S. P. Walker, S. J. Scotti, R. Stephan, NASA Langley Research Center, Hampton, VA; M. E. Melis, NASA Glenn Research Center, Cleveland, OH; W. A. Waters, Lockheed Martin Corporation, Hampton, VA; S. Atmur, T. Russell, Starfire Systems, Inc., Malta, NY

Summary:

The goal of the current study was to identify a method of improving the overall durability of the Orbiter wing leading edge to make it less susceptible to impact damage and obviate the need for on-orbit repair. The approach presented herein consists of hardening the existing Reinforced Carbon/Carbon (RCC) to produce a “Robust RCC” wing leading edge. The RCC is hardened through co-curing/co-bonding a doubler to the inner mold line of the leading edge panel. A pre-ceramic polymer (PCP) carbon/silicon-carbide (C/SiC) material system was chosen as the most suitable doubler material for this effort. The study included a preliminary determination on the feasibility of the hardening approach and an evaluation on the response of the hardened leading edge during critical ascent and re-entry conditions through analyses, sample fabrication and testing. Foam and ice impacts on the leading edge during launch have been identified to be the critical damage threats, and foam impact from the Shuttle External Tank “Bipod” region was identified by the Columbia Accident Investigation Board (CAIB) as the likely cause of the breach in the RCC panels on the Columbia, therefore the efforts focused on these threats. Analyses of the foam impact test sponsored by CAIB on a full scale RCC panel which resulted in a large breach were performed using the tools and models that were “calibrated” to that test. These previously developed impact analysis models of the RCC were modified for evaluating the Robust RCC concept. The impact analyses predicted that a Robust RCC panel would increase the threshold energy for critical damage by 25%. In addition to the impact analyses, preliminary transient thermal analyses were conducted on a wing leading edge panel to identify the effect of the Robust RCC doubler on peak temperatures during re-entry at both the stagnation region and at attachment points. Results revealed no thermal issues to deter from the feasibility. To experimentally validate the improved durability of the Robust RCC, an effort was made to develop the co-curing/co-bonding process and to demonstrate improved impact resistance. Initial processing trials of coupon specimens were performed to ensure an adequate bonding of the doubler to the RCC and specimens were successfully fabricated for further testing. Ballistic impact analyses and testing were conducted on flat Robust RCC plates to replicate previous testing conducted on RCC plates without an attached doubler. Both foam and ice impacts were investigated. The plate impact tests revealed an increase in critical threshold impact energy for the Robust RCC material system compared to the threshold for RCC. In light of the results of the work conducted here, a Robust RCC wing leading edge appears to be a feasible approach which would increase the threshold for catastrophic failure of a wing leading edge panel.