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ENCLOSURE 9
Figure 1. SRB Recovered Debris, RH Aft Segment View from Outboard.
B. STRUCTURAL EVALUATION GROUP.
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The structural evaluation of right-hand (RH) Solid Rocket Motor (SRM) pieces, sidescan sonar contact (SSC) Nos. 131 and 712, established conclusively that propellant fire burned a large hole (about 6 square feet) in the side of the SRM and that the hole was centered approximately about the 305-degree position of the lower field joint. The propellant flame exhausted through the hole in the RH SRM, breached the Liquid Hydrogen (LH2) tank, and destroyed its structural integrity.
It is believed that the fire also separated the aft lower External Tank (ET) attachment strut connection to the ET which partially released the RH Solid Rocket Booster (SRB) from the ET. Following its partial release, the right SRB probably rotated and yawed sufficiently about its remaining ET attachment connections for the base of its frustum to forcefully contact and penetrate ET intertank structure. The losses of structural integrity in the LH2 tank and in the aft lower ET attachment connection led directly to the destruction of STS 51-L.
The propellant fire burned through the RH SRM case most probably because the two O-ring seals in the lower field joint failed to contain internal combustion pressure which permitted the fire to burn abnormally toward and into the joint; however, because of thermal destruction of the seals and joint, this probability could not be conclusively verified by physical evidence.
In late April 1986, two pieces of the RH SRM from Space Transportation System (STS) Mission 51-L were recovered from the Atlantic Ocean and were transported to Hangar "O" at the Cape Canaveral Air Force Station (CCAFS) for evaluation. These two pieces of SRM were identified by the Recovery Support Team as Sidescan Sonar Contact (SSC) Numbers 131 and 712. Because of the evident burn damage on these two pieces, they were evaluated separately from the other 40 pieces of SRM recovered.
Since the pieces of SRM were too large and heavy to maneuver into positions to match fracture surfaces, full size styrofoam templates were made of many pieces of the RH SRM, and the templates were arranged in a two-dimensional pattern to match fracture surfaces. (Reference Photograph 1.)
The RH SRM pieces identified as SSC Nos. 131 and 712 were evaluated at Hangar "O" (see Photographs 2 and 3). The [O421] propellant on these two pieces of SRM was not removed by burning on the Explosive Ordnance Disposal (EOD) Range to avoid any risk of damage to the pieces (see Photograph 4).
The base of the SSC No. 131 piece contained the tang portion of the lower field joint and extended from the 264-degree position clockwise to the 90-degree position, about 238 inches circumferentially. The piece varied irregularly in height from approximately 71 inches at the 264-degree position to about 132 inches at the 90-degree position. The piece was similar to other pieces of SRM recovered with regard to the presence of unburned propellant and the condition of the exterior surface and insulation, except for an irregular oblong-shaped area about 15 inches high by approximately 28 inches long that was missing along its base. Also, sooting and blistered paint were evident on the exterior surface of the SRM surrounding the missing area. The base of the missing area was centered about the 305-degree position, and the steel case around the area was melted and eroded from exposure to heat (see Photograph 5).
A 17-inch crack in the SRM case extended from the upper left corner of the missing area (see Photograph 6). Examination of the crack surfaces disclosed no evidence of thermal erosion which indicates that the crack developed after propellant burning had ceased&emdash;probably on impact with the surface of the Atlantic Ocean.
The piece of RH SRM identified as SSC No. 712 extended circumferentially from about the 226-degree position clockwise to the 320-degree position, and it varied irregularly in height from about 60 inches at the former position to about 72 inches at the latter position. The base of the piece was a tang portion of the forward stiffener-to-External Tank (ET) attachment joint at XB 1577.5. The tang of this piece is used in a factory joint. The upper part of the piece contained portions of the ET attachment ring stubs (see Photograph 3).
The upper left edge of piece SSC No. 712 was burned and eroded from the 291-degree position to the 318-degree position, approximately 33 inches circumferentially (see Photograph 7). This portion of the piece also contained a burn hole about 4 inches long by 1.5 inches high, just above a portion of the ET attachment ring lower stab (see Photograph 8).
Hardness tests were made in the SRM case material surrounding the burned edges of both pieces of SRM. These tests showed normal hardness values for D6AC steel except within about 5 inches of the burned edges where the values showed thermal degradation. Also, the area between the ET attachment ring stubs on No. 712 was thermally degraded.
Full-size templates of the SSC Nos. 131 and 712 pieces were made and placed in their relative two-dimensional positions (see Photograph 9), and a drawing was made of the two pieces in their relative two-dimensional position (see Figure 1). Calculations based on the dimensions of the areas missing from and between the two pieces indicate that the missing areas represented about 862 square inches (about 6 ft2) of SRM case including about 28 circumferential inches of the lower field joint. Based on the evidence provided by these two pieces of the RH SRM, it is concluded that propellant fire burned a hole through the case of the SRM and that the approximate size of the hole was 6 square feet when the burning ceased.
Since the burned-out areas of the RH SRM included about 28 circumferential inches of the lower field joint, the evidence related to the most probable reason for the development of a propellant flame path . . . to and through the SRM case&emdash;a combustion pressure leak past the O-ring seals in the field joint&emdash; was consumed by the hot propellant flame (about 5,600°F). However, because of the thermal destruction of the field joint including the O-ring seals, other possible sources of a combustion pressure leak could not be excluded by the physical evidence. Consequently, additional supporting evidence is needed to conclusively establish that failures of the O-ring seals to contain internal combustion pressure permitted propellant fire to follow a pressure leak path to and through the SRM casing and to initiate the destruction of STS 51-L.
In order to portray the burned-out areas of the RH SRM case, as established by pieces SSC Nos. 131 and 712 in relationship to other SRM structure, an outline of the areas was drawn on the side of a RH aft SRB field joint area that was located in the Vehicle Assembly Building (VAB) at the Kennedy Space Center (see Photograph 10). As can be seen from this portrayal, the lower left portion of the missing areas included case material behind the ET attachment ring in the vicinity of the aft lower ET attachment strut. Also, the lower portion of the LH2 tank in the ET was in close proximity (about 2 feet) to the hole burned in the side of the SRM. Consequently, it is concluded that the LH2 tank was penetrated by the propellant flame exhausted through the hole. Further, the small hole in the SRM case near the lower ET attachment ring stub was burned by fire deflected inward, probably by the attachment strut connection to the ET attachment ring. It is concluded that the ET attachment strut separated at its ET connection. Finally, separation of the aft lower ET attachment strut probably allowed the RH SRB to rotate and yaw sufficiently about its remaining ET connections for the base of the SRB Frustum to forcefully contact and penetrate the ET intertank structure. Therefore, it is concluded that the partial separation of the RH SRB at its aft lower attachment strut connection to the ET and the breach of the LH2 tank lead directly to the destruction of STS 51-L.
1. Hot propellant flame burned a hole through the side of the RH SRM that was centered near the 305-degree and XB 1496 position of the SRM; the hole included about 6 square feet of SRM case and about 28 circumferential inches of the lower field joint when the burning ceased.
2. A 1.5-inch by 4-inch hole was burned from the outside of the SRM case inward and through the case near the lower stub of the ET attachment ring; the hole was burned by hot propellant flame deflected inward probably by the aft lower ET attachment strut connection to the ET attachment ring.
3. Hot propellant flame exhausted from the hole in the RH SRB burned and penetrated the LH2 tank in the ET near the aft lower ET attachment strut connection which destroyed the structural integrity of the LH tank and separated the lower attachment strut connection.
4. With the aft lower attachment strut separated from the ET, the SRB probably rotated and yawed sufficiently about its remaining attachment connections for the base of the frustum to forcefully contact and penetrate ET intertank structure.
5. The hot propellant flame burned through the SRM casing most probably because of a combustion pressure leak across the two O-ring seals near the 305-degree position of the lower field joint; however, because of thermal destruction of the seals and joint, this probability could not be conclusively verified by the physical evidence.
6. Additional supporting evidence is needed to conclusively verify that sealing failures of the two O-rings caused a combustion pressure leak that permitted propellant fire to burn toward and through the case of the RH SRM and to initiate destruction of STS 51-L.
1. The destruction of STS 51-L was caused by propellant fire that burned through the case of the RH SRM and destroyed the structural integrity of the LH2 tank and separated the aft lower SRB-to-ET attachment strut.
2. Propellant fire burned through the RH SRM case most probably because the two O-rings in the lower field joint failed to contain internal combustion pressure which permitted the fire to burn abnormally toward and into the joint; however, because of thermal destruction of the seals and joint, this probability could not be conclusively verified by the physical evidence.