Metallic Concepts for Repair of Reinforced Carbon-Carbon Space Shuttle Leading Edges

摘要

The development of thermal protection systems (TPS) for re-entry vehicles has been a topic of interest since the inception of manned space flight. Re-entry materials systems are critical for a spacecraft to withstand the intense temperature and environmental effects that are associated with the re-entry profile. Recently, intense efforts have been ongoing to develop repair technologies for TPS systems of the Space Shuttle Orbiter to reduce the risk of another catastrophic loss like that of Columbia. In particular, attention has focused on the ability to apply an on-orbit repair of both the wing leading edges and the tiles on the Orbiter's underbelly. Both large and small areas of damage to leading edges allow hot gases to enter and destroy the reinforced carbon-carbon (RCC), thereby leading to catastrophic failure. Repair of large areas of damage, greater than 25 centimeters in diameter, are particularly of interest since that is the widely accepted reason for the Columbia accident. This paper presents results on a metallic overwrap concept that can conform to the contour of the wing leading edges and prevent internal structural damage from the hot gases during re-entry, which can reach in excess of 1649°C in a highly oxidizing plasma environment. Sheet samples (380um thick) of several silicide-coated refractory alloys were evaluated and a series of coated Re-based concepts were down-selected for more rigorous testing in an arcjet testing facility. The Re sheet was coated with an R512E silicide coating to mitigate oxidation. Two additional concepts using Ir were examined. The first examined Ir as a surface layer without the silicide coating while the second added a layer of Ir below an outer layer of Re which also received the R512E silicide coating. The R512E coating, consisting of Fe, Cr, and Re silicides, is known to be brittle and was found to contain numerous fine cracks both perpendicular and parallel to the coating surface. Consequently, a "Type A" coating consisting of a mix of sodium silicate and SiC was applied to the surface to seal fine cracks 2in the R512E coating as well as for emissivity considerations. The concepts with the R512E coatings (Re+R512E+Type A and Re+Ir+Re+R512E+Type A) survived the arcjet exposure that simulated atmospheric re-entry conditions forming a porous silca scale. However, the Ir coating alone did not sufficiently protect the Re. Part of the failure of the Ir coating, deposited by CVD, may have been due to poor bonding with the Re substrate observed in test samples. Although handling and bending of the coated Re sheet remains a concern, the coated Re materials look promising for repairing Orbiter wing leading edges exposed to temperatures exceeding 1649°C.