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segments that allow for thermal growth and provide an interface with the adjacent tiles. The key to successful use of CC in this application is with the development of an effective oxidation-resistant coating system. Silicon is diffused into the outer layers of the CC fabric reinforcement in an argon atmosphere at a temperature of 3000°F (1649°C). This diffusion yields an oxygen barrier that is further enhanced by a sealant consisting of tetraethylorthosilicate (TEOS), sodium silicate, and graphite fibers. Details of these CC applications are illustrated in figures 1-3.
Figure 1. CC areas on Space Shuttle orbiter.
In a related but more demanding application, the National Aero-Space Plane (NASP) Program is considering the use of CC in thermally critical areas. The anticipated temperatures of 5000°F (2760°C) on the nose cap and approximately 3500°F (1930°C) on the wing- and tail-leading edges, coupled with minimum weight and high-strength requirements, make CC one of the more attractive candidate materials (ref. 4). A specific NASP application under development at LTV Corporation involves the fabrication of a CC elevon control surface (ref. 5). This control surface will be based on an advanced version of the CC used on the Space Shuttle, and it will also use a silicon carbide-type of coating for oxidation resistance to 3000°F (1649°C). NASA Langley Research Center is sponsoring this effort, with the deliverables scheduled for 1990. The European spaceplane
Figure 3. Space Shuttle orbiter CC nose cap.
Hermes is also using CC, with appropriate oxidation-resistant protection for the nose cap, leading edges, and underwing areas (ref. 6).
The fabrication, by The B.F. Goodrich Company, Super-Temp, of a 10 ft x 4 ft, unidirectionally, rib-stiffened curved panel to simulate a structural component suitable for use on a hypervelocity vehicle illustrated the application of CC to large-area, structural panels. The panel was based on a two-dimensional (2-D) layup of a balanced-weave, T-300 graphite fabric with a phenolic matrix precursor. The panel (fig. 4) was designed to achieve a uniform fiber volume across the width with strength ranging between 50 ksi to 60 ksi and a tensile modulus of 16 msi in the warp direction (ref. 7). A General Dynamics X-30 vehicle design exemplified the potential use of this structural panel, suggesting that a primary CC structure be used for the entire vehicle. This CC design offered a significant savings in structural weight (ref. 8).
Using CC in jet-engine rotors and stators to permit high-combustion temperature operating conditions holds the promise of reduced engine size, weight, and fuel consumption. Higher temperature capabilities reduce the need for engine bypass cooling, thereby increasing engine efficiency. Carbon-carbon offers this potential capability plus a significant weight advantage over current high-temperature alloys. Operating temperatures that are well in excess of 1000°F hotter than temperatures used in conventional engines are being sought. The LTV Missiles and Electronics Group is developing CC jet-engine components under the Air Force Extended Long-Range Integrated Technology Evaluation (ELITE) program. This includes turbine wheels, combustion chambers, and exhaust nozzles. A CC turbine wheel that is 14.4 in. in diameter and weighs only 7.5 lb was successfully spun at 28000 r/min at a temperature of 3200°F (1760°C) (ref. 9). The pretest turbine rotor is shown in figure 5.
An innovation in CC turbine rotors and disks was also demonstrated by the LTV Missiles Division through the use of a polar-weave-based CC in test rotors. Fill fibers radiated from the rotor hub plies in this arrangement with hoop or warp fibers oriented in concentric circles. Spin tests on polar-weave CC rotors achieved approximately 40 000 r/min before failure (ref. 10).
Potential applications for CC components are found in conventional high-performance jet engines. Afterburner flaps and seals used to provide thrust control have had prototypes designed in CC. The F-100 jet-engine afterburner nozzle uses coated CC (ref. 11). These CC applications offer weight reduction and higher temperature capabilities than those of conventional metallic materials. A CC prototype jet aircraft turbine engine flap fabricated by HITCO* is illustrated in figure 6.
Piston-driven engines such as gasoline and diesel engines could operate at increased efficiency and reduced weight if cooling requirements were minimized through the use of high-temperature capability materials. Carbon-carbon has been proposed and is under evaluation for many applications in this area. A CC piston has been tested successfully at NASA Langley Research Center (ref. 12), and a U.S. patent has been issued covering this composite piston concept (ref. 13). Carbon fiber reinforcements of four-dimensional (4-D) angle interlock and a knitted multilayer warp sock were evaluated with resin infiltration and chemical vapor deposition (CVD) carbon densification to complete the CC processing. The CC piston has a lower density than the conventional aluminum lightweight piston. This reduced weight improves internal combustion engine efficiency. However, the very low thermal expansion of CC, relative to aluminum, and the high-strength retention of CC at elevated temperatures allow a pistonto-cylinder wall clearance that is small enough to eliminate the need for piston rings. Lightweight CC pistons permit the use of lighter weight reciprocating components, allowing possible higher engine speeds and improved
*Now called BP Chemicals (HITCO).