With numerous repair operations ongoing with Discovery’s External Tank (ET-137), managers are creating a number of options to provide some additional flexibility in launching STS-133. As part of the evaluations, the Space Shuttle Program (SSP) have requested input from the International Space Station (ISS) on the option of a launch window which opens on December 17. Managers have already decided to slip the launch date to NET December 3.
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Discovery’s tank is on the right path to complete the required repairs on both the Ground Umbilical Carrier Plate (GUCP) and the cracked stringers/foam, although the schedule is “tight” to fully complete the procedures in time for the start of the countdown – which was to include either an embedded Tanking Test or begin nominally on November 27.
Due to the progress made on the GUCP side of the repairs, a decision has already been made to cancel the Tanking Test which would be part of a six day countdown, with a telecon on Wednesday noting confidence in the GUCP performing as advertised during the nominal tanking on launch day. An option remains for a Tanking Test during S0007 (Launch Countdown Operations), followed by a 48 hour scrub turnaround.
The last time a Tanking Test was conducted came during STS-127’s troubleshooting on what was a second successive GUCP leak during tankings.
Following the GUCP related scrub suffered by STS-133 on November 5, engineers and technicians have successfully reinstalled a new flight seal, along with an alternative carrier plate.
“Disassembly was performed incrementally, taking measurements along the way to compare to installation and historical data. The replacement GUCP and flight seal were installed on Friday and concentricity offset measurements taken,” noted an update to MOD managers (L2).
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The Quick Disconnect (QD) hardware has also been “clocked”, following the known misalignment found via concentricity measurements.
“OV-103 / SRB BI-144 / RSRM 1 112 / ET-137 (Pad-A): Ground Umbilical Carrier Plate (GUCP) leak: GUCP 7” QD installation was completed. The QD was clocked as advertised to take advantage of its known offset. Measurements were taken following QD mate and no movement of the carrier plate was detected,” noted the NASA Test Director (NTD) processing report (L2).
“Because the QD had been reclocked, the spotface on the QD for the vent line bond jumper was in the wrong position for installation of the ice suppression shroud. The jumper was re-spotfaced from the 7 o’clock to the 10 o’clock position, and the Aerogel blankets and shroud were installed correctly.
“Installation of the 3/8” QD’s at the top of the GUCP and GH2 vent line mate were completed. GH2 vent line alignment measurements were good. Vent line leak checks for GUCP retest – No leaks were detected. GUCP retest was successfully completed last night.”
The four cracks in two of the tank’s stringers continues to be the main driver of the schedule, with a repair schedule (L2) showing work will be carried out on the flange area between the Intertank and LOX tank through until later this month. It is also hoped that no further defects are found via the high resolution backscatter and computed radiography x-ray scans that are being conducted on a portion of tank.
“ET Foam Crack: The area of cracked foam was dissected and the underlying structure exposed to gain insight into the cause of the crack. Two cracks on the underlying stringer and two cracks on an adjacent stringer have been identified. The plan is to replace the cracked section of the stringers and utilize a doubler plate to cover the area where the new section and existing section meet,” informed the MOD update.
“Two types of non-destructive evaluation (NDE), high resolution backscatter and computed radiography x-ray, are being discussed as potential data gathering efforts. The repair plan and procedures are being finalized. Investigation into root cause and determination of flight rationale is in progress.”
With the removal of the cracked stringers and the fabrication of the doublers completed, preparations were in work to begin the installation of the doublers on to tank during Wednesday.
“ET LO2/IT stringer cracks: The final fasteners from stringers S6 and S7 have been removed. ET surface preps for doubler installations are in work. Borescope of the remaining 3-4 holes on stringers 6 and 7 is complete. Damage was found on 2 upper holes on both stringers. As a result, ET Mechanical initiated a Problem Report (PR) for drill witness marks on the LOX aft dome,” added the NTD report.
“NDE dye pen on the grinder nicks on stringer S7 were performed with no cracks found. Mold impressions of the nicks on stringer S6 were taken and the nicks were sanded. NDE Backscatter x-ray scanning of the +Y area of the LO2 interface flange picked up and will continue through Thursday on a non-interference basis to ongoing repair work.”
While Stringers 6 through 19 have been scanned, Stringers 1 through 5 were not accessible. The results of the inspections are still being checked as of Thursday morning. Also, one of the doublers required some additional work, after it was found to be too long for its installation on to the tank.
“The S6 doubler was found to be too long, so it was sent to the shop for trimming and is now dimensionally correct and ready for installation,” noted the NTD report on Thursday.
Once the doublers have been installed, BX foam will be sprayed on to the area, which will then mark the start of a three/four day cure time. The required environmental conditions have been satisfied for the spray and cure, via a tent hood being installed around the repair area.
In tandem with the ongoing repair work, managers and engineers are being tasked with the ultimate requirement of creating flight rationale – something which may be very challenging to achieve in time for the upcoming launch window.
It is understood that ET-137’s stringer cracks do not share commonality – or suffered from a “different mechanism” – compared with the history of 38 cracks that have been observed during the production of the tanks at the Michoud Assembly Facility (MAF) – all repaired at the New Orleans factory – an issue that has been suffered by 20 tanks since the switch to a lightweight aluminum-lithium alloy as part of the transition to the Super Light Weight Tank (SLWT).
ET-137 is also understood to be the first tank to suffer from stringer cracks at the pad, at least to the point where it caused a visible crack to the Thermal Protection System (TPS) foam. Sources note that the crack was observed on a camera during tanking/de-tanking on November 5, with the crack instantaneously “popping” into view.
Daily meetings are being conducted to discuss both the repair effort and the root cause, building a knowledge base towards flight rationale.
A joint Shuttle Engineering Review Board (SERB)/Systems Integration Control Board (SICB) was conducted on Tuesday, with Thursday’s all-powerful Program Requirements Control Board (PRCB) meeting likely to provide a major update. (10 Presentations from the meeting now uploaded into L2 during).
Managers will then meet in a Flight Readiness Review (FRR) style Launch status briefing, (UPDATE: Now delayed to Monday, November 29), prior to updating reporters on the state of play.
Discovery was aiming for opportunity to launch in a window which ranges from November 30 to December 6 – although there may be refinements available over the coming days. Those refinements came on Thursday, with a slip (although no real date had been previously set) of the launch date to NET (No Earlier Than) December 3 through to December 7.
“If we slip 133 past the December window, then we will be in February, because there is a big beta cutout and cutout for HTV,” noted MOD Director Paul Hill in recent notes, before conversations this week spoke of what is currently only a potential option to create another window in December.
STS-133 Specific Articles: http://www.nasaspaceflight.com/tag/sts-133/
Created by the SSP, ISS managers have been asked to assess the preliminary option of a window opening on December 17. Challenges would include the need to offload around 600 lbs of payload from the mission, in order to provide the required ascent performance margin during such a window.
It is not clear how long such a window would last, which would be key for the potential of Discovery being on orbit on through to New Years Day, as this would trigger the YERO (Year End Roll Over) consideration. YERO is a problem which was first highlighted when Discovery’s STS-116 launch window was restricted to launching before the mission could overlap the change of year.
The problem relates to the shuttle orbiter’s computers needing to reset through a change of year, which could cause a glitch on orbit. While a contingency was already in place – involving the orbiter remaining docked to the International Space Station (ISS) during a YERO event – a final solution was requested by previous shuttle manager Wayne Hale.
“When (the shuttle computers) were originally programmed – the basic operating system was built back in the 1970s – it was not envisioned that we would fly across the end of the year, so we didn’t handle it very well,’ noted Hale during the pre-flight briefing three years ago.
“We’re making some changes to the way the software works, which will be fairly simple and easy. So if we need to at the end of this year, fly across the end of the year, we will be able to do that.”
Despite the solution being found, as documented by the PRCB, it has always remained a contingency in the unlikely event an orbiter was on orbit, and undocked from the ISS, during the change of year. It is possible that managers will plan a mission – should the notional second window in December be taken – to avoid such a scenario.
A range of options is always the norm for NASA, with the goal continuing to be one which aims for launching Discovery in the initial December window.
(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, via NASA.gov and L2 documentation).