Analysis of Factors Affecting the Performance of RLV Thrust Cell Liners

The reusable launch vehicle (RLV) thrust cell liner, or thrust chamber, is a critical component of the Space Shuttle Main Engine (SSME). It is designed to operate in some of the most severe conditions seen in engineering practice. This requirement, in conjunction with experimentally observed “dog-house” failure modes characterized by bulging and thinning of the cooling channel wall, provides the motivation to study the factors that influence RLV thrust cell liner performance. Factors or parameters believed to be directly related to the observed characteristic deformation modes leading to failure under in-service loading conditions are identified, and subsequently investigated using the cylindrical version of the higher-order theory for functionally graded materials in conjunction with the Robinson’s unified viscoplasticity theory and the power-law creep model for modeling the response of the liner’s constituents. Configurations are analyzed in which specific modifications in cooling channel wall thickness or constituent materials are made to determine the influence of these parameters on the deformations resulting in the observed failure modes in the outer walls of the cooling channel. The application of thermal barrier coatings and functional grading are also investigated within this context. Comparison of the higher-order theory results based on the Robinson and power-law creep model predictions has demonstrated that, using the available material parameters, the power-law creep model predicts more precisely the experimentally observed deformation leading to the “dog-house” failure mode for multiple short cycles, while also providing much improved computational efficiency. However, for a single long cycle, both models predict virtually identical deformations. Increasing the power-law creep model coefficients produces appreciable deformations after just one long cycle that would normally be obtained after multiple cycles, thereby enhancing the efficiency of the analysis. This provides a basis for the development of an accelerated modeling procedure to further characterize “dog-house” deformation modes in RLV thrust cell liners. Additionally, the results presented herein have demonstrated that the mechanism responsible for deformation leading to “dog-house” failure modes is driven by pressure, creep/relaxation and geometric effects. Since creep and relaxation are strong functions of temperature, the mechanism is temperature driven as well, and thus thermal barrier coatings have the potential to mitigate this failure mode. Analyses of RLV thrust cell liners employing thermal barrier coatings have shown that their influence on temperature and deformation fields was dependent on coating thickness. However, grading of metallic substrate and coating materials has not demonstrated improvement in terms of overall thrust cell liner performance. This suggests that the cost of grading the considered constituents likely outweighs the benefits. Furthermore, grading is potentially detrimental due to likely roughening of the thrust cell liner wall in the combustion chamber as a result of local deformations around relatively coarse inclusions. The homogenized approach to analyzing the graded coatings has been shown to be incapable of capturing these localized effects. 1 https://ntrs.nasa.gov/search.jsp?R=20040066097 2019-05-22T08:47:50+00:00Z