Silicon Carbide based ceramic matrix composites (CMCs) offer the potential to fundamentally change the design and manufacture of aeronautical and space propulsion systems to significantly increase performance and fuel efficiency over current metal-based designs. Physical Sciences Inc. (PSI) and our team members at the University of California Santa Barbara (UCSB) are developing, designing and fabricating enhanced SiC-based matrices capable of long term operation at 2750oF to 3000oF in the combustion environment. Our approach is successfully building upon PSI's and UCSB's previous work in incorporating refractory and rare earth species into the SiC matrix to increase the CMC use temperatures and life-time capabilities by improving the protective oxide passivation layer that forms during use. As part of this work we are creating physics based-materials and process models that qualitatively define methods of improving matrix properties and the interaction of the fibers, interphases and matrix with each other. In the Phase I program the PSI team developed and experimentally demonstrated CMC's capable of withstanding 100's of hours of oxidation at 2700oF with no degradation. We have focused predicting the effect of phase distribution, grain size, chemical composition, matrix density, and surface flaws on the oxidation behavior of the CMC matrix. During the Phase II program we will iteratively improve the CMC performance by optimizing the composition and characteristics of the additives based on oxidation and mechanical test results and burner rig exposure testing.