BAA-11-04-PKV-SOO2.doc

TRIAD SOO #2

High Temperature Structure Concept Development AFRL is engaged in the development of high temperature materials and structures for application to global strike vehicles that fly at hypersonic velocities for extended periods of time. This harsh flight environment will require novel concepts for acreage and leading edge structures and thermal management. Extended flight at hypersonic speed requires that the air vehicle possess a very efficient airframe. The airframe of the most efficient vehicle would likely include hot structure that can withstand both harsh aerothermal heating and flight loads. Such a structure would not rely upon thick external thermal protection materials which increase the vehicle’s frontal and wetted areas, raising drag. Without external-surface thermal protection, the skin of a hot structure must be made of a material that can withstand high levels of heating while maintaining stiffness and strength. Carbon based composite materials are the leading candidates for service temperatures between 2000 and 3000°F, a range likely to be of interest to hypersonic global strike. Many issues remain for design, fabrication, test, and operation of structures constructed of these materials. This research effort will look at several inter-related facets of hot structure design to include the following tasks:

1. Tensile shear-out strengths of CMCs. Data are desired for net tensile and shear-out strengths of leading C-C-SiC, SiC/SiC, and SiC/SiNC composite materials (see Reference 1). Each material must be tested per the specimen configurations described in Table 1. The data should be analyzed for trends and any outlying data points should be investigated and evaluated for their implication to the data set as a whole. Evaluation of the materials for their effectiveness in hypersonic prompt global strike vehicle applications should also be considered, particularly in the areas of leading edges and acreage hot structure.

2. Carbon and silicon carbide foam evaluation for structural applications

a. Fabrication of complex geometries. Investigation into novel methods for fabricating curved panels using high-temperature, carbon-based composites and foams should be conducted as large hypersonic vehicles may require curved hot-structure airframe panels. Current practice of machining carbon or SiC foam to match a curved composite shell has inherent problems, including difficulty maintaining a uniform thickness bonding layer. Research is desired into methods to more efficiently form curved foam-core structure using carbon or SiC foam. An example is creation of the foam on a curved carbon-carbon composite panel, machining of the free surface to shape, followed by application of close-out plys of composite material. This activity will focus on process development. Coupon or subcomponent level trials are expected to validate fabrication concepts but large or moderate-sized components are not a requirement. A maximum target curvature radius of 120 inches is desired with an objective of 30 inches. Successful adhesion must be demonstrated via ASTM standards for surface adhesion of foams to a surface, and values must be recorded for documentation.

Table 1. Test Matrix with Specimen Sizes

d = Hole diameter w = specimen width e = Hole distance from hole center to nearest edge in loading direction

L = specimen length

b. Shear properties enhancement. Research into potential methods for increasing the shear properties of carbon and/or SiC foam. Hot structure fabricated from foam core panels will likely depend upon the foam’s shear strength for component strength and rigidity, and a foam core sandwich structure represents an attractive alternative to current hot structure concepts from a weight and thermal management standpoint. Methods to enhance shear strength and modulus of carbon and SiC foam are desired The largest possible shear properties increase is desired without proportional increase in foam density. A minimum increase of 50% in the properties is desired. Coupon-level trials of the modified carbon and SiC foams, tested according to ASTM standards, are expected but large or moderate-sized components are not.

References

1. Thermal Protection System Materials Integration Evaluation: Methodology and Results; Sullivan, Brian J., et al.; 35th Annual Conference on Composites, Materials, and Structures, 24-28 January 2011; Cape Canaveral, FL.