Low-cost access to space demands durable, cost-effective, efficient, and low-weight propulsion systems. Key components include rocket engine nozzles and nozzle extensions for boost and upper stages. Options for such nozzles include actively cooled alloys, ablatives, and radiation-cooled composites and metals, each of which has known limitations. Actively cooled structures are complex and costly. Ablatives are heavy and limit performance due to shape instability. Radiation-cooled composites are costly, have a limited production base, and are size-limited. Radiation-cooled metals face low temperature limits, require significant machining for acceptable weight, and require protective coatings. These limitations are highlighted by the J-2X nozzle extension, which uses a highly machined metallic structure to minimize weight and requires an emissivity coating to maintain safe operating temperature. Carbon/carbon (C/C) provides an attractive alternative, but has joining ability, oxidation resistance, and manufacturability limitations. Ultramet previously developed and demonstrated carbon fiber-reinforced refractory ceramic matrix composites (CMC) for liquid propellant applications up to 4300¿F. Ultramet has also demonstrated the integration of lightweight C/C with CMCs in a unique system comprising a C/C primary structure with an integral CMC liner or jacket. This system bridges the weight and performance gap between C/C and CMCs. The CMC provides enhanced mechanical properties and environmental resistance while the C/C provides a lightweight and cost-effective structure. In this project, the feasibility and benefits of applying this integrated material system for large liquid rocket engine nozzles will be validated. Subsequent work will address scaleup and will include a C/C producer. The fully developed system will combine the low weight and cost-effectiveness of C/C with the strength and durability of CMCs to support a range of potential NASA missions.