STS-132: DAT Clear Atlantis’ Thermal Protection System for Landing

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As Atlantis’ crew complete final checkouts for entry and arrival back home at the Kennedy Space Center, the Damage Assessment Team (DAT) have successfully concluded their thorough review of Atlantis’ fragile Thermal Protection System (TPS) – clearing the vehicle’s heat shield for entry. Two landing attempts available for Atlantis on Wednesday morning – the first at 0844 EDT and the second at 1023 EDT.
TPS DAT Overview:

The Mission Management Team (MMT), with information from the DAT and OPO (Orbiter Project Office), officially cleared Atlantis’ TPS for reentry – following a long and arduous process of gathering all the necessary inspection data via Atlantis ‘Orbiter Boom Sensor System (OBSS), R-bar Pitch Maneuver (RPM) photography from ISS, and EVA (Extravehicular Activity) photography opportunities.

In all, the complete inspection of OV-104′s TPS revealed an extremely clean vehicle – aided in large part to the excellent performance of Atlantis’ External Tank which experienced very minor foam liberations during the May 14 launch of STS-132/ULF-4.

“Imagery review complete: 4 items in TIIMS were evaluated,” notes the short, 19-page comprehensive DAT presentation – available for download on L2.

In particular, the DAT identified one area of OMS (Orbital Maneuvering System) Pod blanket damage, two lower surface (underbelly) tile damage locations, and one AMES Gap filler protrusion.

Furthermore, no damage was identified at Atlantis’ port and starboard T0 umbilicals.

The presentation moreover notes the tremendously successful effort by Atlantis’crew to obtain all TPS imagery data in a timely manner given the OBSS’s PTU’s (Pan-Tilt Unit’s) inability to maneuver to the proper positions during initial TPS inspections on FD-2 because of an errant cable.

“Missed port imagery Upper LESS carrier panels 7-9 and Chine tiles, ~150 upper surface tiles.”

The errant cable was removed as an obstruction to the OBSS’s PTU during EVA-2 and all late-inspection imagery was obtained without issue on FD-11.

However, Station and EVA assets that were used to complete the necessary coverage of the TPS tiles by FD-5 of the mission, which resulted in a DAT and OPO recommendation to clear the TPS (save the RCC – Reinforced Carbon-Carbon – panels) for reentry.

STS-132 Specific Articles: http://www.nasaspaceflight.com/tag/sts-132/

Specific DAT TPS Clearance Review:

As is standard in the post-Columbia era, Atlantis’ R-bar Pitch Maneuver (RPM) just prior to docking with the International Space Station provided valuable data on the health of her underbelly TPS – specifically, verification of ET Umbilical door closure.

“Lighting and viewing angles sufficient for assessment of forward edge,” notes the presentation. The paint stripe used to verify ET door closure was – as expected – not visible from RPM photography, confirming Atlantis’ computer data that the ET doors are closed and properly sealed for entry.

“Closure indicators not visible in imagery and no evidence of off nominal steps or thermal barrier protrusions [are present]. ET Doors are closed.”

Interestingly enough, all TPS damage locations for STS-132 are located on the port (or left-side) of Atlantis, with the two tile damage areas located on the port Nose Landing Gear Door (NLGD) and just outboard of the left-aft corner of the Port ET Umbilical door (and just inboard of the forward, starboard corner of the inboard Elevon), respectively.

For the damage location on the NLGD, the DAT summary notes that the tile in question is 1.7-inches thick with a missing “putty repair” of 1.2-inches in length, 0.6-inches in width, and 0.1-inches in depth.

Resulting damage dimensions are estimated at 1.89-inches in length and 0.49-inches in width with “photogrammetry determined depth [at] 0.2-inches.”

This damage area is of no concern for entry and was cleared using P-DAT.

The second tile damage location was similarly cleared using the Port Wing Tile Damage Analysis Tool, which includes the Peer Reviewed categories of Baseline Heating, Cavity Heating, Thermal Analysis, Tile Stress, and overall Stress.

These tools yielded “high” confidence in the damage analysis to the tile in question. Overall, this tile is 1.15-inches thick with photogrammetry damage estimated at 2.25-inches long, 0.97-inches wide, and 0.15-inches deep (+/- 0.05-inches).

However, clearance of this tile damage could not be had without consideration of the upstream AMES Gap filler which could affect the airflow and local heating environment around the damaged tile.

“Thermal analysis performed with STS-132 EOM M18 (Mach 18) heating due to upstream AMES gap filler,” notes the presentation.

For this protruding AMES Gap filler, “Thermal bounded by previous analysis that showed transition at M=18 (keq=.25) acceptable in this area,” notes the DAT summary.

Furthermore, a “Partial transition load comparison based on aeroheating heat loads” was performed as well.

A reference point to a previous analysis “which used partial transition load cases” developed for nonstandard processes was also used to clear the gap filler itself.

In all, the AMES Gap filler is protruding 0.24-inches (+/- 0.15-inches) in a non-reworked area of Atlantis Gap fillers located inboard of the Port Main Landing Gear Door (MLGD).

Since most of the Gap filler remains in the gap, liberation during reentry is not expected as ascent loads on the Gap filler are “several times higher than entry loads.”

Furthermore, considerable flight history with protruding Gap fillers is on file and was used to clear the Gap filler.

In fact, a chart of the historical Gap filler protrusions since Return to Flight in July 2005 showed three Gap filler protrusions with comparable or greater protrusion Height Maximums (Hmax); STS-124: Hmax= 0.23-inches; STS-120: Hmax= 0.29-inches; and STS-121: Hmax= 1.04-inches.

However, given the range of height uncertainty (+/-0.15-inches) with this particular Gap filler, it is possible that the Gap filler could trip the Boundary Layer at Mach 18 during reentry, creating a potential off-nominal heating effect on downstream areas of the vehicle, most notably the damage site of damaged TPS tile #2.

Therefore, while the Gap filler itself was cleared for entry, its affect on the lower surface aft TPS tile damage location had to be evaluated before that TPS tile damage site could be cleared for entry.

In all, it was determined that if the boundary layer is tripped at March 18 by the protruding Gap filler, the structural area directly below the damaged tile would see temperatures below 350˚F with just a small potential RTV overtemp: 2.79in2 normal flow.

Maximum structural temperature expected in the region is 246˚F, only 6˚F above the standard 240˚F baseline temperature.  Therefore, positive structural margin is maintained and no concern exists for the damaged TPS tile due to upstream affects from the protruding Gap filler.

Finally, the last area of Atlantis’ TPS to come under scrutiny from the DAT was a partially frayed outer layer on one FI blanket on the Port OMS pod.

The blanket in question is 1.6-inches thick and all blanket insulation remains intact – thus, thermal performance is maintained. Liberation is not expected due to previous experience with “frayed outer covers and loose patch repairs.”

Currently, it is believed that the blanket was damaged on the ground during final vehicle configuration for flight as the blanket is located next to a door that is removed each flow for access inside Atlantis.

In all, the extremely clean nature of Atlantis’ TPS tiles has led to the unanimous decision by the OPO and DAT to recommend the TPS tiles and blankets cleared for reentry.

Final clearance of the entire TPS (including the all-important RCC panels and nose cap) was approved earlier this morning by the MMT following a complete review of the RCC Wing Leading Edge panels and nose cap imagery from the OBSS late-inspections yesterday.

Thus, weather permitting, Atlantis has been cleared for her final landing at the Shuttle Landing Facility (SLF) Wednesday morning.

If all goes according to plan, Commander Ken Ham will ease Atlantis onto runway 33 (the southeast to northwest approach to the SLF) at the Kennedy Space Center at 0844 EDT, capping a near 12-day flight for Atlantis and the STS-132/ULF-4 mission.

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