Thin, flexible plugs conform to surfaces.
In response to the Columbia Accident Investigation Board report, a plug repair kit has been developed to enable astronauts to repair the space shuttle’s wing leading edge (WLE) during orbit. The plug repair kit consists of several 17.78-cm-diameter carbon/ silicon carbide (C/SiC) cover plates of various curvatures that can be attached to the refractory carbon-carbon WLE panels using a TZM refractory metal attach mechanism. The attach mechanism is inserted through the damage in the WLE panel and, as it is tightened, the cover plate flexes to conform to the curvature of the WLE panel within 0.050 mm. An astronaut installs the repair during an extravehicular activity (EVA). After installing the plug repair, edge gaps are checked and the perimeter of the repair is sealed using a proprietary material, developed to fill cracks and small holes in the WLE.
In developing the plug repair concept, several issues had to be addressed including material, design, performance, and operability. An oxyacetylene torch was calibrated to heat a specimen to WLE entry temperatures and was used to screen candidate repair materials. Promising materials were then tested in the Johnson Space Center arc-jet test facility to determine their resistance to oxidation in a hypersonic environment. C/SiC was selected as the cover plate material because of its superior strength and resistance to oxidation. In order to raise its operational temperature limit, a proprietary oxidation barrier coating was developed. TZM was selected as the attach mechanism material because of its manufacturability and structural performance as well as its ability to withstand the plasma environment when coated with the oxidation barrier coating.
Prior art involved square, inflexible plugs that were too small to provide complete WLE coverage and would also create protuberances into the hypersonic flow that would overheat during operation. This innovation is an evolution of the prior art in that the current plug is a thin, flexible double-curved shell with rounded edges, which facilitates a deformed shape that conforms to the WLE geometry with minimal disturbance of the flow.
The repair plug is designed to capitalize on flexibility and curved shape in order to minimize the number of cover plates required. The current design reduced the number of unique plug shapes required to cover critical WLE regions from over 1,300 to less than 20. Non-linear finite element analysis, including contacting surfaces, was used to model the plug during installation and operation. Computational fluid dynamics and thermal analysis were used to predict plug temperatures during entry.
This work was done by Charles J. Camarda, Joseph Sikora, Russel Smith, H. Rivers, and Stephen J. Scotti of NASA; Alan M. Fuller of United Space Alliance; Robert Klacka of General Electric Ceramic Composites, LLC; Martin Reinders of The Boeing Company; Francis Schwind of Carbon-Carbon Advanced Technologies, Inc.; Brian Sullivan of Materials Research and Design, Inc.; and Dean Lester of ATK Thiokol for Johnson Space Center. MSC-24347-1