Excellent External Tank TPS performance reduces pull test requirements

by Chris Bergin

A multi-stage approach to the mitigation of foam liberations from the intertank region of the External Tanks (ETs) continues to pay dividends, as STS-132’s In Flight Anomaly (IFA) review pointed to yet another clean tank – with only one area of notable foam loss from Atlantis’ tank. The results have removed the requirement for additional plug/pull tests on STS-133’s ET.

STS-132’s ET-136 Review:

The performance of the foam Thermal Protection System (TPS) on the ETs – specifically the liberations that occur during the ride uphill – has been one of the key drivers for the successful and safe completion of the Shuttle manifest.

Noted as a science by former Space Shuttle Program (SSP) Wayne Hale, the protection against serious foam liberations – from critical areas, and at critical times of ascent – is the ultimate driver for protecting the orbiter’s heatshield. The result has been a run of orbiters returning to Earth with nothing more than cosmetic damage.

Return To Flight modifications to the tank continued throughout the second half of the last decade, with further lessons learned on areas such as the Ice Frost Ramps (IFRs) – which have seen hardly any foam shedding since redesigns were implemented.

However, before a tank leaves the pad with the rest of the stack, the three hour process of loading the ET with LH2 and LOX requires the hardware to perform as advertised, notably of late with the Ground Umbilical Carrier Plate (GUCP) – the cause of scrubs during both STS-119 and STS-127.

Since a redesign to a two-part seal – and the correction of misalignments on the External Tank Carrier Assembly (ETCA) mounts, or feet – no leaks have been detected during topping operations, as was the case with STS-132.

“Pre-launch Performance Assessment: Electrical and Propulsion systems performed nominally. No leakage detected at the GUCP during the transition to topping. Total of ten (10.3) pre-press cycles as predicted,” noted the STS-132 External Tank IFA Review presentation – all 16 IFA presentations available on L2.

For all 17 NASASpaceflight.com articles on the GUCP, click here: http://www.nasaspaceflight.com/tag/gucp/

Several observations were logged after tanking by the Final Inspection Team (FIT), who photograph the tank for issues such as ice build up and any cracks caused by the cryo conditions associated with tank loading. However, all were cleared during the business end of the countdown by the Mission Management Team (MMT).

“TPS performance acceptable per (requirements): Typical TPS crack on -Y Vertical Strut. Thick shell ice ball at LH2 Feedline inboard closeout near LH2 tank aft dome. LCC (Launch Commit Criteria) Ice-01 violation – Crack at diagonal strut fitting closeout inboard near interface to the tank,” added the presentation. LCC Waiver processed to accept condition. Explained condition – same mechanism that induces crack at Vertical Strut.

“Post Flight Performance Assessment Complete: Electrical and Propulsion systems performed nominally. Structural System performance nominal. ET/Orbiter separation and ET Disposal nominal. TPS imagery assessment complete. Umbilical stills and video, Handheld, Cockpit camera and SRB imagery reviews complete.”

Only three areas of foam loss – four events in total – were noted on the IFA presentation, all deemed as minor. None of the liberations are believed to have impacted the orbiter – although none would have threatened any damage of note on Atlantis’ TPS due to their small mass.

“Post flight performance assessment identified the following ET TPS debris events of note in critical debris zone for further scrutiny,” the presentation continued.

“XT 1593, LH2 Ice Frost Ramp: Two Events: Mass 1: ~0.080 lbm total (likely multi-piece event). Mass 2: ~0.001 lbm MET (Mission Elapsed Time): Indeterminate (possible 323 or 374 event). Failure mode: Cryo-pumping (loss 1) Risk Assessment Mass: ≤ 135 Seconds: 0.25 lbm > 135 Seconds: 0.25 lbm.

“XT 1129, LO2 Feedline Base Closeout: Mass: ~0.050 lbm MET: ~160 sec. Failure mode: Cryo-Pumping Risk Assessment Mass: ≤ 135 Seconds: 0.077 lbm > 135 Seconds: 0.077 lbm.

“XT 1102, Intertank Acreage: Mass: ~0.016 lbm MET: ~129 sec. Failure mode: Bond Adhesion Risk Assessment Mass (Void delta P): ≤ 135 Seconds: 0.004 lbm > 135 Seconds: 0.015 lbm.”

Only the intertank loss occurred near to the time foam liberations may have enough energy – via the remaining air resistance as the stack leaves the atmosphere – to be a threat to the orbiter, although it would have had to been of a higher mass in such an event.

Encouragingly, it was only a single area of intertank foam loss, which adds additional confidence that the mitigation procedures – set up to reduce the run of numerous intertank foam losses – have worked to plan.

That mitigation effort involved a change to the platforms used to clean the intertanks at the Michoud Assembly Facility (MAF) ahead of foam application, which reduced the possibility for dust to remain on the structure, in turn reducing the adhesive properties between the aluminium and the foam.

“Intertank imagery reveals only one loss similar to those as observed on ET-131 (low bond adhesion induced losses with stringer top primer visible), and no losses observed from bond adhesion test repair sites,” the presentation noted.

While bond tests – sometimes known as plug/pull tests – will continue to be carried out in the Vehicle Assembly Building (VAB), engineers are not required to carry out additional tests on additional areas, as previously called for.

“Intertank acreage foam loss at Xt 1102. Failure mode not documented – IFA recommended (IFA-STS-132-T-002). This loss is consistent with our understanding of this failure mode and within expectations since this failure mode was identified on ET-131.

“No additional bond adhesion tests are planned for ET-137 based on process similarity and performance of ET-136.”

STS-132 Specific Articles: http://www.nasaspaceflight.com/tag/sts-132/

All other areas of the tank, which have suffered from foam liberations during flights in recent years, were classed as clean, confirming the excellent performance of the tank. These results will next become part of the baseline for the STS-133 Flight Readiness Review (FRR).

“All LH2 Ice Frost Ramps intact except for loss as noted at Sta 1593. Nominal popcorning/charring observations from LO2 Ice Frost Ramps,” added the presentation in conclusion. “Nominal popcorning/charring/ablation observations from aft hardware closeouts.”

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