XT900 location.
Evidence of contact between the ET and the Orbiter was indicated by a small piece of TPS gap filler material from the Orbiter which was lodged into the ET near the 358-degree and XT1130 location. The appearance of the TPS gap filler material indicated that it was from around the nose landing gear door frame area of the Orbiter. The area of the ET where the TPS gap filler material was found would be near the vicinity of the nose landing gear doors of the Orbiter when in its normal attachment configuration to the ET. Small bits of material identified as pieces of TPS tile from the Orbiter were found embedded in the ET foam insulation at various locations. The majority of locations were toward the top (+ z) side of the intertank; however, there was no discernible pattern to the embedded tile locations, and the possibility could not be excluded that some of the tile pieces may have been embedded into the ET foam insulation during wreckage recovery operations.
2.2. Probable Failure Mode
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.
The separations 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.
3.0. STRUCTURAL EVALUATION OF SOLID ROCKET BOOSTERS (SRB's)
3.1. General
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 frustums of both the left and right SRB's were found floating on the ocean surface shortly after the accident. The remaining pieces of SRB recovered were found on the bottom of the ocean. All recovered pieces were visually examined and selected areas were extracted for chemical and metallurgical analysis. The remainder of this report concerns all SRB pieces recovered except the two pieces of right SRB mentioned in the preceding paragraph-the evaluation of these two pieces is contained in Volume 4, Enclosure 9, of the report.
The exterior surfaces of the SRB's are normally protected from corrosion by an epoxy resin compound. There were several areas where this protective coating was gouged or missing on the pieces recovered and, as a result, the exposed metallic surfaces in the areas were corroded. The damage to the protective coating was most likely the result of detonation of the LSC's and of water impact. There was no obvious evidence of major flame impingement or molten metal found on any of the SRB pieces recovered. All fracture 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 LSC's.
Most of the SRB case material recovered contained pieces of residual unburned propellant still attached to the inner lining of the case structure. The severed propellant edges were sharp with no unusual burn patterns. Propellant recovered with a forward segment of SRB exhibited the star pattern associated with the receding shape of the propellant at the front end of the SRB. There was no evidence found of propellant grain cracking or debond on the pieces recovered. Casting flow lines could be distinguished on the propellant surfaces in several areas, which is a normal occurrence due to minor differences in the propellant cast during the installation of the propellant to the SRB case structure.
The unburned propellant which remained attached to the recovered SRB pieces was considered highly volatile and, therefore, a serious safety hazard to the evaluation process. In order to reduce this hazard, the propellant was removed by burning under controlled conditions. Thermal protection features such as vacuum putty, insulation blankets, water spray, and temperature indicators, were used to ensure that areas critical to the investigation were preserved. Photographs 19 and 20 show the condition of a recovered portion of SRB case before and after the propellant was removed.
Hardness tests of each piece of the steel casing material were taken before the propellant was burned from the piece. All of the tests showed normal hardness values.
One of the pieces of SRB casing showed evidence of O-ring seal tracks on the tang of a field joint-see Photograph 21. The tracks were cleaned with hexane to remove the grease preservative that had been applied after recovery of the piece, and samples of the track material were removed for analysis. Chemical analysis of the track material did not contain either barium sulfate or fluorinated hydrocarbon. Therefore, the tracks were not composed of degraded O-ring seal material.
3.2. Left SRB
3.2.1. Left SRB Frustum
The left SRB Frustum, which is shown in Photograph 22, was relatively undamaged except along the base where the forward [O407] skirt attached; the nose cap was missing and was not recovered. The forward booster separation motors (BSM's) were intact and unfired. The lower ring of rivet fasteners along the inner circumference of the base of the frustum were failed in a shear-type manner. Reinforcing structure, which attached to the rivets, was separated from the frustum. The shear failure of the rivet fasteners was probably the result of a pressure pulse generated by detonation of the LSC's. The drogue parachute was recovered still attached to the frustum, in good condition, and in the deployed configuration. Deployment of the drogue chute during the frustum's descent had obviously minimized water impact damage to the frustum. A 2-foot long crack at the base of the frustum was probably due to water impact and missing reinforcement structure in the frustum. There was no evidence of heat or fire damage, and all fractures on the left SRB frustum appeared to have been the result of overload forces.
3.2.2. Left Solid Rocket Motor (SRM) (See Figure 22)
Pieces of left SRM casing recovered included portions of the aft segment with a length of the ET attachment ring still attached. The aft center segment pieces consisted of two pieces of adjacent case structure from the 110-degree location of the forward case cylinder-see Figure 23 for SRB coordinate system. The combined dimensions of the two pieces were approximately 12 feet by 12 feet. The section of aft segment recovered was approximately 11 feet by 8 feet in dimension and contained a 180-degree section at the ET aft attachment ring-see Figure 24. The section of the ring contained none of the three ET attachment struts. A section of tang and clevis field joint was partially intact on the section of aft segment recovered. Visual examination of the fracture surfaces indicated that all breaks were the result of overload forces. Evaluation of the fracture and deformation patterns indicated that the overload forces were the results of the linear-shape charges detonated by the Range Safety Officer and of high-energy impact with the surface of the water.
3.3. RIGHT SRB
3.3.1. Right SRB Frustum
The right frustum had sustained more nose damage than the left frustum; the nose cap was missing and was not recovered. Also, the forward BSM's and housing were missing and were not recovered-see Photograph 23. The BSM and housing probably separated at impact with the water since the housing fracture surfaces were clean with no evidence of heat or erosion. The interior of the frustum sustained moderate damage to the struts and other surfaces. The fasteners at the base of the frustum which attach the frustum to the forward skirt had failed in shear similar to the failures of the fasteners on the left frustum.
There was evident and unique damage to the base of the frustum at the 240- to 255-degree circumferential location. Chemical analysis of deposits in the damaged area established the presence of polyurethane of the type used for outer insulation on the ET intertank. Also, indentations in the damaged area were spaced about 5 inches apart which corresponds to the spacing between the integral stiffeners in the ET intertank-see Photograph 24.
3.3.2 Right Solid Rocket Motor (SRM)
Twenty-one pieces of the right SRM were recovered, including the two pieces near the lower field joint that are evaluated in Volume 4 of the report. All of the fractures of the other 19 pieces of right SRM were typical of fractures produced by overload forces or LSC detonation. Hardness tests of these pieces indicated no abnormalities in the D6AC steel cases.
3.4 Probable Failure Mode
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 systems tunnel of each SRM.
1.0 Findings
a. 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.
b. About 20 percent of the ET was recovered; the structure vas predominantly from the intertank and LH2 tank of the ET.
c. All fractures examined on the STS structure had failed from overload forces; there was no evidence of fatigue, stress corrosion cracking, or manufacturing defect.
d. 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.
e. There was no evidence of internal fire or explosion in the Orbiter preceding its disintegration.
f. The Orbiter was destroyed predominantly by high acceleration loads, high inertial load, and adverse aerodynamic forces.
g. 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.
h. The aft right side of the Orbiter was burned by hot pro pellant gases exhausted probably from the hole in the inboard circumference of the right SRB.
i. The SSME's were functioning properly until the LH supply was terminated by the failure of the LH2 tank and/or tank connections of the Orbiter.
j. 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.
k. The crew module separated from the Orbiter at frame 582; the aft pressure bulkhead (576) remained with the crew module.
1. 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 steel left bank attitude.
m. The crew module and forward fuselage structure disintegrated from high deceleration and inertial forces associated with water impact.
n. The base of the right SRB frustum penetrated the inter tank structure of the ET following release of one or more of the SRB's lower attachment fittings to the ET.
o. The LO2 tank in the ET probably was breached by intertank structure which permitted LO2 to escape and vaporize in the relatively warm atmosphere.
p. Fragmented LO2 structure may have been burned by vaporized LO2 following failure of the LO2 tank.
q. The LH2 tank in the ET probably failed from external forces and thermal damage near the aft ellipsoidal dome
r. The pieces of SRB evaluated in this report were unremarkable; all fractures were from overload or from detonation of LSC's.
2.0. Conclusions
a. There is no evidence that structural failures in either the Orbiter or its payload package preceded the destruction of STS 51-L.
b. The Orbiter's MPS and SSME's functioned properly and did not contribute to the loss of STS 51-L.
c. 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.
d. The ET probably failed from thermal and structural damage near the base of its LH2 tank and from over pressurization of its LO2 tank following partial separation of the right SRB from the ET.
|
LINE |
ENGINE |
VALVE |
POSITION |
REMARKS * |
|
. | ||||
|
LH2 |
1 |
PREVALVE (PV4) |
CLOSED |
DAMAGED |
|
RECIRC (PV14) |
CLOSED |
- | ||
|
2 |
PREVALVE (PV5) |
OPEN |
- | |
|
RECIRC (PV16) |
UNDETERMINED |
DAMAGED | ||
|
3 |
PREVALVE (PV6) |
OPEN |
- | |
|
RECIRC (PV16) |
CLOSED |
- | ||
|
N/A |
FILL/DRAIN (PV11) |
80% CLOSED |
- | |
|
. | ||||
|
LO2 |
1 |
PREVALVE (PV1) |
CLOSED |
DAMAGED (ACTUATOR OPEN) |
|
2 |
PREVALVE (PV2) |
OPEN |
- | |
|
3 |
PREVALVE (PV3) |
OPEN |
- | |
|
N/A |
INBOARD FILL/DRAIN |
OPEN |
DAMAGED (ACTUATOR CLOSED) | |
|
FLAPPER (PD-1) |
80% CLOSED |
DAMAGED | ||
1. Loads, forces, and directions are described with
respect to the Orbiter coordinate system wherein x represents the
longitudinal axis, y represents the lateral axis, and z represents
the vertical axis (Figure 1).