STS-125: Eight hour EVA-4 works STIS repair – Atlantis’ TPS overview

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For Flight Day 7 of STS-125, Atlantis’ crew turned their attention to the forth EVA of the mission, focusing on the repair of the STIS instrument. Mike Massimino and Mike Good beat EVA-2’s duration by recording an eight hour EVA-4, which was forced to delete one item from the timeline. Meanwhile, the Damage Assessment Team (DAT) have overviewed Atlantis’ TPS to the Mission Management Team (MMT).

EVA-4:

With the highly successful EVA-3 behind them – and only two EVAs to go – astronauts Massimino and Good ventured outside of shuttle Atlantis to undertake the forth spacewalk of the mission.

The first item on the list for EVA-4 was the repair of the Space Telescope Imaging Spectrograph (STIS), with the second task – the installation of the New Outer Blanket Layer (NOBL) 8 – due to follow on the schedule, before being deleted due to timeline constraints.

Given the expediency of Saturday’s Advanced Camera for Surveys (ACS) repair – a repair effort that saw John Grunsfeld and Drew Feustel not only finish the timelined repairs of ACS but also complete several get ahead repairs of ACS – Massimino and Good were tasked with another complex spacewalk.

First installed on February 14, 1997 during the STS-82 SM-2 (Servicing Mission 2) mission of shuttle Discovery as a replacement for the Goddard High Resolution Spectrograph, STIS stopped working on August 3, 2004, two years after its expected end-of-life service.

The instrument was placed into a “safe” mode, pending EVA-4’s repair.

a215During its operations, STIS provided scientists with spectra and images at ultraviolet and visible wavelengths, and conducted the first spectrum analysis of the atmosphere of an extra-solar planet.

Due to an issue with a troublesome bolt on the STIS handrail, a plan was created for its removal, which resulted in Massimino being instructed to snap the handrail off.

This plan – simulated by engineers at the Goddard Space Flight Center as the plan was being created – called for 60 lbs of pressure on to the handrail to snap it off. This was a potentially dangerous operation on orbit, as the force will be translated through to Massimino, and debris risks were threatened via the unorthodox removal.

However, the plan worked, with no debris of note liberating from the hardware, and all 111 fasteners were successfully removed, freeing the electronics box for its repair to take place.

Due to the task taking over two hours longer than expected, the installation of NOBL 8 – designed to help maintain thermal conditioning of the telescope – was deleted from EVA-4. The spacewalk ended after eight hours and two minutes, the six longest EVA in history.

Following these activities, Hubble was rotated to the +V3 forward position in preparation for the fifth and final EVA dedicated to servicing of the telescope.

The EVA – as with all flight days – was covered live on the LIVE UPDATE pages – link above.

TPS Assessments:

Following a spectacular launch on Monday afternoon, Atlantis’ Flight Crew went right to work inspecting their vehicle for any damage incurred during liftoff.

During the early hours of FD-1 (Flight Day 1), the crew used the End Effector (EE) camera on the Shuttle Remote Manipulator System (SRMS) to observe the upper surfaces of the crew cabin – one of the survey techniques added to the STS-125 timeline due to the lack of R-bar Pitch Maneuver photography from the International Space Station crew.

A report from the Mission Management Team (MMT) on FD-2 confirmed that this inspection was complete, stating “upper canopy inspections complete. No damage failing criteria was identified.”

Thus, the DAT was able to clear this portion of Atlantis’ TPS (Thermal Protection System) for reentry while the crew performed the standard FD-2 inspections with the OBSS (Orbiter Boom Sensor System).

Also noted by the MMT report on FD-2 was a clean T-0 umbilical area on Atlantis, no obvious damage to the lower TPS acreage (another inspection technique added to STS-125 from nominal Space Station missions), and a damaged area of chine tiles.

a312The area of chine damage was most likely caused from an impact by a piece of liberated Ice Frost Ramp foam from the External Tank (ET-130) during launch, an impact which was recorded by the Wing Leading Edge sensors 106-seconds after liftoff.

A FD-3 TPS DAT summary presentation – available on L2 – noted that the OBSS inspections showed that the largest area of chine damage was 2.64” x 1.2” with an imagery uncertainty of +/-0.25”. Initial estimates place the angle of ET foam impact on the chine tiles at <10-degress.

Nevertheless, while all the chine area damage sites are within acceptable margins for a safe performance of the damaged tiles during reentry, the DAT requested an analysis of the interaction effects of the damage sites.

This assessment was called for due to the close proximity of the damage sites and their potential effects on heating increases to downstream chine damage sites from upstream chine damage sites, heating increases on the RTV bond line interfaces, and structural temperatures/margins.

For a better understanding of the region in question, the DAT summary included information on the underlying structures near the chine damage locations.

“Damage sites are located in close proximity to structural splice between honeycomb leading edge panel and lower skin panel,” notes the DAT presentation.

The area underneath the TPS chine also includes an internal rib structure with upper and lower caps and vertical posts at the leading edge and “thinwalled aluminum tubes.”

a62“Upper and lower skins are ‘skin/stringer’ configuration. Stringers run in the forward and aft direction.”

The first analysis conducted at the request of the DAT was a test of the downstream heating effects of upstream damage sites.

For this analysis, engineers examined chine damage site -002 (the forward most significant damage site) and its effect on downstream, adjacent damage site -001.

Engineers, using wind tunnel testing, determined that downstream “heating from -002 will have decayed by 85 percent” by the time it travels 0.66” – the distance between chine damage sites -002 and -001.

Furthermore, the damage cavity interaction analysis yielded positive results for reentry.

“Aeroheating team assessed impacts of the -002 damage site and the transition time and bump factors for the -001 damage site,” notes the DAT summary. “Size of the -002 damage site was not large enough to impact transition time.”

Nevertheless, the DAT noted that some minor influences could occur to bump factors downstream of -002 during reentry. However, these influences would be of no concern for crew or vehicle safety.

Additionally, the assessment team also analyzed the impacts to the RTV bond line margins due to the chine TPS damage.

Again, this analysis yielded positive results for reentry, with the most badly damaged tile maintaining a safety margin of +1.75. Similarly, the impacts on the structural margins of Atlantis are well within the design limits of the vehicle.

The analysis team, using a conservative dataset (assumed a 25 percent greater impact angle than that observed on orbit), determined that the maximum structural temperature Atlantis will experience due to the damaged chine tiles would be 190 degrees F – well below the 350 degrees F maximum temperature limit.

As such, structural integrity of the vehicle has not been compromised in any way because of the chine damage.

a56Furthermore, the DAT also relied on past flight experience and flight history with underbelly and chine tile damage.

As was seen during the analysis of the tile damage on STS-118 in August 2007, engineers poured through the vast flight history of Shuttle TPS damage to determine what ground crews can expect once Atlantis’ returns home.

Past examples of significant chine damage occurred on STS-41G, STS-27, STS-73, and STS-87 – with the worst such damage being Challenger’s STS-41G mission.

On that particular flight, the damage area extended for eight TPS tiles and had a maximum depth of 1.5”. Challenger returned without incident and the effected tiles were replaced during the vehicle’s turnaround processing flow.

Finally, further analysis of the imagery obtained during the FD-2 inspections showed that all damage sites identified passed the inspection criteria agreed upon before the mission.

a71No lower surface protrusions were observed, the ET doors were verified closed with positive seal margins, no protruding gap fillers or lifted blankets were observed on the OMS pods, and the upper LESS carrier panel was verified to have all its tiles in place.

As was noted prior to the mission, the base heat shield around the main engines and OMS pods was not imaged due to length limitations of the OBSS.

The inability to image this area of the vehicle was accepted during pre-flight meetings and reviews mainly due to the fact that the base heat shield experiences higher temperatures during ascent than it does during reentry.

Furthermore, with the pick-up inspections of a 40-tile region missed during the FD-2 inspections, the DAT team has officially cleared Atlantis’ TPS for reentry.

Atlantis’ crew will re-inspect the vehicle’s Wing Leading Edge and Nose Cap RCC panels on FD-9 to ensure that no damage occurred after their initial inspections.

L2 members: Documentation – from which most of the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.

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