STS-133: ET-137 flight rationale progress – Two launch date options prepared

by Chris Bergin

Progress continues to be made in returning Discovery’s External Tank (ET-137) back to flight status, as nearly all of the physical repairs are completed on what were two cracked stringers on the LOX/Intertank flange area. With flight rationale work building, December 3 and December 5 – the latter including a Tanking Test – are being considered as preliminary launch date options.

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While the work is being finalized on ET-137, Discovery has re-entered preparations for the launch countdown (S0007 operations), although no date has yet been set for when the clocks will start ticking down for Discovery’s swansong.

Two options have been prepared, with S0007 launch countdown barchart presentations (available on L2) for a December 3 launch attempt – currently the No Earlier Than (NET) date – and a December 5 option, which includes a Tanking Test built into the timeline. 

The requirement of a Tanking Test was classed as unlikely ahead of the slip from the November 30th NET, thus remains only as an option that has been provided by the Kennedy Space Center (KSC) team.

UPDATE: However, NASA managers have since decided to slip the NET to December 17. New article pending.

“OV-103 / SRB BI-144 / RSRM 112 / ET-137 (Pad-A): CTS (Call To Stations) for launch countdown is scheduled for NET 11/30. No formal launch date has been selected. The option to perform a tanking test is still under discussion,” noted the NASA Test Director (NTD) report (L2) on Tuesday. “S0007.100 launch countdown preparations continue.”

Those countdown preparations continue to follow a preliminary timeline for entering the countdown on November 30, with pressurization tasks already in work on Discovery’s Main Propulsion System (MPS) according to the NTD.

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“Preparations for hyper/MPS pressurization operations (have begun). Press to flight mass is scheduled for Sunday, 11/28. Preparations for ordnance disconnect and reconnects worked Monday. Range safety battery R&R was completed. The associated retest, which requires ordnance to be disconnected and reconnected, is planned for Wednesday.

“Flight Crew Systems (FCS) mid-deck stow operations were worked Monday; with more to come.”

With work on the Ground Umbilical Carrier Plate (GUCP) completed, the main focus on setting a launch date for STS-133 remains with the stringers on the tank.

Thanks to some fine work from the techs and engineers tasked with replacing two cracked stringers with two doublers, along with the reapplication of BX-265 foam to the area, work is coming to a close on the damaged area.

“IPR 0072 ET LO2/IT stringer cracks: Final inspections were completed and the enclosure built to support foaming operations on the -Y ET platform was removed. Two minor non-conformances were identified but are acceptable for flight,” added the NTD.

The last “hands on” work to be carried out includes the potential to conduct some final trims to the newly applicated TPS foam, along with the completion of scans on the remaining stringers – pending access – to ensure no further defects are hidden under the foam on the flange.

“Backscatter scanning of the +Y LH2 flange was completed Monday. Computed Radiography scanning of the LO2 flange is in work. 15 shots have been completed and 8 more will be taken.”

The scanning of the tank is a key element towards approving flight rationale – itself the main outstanding element in approving a new launch date.

With the “additional tests, analyses, and inspections for potential flight rationale,” now in work for the past several days in relation to the defects “observed on LO2-Intertank Flange Closeout during loading for STS-133 (1st occurrence in history of program)”, a “Preliminary Working Failure Scenario” provided a baseline for engineers to work with.

“Cracks possibly due to interaction of pre-existing flaw and bending stresses during cryogenic loading Cryo shrinkage of Xt 852 (station/location) ring frame (~0.5”) induces stress in stringer-to-skin interface,” noted an expansive Program Requirements Control Board (PRCB) presentation from Lockheed Martin (available on L2).

“Same mechanism causes stringer valley TPS cracks typically observed at LH2/Intertank interface. Interface forces between attach fasteners and stringer induces bending in stringer feet.”

Managers requested a plan was put into action, in order to identify the most likely cause of cracks, implement necessary corrective actions, and develop data necessary to support flight rationale – opening with a two-fold approach to finding the root cause, followed by repairs and the construction of flight rationale, the latter including the “in-situ” NDE (Non Destructive Evaluation) techniques employed over recent days.

With procedures and documentation investigated at the tank’s base of manufacture – the Michoud Assembly Facility (MAF) – resulting in “no findings/off-nominal processing identified specific to ET-137 stringer crack locations”, ET-137’s issue could not be classed as a defect which had been missed in New Orleans, notable given a total of 37 stringers have suffered issues during production in recent years.

“Review of panel processing at MAF has identified potential for stringer damage,” added the presentation. “37 stringer cracks identified due to various causes (impacts, fastener installation, etc.) Cracks detected by planned visual inspections. No evidence of similar cracks as observed on ET-137.”

The root cause also looked further back into the life of the stringers used on ET-137, ruling out the possibility they may have been suffering from a defect prior to their installation on to the tank at MAF.

“Stringer cracks prior to installation considered unlikely. 100 percent penetrant inspection performed prior to primer application (at formed stringer stage) verifies no detectable surface cracks,” the presentation continued, noting some evaluation work was still outstanding as of late last week.

“Failure analysis of remnant and control parts in-work at MAF and MSFC (Marshall Space Flight Center) labs. Initial measurements (stringer 7) confirm part thickness within requirements. Initial hardness data (stringer 7) shows material is within spec. Additional mechanical tests and inspections planned to verify material properties.

“Detailed stress analysis FEM (Finite Element Model) and component structural testing in-work to further refine design margins with and without flaws. Analysis models complete and results in-review with MSFC engineering. Component testing in-work – Initial data shows similar failure mode and nominal design capability consistent with engineering analysis. Dynamic / shock loading analysis in-work to determine if failures are related. FEM complete – Analysis in-work.”

With the physical repair of the tank – involving the removal of the two cracked stringers (S6 and S7) from the tank, prior to their replacement with two doublers – proceeding to plan, specialist engineers moved on to the application of a conathane primer ahead of the BX-265 TPS foam spraying – all of which were completed ahead of schedule.

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While the repairs were ongoing, engineers began their inspections on neighboring stringers, in stages due to the limitations of access at the pad. These additional inspections – tied in with additional analyses – is aimed at providing the required flight rationale Space Shuttle Program (SSP) managers insisted on from the moment the defect in the tank was observed.

“Completed detailed 2D NASTRAN FEM of skin/stringer panel structure for prelaunch and ascent load conditions,” added the presentation. “Goal is show level of ‘proof’ demonstrated during cryogenic loading and define stress levels / critical flaw sizes for NDE inspections.

“Coordinated backscatter x-ray and computed radiography x-ray NDE capability / access with KSC. Both techniques have adequate capability to detect cracks similar to observations on ET-137.”

With no notes of any additional defects being observed during the scans thus far, the second main element relating to gaining flight rationale was – or indeed remains – on evaluating fail-safe ratios for a tank flying with undetected cracks in its stringers. Initial notes from the PRCB meeting proved to be edging towards a positive conclusion late last week.

“Performing stress analysis to demonstrate fail-safe capability (i.e. FS > 1.0) for structure assuming undetected cracks. Initial analysis results are promising assuming critical stringer is ‘ineffective’ (i.e. Ult. FS > 1.40). Analysis planned to confirm results. Model complete. Analysis pending definition of assumptions for failed conditions,” noted the presentation, which also covered foam liberation events from this area of the tank.

“Performing basic stress analysis of TPS assuming condition observed on ET-137 occurs post T-0. Analysis would assume nominal foam material properties and would not account for defects (i.e. voids, cracks, etc.). Results are expected to show positive margins with stated limitations.”

In summary, an impressive shopping list of requirements had already been built by the time the PRCB met late last week, with the aim of achieving a greater understanding to aid a flight rationale decision by this week.

Not only is the aim to clear ET-137 for flight, but to also baseline the inspection and observational procedures for the two remaining tanks set to fly (ET-138 and ET-122) – the latter of which is due to mate with the Solid Rocket Boosters (SRBs) for STS-134 at the end of the month.

“Critical Elements of Flight Rationale Development for ET-137: Determine if event can be isolated to S6 and S7 (i.e. root cause investigation). Tests and inspections in-work on remnant parts to verify material and configuration. Fractography planned to determine most likely failure mode / crack starter type evidence,” listed the main flight rationale requirements page.

“Build paper review for off nominal processing. Show by stress analysis that similar failures most likely to occur during cryogenic loading due to applied loading and induced stress levels. Ascent loads expected to be second-order effect for bending stress in stringer feet. Leverage investigation results to target NDE inspections on ET-137.

“X-ray and backscatter NDE provide engineering data / confidence that similar conditions not present at locations of interest. Baseline NDE inspections planned on ET-122 and ET-138 to confirm nominal condition prior to loading. All observed cracks removed and repaired with standard, proven methods (structural and TPS).

“Demonstrate by analysis that structure is fault tolerant (i.e. fail safe, FS > 1.0) for potential undetected cracks at stringer ends. Characterize debris potential (foam and structure) for potential undetected cracks at stringer ends. FIT (Final Inspection Team) inspections (OTV (cameras), IR (Infraed), visual) will verify nominal TPS configuration prior to T0.”

A full presentation was also dedicated towards the latter paragraph, ensuring all observational assets were able to monitor any off-nominal conditions occurring on the tank during loading. This requirement may in turn be the decision maker on whether or not to conduct a Tanking Test, in order to provide a dress rehearsal for such a utilization of cameras and additional FIT inspections on the tank.

As always, NASA managers aren’t setting themselves a deadline for the completion of flight rationale, although the PRCB noted that “Flight Rationale Plan Defined but will Evolve and be Guided by Data – Majority of Scope Planned to be Complete Early Next Week.”

Although meetings have been ongoing since the PRCB, a good understanding on where the flight rationale stands is expected to be overviewed at the Johnson Space Center (JSC) during Wednesday.

UPDATE: NASA managers decide to slip launch date to December 17 NET.

(Further updates and articles will follow. Refer to live coverage threads linked above. L2 members refer to STS-133 live coverage sections for internal coverage, presentations, images and and updates from engineers and managers. Images used, Lead: Within the article: via L2 acquired PRCB presentations).

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