Endeavour preparing for the trip home – TPS clearance overview

Endeavour preparing for the trip home

        After nearly 12 days of docked operations with Space Station Alpha, the Space Shuttle Endeavour, NASA youngest Shuttle orbiter, departed the international outpost to begin her final journey home. Thanks to the superb work by her flight crew and her thousands of support staff on terra firma, Endeavour’s Thermal Protection System (TPS) was formally cleared for reentry thanks in large part to Endeavour’s former Orbiter Boom Sensor System.

Post Undocking TPS Damage Assessment Team (DAT) clearance:

       Following the preliminary clearance of all areas of Endeavour’s TPS on FD-5, less the multi-tile gouge area between Endeavour’s Right Hand MLGD (Main Landing Gear Door) and Right Hand ET Umbilical door, Endeavour’s flight crew was instructed to proceed with a focused inspection of the damage location.

         Prior to that, all Damage Assessment Team (DAT) assessment of OBSS (Orbiter Boom Sensor) data and FD3 RPM (R-bar Pitch Maneuver) photography from the ISS confirmed that Endeavour’s two ET umbilical well doors were closed, that there were no upper flight surface protrusion of any kind, and that six of the seven lower damage sites had been cleared without the need for a Focused Inspection (FI).

Following the FI of the three-tile damage location, lovingly dubbed “The Maine” damage because of its remarkable resemblance to the U.S. state of Maine, a detailed characterization of the damage was compiled.

        From this, it was confirmed that all tile material was still intact in all areas with no exposed filler bar material; no cracks were identified; the dark area of TPS as seen from RPM imagery was an area of abrupt damage depth change – as expected; all thermal stress assessments revealed no structural overtemp issues for reentry; a “small area” of TPS bondline overtemp would occur during reentry but is acceptable for reentry due to its distribution over three tiles; all TPS and structural margins were well within safety limits for reentry.

        Thus, the TPS DAT unanimously recommended clearing Endeavour’s entire Thermal Protection System for reentry in emergency return cases and for nominal EOM (End of Mission) reentry pending the completion of the Docked Late Inspection (DLI).

Detailed Focused Inspection damage site clearance overview:

    Following the detailed and fascinating Focused Inspection of the “Maine” damage site, photographic and laser measurement data revealed that the damage was 0.89 inches in depth, +/-0.04 inches, and remained above the filler bar.

This means that tile margin exists in all areas of the damage cavity, or what the post-FI inspection presentation – available for download on L2 – classed as a “dense layer.” (View Anaglyph slide left with 3D glasses).

In all, the damage location is 0.89 inches in depth, 2.95 inches in length, and 2.43 inches in width.

In fact, the TPS clearance presentation notes that there are very few areas of missing material and no cracks radiating outward or downward from the main damage site.

Furthermore, views of the damage location obtained from the OBSS’s Laser Dynamic Range Imager (LDRI) confirmed that sections – but not all – of some AMES gap fillers were missing, indicating that the “Impactor had enough energy to damage multi-layer AMES gap filler.”

However, it appears that the AMES gap fillers had an unintended positive consequence as “Adjacent tile damage size [was] reduced by the presence of AMES gap filler.”

In comparison to a similar tile damage event on the STS-118/Endeavour mission, the STS-118 damage was 3.48″ x 2.31″ x 1.12″, was located at Xo 1260 Yo 123 Zo 269, and carried a tile depth of 1.12 inches.

STS-134/Endeavour’s damage was 3.22″ x 2.49″, was located at Xo 1243 Yo 106 Zo 267, and was located on a tile with a thickness of 1.04 inches.

Based on analysis conducted during the STS-118 mission, that mission’s damage was found, based on on-orbit information analysis, to have a Mach 16.7 Boundary Layer Transition (BLT) time. STS-134′s damage is predicted to have a Mach 12 (nominal) BLT time.

All STS-134 FI damage was further found to have baseline aeroheating, BLT, Boundary Layer Wedge, Cavity Heating, Thermal Analysis, Tile Stress, and Stress indicators in Model Category “A” – indicating that “Baselined model used within model limitations or intended use.”

A maximum temperature calculation for the FI damage locations was created, and “Due to flow orientation, assessment [was] performed in both the aligned and cross-flow orientations of the simplified cavity,” notes the TPS DAT clearance overview presentation.

Based on these parameters, a maximum structural temperature of 219-degrees F and a maximum RTV bondline local temperature of 1,194-degrees F are expected during entry on Wednesday morning. The maximum structural temperature allowed is 350-degrees F, leaving high structural margin.

Nonetheless, the total RTV bondline temperature does exceed nominal limits; however, RTV bondline temperature “over 625°F is limited to 6 inches squared distributed over 3 tiles.” For the damage site on STS-134/Endeavour, the aligned flow max temperature is predicted at a modeled rate of 5.75 inches squared with an actual in-flight temp on the critical tile of 4.06 inches squared – both meeting the design requirements