1、Public Lessons Learned Entry: 6816 Lesson Info: Lesson Number: 6816 Lesson Date: 2012-6-7 Submitting Organization: MSFC Submitted by: Jennifer Stevens Subject: Space Transportation System (STS)-133/External Tank (ET)-137 Intertank (IT) Foam Crack and Stringer Abstract: Thermal Protect System (TPS) f
2、oam cracking on the ET was discovered during the STS-133 post-drain walk-down after the scrubbed launch attempt on November 5, 2010. An investigation was conducted to determine the root cause of the stringer cracks and corrective actions to return the ET IT to flight status. The root cause was deter
3、mined to be the result of a failed aluminum-lithum (Al-Li) stringer, an element of the exterior metal surface of the Intertank. 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 the
4、rmal 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
5、. 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 de
6、fects, 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 evolut
7、ion 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
8、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 t
9、o 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. A
10、nalysis 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 oper
11、ational 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 c
12、oncerns 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 th
13、e 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),
14、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 significan
15、t 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,
16、 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 m
17、eet 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.
18、Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-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/sur
19、face 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 rat
20、ionale 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 ta
21、nk, 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. Lesson(s) Learned: When material substitutions a
22、re being considered for an existing design (development or operational), a comprehensive systems cost/benefit analysis assessment of critical design factors (e.g., performance, manufacturing, operations) should be conducted to minimize the potential for unintended consequences. Recommendation(s): Al
23、loy 2090 T83 sheet (less than 0.25 inches thick) should not be used for stringer applications attached by fasteners. Stringers can develop complex, concentrated through-the-thickness bending and shear stresses near the fasteners. Marginal alloy 2090 through-the-thickness properties (i.e., low ductil
24、ity) combined with allowable variation of material processing parameters can alter the microstructure and properties resulting in unexpected fracture initiation and propagation under bending and shear stress stringer environments. Evidence of Recurrence Control Effectiveness: N/A Documents Related t
25、o Lesson: Click here to view ET Stringer BackUp Material Mission Directorate(s): Exploration Systems Space Operations Additional Key Phrase(s): Program Management.Acquisition / procurement strategy and planning Program Management.Affect of Cultural factors on long-term human and robotic missions Pro
26、gram Management.Communications between different offices and contractor personnel Program Management.Configuration and data management Program Management.Cross Agency coordination Program Management.Program level review processes Program Management.Risk management Program Management.Role of civil se
27、rvice technical staff versus contractor staff Program Management.Science integration Missions and Systems Requirements Definition.Configuration control and data management Missions and Systems Requirements Definition.Level 0/1 Requirements Missions and Systems Requirements Definition.Mission concept
28、s and life-cycle planning Missions and Systems Requirements Definition.Requirements critical to costing and cost credibility Missions and Systems Requirements Definition.Review boards Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,- Systems Engineeri
29、ng and Analysis.Engineering design and project processes and standards Systems Engineering and Analysis.Level II/III requirements definition Systems Engineering and Analysis.Long term sustainability and maintenance planning Systems Engineering and Analysis.Mission and systems trade studies Systems E
30、ngineering and Analysis.Planning of requirements verification processes Systems Engineering and Analysis.Systems analysis - cost analysis Engineering Design (Phase C/D).Propulsion Manufacturing and Assembly Integration and Testing Safety and Mission Assurance.Early requirements and standards definit
31、ion Safety and Mission Assurance.Product Assurance Safety and Mission Assurance.Quality Safety and Mission Assurance.Review systems and boards Additional Categories.Configuration Management Additional Categories.Cryogenic Systems Additional Categories.Hardware Additional Categories.Independent Verif
32、ication and Validation Additional Categories.Launch Vehicle Additional Categories.NASA Standards Additional Categories.Parts, Materials, & Processes Additional Categories.Policy & Planning Additional Categories.Pressure Vessels Additional Categories.Procurement, Small Business & Industrial Relations
33、 Additional Categories.Program and Project Management Additional Categories.Research & Development Additional Categories.Spacecraft Additional Categories.Test & Verification Additional Categories.Test Article Additional Info: Project: Space Shuttle Program, External Tank Project, ARES, SLS Approval Info: Approval Date: 2012-11-16 Approval Name: mbell Approval Organization: HQ Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
