ET-131 repair proceeding to plan – confidence root cause has been found
Preparations for next Wednesday’s tanking test are proceeding well, with the flight seal now replaced ahead of weekend work to reinstall the Quick Disconnect (QD) on the Ground Umbilical Carrier Plate (GUCP). A successful tanking test is expected, due to “high confidence” a misalignment was responsible for at least STS-127’s scrub.
Engineers have been carrying out a complex procedure to replace the flight seal, compared to the first scrub turnaround of STS-127, in an attempt to prove – and subsequently mitigate – the root cause of the leaks suffered by ET-131’s two tankings.
“Planning continues to progress for a tanking test (S0037), targeted for next week, to assess the effectiveness of the GUCP troubleshooting effort,” outlined confirmation the current plan is on track, via processing latest (L2).
“Pending successful installation of the new seal (complete) and QD (in work), the call to stations is currently scheduled for 20:30 EDT, Monday, 29 June 2009. Filling of the External Tank should begin at 0700 EDT on Wednesday, 01 July 2009.”
It’s been a busy couple of days at Pad 39A, with engineers working through the three shifts on a dual approach of replacing the GUCP hardware, whilst taking numerous measurements of the system, in order to build the database of information for engineers working on the mitigation plan.
“Numerous measurements and observations were made on the hardware during disassembly and that data is being compiled and evaluated by the respective teams,” added information via L2.
“The development of a concentricity tool is complete and the offline process development test in the VAB (Vehicle Assembly Building) with the GSE (Ground Support Equipment) test panel and two part seal was successfully performed.
Following the removal of the GUCP QD this week, strain gauges were installed on to the hardware – which in turn will provide data during Wednesday’s tanking test. Also, several elements of the removed GUCP hardware are being examined, such as the pyro bolt – now enroute to the Marshall Space Flight Center (MSFC).
“The ordnance (pyro) bolt R&R was completed and the bolt is expected to arrive at MSFC for NDE (Non Destructive Evaluation) testing on Saturday,” added Friday processing information.
With data comparisons between the first and second scrubs now fully evaluated, confidence is high that the previously noted misalignment issue – related to the External Tank Carrier Assembly (ETCA) mounts, or feet – was the cause of both scrubs.
However, there appears to be a lack of reference to the root cause of ET-127’s scrub during STS-119, other than Flight Readiness Review (FRR) documentation noting alignment issues during the GUCP’s installation inside the VAB on both STS-119 and STS-127’s tanks. For the interim, engineering notes only refer to Endeavour’s ET-131.
“The engineering teams, after much analysis of the measurement data between the 2nd scrub disassembly and the 1st scrub disassembly have high confidence that misalignment is the issue,” added engineering notes (L2).
“The carrier plate was realigned using the high fidelity 0.515” alignment pins and a check of the pivot assembly feet indicated that an interference condition exists between the feet and the pivot support brackets.
“Faro Arm optics measurements were performed Thursday night on the gap between the pivot support bracket and the bracket feet with the feet placed in the nominal centered position on the pivot pins.
“These gap measurements were then used to develop a CAD drawing that will used to machine new feet specifically tailored for ET-131. In addition, concentricity measurements were also performed to verify that the plate, once aligned, will indeed result in acceptable alignment of the flight seal and QD bellows probe when the QD is reinstalled.”
With the alignment issues understood, a new flight seal (a new two-part flight seal) installed, and an array of instrumentation available to monitor the GUCP hardware during tanking, engineers are confident the tanking test will prove to be successful – allowing STS-127 to realign for a July 11 launch to the International Space Station.
Engineers will continue to work out at the pad over the weekend, installing the GUCP QD and remating the vent line – prior to conducting leak checks, measurements, and inspections on the tank.
For all 17 NASASpaceflight.com articles on the GUCP, click here: http://www.nasaspaceflight.com/tag/gucp/
ET-131’s PRCB Note Of Interest:
ET-131’s GUCP hardware also gained a section of the “Quarterly Process Escape Summary, 2nd Quarter Fiscal Year 2009 (Reporting Period: January through March 2009)” presentation to the all-powerful Program Requirements Control Board (PRCB) meeting on Thursday. (Presentation available on L2).
The “Incorrect Washer Stack Between GUCA Foot and Leg Assembly” report is not understood to be related to the root cause of the tank’s scrubs, despite the that specific area of the tank being of interest during the investigation.
“Process Escape Description: Ground Umbilical Carrier Plate (GUCP) pivot assemblies were incorrectly assembled with one washer, instead of three washers as required by the drawing, in each pivot assembly bolt stack,” outlined the presentation, dated June 25.
“Proximate Cause: In February 2005, the GUCP post-launch refurbishment procedure was updated to reflect the complete design modification of the GUCP Leg assemblies. Part of the design change included the use of three (3) washers versus one (1) washer to reduce thread protrusion due to the lengthening of the bolts.
“The change to the number of washers was inadvertently omitted from the procedure update. Subsequent review in place at that time failed to identify the omission.”
Despite the issue being classed only as a “green” risk, NASA managers take all design issues – even a few missing washers – very seriously.
However, an interesting reference is made to modifications that have been carried out on the Ground Umbilical Carrier Assembly (GUCA) – central to the recent leaks and scrubs – and the approval of flight rationale from ET-127, the tank that leaked on STS-119’s first attempt.
“It should be noted that this change was implemented with a full compliment of other modifications done to the GUCA’s which included multiple changes to the drawing. All other drawing changes were successfully implemented, however the washer additions were inadvertently missed,” added the presentation.
“Rationale for Flight: Approval was obtained for ET-127 since vent line was in flight configuration and access was limited. The GUCPs for External Tanks (ET-130, ET-131 and ET-132) in process since discovery of this condition were corrected to reflect proper drawing configuration. The GUCA, GUCP, and legs are cleaned, processed, and refurbished after each flight.
“Lessons Learned: When major design modifications are incorporated into work instructions, attention to detail is required to ensure all changes are included. Controls that rely on human behavior require constant monitoring through measurement, reporting and investigation of any errors to understand possible improvement opportunities to the process.”
Given no reference is made to the leak issues with either ET-127 or ET-131, the incorrect hardware configuration is likely to be coincidental, as opposed to linked to the recent scrubs.
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.