The Tiger Team set up to investigate the root cause of a potential issue with the shuttle’s Reinforced Carbon-Carbon (RCC) leading-edge panel has released some of their preliminary findings.The concern – which was originally highlighted at STS-120’s Flight Readiness Review (FRR) – is with the joggle surface spalling, where small pieces flake off and possibly compromise the integrity of the heat shield.
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RCC Investigation Latest:
The group was established last fall, led by a three member group from NASA, USA, and Boeing, and asked to determine the mechanism and root cause of RCC Silicon Carbide (SiC) coating liberation at the slip side joggle areas.
Its charter: to determine the mechanism responsible for the coating liberation, to establish pass/fail criteria for Orbiter RCC, and identify a plan to replenish the stockpile for future missions.
The current situation involves four RCC components being removed from service: Endeavour’s nose cap – which was damaged during refurbishment, and a panel following each of STS-102, STS-103 and STS-114’s flights.
Ten more panels have been pulled offline due to defects that exceed the 0.2 line scan critical limit.
Six of 17 planned tests have been fully completed, with the remaining finished and awaiting final analysis or in progress at this time. They range from “thermomechanical testing of joggle in argon” to “microscopy of RCC joggles” and “vibro-acoustic testing of RCC panel ascent / re-entry loads.”
Many samples have been analyzed so far, including material from Columbia. All panels with line scan values greater than 0.05 did show subsurface separations (except for the Columbia pull-test material.) There is a correlation between line scan amplitude and separation width, but there is no connection to separation span.
Separation spans average around one-third of an inch long, with the range between 0.23 and 0.41 inches. Oxidation is not a major factor in the coating separation issue, and the separation was not enough to cause pieces to flake off.
Six joggle test articles have been exposed to a 2800 degrees F arc jet environment. One panel did develop a coating interface separation, when it had not shown a detectable underlying defect before testing.
Thermal cycling was found to increase the line scan amplitude, but even for panels with very high line scan indications – no coating spallation ever occurred during any arc jet test performed.
Work next month will test panels at a lower temperature (2500F), and to see if a spallation event can be induced on the article with a very high 1.05 line scan indication.
In mid-September, they will test a panel that has been through two refurbishment cycles to see what effect – if any – that process has on the formation or growth of subsurface defects.
Vibration-acoustic tests were performed to simulate loads placed on the joggle during liftoff, ascent, and re-entry. Panel 13L (line scan value of 0.11) was a pathfinder sample for panel 16L (line scan 0.42) which will be tested later. The test was performed with nominal vibrations and a +3dB sound environment – to cover uncertainties in the surrounding environment.
Post-test examinations showed no material change before and after the vibration test, and no visual difference following the +3dB test – with a full NDE (Non Destructive Evaluation) review still ongoing. Strain gage results showed higher than predicted strain, which was classed as quite significant in some cases.
In general, damage will begin when the local stress exceeds the local material strength. Due to curvature of the joggle panel, the local stress induced from elevated re-entry temperatures may exceed the material strength.
Further, the RCC material has numerous inherent features such as porosity and microcracks, so damage growth will occur when local forces exceed the local material toughness at the defect site.
Forces can exceed that limit at both high temperatures during re-entry, and with low temperatures on-orbit.
Growth of damage along the joggle has been observed as a result of re-entry temperatures, while expected variations within the RCC material could explain why the damage dooesn’t spread.
Delamination in the joggle regions has been observed directly adjacent to areas where spallation has occurred on flight panels, and a decrease in coating adhesion has been experimentally measured when delamination is present in the joggle.
“It is assumed that damage (delamination) at or near the SiC/CC interface is a significant contributing factor to the occurrence of spallation; however the exact mechanism leading to spallation, or the conditions under which it can occur, have not been identified,” noted the associated presentation.
For the primary objective, root cause determination, testing has shown the following to have a negligible role in spallation and joggle damage: ascent loading, MMOD and atomic oxygen exposure, landing loads, ferry flight loading, pad contamination, panel age, ground handling, vacuum heat cleaning and removal during refurbishment, and local step/gap augmented heating.
The fault tree closure rationale is 30 percent complete, with future work including: testing with on-orbit temperatures, investigating internal pressures from volatiles remaining after refurbishment process.
More cases will be used to finalize global stress assessment, microscopy of joggles – flown and unflown – more re-entry mission profile testing, examining flight history to explain why no spallations are seen in the lock side joggle.
Engineers will need to explain panels 8L, 10, 8R, and Endeavour nose cap spallations, in order to determine if the root cause is really known, and assess loads that will cause spallation after coating separation develops.
The next objective, establishing pass/fail criteria for suspect areas, is expected to be completed by October 1, with safe-to-fly limits established through remaining testing.
Two forms of measuring are used, a 13mm line scan and 25mm line scan. Up to 25 percent variations have been found when using the 13mm data, with just a minimal error percentage when using 25mm data, so the latter set should be used for watching flight-to-flight trends.
Sources of that variation are from within the RCC material, related to the orientation of the test hood, the angle of the hood causing differences in illumination, and where the actual weakness is placed in the field of view.
Forward work in this objective includes: 2D/3D fracture mechanic modeling, thermal testing in arc-jet and Langley’s thermal facility to provide defect growth rate data, assessing loads that will spall Sic chip after a subsurface separation is formed, continuing microscropy, lift-off and ascent testing of panels 13L/16L.
The third objective was to determine a strategy for having spare panels available. The recommendation is to purchase six additional RCC panels to insure one spare panel is available at each location. Currently two each of 9R, 10R and 9L are in production at Lockheed Martin.
The final strategy is to complete all testing and analysis by September 1. All work will be reviewed at a September 3 Tiger Team meeting.
Engineers will go over the “defense” briefing with the Orbiter Configuration Control Board (OCCB) on September 16. A 2-day Technical Interchange Meeting (TIM) with the peer review board is scheduled for September 18 and 19, conducted at JSC, with all Tiger Team members to attend and present their own work.
The Peer Review Board will produce a report of recommendations and findings, and a PRCB (Program Requirements Control Board) review will follow.
“Tiger Team has made tremendous progress in determining root cause of RCC SiC damage initiation, growth and spallation,” noted the presentation in conclusion. “Forward work is clearly defined and the team is working to completion by October 1.”