On-orbit repair concepts for the space shuttle orbiter wing leading edges have been actively pursued since the Space Shuttle Columbia accident. Several teams have collaborated to design "kits" to repair damage to the reinforced carbon/carbon (RCC) leading edges, which can range from scratches to holes 16 in. in diameter. Atmospheric reentry of the orbiter imposes extreme requirements on the RCC as well as on any repair concepts. These requirements include temperatures up to 3000 °F for over 15 min in the presence of an extremely oxidizing plasma environment.
Overwrap concept.
One proposal for on-orbit repair of large damage areas along RCC leading edges is an overwrap concept utilizing a thin (0.010- to 0.015-in.) rhenium (Re) sheet measuring 24 in. wide by 24 in. long (see the preceding photograph). Re possesses excellent high-temperature strength, good low-temperature ductility, and good fabricability. A commercial R512E coating (made of silicon, iron, and chromium: 60Si-20Fe-20Cr) was chosen to protect the Re substrate from the harsh oxidizing environment of atmospheric reentry since coatings that form silicides with the refractory metal base material are particularly effective against catastrophic oxidation. The following figure shows that the application of the R512E on Re produces three distinct silicide layers in the coating. The innermost layer is (Re,Cr,Fe)2Si, the next columnar layer is (Re,Cr,Fe)Si1.8, and the outermost layer is a silicide with Cr and Fe as the major constituents.
Re+R512E forms three distinct layers.
One poor characteristic of the R512E coating is that it is inherently brittle and could crack when the thin-gauge metallic overwrap is bent to follow the curvature of the orbiter leading edges. In anticipation of this situation, a "Type A" sodium silicate coating is applied over the R512E; this coating effectively flows into the cracks at the reentry temperatures, providing additional protection for the refractory metal substrate. This coating performs the same function on the current RCC material.
Arcjet testing of 2.8-in.-diameter coated Re disks simulated the high-temperature oxidizing conditions experienced by the leading edge during reentry (greater than 3000 °F over the 15-min test duration). Both coated Re concepts (Re+R512E+Type A and Re+Ir+R512E+Type A) passed this arcjet testing. The following photomicrographs show cross sections of the test article in both the as-processed and postarcjet conditions. The coating effectively protected the substrate, and a heavy oxidation layer exists on the post-test surface. However, the other silicide layers are intact, and the Re substrate is essentially untouched.
Cross section of Re+R512E. Left: As-coated. Right: Post-arcjet test.
The final photographs show both pretest and post-test conditions for the Re+R512E+Type A. Although the front side of the specimen shows evidence of attack, no penetration or failure occurred. That photograph also shows that 93 percent of the original Re substrate survived the exposure, a substantial safety margin. The degree of attack on the backside of the specimen was negligible. This is significant because the backside is adjacent to damage that is targeted for repair. Results for the Re+Ir+R512E+Type A concept were similar to those for Re+R512E+Type A.
The Re/R512E concept passed arcjet testing, with no sample burn through after 1000 sec.
Given the successful arcjet test results, the Re-based concept was selected by the development team of the NASA Langley Research Center and the NASA Glenn Research Center for additional testing and scale up. Scale-up activities underway to evaluate the Re+R512E+Type A concept include larger panel fabrication, more arcjet testing, and attachment concepts.
Glenn contacts: Frank J. Ritzert, 216-433-8199, Frank.Ritzert@nasa.gov; and Dr. James A. Nesbitt, 216-433-3275, James.A.Nesbitt@nasa.govLast updated: October 16, 2006
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