STS-134: Mammouth EVA-2 completed – Endeavour TPS clearance overview

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Endeavour’s flight crew have completed the second of four planned EVAs/spacewalks – focusing on port SARJ and Dextre lubrication and ammonia coolant line and tank work – lasting an epic eight hours and seven  minutes. The spacewalk came after mission managers officially cleared Endeavour’s Thermal Protection System for reentry, following a multi-day review process of the vehicle’s ascent data, on-orbiter inspections, RPM photography, and Focused Inspection data.

EVA-2: SARJ, Dextre, and ammonia coolant work:

After beginning their day at 2126 EDT, Endeavour’s flight crew got right to work preparing for the mission’s second EVA.

After a 50-minute hygiene break for Drew Feustel (EV-1) and Mike Finke (EV-2), the two returned to the ISS’ Quest Airlock for EVA preps, spacesuit purge, and spacesuit prebreathe activities.

Following Crew lock depressurization, Feustel and Finke took their EMUs (Extravehicular Mobility Units) to battery power – officially beginning EVA-2 for the STS-134 flight, marked at 1:05am Central Time.

After egressing the International Space Station, Feustel and Finke spent the first 30mins of the EVA performing EVA setup activities and translating out the P3/P4 truss. Once there, the spacewalking spent approximately 15mins performing re-routing operations on the P3/P4 truss.

At this point, Feustel and Finke went their separate ways, with Feustel beginning the hour-long process of filling the P5/P6 EAS jumper.

Meanwhile, Finke spent 20 minutes filling the Ammonia Tank Assembly before moving on to the one hour-long task of removing covers on the port SARJ (Solar Alpha Rotary Joint).

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This cover removal, for which Drew Feustel will join Mike Finke after he finishes his P5/P6 EAS jumper work, will precede lubrication activities of the SARJ itself – which is a large rotary wheel designed to turn the outer portions of the Station’s truss so that the eight solar array sets can track the sun and collect as much solar energy as possible to power the Station.

During cover removal, Finke noted at least two bolts flew off the cover when they were removed with his Pistol Grip Tool (PGT). Amazingly, Finke caught both bolts as they popped out. “The bolt flew off, I have it in my hand,” noted Finke, with Feustel responding with a “Wow!”

A loose washer was also observed, floating undernearth the cover. Efforts are being made to secure the washer, which – had it entered the SARJ mechanism – could have caused potential problems for moving hardware associated with the Race Ring.

As a result of the observations – which may point to the bolts and washers on the covers being worn down by removal – only half of the covers are being removed for what became partial lube task, and the PGT was used for only part of the cover removal, with the bolts then being removed by hand.

As first noted in mid-2007 on the starboard SARJ, evaluations into problems with the starboard SARJ began after vibrations and power fluctuations were noted by ground controllers and ISS crewmembers, which led to an inspection of the hardware during STS-120’s EVAs in October 2007.

With observations of metallic shavings – consisting of 1505 Nitride material – on the hardware, engineers concluded the debris was the result of grinding on the Race Rings.

With a similar issue then noted on the port SARJ, concerns grew with the starboard SARJ when Mike Fossum observed a depression, or pit, on the Race Ring during his EVA inspection on STS-124 in June 2008.

Plans were then created to replace the Trundle Bearings Assemblies (TBAs) and grease/lube the Race Ring during STS-126′s EVAs. The results were highly encouraging.

As noted by managers in an update after the STS-126 TBA replacement and Race Ring lubrication, “Status briefing on the initial quick look results of the Starboard SARJ rotations and disturbances seen after the ring was lubricated and the TBAs were changed out: The disturbances seen by starboard SARJ motion have greatly decreased based on the two orbits of autotrack that were performed immediately post R&R.

“The team requested an extended autotrack to obtain more data and determine if the disturbances drop even lower after a long period of operation and distribution of the grease due to rotation.”

Managers then created a forward plan to lubricate the Race Rings of both SARJs over Shuttle missions throughout 2009 and 2010 – with one of those planned flights (STS-134) eventually finding itself in 2011.

After cover removals are complete, Feustel was tasked with taking detailed photographs of the SARJ and collect samples for analysis once Endeavour returns to Earth. However, camera issues were noted, so it’s unknown how many useful photos were taken.

Following directly from the SARJ cover removals, Mike Finke was tasked with the first of two SARJ lubrication activities for the EVA. By now, the EVA was running late due to the bolt issue with the covers, leading to a “big picture” plan to extend the EVA by another hour – as both spacewalkers said they felt good to continue.

Meanwhile, Feustel left Finke to his work and move on to his EAS setup/vent operation. After this, Feustel turned his attention to cleanup operations for the ammonia vent tools.

Finke completed the first SARJ lubing five minutes as Feustel worked the ammonia tool cleanup task. Finke then moved on to stow the P3/P4 jumpers – during which Feustel joined him for the remainder of the activity.

Feustel moved to the ATA and performed a venting operation while Finke tackled the S1 truss Radiator Grapple Bar Stow Beam operation.

While Finke was performing this 1hr 10min operation, Feustel installed a CLA cover on Dextre, or the Special Purpose Dexterous Manipulator. He then performed a lubing operation on Dextre with the aide of the Station’s Remote Manipulator System arm.

During the S1 Radiator Grapple Bar Stow Beam and Dextre operations, the port SARJ was rotated 200 degrees, a 45 minute operation.

After finishing their two previous activities, Finke and Feustel moved back to the port SARJ and perform the second lubrication of the joint for the EVA.

The two astronauts spent the final part of the EVA reinstalling the SARJ covers, cleaning up their worksite and tools, and translating back to the Quest Airlock to re-enter the ISS.

“That was an awesome EVA,” Commander Kelly noted, as the duo came to the end of their epic EVA, ahead of preparations to get the duo out of their spacesuits. Due to the length of the EVA, the crew moved into their pre-sleep shortly after they rejoin the rest of the crew.

Flight Day 8 will be mainly an off duty day. Monday will also mark the historic Soyuz “flyabout”, with the next article focusing on the once-in-a-lifetime event.

TPS Clearance Overview:

Following Endeavour’s brilliant ascent on Monday morning, information on the health OV-105’s Thermal Protection System (TPS) began pouring in from Endeavour’s computers, with the first indication of health coming from the Launch +12hrs (L+12 hours) Wing Leading Edge Impact Detection System (WLEIDS) report.

According to the L+12hrs WLEIDS executive summary (L2), “All WLEIDS ascent summary data was downloaded and down linked successfully. Ten (10) half second windows of detailed G time histories were downloaded in order to confirm the implication of cases above 1 Grms.

“In total, there are two indications; one Category IV indication on the starboard wing, and one Category III indication on the port wing. Both indications occurred on the 11/12 interface” – meaning they occurred at the interface region between Reinforced Carbon Carbon (RCC) Wing Leading Edge panels 11 and 12 on both the port and starboard wings.

Furthermore, “Overall, background levels for STS-134 (were) very similar to background levels of previous missions. No data anomalies (were) identified, and all (WLEIDS) units triggered on Main Engine Ignition within 0.13 seconds of each other.”

Both WLE impact indications met all six impact reportable criteria to be considered areas of interest/potential impact indications and not just normal RCC WLE settling “noises” created as the vehicle accelerates through Earth’s dense lower atmosphere.

Of the two potential impact indications, the largest was the indication from the starboard wing – which registered a Grms indication of 2.54 in Damage Likelihood Category IV.  Damage Likelihood Category IV is defined as a damage probability ratio between 1/200 and 1/100.

The second potential impact, the one on the port wing, registered on the WLEIDS with a Grms of 1.34 in Damage Likelihood Category III. This was a double transient event with the first transient falling in Damage Likelihood Category I: a damage probability ratio greater than 1/1000.

The second transient contained a damage probability between 1/500 and 1/200.

Flight Day 2’s (FD-2’s) inspection of the WLE via the Orbiter Boom Sensor System (OBSS) observed absolutely no areas of damage on the RCC at the locations identified as potential impacts sites by the WLEIDS.

The following day, on FD-3, Endeavour performed her customary R-bar Pitch Maneuver (RPM) to allow the crew on the International Space Station to photography her underbelly TPS. (Image used, 125mb “super belly” RPM photo stitch (L2))

Following this maneuver, the TPS Damage Assessment Team (DAT) released a preliminary report on the vehicle’s TPS health during FD-4. “Imagery review complete: ET doors verified closed. Five items on the lower surface have been evaluated – all QA completed. All lower surface tile (superficial) damages – four cleared by PDAT and one by comparison with OOIC.”

No gap filler protrusion were noted during RPM photography image review, and no upper surface discrepancies were identified either. FD-2 inspections confirmed that the T0 umbilical region of Endeavour suffered no damage during T0 umbilical release at liftoff.

However, the FD-4 presentation noted two areas of TPS underbelly damages that could not be cleared by the DAT and might require a Focused Inspection by Endeavour’s crew.

The first location was a multi-tile damage site on the inboard elevon with an estimated 3D damage depth of 0.3 +/-0.1 inches. The damage location itself was 6.52 inches in length x 2.32 inches in width +/-0.15 inches. Total tile thickness in this region is 1.636 inches.

This area, while initially a Focused Inspection candidate, was cleared for reentry by the DAT through an amazing series of analyses that proved that positive structural and thermal margins would be maintained in this region during entry.

Thus, the need to perform a Focused Inspection on this area was eliminated.

(Animated GIF created by NSF member Lee Jay Fingersh via the several hundred hi res RPM images available in L2).

The second area of damage that did eventually require a Focused Inspection was classed as a “large damage site.”

According the FD-4 TPS Subsystems Status presentation – available for download on L2 – “RPM Imagery: Visual indications of RTV (filler bar) and dark line (abrupt change in depth).”

The RPM imagery showed indications of “ledges/drop offs” in the TPS cavity with decreasing TPS tile thickness close to the edge of the tile.

STS-134 Specific Articles: http://www.nasaspaceflight.com/tag/sts-134/

Based on RPM imagery, the TPS tile damage depth was estimated at 0.6 inches +/- 0.1 inches with reduced confidence toward the tile edge.

This estimated damage depth corresponded nearly identically to the damage depth prediction on STS-118/Endeavour – the only other occurrence of deep tile damage on an orbiter post-Columbia.

On STS-118, RPM imagery yielded a predicted tile damage depth of 0.5 inches. Upon Focused Inspection of the area, the damage depth was revealed to be around 1.0 inch.

Due to these factors, the DAT and imagery support personnel were split on their interpretation of the RPM data. As such, the DAT formally recommended proceeding with a Focused Inspection of the “large damage site” on Endeavour.

Following the Focused Inspection on Saturday, the TPS DAT and Mission Management Team officially cleared Endeavour’s entire TPS for reentry.

(Extensive coverage is being provided on the news site, forum and L2 special sections – the latter of which is the world’s best front row seat to Shuttle missions. With specific and extensive flight day coverage, from interactive “one stop” FD live coverage in the open forum, to internal documentation, photos, videos and content in the specific L2 FD areas).

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