STS-133: Stringer repairs to be completed on ET-137 this week

by Chris Bergin

A group of United Space Alliance (USA) and Michoud Assembly Facility (MAF) technicians and engineers have finished work on modifying the stringers on Discovery’s External Tank (ET-137), with only foam application tasks outstanding. With the root cause evaluations and subsequent modifications ensuring there is “no uncertainty” on all flight tanks, STS-133 remains on track for rollout to Pad 39A next Monday.

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With the work ongoing on ET-137, Discovery is now being warmed up for her return to Pad 39A via repressurization and decay checks, ensuring the orbiter’s unexpected stay in the Vehicle Assembly Building (VAB) held no adverse effects on her systems.

This saw Discovery powered up at the end of last week, allowing engineers to conduct a repressurization of an Auxiliary Power Unit (APU) gearbox.

“OV-103/SRB BI-144/RSRM 112/ET-137 (VAB HB-1): GSE (Ground Support Equipment) required for APU gearbox pressure adjustment completed calibration and the task was successfully completed. Discovery was powered up briefly to support the test,” noted the NASA Test Director (NTD) report (L2).

Also a clear sign of Discovery’s upcoming launch window – which opens on February 24 – is the replenishment of Liquid Oxygen (LOX) at the storage tanks located near the pad, along with preparations being conducted on the Rotating Service Structure (RSS), which will rotate to protect Discovery shortly after her return to the pad.

“Pad A: LOX storage tank replenishment continued; two waves of tankers (10 tankers total) were offloaded. A SCAPE (Self-Contained Atmospheric Protection Ensemble) operation to R&R (Remove and Replace) hyper GSE flow meters on the 107′ and 207′ levels of the RSS was successfully completed.”

STS-133 Specific Articles (click for numerous background content on the ET and relating to this article):

By the time of last Thursday’s Shuttle Standup/Integration meeting, KSC Ground Operations had noted 50 percent of the radius block installations had been completed. By Sunday, workers were closing in on the final set of 94 stringers around the circumference of the LO2/Intertank flange, notably ahead of schedule.

UPDATE: However, by Monday, the NASA Test Director noted that all mechanic modifications had been completed, with only foam application tasks outstanding.

The methodical approach to the work on the stringers shows an excellent level of planning, with a process of working on several sets of panels in tandem – each of which contain a group of stringers – by removing the BX-265 Thermal Protection System (TPS) foam, accessing the stringer, removing fasteners (match drilling), installing the small radius block, before preparing the surface for the reapplication of foam.

Up to six panels around the tank were at various stages of modification via the latest reports, with foam curing completed on a large section of the tank – allowing for trimming operations to remove excess foam and shape the newly applicated foam as per specifications.

A total of 94 stringers are receiving radius blocks, while 105 – including the stringers which suffered cracks and thus already have doublers in place – of the support beams are receiving a new coat of foam. Only stringers 6 and 7 on -Y Panel 6 have been a problem, when the gun used to applicate the foam suffered an unspecified malfunction.

Confidence is high that the tank will pass through the upcoming Delta Flight Readiness Reviews (FRRs), with the flight rationale to be based heavily on the root cause evaluations which has pinpointed issues which range from the observation of a “mottled” appearance on stringers which have reduced fracture toughness, through to assembly procedures.

As such, a large part of the current evaluations are being based on potential work which may be required on the next tank to fly – ET-122 with STS-134.

“Status on ET-137 progress, and a status of how the TIM (Technical Interchange Meeting) went. There is a lot of work still ahead from an analysis, test and digesting standpoint. The larger team is in agreement with the direction with the work required,” noted the Shuttle Standup/Integration report (L2).

“The testing going on at MSFC (Marshall Space Flight Center) has been going very well. It has been enlightening us with how this material performs, and how the radius block modification performs. We will also talk a little about the forward plan for ET-122.

“(Space Shuttle Program manager) Mr. (John) Shannon heard that and believes it is a good, well thought-out plan to ensure there is no uncertainty on that tank.”

Currently, managers have not yet called for radius block modifications to be carried out on ET-122, with the forward plan opening with X-Ray and Backscatter scans to check for any signs of cracks. ET-122 is an older tank, returned to flight status following the repair of damage sustained during Hurricane Katrina.

While that tank may have avoided reduced fracture toughness stringers seen on ET-137, STS-135’s tank – ET-138 – is likely to also gain an inspection sweep. Given ET-138’s close relation – by way of manufacturing dates – with ET-137, radius block modifications can’t be ruled out.

However, even if modifications were carried out on both ET-122 and ET-137, the launch dates should remain unaffected.

More information on the forward plan is likely to be known next month, when the two main FRRs are held at the Johnson Space Center (SSP FRR) and Kennedy Space Center (Agency level FRR) respectively – the latter on February 18, which will confirm the launch date.

February 24 remains the launch date NET (No Earlier Than) despite STS-133 losing one of its crewmembers to a biking accident. Lead spacewalker Tim Kopra sadly lost his seat to Steve Bowen as result, and despite the amount of training the US Navy Captain has to catch up on, managers are confident the change of crewmember can be installed into the mission without impacting the schedule.

“Flight Crew (NASA/JSC) Last Saturday STS-133 Mission Specialist Tim Kopra had an accident on a bicycle, so Steve Bowen has been assigned to fill in for him,” outlined notes included in the Standup report. “With the current launch date, Steve will be the one who flies. Steve Bowen is training for the EV1 tasks, and most likely the MS1s will swap over and do the MS2 rolls, taking turns doing MS2 for ascent and entry.”

“Been working hard to get a stowage plan for Bowen and get the right EMUs (Extravehicular Mobility Units) down there,” added Flight Operations & Integration. “They have a plan in place that supports the crew swap from a stowage standpoint.”

The fact the injury to Kopra hasn’t caused an untimely impact to STS-133’s launch date pleasantly surprised Mr Shannon, who praised the Crew Office and the Mission Operations Directorate (MOD) for coming up with a plan which protected the schedule for Discovery.

“Tim had his accident, which is heartbreaking for him, but the team picked up and came up with a very creative plan,” added notes taken from the Standup meeting. “If anyone had told Mr. Shannon hypothetically that this close to launch we would lose Tim as EV1 and asked if we could still make our launch date, he would probably say no.

“The Crew Office and MOD came up with a really good plan to make sure that we will have 100 percent mission success.

“We keep getting these little bits of adversity hitting us, and the team has been reacting very maturely and just going about business and making sure that we are safe and set up to be very successful to finish the Program very strongly. Mr. Shannon just appreciates everyone’s efforts over the last couple of weeks.”

(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 image. In the article, via L2 acquired PRCB presentations).

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