Another status update has been issued by the NASA Tiger Team which is investigating the cause of a potential problem with the orbiter’s Reinforced Carbon-Carbon (RCC) leading edge panels.
About a year ago this group was established to determine the circumstances behind an issue where the RCC’s Silicon Carbide (SiC) coating can flake away, and to find out what methods should be used to test the panels and how to go about getting replacement panels for future missions.
The focus of this team is based upon concern about a loss of critical SiC coating, which could result in the loss of crew and vehicle. To reduce the risk, the Space Shuttle Program (SSP) inspects, removes and replaces panels with a line scan magnitude (Wf) of 0.2. That criteria has resulted in 10 panels being removed from the fleet.
Six new panels have been procured to maintain a spare inventory of at least one panel per location. Three have been ordered, the first of which (10R) is expected to be delivered in December. A mid-summer progress report detailed tests that had been completed along with early results while the investigation continues ahead of a Technical Interchange Meeting (TIM) scheduled for October 28-30.
Fifteen of 21 scheduled tests have been completed at this point, with five more in progress and one additional (Hot Gas Testing) still in the planning stage. The fracture mechanics summary has shown that on-orbit cold soak will not propagate any existing defects, but it’s possible that forces from re-entry could exceed the capability of the RCC coating. Three-dimensional fracture analysis is in work to reduce the conservative nature of that test.
Some of the arc jet testing has resulted in major changes in panels’ Wf values. Two samples of panel 10L have show dramatically different behaviors than other test samples and flight history, with one climbing from a Wf of 0.3 to as high as 1.3.
Changes in temperatures from a so-called “Stress Free Temperature” have been shown to cause stress on the panels. At this SFT (usually around 1350 degrees F, but can range from 700-1700 degrees F) the craze crack surfaces are just touching. There are no stresses in the SiC coating or at the SiC/C-C interface.
If the temperature is below the SFT (for example, during on-orbit cold soak) then cracks can open because the SiC contracts at a faster rate than the C-C, generating stress in the interface – a mode II fracture. If the temperature is higher than the SFT (during re-entry or panel refurbishment) the SiC expands at a higher rate than the C-C, so the islands are forced together and upward – mode I.
The fault tree for a root cause mechanism has been about 40 percent completed. The hypothesis is related to the local curvature along the joggle, with indications that stress induced from re-entry temperatures may exceed the material strength. Damage at or near the SiC/CC interface has been observed to grow during arc-jet and thermal tests.
Variations in the material could explain why damages are not more widespread, and subsurface damage near the interface is a significant contributing factor to the coating falling off (spallation.) However at this point, the exact mechanism leading to spallation, or the conditions under which it can occur, have not been identified.
The thermal loads of re-entry do not appear to be sufficient to cause spallation based on arc-jet conclusions, so questions remain if vibration and pressure loads are enough to spall the coating.
An additional 190 re-entry cycles are to be simulated in arc-jet testing, with a statistical evaluation of data to determine the safe-to-fly limit. Analysis of defect flight history and test data will help provide confidence in the expected growth rates of defects. Global stress assessment sensitivity cases are also to be completed.
Microscopy of flown and unflown joggles will continue, in order to support closing additional fault tree blocks. It may help explain the cause of reduced material strength by looking at the coating thickness, porosity of the material, or other parameters.
The stress free temperature (SFT) needs to be refined, and 2D/3D fracture mechanics need to be understood. The 3D models will reduce conservatism of the current 2D analysis. The refurbishment fault tree block is hoped to be closed with arc-jet and multi-parameter tests of refurbished joggles.
RCC joggle testing at Sandia National Lab using acoustic and infrared techniques should improve the understanding of when joggle damage initiates or grows, which would help determine the criticality of damage or spallation.
Three existing data points show damage occurs at peak heating (IR data analysis, arc-jet thermal response, and “off-gassing” during arc jet tests.) If SiC falls off during re-entry, the Carbon-Carbon is exposed for less than a full re-entry thermal cycle, so the time and location will dictate the extent of oxidation and/or burn-through time.
The arc jet needs to be tested to see if the environment matches the actual state from entry interface to 1100 seconds. If it is not representative, then wind tunnel testing of test coupons might be necessary at Ames Research Center or Texas A&M to simulate pressure loads.
It is estimated that the ascent vibration is an order of magnitude greater than re-entry, but that needs to be formally assessed. A determination then can be made of how many equivalent re-entry vibration cycles panel 16R has been exposed to during testing. They may then need to clean that panel and re-vibe it with an existing 0.46 Wf defect.
Analysis will be performed to determine local pressure effects on a degraded piece of joggle SiC, as aerodynamic loads may be sufficient to spall a chip during re-entry. Along the same lines, hot gas testing of a panel with a large defect (over 1.3 Wf) might help determine what loads are necessary to induce spallation.
Arc jet cycles will continue on specimens until spallation occurs, and other pieces will be vibration tested until SiC spallation.
Much progress has been made in understanding the mechanism for SiC damage initiation and growth. The current focus is on the timing and cause for SiC spallation. At the TIM (Technical Interchange Meeting) later this month, the team will review data, refine a forward plan for root cause investigation, and discuss raising the 0.2 Wf “go/no-go” limit for RCC panels.
All testing and analysis is expected to be completed by mid-December, with peer review scheduled for the late February/early March 2009 timeframe.
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.