DAT role – Examining every inch of Endeavour’s TPS for safe return

by Chris Gebhardt
The engineering review of Endeavour’s Thermal Protection System (TPS) – which earlier this week cleared the orbiter for re-entry – has once again shown the amazing level of detail the Damage Assessment Team (DAT) engineers undertake to ensure orbiters can return home safely. 

Overall, Endeavour’s TPS is very clean, owed in part to the superb job by Michoud Assembly Facility (MAF) workers in preparing Endeavour’s External Tank (ET) for flight – a tank which appears to have had the cleanest flight in the history of the program.

Nevertheless, there were a total of 13 Areas of Interest (AOIs) identified during Endeavour’s stunning Rbar Pitch Maneuver (RPM) that were referred to the Damage Assessment Team (DAT). These areas consisted of dings to Endeavour TPS tiles as well as frayed and protruding gap fillers and thermal barriers.

Of particular interest to imagery experts were the RPM photos of Endeavour’s two ET umbilical doors. The doors cover the umbilical wells – the area at the aft end of the shuttle where it connects to the ET, thus allowing propellants to flow from the ET to the Orbiter’s three main engines during liftoff – after ET separation to provide complete thermal protection for the vehicle during reentry.

As such, one of the goals of the RPM photography is to ensure that the doors are indeed fully closed for entry.

“ET doors are closed. No paint stripe visible,” notes the DAT presentation available for download on L2.

The first area of TPS damage noted by the DAT presentation pertains to a small area of tile on Endeavour’s chine. “Observed chine damage – failed standard OOIC due to small SIP footprint,” notes the DAT presentation.

A standard mission specific end-to-end analysis was conducted, revealing that the area of damage on the irregularly shaped tile is acceptable for reentry – with maximum structure temperature in the area coming nowhere near the maximum allowable limit.

Similarly, RPM imagery identified damage to one of Endeavour’s tiles near the port ET umbilical door. The damaged area is approximately 2.3” in long and 0.8” wide. The area is not a safety threat to the tile’s bond to Endeavour, maximum allowable temperature to structures, or downstream areas of the vehicle.

In addition to these areas of tile damage, there are also a few frayed and protruding gap fillers on the underside of Endeavour.

“Port MLGD (Main Landing Gear Door) gap filler fray (has a) max height around 0.4 (inches),” notes the presentation. The fray is within previous flight experience. STS-114 had a frayed gap filler with a maximum height of 0.8 inches.

The current protrusion does not present any downstream structure concerns and the gap filler continues to seal the sidewall of the adjacent tiles.

Moreover, there are two protruding gap fillers on Endeavour’s right hand elevons. One protrusion is on the inboard elevon – with a maximum height of around .75” – and the other protrusion is on the outboard elevon. No height measurement is available for the outboard elevon protrusion.

The presentation notes that the “elevons (are) considered fully turbulent at Mach 25. No damage concerns downstream of protrusions.”

In addition to these blemishes, the RPM photography noted two AOIs near a couple of Endeavour’s left-hand Reinforced Carbon Carbon (RCC) panels.

Two Horse Collar Thermal Barrier protrusions were identified at left-hand RCC panels 15 and 18.

“Aeroheating evaluation of a 0.25” protrusion at Panel 15/18 predicts a Mach 16.8 boundary layer transition,” notes the DAT presentation.

Previous testing in December of last year shows that the protrusion at LH RCC panel 18 will not cause damaging heat levels to adjacent RCC panel 19.

According to the DAT, “Generic BLT (Boundary Layer Transition) assessment using cert heating for LESS at Mach 18.6 demonstrates temperatures at Panel 19 will not exceed 2600F.”

The protruding Horse Collar is neither a threat to downstream TPS and vehicle structure nor a concern for partial transition.

Also captured by RPM photography is a chipped tile on the Base Heat Shield, occurring in virtually the same location as damage observed on STS-124 in June.

“Broken corner approximately 2.5” long by 1.0” wide,” notes the presentation. This damage is not of high concern as the Base Heat Shield experiences higher heating loads during ascent than descent.

“Open hole analysis performed during STS-124 showed that the structure temp is less than 350 degrees on descent.” Additionally, post-flight assessments of Discovery (STS-124) yielded no evidence of heat damage from reentry.

Finally, the DAT presentation documents the damage observed to Endeavour’s two Orbital Maneuvering System pods. In all, four AOIs have been identified.

Three of these areas are chips to the white TPS tiles – all of which are no concern for entry. Additionally, there was one gap filler protrusion on the port OMS pod.

Since Return to Flight operations, a total of 16 protruding gap fillers have been identified on the OMS pods during flight.

While the protrusion length of the gap filler on Endeavour is beyond the limit previously experienced during flight, the DAT presentations states, “The protrusion height is comparable to previous flights (STS-117 and STS-118). Condition of the gap fillers post flight were unchanged. Engineering has high confidence that the current gap filler remains bonded in place (for STS-126).”

However, in the unlikely event that the gap filler dislodges during entry, the DAT has requested a complete analysis of the gap filler’s likely trajectory away from the vehicle. Mainly, a liberation during reentry is unlikely because the gap fillers experience three times more load during ascent than they do on descent.

“Entry Attitude and RSB (Rudder Speed Brake) schedule preclude impacts below Mach 3 for Nominal Entry Trajectory,” notes the presentation. As of now, it appears that a potential impact to the Vertical Tail and RSB would only occur between Mach 6 and Mach 3.

The possibility that the gap filler could strike the OMS pod or OMS engine has not yet been ruled out. As always, the gap filler’s potential path away from Endeavour, should a liberation occur, is dependent on the time of liberation and Endeavour’s position in the reentry sequence.

In the event that the gap filler does hit and damage Endeavour’s RSB, analysis shows that the RSB would still provide acceptable performance with a damage area of 12.5 square feet.

With this comprehensive review, the DAT has cleared and closed all TPS items on Endeavour with no dissenting opinions.

L2 members: All documentation – from which 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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