STS-133: ET-137 stringer investigation boosted by potential root cause find

by Chris Bergin

The root cause of the cracked stringers on STS-133’s External Tank (ET-137) may have been found, following the investigation team’s findings that the material used for the tank’s intertank support beams was found to be “mottled”, when compared to standard material. Meanwhile, the ongoing evaluation into the modification plan for the stringers may slip the launch to late February.

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Discovery is patiently observing the ongoing work that is being carried out on her External Tank inside the Vehicle Assembly Building (VAB), as repair and modification work – which is “currently” scheduled to be completed in time for a roll back out to Pad 39A sometime around January 13 – continues.

Discovery herself is next scheduled to be powered up on Thursday, for the work relating to the removal and replacement of a GPS Battery. A proficiency run of S0044 Launch Countdown simulation is planned for this Friday.

Given Discovery’s STS-133 payload has remained installed insider her Payload Bay, along with the reduced turnaround work required at the pad – an opening launch attempt on February 3 remains possible, although this date may suffer from an additional slip later this week.

The current plan – approved this week – calls for the installation of Radius Blocks (mini Doublers) on 34 of ET-137’s stringers. This modification would add strength to the support beams on the intertank, thus helping to mitigate the potential for additional cracks, which would in turn potentially threaten foam liberation events during launch.

“OV-103 / SRB BI-144 / RSRM 112 / ET-137 (VAB HB-1): The program has decided to fix our existing cracks in panel 6 and proceed with modification of at least 34 stringers using doublers and radius blocks,” noted the NASA Test Director (NTD) status on the repair work.

Currently, technicians have a full circumference review of the LO2 flange via the use of an X Ray machine, which found three additional cracks on the ‘backside’ of the tank. Aided by the arrival of a second backscatter machine from the Marshall Space Flight Center (MSFC), additional scans are being taken on the LH2 flange, with an aim to complete the data gathering effort by the end of the week.

Lockheed Martin specialists have also arrived from the Michoud Assembly Facility (MAF) to assist with the repairs and modifications, as foam from the S-6-6 and S-6-7 stringer tops was successfully removed ahead of preparing all three stringer tops for the mechanical removal process, set to take place this week. 

“Forensic measurements on panel 6, stringers 6, 7 and 11 are complete. Backscatter non-destructive evaluation of the LH2 stringers around the circumference of the tank began Monday and is expected to continue for the remainder of this week. Panel 6, stringers 6, 7, 8, and 11 are complete,” added the NTD.

“ET panels 2, 3, 6, and 7 foam removal is complete. ET panels 2 and 3 foam trim for spray preps were completed Tuesday morning. Final cleanup is in work.”

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In preparation for the fastener removal operations – which began Tuesday – the goal is to have the ET-137 S-6-6, S-7-6, and S-11-6 stringer remnants routed to MAF for failure analysis by Wednesday.

While the work on the 34 stringers is already approved, Thursday’s Program Requirements Control Board (PRCB) will discuss an option to install radius blocks on all 108 remaining stringers, which in turn would slip STS-133 out of the early February launch window.

Sources claim a planning date of February 27 or 28 is being noted, should the PRCB decide the full modification option. No official manifest decisions have been made at this time.

“The investigation team was also directed to continue to plan for executing the full radius block reinforcement mod on all 108 stringers on ET-137 (minus the 5 cracked/repaired stringers),” added the NTD.

“A comprehensive overview of all the various investigation activities and findings will be brought forward to the SSP (Space Shuttle Program) PRCB this Thursday and additional direction from SSP management may come at that time.”

That meeting will also overview the investigation team findings into the root cause of the cracked stringers, which has remained as an “unexplained anomaly” on all documentation at previous PRCB overviews.

According to notes, the investigation team believe they have found evidence that the material on ET-137’s S7 stringer – one of the stringers which cracked during the scubbed tanking back in November – was made from a material which is being classed as “mottled”, when compared to standard stringer material.

“ET-137 Stringer Investigation/Repair: The investigation team brought forward a post-holiday progress update to SSP senior managers. The team has made significant progress in determining the possible cause of the cracks,” added investigation notes (L2).

“Some material used for the stringers was found to be “mottled”, with a different surface appearance than the standard material. Testing revealed this mottled material had lower fracture toughness than the nominal material and exhibited unstable crack growth.

“All of the cracks found during tanking as well as cracks fixed during manufacturing were located on stringers made with this mottled material.”

Such a finding will provide a boost to the mitigation plan, allowing managers to consider if all of ET-137’s – or indeed ET-138’s – stringers are made from this same “mottled” material. Such a finding would raise the possibility that a decision will be taken to add the radius blocks to all of the stringers. It is unknown if ET-122, due to its much earlier production date, may have avoided the installation of ‘defective’ stringers.

Notably, stringers from the part-built ET-139 are also being evaluated, via their use as test articles at MSFC, with notes adding Stringer 7 from ET-139’s panel 7 “very closely replicated” the failure seen at the pad on ET 137 panel 2 stringer 7 during testing.

ET-139’s stringer is also classed as suffering from the mottled appearance, and matched its sister stringer failure on ET-137 when it failed within the predicted “on pad” loading conditions for the LOX flange displacement and rotation, well below the strains that were measured during the pathfinder testing, and well below the strain values predicted by the analysts for failure of a Al 2090 stringer.

It is understood that there is a high level of confidence that the tanks will pass flight rationale evaluations, once their stringers have received the radius block modifications.

(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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