scope:

Description of Driving Event:

During the Space Transportation System (STS)-133 post-drain walk-down following the November 5, 2010 scrubbed launch attempt, an anomalous thermal protection system (TPS) crack was observed in the External Tank (ET)-137 insulation, adjacent to the intertank (IT) to liquid oxygen (LOX) tank flange. The TPS crack was subsequently determined to be the result of a structural failure of the underlying aluminum-lithium (Al-Li) 2090-T83 stringer. Later inspections detected a total of five cracked stringers. The investigation determined that the cracks originated along the bottom of the stringer feet through combined failure in multiple linked initiation locations. Failed stringer fractographic analysis indicated no pre-existing material defects, although a wavy refractive pattern that was not a surface contour condition was noted on the two lots of stringers prone to cracking. Other lots of stringers did not have the wavy pattern.

The investigation determined that the material had low fracture toughness due to a microstructure evolution process, termed recovery, which occurred prior to stringer processing (i.e., material processes were changed prior to stringer processing). Recovery in the two affected lots resulted in material with higher yield and ultimate stresses, and lower fracture toughness (i.e., more brittle). The time of stringer/TPS crack occurrence suggested a thermally induced load contribution to the structural failure. The stringers were subject to assembly strains caused by initial installation on the IT panel as well as other mechanical events. Transient thermal loading occurs as the LOX tank fills prior to launch. All configurations tests exhibited high plastic hoop strains in the stringer feet due to the transient thermal loading, especially at the first three fastener locations. Cryogenic shrinkage during tanking causes rotation of the IT flange radially inward, which loads the IT stringer feet. Analysis suggests that the strain levels in the first three fasteners/bolt holes remain sufficiently high that a failure may occur. The bounding loading event is thermal loading. Analysis confirmed that installation of radius blocks over fasteners 2 through 7 reduces the peak hoop strain for the operational loading events, but that they have a minimal effect on the assembly strain at fastener 1.

During the investigation, the history of the implementation of Al-Li use in the ET was evaluated, including selection criteria, programmatic goals, and technical assessment review. Schedule and budget concerns drove the ET Project to seek technical ways to significantly reduce the weight of the ET early in the Space Shuttle Program (SSP). In 1993, the ET Project was tasked with reducing ET weight by 10% in 48 months or less. Earlier, the weight was reduced from the Standard Weight Tank (SWT) to the Light Weight Tank (LWT; 1983-1998) by eliminating the paint, redesigning the feed system, and reducing margin of safety requirements on structural and load bearing parts wherever possible (i.e., Factor of Safety was reduced from 1.4 to 1.25 where possible). For the Super Light Weight Tank (SLWT), the primary methods for reducing weight include use of Al-Li, a lighter, stronger replacement for aluminum-copper alloys, new welding techniques, and an orthogrid structure. Although Al-Li 2195 material cost 2 1/2 times that of the Al-Cu 2219 alloy used in the LWT, the weight savings were significant enough to justify the replacement. In addition, some areas of the ET that had low Factors of Safety were able to be modified to increase margins because of overall weight savings. The IT design verification included a combination of tests and analyses. To mitigate potential tank buckling concerns, designers maintained the structural ringframe stiffness, thrust panel material, and solid rocket booster (SRB) crossbeam design. Many of the subsystems, such as the skin stringer/joint interface, the beaded web, and the thrust panel, were tested to failure. The skin stringer/joint was improved to meet buckling prevention requirements. Analytical model verification was performed by comparing predicted structural load conditions to load conditions measured during test programs and by comparing the output prediction of the structural model with the output of other independent analytical models.

The IT skin/stringer/foam interaction was never measured or modeled in the assembled state. Residual assembly stresses were never measured or modeled prior to STS-133 investigation. The lack of skin/surface metal/foam models was noted during the post-STS-107 accident investigation (2003-2004). For the SLWT, the risks for unintended consequences were seen as stemming from the risk of using a material, a process, and a structural design that had never flown in a launch vehicle. Confidence in the rationale against fracture issues was increased by reliance on dye penetrant nondestructive evaluation (NDE) to detect flaws. Dye penetrant NDE flaw detection was not conducted on assembled IT stringers or after TPS application. The components considered critical on the ET were the LOX tank, the LH2 tank, the feedlines and joints, the crossbeam, and buckling prevention. As was observed during the STS-107 investigation, the skin and skin/foam interfaces were of less concern although interactions between all components led to unintended consequences.