Following the multiple Intertank foam debris liberations from Endeavour’s STS-127 External Tank (ET-131) in July, engineers at the Kennedy Space Center and the Michoud Assembly Facility (MAF) in Louisiana undertook an ambitious campaign of Non Destructive Evaluations and Pull Tests on downrange tanks to ensure that these kinds of liberations would not occur again. Those efforts have paid off, with MAF reporting only five areas of interest on Discovery’s STS-128 tank.
ET-132 Pre-Flight Performance:
STS-128’s ET-132 In-Flight Anomaly (IFA) review by MAF – available for download on L2 – reveals that all ET-132 systems performed nominally during the pre-launch and countdown campaigns for STS-128.
During the three tanking cycles performed on ET-132 during STS-128’s multiple launch attempts, the GUCP (Ground Umbilical Carrier Plate) and related ET and Ground Support Equipment hardware performed as expected, with no abnormal or out-of-spec leaks detected.
The IFA presentation notes that the launch vehicle was exposed to “light rain during final inspection” on the day of actual launch; however, this rain was well within limits set by NSTS (National Space Transportation System) safety and control standards.
During Final Inspection checks following LH2 (Liquid Hydrogen) and LO2 (Liquid Oxygen) cryo-loading, a crack on the -Y Vertical Strut Fairing TPS (Thermal Protection System) was identified. This crack was analyzed by launch team personnel and the Mission Management Team, with the ultimate decision being that the crack was acceptable for flight.
Similarly, frost on the LH2 Aft Manhole Cover Closeout and LH2 Feedline were also cleared for launch, as was “froth” noted on the LO2 Feedline outboard base fitting closeout XT1623.
Furthermore, the presentation notes that both of STS-128’s post-tanking scrubs were not the result of a problem with the tank. The first launch attempt was scrubbed because of weather violations with the second scrub being called due to a problem with the PV-12 inboard fill and drain valve on Discovery herself.
ET-132 Post-Flight Performance Review:
Overall, ET-132’s flight performance can be classed as excellent based on the MAF IFA presentation.
According to the preliminary look at ET-132 based on post-ascent imagery from Discovery’s ET Umbilical Well camera, ET-132 observations are “Consistent with performance expectations; no observed losses on Intertank acreage similar to ET-131; no observed losses related to tensile test repair (Pull Tests),” notes the IFA presentation.
Furthermore, all Protuberance Air Load ramps and Aft hardware performed nominally; all LH2 Ice Frost Ramps (IFRs) were intact, as were all LO2 IFRs which only sustained nominal popcorning/charring/erosion as a result of ascent loads and temperatures.
Also, all Orbiter/ET separation and ET disposal events were nominal, as were ET Structural Systems and Electrical and Propulsion Systems.
Nonetheless, the most rewarding aspect of ET-132’s performance for the dedicated engineers at MAF is the lack of Intertank foam liberation – as well as the lack of foam liberation overall.
Well over 100 Pull Tests of ET-132’s Intertank foam were conducted during the tank’s stay in the Vehicle Assembly Building and at Pad-39A. Those Pull Tests revealed positive adhesion margins of the TPS foam to the tank’s underlying structure.
Based on Umbilical Well imagery obtained by a camera and flash system installed on one of the ET Umbilical Wells of orbiter Discovery, “ET-132 did not demonstrate ET-131 (STS-127) like issues with respect to debris formation or failure modes,” notes the MAF IFA presentation.
Furthermore, in light of recent LO2 IFR liberations (seen on STS-125 and STS-127), STS-128 debuted a modified +X maneuver designed to increase Discovery’s lateral translation over the ET after separation.
The new +X maneuver resulted in a 20-second firing of Discovery’s aft RCS thrusters instead of the previously approved 11-second firing that has been performed for several previous flights.
As a result of this modified +X maneuver, Discovery was eight feet closer to the ET during her pass over the LOX portion of the tank.
This allowed imagery analysts to have sharper images at their disposal during post-launch analysis.
This closer pass allowed clear and more detailed resolution of the bi-pod and LOX tank of ET-132, allowing imagery analysts to determine that no adverse erosion or liberation of the LOX IFRs occurred during Discovery’s ascent.
Furthermore, the +X maneuver images, as well as images from the onboard SRB engineers cameras, revealed only five areas of ET TPS damage/liberation, making this the cleanest External Tank in terms of foam liberation in the history of the Space Shuttle Program and highlighting the supreme work by all MAF and KSC workers involved in processing the External Tanks for flight.
In all, one of the TPS liberations was discovered of the LH2/Intertank Flange closeout, two on the ET Bipod area, and two “outside of the debris zone on the -Y/-Z thrust panel” of the Intertank.
Based on images/video obtained from the SRB engineering cameras and the ET Umbilical Well camera, the LH2/Intertank Flange foam liberation event occurred after SRB separation and contained a maximum mass of ~0.040 lbs.
MAF has determined the failure mode for this liberation as “cryo-ingestion.”
This foam loss exceeds the allowable mass limit set by the NSTS 60559 safety rule. As such, this foam liberation has been identified for inclusion into the STS-128 Integration In-Flight Anomaly review.
As such, MAF is recommending an update the NSTS 60559 table risk assessment for “LH2/Intertank Flange for Cryo-ingestion” to update the table for failure modes “in accordance with flight history, debris cloud, and debris expectation.”
As for the two Bipod foam liberations, they occurred on opposite sides of the Bipod closeout.
The first, occurring on the -Y Bipod closeout had a mass of ~0.001 lbs and was caused by either a “Void delta P, or cryopumping/ingestion.” The exact time of release is unknown, though its mass would not have posed a hazardous risk to Discovery’s TPS regardless of its liberation time.
The second Bipod foam liberation, occurring on the +Y Bipod closeout, had a mass of ~0.005 lbs. Like its counterpart on the -Y Bipod, the +Y Bipod liberation time is unknown and was likely caused by “Void delta P, or cryopumping/ingestion.”
The final two foam liberations from ET-132 occurred on the -Y/-Z side of the tank near the GUCP on the thrust panel of the ET.
The mass of these pieces was estimated to be 0.003 lbs with a release time of ~123-seconds (2-minutes 3-seconds) into flight.
The primary liberation cause was determined to be either popcorning/Void delta P/or crush.
These liberations were outside of the critical debris zone and are thus not a concern for Orbiter TPS damage.
Overall, ET-132 performed exceptionally well, a true testament to the care and dedication of the MAF workers in Louisiana.
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.