STS-128: Completed ET-132 intertank pull tests show positive results
The foam liberation from Endeavour’s External Tank (ET-131) intertank may have been a one-off event, as all 26 pull tests on ET-132’s intertank show no signs of weakness in the tank set to fly with Discovery next month. However, minor damage was sustained on the tank after a tube-lock broke loose and fell from scaffolding being erected for the remaining pull tests.
ET-131 Event – ET-132 Investigation:
Foam on the intertank region of the ETs is added via an automated application process at the Michoud Assembly Facility (MAF) in New Orleans.
Due to its placement in-between the LH2 and LOX tanks, the region usually avoids the large foam liberation events due to the relatively benign thermal conditions – compared to foam directly associated with the super cold temperatures of the propellant tanks.
A condition known as “popcorning” can occur, but holds no threat to the orbiter, due to the small mass associated with such liberations, and the late stage of ascent during which this condition usually occurs.
The mechanics of foam loss relates to when TPS cell pressure at substrate exceeds bond adhesion strength. Voids in the foam can also suffer from “cryopumping” – although such events have not been seen on the intertank.
What is for certain is the complex material sciences associated with the control of TPS foam loss during the pressures of launch, which is near-impossible to avoid. Modifications to the tanks since Columbia have mainly focused on reducing the most “at risk” areas of foam, thus removing the debris threat.
Although it was late in ascent when Endeavour’s ET-131 intertank started to shed foam - thus holding no serious debris impact threat to the orbiter due to the lack of atmosphere that would result in a high velocity impact – engineers are looking into its root cause.
Currently, that root cause remains elusive, with the initial conclusions focused on a potential issue with the adhesive/bond properties between the foam and the intertank’s structure. That in turn placed doubts on the next tank set to fly, ET-132 – kick-starting an investigation to gain confidence the same event was unlikely to occur during next month’s launch.
“ET-132 Bond Adhesion Evaluation: Purpose: Provide plans and obtain approval to inspect ET-132 / STS-128 Intertank TPS (Thermal Protection System – Foam) at KSC. Additional data needed to provide confidence in the as-sprayed TPS integrity,” noted an overview presentation available on L2.
Interestingly, the presentation appears to cover other foam loss events from ET-131, such as the debris shower noticed at T+104 – which was initially claimed as not originating from the intertank, when it caused cosmetic damage to Endeavour’s starboard Chine area. However, this is now unclear.
“Background: Multiple / unexpected Intertank TPS loss events observed on ET-131. 1st observation occurred at around 104 seconds MET (Mission Elapsed Time). Losses occurred around entire periphery of tank. Majority of foam loss events appear to extend to the primed substrate.
“Indication of inadequate TPS adhesion due to unique processing anomaly. Timing (> 100 secs. MET) consistent with physics-based understanding of bond adhesion failure mode. Losses occur due to vacuum / ascent heating load affects at TPS / primer bond.”
Root cause may have been easier to pinpoint had a manufacturing change been noted by MAF in relation to the intertank. However, the presentation confirms no changes to the application of the foam have been made.
“No engineering changes (design or process) to Intertank acreage for ET-131 / STS-127 or ET-132 / STS-128. NCFI 24-124 TPS applied using automated spray process. Majority of material power machined (optimize weight) and perforated (+Z, debris minimization).
“Majority of material thickness around 0.5” (nominal). Nominal as-built density around 2.6 pcf. On-part lead-in/out bond tension tests performed in cryo regions near flanges (>30 psi required).”
ET-132 Pull Tests:
Already inside the Vehicle Assembly Building (VAB), mated with the twin Solid Rocket Boosters (SRBs) and awaiting the arrival of Discovery for mating, access to ET-132 is already in place.
This has allowed the majority of the pull tests – used to check the adhesive properties on ET-132 – to take place, ahead of scaffolding being installed for access to the remaining test areas on the intertank.
“Plan focused on determining root cause in parallel with performing tests/inspections of as-built hardware,” added the presentation. “Planning bond adhesion pull tests on ET-132 / STS-128 to provide confidence in Intertank TPS application integrity.
“Twenty-six (26) tests planned with samples from each panel (8 panels). Majority of tests (12 of 26) being performed on three (3) +Z panels. Locations selected based on observed ET-131 performance and debris potential (i.e. CDZ).
“Tests planned on stringer top and sidewall leading / trailing edges. Tests provide best method to determine integrity of bond strength. Additional testing may be required based on ET-132 process data review.”
As of Tuesday morning, 18 of the 26 pull tests have been conducted, with results showing encouraging signs that ET-132’s intertank has the required adhesive strength between its structure and TPS foam.
“Of the 26 test sites 18 were completed (all the walk up access). All tests were 100 percent cohesive failures (broke in the foam – which is good news) and the pull numbers ranged from 14.25 to 84.9 psi with only two values below 43.77,” noted engineering logs on the test (L2).
“The two that broke between 14 and 16 and these had some issues during the testing (the cores were not completely perpendicular, and the plug was bonded slightly crooked) and could have technically been declared a no test.”
Access was being built via scaffolding to conduct the remaining tests – now expected to show the same positive results – while repairs on the previous test areas are soon to take place.
“High Crew is in the process of completing installation of scaffolding this morning. 13 of the 18 locations are being prepped for repair,” added STS-128 processing information for Tuesday morning (L2). “Three require additional photo work and two others are under Engineering Evaluation.
“USA (United Space Alliance) will begin repairing the 18 locations starting with any primer touch up that is required, less the two areas that had lower pull values.”
Should the tests continue to show good results, shuttle managers will have the confidence to clear the tank to fly near the end of August.
UPDATE: All 26 Pull Tests now conducted by Tuesday night, all with good results.
“Path to Flight Rationale for STS-128 / ET-132: Root Cause is isolated to ET-131 with positive test / inspection results. Similar foam loss not expected during STS-128,” noted the presentation based on good pull test results.
However, even the worst case results would have only incurred around a few weeks worth of delay to the schedule, due to the low threat from such foam liberations from the intertank during ascent.
The only requirement would have been to “remove and replace suspect NCFI (foam) with BX manual spray,” which can be carried out inside the VAB.
Other work taking place relates to the Damage Assessment Team (DAT) creating models to show the main flow paths of foam liberations from the intertank, and the worst case damage such a liberation could threaten.
The results will be overviewed at the STS-128 Space Shuttle Program (SSP) Flight Readiness Review (FRR) next month.
ET-132 Damaged During Tests:
As luck would have it, the tests caused new damage to ET-132, when part of the new scaffolding fell on to the tank. Thankfully the damage is minor.
“During build up of scaffolding on platform E around the external tank, a technician was tightening a tube-lock for a handrail,” added Tuesday processing information. “The tube-lock broke and fell from the scaffolding, impacted the ET and landed on B platform roof directly.
“In the area contacted, there is minor damage to a stringer just below the flight door. Dimensions and repair technique to be addressed in PR (Problem Report).”
This incident should not cause additional delays to the rollover of Discovery, which is tracking both the finalization of testing and subsequent repairs to ET-132, and the completion of repair work that is taking place on Pad 39A’s flame trench, which lost some of its refractory concrete during Endeavour’s launch.
“Pad-A: MLP-3 (Mobile Launch Platform) transfer to the East Refurb Site is complete. Flame trench repairs continue; completion is expected by the end of the week,” noted processing information, which confirms the Pad will be ready well ahead of Discovery’s current date for rollout on July 30 – based on a Thursday rollover to the VAB for Discovery.
“The STS-128 Shuttle Final Countdown Phase Simulation (S0044) for team certification is scheduled for today,” added Tuesday processing information.
“The following operations have been rescheduled to support inspections resulting from the STS-127 launch debris events: Rollover to VAB for integration is scheduled NET July 23. Shuttle Interface Testing will pickup immediately following Orbiter/ET mate. Payload transport to Pad-A on July 27. SSV (Space Shuttle Vehicle) rollout to Pad-A on July 30.”
Discovery is being prepared for a 13 day, 3 EVA, mission to the ISS that will include the flight of the TriDAR AR&D Sensor Detailed Test Objective (DTO), the next Boundary Layer Transition (BLT) DTO, and the first flight of the OI-34 flight software. The mission was swapped from Atlantis to Discovery, following the changes in the 2009 manifest.
Also riding uphill with Discovery will be a new crew member for the ISS – Nicole Stott – as well as MISSE (Materials on International Space Station Experiments) 6A and 6B, SIMPLEX, MAUI, and SEITI.
However, the primary payload for Space Shuttle Discovery will be the MPLM (Multi-Purpose Logistics Module) Leonardo, and the Lightweight Experiment Support Structure Carrier with an Ammonia Tank Assembly.
L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.