Tiger Team set up to find root cause of RCC concern

by Rob Dale

NASA has set up a special Tiger Team to investigate the root cause of a potential issue with the shuttle’s Reinforced Carbon-Carbon (RCC) leading-edge panels, which was highlighted at STS-120’s Flight Readiness Review (FRR).The process will work in two stages – near term – with opening results expected ahead of next year’s STS-123, and long term – with the full findings to be presented in nine to 12 months time.

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The issue was uncovered following the STS-114 mission, after a new method of inspecting the panels was developed after the loss of Columbia.

Called Flash Thermography, the results produced anomalous readings near the junction between panels along the edge of the T-seal. The panel was not damaged, but based on NESC (NASA Engineering and Safety Center) concerns about possible spalling, where small pieces could flake off and possibly compromise the integrity of the heat shield, that panel was replaced.

Objections were raised during preparations for STS-117, and concern escalated prior to STS-120 when the NESC recommend replacement of three panels based solely on the thermography testing.

The RCC panels have always been examined with x-rays and ultrasound techniques when delivered from the factory.

Between flights, a technician physically – and painstakingly – tapped every inch of over forty panels, watching with a 10-power jeweller’s eyeglass, subjectively judging the response of the ‘tap.’

The recently developed – and not fully certified – thermography technique utilizes a split-second flash from a heat-lamp. As the panel cools, any problems below the surface (manufacturing defects, air pockets, etc.) would be visible as changes in the cooling rate. Roughly 320 pictures of each wing and 85 pictures of the nose cap give NASA over a terabyte of infrared imagery data, which is then reviewed for sub-surface flaws.

Those images are converted into digital line scans, using the brightness values of individual pixels. An Indication Amplitude, greater than 0.2, was utilized as a number to be ‘concerned’ with. That particular number is fairly arbitrary, with data showing Atlantis has one panel (16L) with an IA of 0.31.

The Silicon Carbide (SiC) coating has only broken off two panels during shuttle flights. STS-102 saw the loss of a 2.0” x 0.3” piece – while STS-103 was observed to have lost a 1.6” x 0.4” piece.

Inspections didn’t show any heat damage, which indicates that the pieces came off after re-entry, during higher aerodynamic pressures and after the point where the SiC coating is needed. Any piece that might break off during launch would be visible during the FD2 inspection process, and could be repaired via the NOAX tile repair capability.

Despite FD2 and FD14 OBSS (Orbiter Boom Sensor System) imagery and data showing no issues with Discovery’s panels during STS-120, NASA is determined to gain a better understanding of the problem.

This has led to the formation of a Tiger Team – the name for a high level investigation team – formed to determine the root cause of the coating liberation, establish pass/fail criteria for areas where the coating is possibly compromised, and recommend a replenishment strategy to USA for obtaining spares.

The SiC Liberation (SiCL) team consists of personnel from various NASA centers and contractors, and a wide range of discipline experts. NESC will also assist with an independent assessment.

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A 62 page presentation (on L2) presented to shuttle management last week, outlined the team charter, which the Tiger Team is tasked with, ‘determining, to the extent possible, the mechanism and root cause of RCC SiC coating liberation at the slip side joggle areas,’ noted the presentation.

‘Through inspection, test and analysis establish pass/fail criteria for suspect coating areas that allow removal and replacement of Orbiter RCC before flight.

‘Identify threats to spares and recommend a replenishment strategy to USA logistics based on the root cause identification and inspection strategy.’

Unfortunately there is a very limited supply of RCC joggle material, and even fewer pieces with high (>0.2) IA scan values. So structural modeling, combined with thermo-mechanical tests using small ‘joggle coupons’ will be needed.

The highly-detailed fault tree – which even noted evaluations into the possibility of foreign object damage to the shuttle while on the cross-country trip back from west coast landings – includes the following for potential causes: ‘Fabrication Defects. Ferry Flight Operations. Flight Environment. Launch Pad Environment. Repairs. Refurbishments. Ground Handling.’

The investigation is expected to take nine months to complete, and since the shuttle will continue to fly as this the evaluations are ongoing. Some results that can be determined rapidly are expected to be presented at the STS-123 OPO (Orbiter Project Office) FRR on January 24th 2008.

The preliminary goal is to identify any panels that need replacement within a month of the vehicle entering the VAB (Vehicle Assembly Building), and setting a final decision point around two weeks prior to rollout.

Currently, as listed in the presentation, the current candidates for the possible contributing factors of the RCC issue are wide-ranging.

‘Subsurface oxidation resulting in loss of SiC coating adherence to the C/C substrate. Pressure build-up below the SiC coating as a result of volatiles formed from the Type A/TEOS in the C/C voids and sealed cracks.

‘Modifications in the refurbishment cycle; Type A being applied before TEOS, filling the voids and volatiles formed (similar to 2). Thermomechanical stresses/fatigue between the SiC coating and C/C substrate resulting in degraded coating/substrate adherence.

‘Thermal cycling (fatigue) while on-orbit and thermal differential during entry between RCC panel, attachments and front spar. Increased heating in the joggle region due to gap/step dimensions being out of specification. Fabrication induced separation between SiC coating and C/C substrate as a result of convoluted plies during initial lay-up.’

Any combination of the above, the presentation noted, is a possible contributing factors that could lead to a SiC liberation from the RCC.

L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, updated live.






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