ET-137 stringers to be modified – LO2 flange foam liberation assessed

by Chris Bergin

NASA managers have made a key decision to modify 34 stringers on the LO2 flange area of the tank – following the observation of several cracks around its circumference. The radius block modification is designed to mitigate potential foam liberations from the flange, an incident that has never been seen before via flight history.

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The Space Shuttle Program (SSP) was brought up to speed on the status of Non Destructive Evaluation (NDE) scans on the two flanges at the top (LO2) and bottom (LH2) of the Intertank, conducted by technicians via the use of Backscatter and X-Ray scanners.

A handful of additional – albeit smaller – cracks were found on the backside of the tank, near-opposite to the original four cracks found on Stringers S6 and S7, in turn showing the decision to rollback the STS-133 stack to the Vehicle Assembly Building (VAB) was the right decision, due to the lack of access out at Pad 39A.

While those stringers are being repaired with doublers – a task routinely carried out at the Michoud Assembly Facility (MAF) in New Orleans when tank defects are detected – a decision to install additional small doublers, known as radius blocks, was the outcome of a meeting on Monday.

Even with the upcoming modification work, STS-133 still expected to achieve the current targeted launch window, which opens on February 3.

The purpose of adding the radius block doublers to the stringers relates to mitigating the ascent hazard of foam liberation from the LO2 flange area, which – at least in the location of the original cracks – would threaten an impact with the orbiter’s Thermal Protection System (TPS). It would still take a large liberation of foam to actually hold any threat of causing damage to Discovery’s heatshield.

Such an incident is unlikely, based at the very least on flight history, which shows no recorded divots (areas from which foam has liberated) from the LO2 flange via the array of imagery data gained over a large number of flights since the implementation of the Super Light Weight Tank (SLWT).

These findings were outlined in a Program Requirements Control Board (PRCB) document titled “Imagery Review of ET Super Light Weight Tank (SLWT) Intertank Flange TPS performance,” which was presented to managers as part of their risk assessments into ET-137’s upcoming ride uphill.

“ACTIONS: 1. Imagery Integration community to re-evaluate post separation imagery specifically related to intertank flange TPS performance (gaps and/or divots),” opened the presentation (available on L2).

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“2. Of the SLWT missions, for how many of those do we have imagery that allows us to say conclusively that there is no foam loss that could be attributed to the stringer crack failure mechanism? Which from an imagery analysis perspective equates to: Assess image quality and coverage capabilities for detecting intertank flange divots.

“Imagery review by JSC, KSC, and MSFC Imagery Analysis Teams (IATs) consisting of 40 missions where SLWTs were manifested: STS-91 to STS-132 (excludes LWT missions – STS-107 and STS-99).”

The presentation employed criteria for annotating ET TPS gaps and/or divots, looking for imagery of visible defects on the flanges, visible defects on stringer side close to flanges, all whilst utilizing historical documentation for records of intertank flange conditions post ET sep.

Although the amount of imagery that has been gained on the condition of the ET during ascent and post sep at MECO (Main Engine Cut Off) has increased dramatically since Return To Flight, the presentation’s findings combined both the pre-RTF KSC Foam Loss Matrix information with the MSFC Historical Foam Observed Loss Database (FOLD).

For the post-RTF missions, the NASA Imagery Reporting Database (NIRD) and ET Post Flight Assessment Team (EPAT) data was also used, along with imagery findings coordinated with and reviewed by Debris Integration Group (DIG) and ET Project.

As expected, all the imagery reviewed showed no issues with the LO2 flange, with all the defects found on the LH2 flange.

Examples provided in the presentation showed how photographic evidence at various stages of tanking, all the way through to post ET sep, can be compared to find root cause – such as STS-125’s ET-122, which suffered from a “TPS Crack Forward of Port Bipod on LH2 Intertank Flange.”

In this example, the post ET-sep image shows a raised edge to a section of flange foam on the LH2 flange – also known as an offset – which was also observed on ET-137’s LO2 flange, although that defect was surrounded by a crack on closer inspection.

Interestingly, a Final Inspection Team (FIT) image and an Infra Red image are both provided, showing no sign of a defect ahead of launch, unlike ET-137’s offset, which was observed by IR cameras during the scrubbed November tanking.

Other examples are also shown from post-RTF missions, all showing a level of commonality – although most defects actually resulted in a small liberation from the LH2 flange. Crucially, all of the examples shown have a root cause, known as Cryo Ingestion/Pumping, which can’t be cited to ET-137’s incident.

“All LH2 flange and adjacent stringer divots can be attributed to previously understood phenomena such as cryoingestion/pumping, popcorning, etc,” added the presentation. “However, based on imagery alone, underlying structural conditions or contributions to the observed features can not be determined.”

As noted, the LH2 flange defects all occur during ascent, with no way to prove any of the incidents have a cracked stringer as a contributor to the cause. However, the findings at least suggest that no foam liberation incidents can be attributed to the stringers.

“Imagery for all SLWTs from STS-91 to STS-132 was reviewed. When comparing the LH2 and LO2 intertank flanges, the LH2 flange has experienced essentially all the significant foam loss events. No divots near the LO2 flange are considered related to intertank structural issues,” added the presentation.

There are some unknowns, such as the quality and lighting of the imagery from post-sep photographs, along with areas that may be obstructed by over laying hardware (LO2 feedline, press lines, cable tray). However, the overriding feature of the presentation is angled towards the observation that there is no history of foam liberations from the LO2 flange, regardless if there had actually been an issue with an associated stringer.

It is likely that area of uncertainty led to managers need to debate is whether or not such flight history could provide the required boost to a potential decision to fly the tank “as is”, or if additional confidence in the LO2 stringers is required – via modifications – as part of the tank’s flight rationale.

With the modifications now being approved, additional confidence in the tank’s strength will aid flight rationale at the Delta Flight Readiness Review (FRR) meetings, which will be used to clear the tank for the ride uphill.

(Images: All via L2 Presentations).

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