PROPULSION MATERIALS MODELING TO IMPROVE PERFORMANCE AND REDUCE COST

MDA is developing materials for several applications including hypersonic missiles, ma-neuvering reentry vehicles, advanced solid rocket motors, and divert and attitude control sys-tems. All of these applications employ components that must operate at temperatures above 3000°F. Viable structural materials for these conditions can be loosely grouped as graphite, ce-ramics (e.g. oxides, carbides), or refractory metals (e.g. tungsten, rhenium). Ceramics tend to exhibit attractive thermochemical stability but suffer from low ductility and poor thermal shock resistance. Refractory metals and graphite, on the other hand, perform better in a thermal shock environment but oxidize too rapidly to be useful even in short term applications. One material development approach that addresses these issues is to create a composite that combines the benefits of two constituents while accommodating their deficiencies. For ex-ample, a potentially attractive composite may employ a ductile metal reinforcement in a thermo-chemically stable ceramic matrix. The properties of such a composite can be tailored to meet the needs of the application by varying the properties, form, and volume fractions of the constitu-ents. The flexibility offered by composites, however, gives rise to a more complex design and development process. Composite properties will be anisotropic, temperature dependent, and nonlinear, and can occur in infinite combinations as the composition is adjusted. Applications of interest to MDA typically experience severe transient temperature fields and complex stress dis-tributions. While it is possible to identify generally attractive composite properties, it is nearly impossible to select the appropriate composition without understanding the interaction between the material and application. In short, MDA propulsion applications will benefit from refractory composite materials, but the development process is faced with an infinite number of potential material combinations coupled with complex design requirements. Therefore a pure "cut and try" approach is not technically sound or economically feasible. Fortunately there are analytical models that can guide the material development process. Micromechanical theories based on fundamental solutions of elasticity and plasticity can be used to compute the properties of refractory composites reinforced with particles, whiskers, or con-tinuous fibers. The predicted composite properties can be subsequently auditioned in ther-mostructural models of any MDA propulsion component. The computed results can be reviewed to select one or more attractive materials, identify the critical properties, and recommend a fabri-cation and testing plan. The goal of the proposed Phase I program will be to prove the feasibility of a modeling procedure that identifies attractive refractory composites for MDA propulsion ap-plications. The proposed program will be performed by a team of Materials Research & Design, a small business specializing in modeling of composites and propulsion materials, and Southern Research Institute, a non-profit research center well known for testing materials at high tempera-tures. The technical tasks will include 1) literature review, 2) micromechanical model develop-ment, 3) composite property prediction, 4) thermostructural analysis, 5) material property testing, and 6) data correlation. In Phase I the feasibility of the proposed approach will be tested by comparing predicted composite properties with measured data.