The objective of the proposed research is to develop functionally graded polyimide foams as light-weight, high performance thermal protection systems (TPS) for ablative and inflatable/flexible/deplorable structures used in space exploration missions. In the first phase of this project, we will demonstrate the feasibility of fabricating graded polyimide micro/nanocellular foam structures and characterize their insulation and ablative properties. We propose to fabricate graded polyimide foams with density gradient that are similar to those of functionally graded PMMA foams produced at UT Austin recently, using melt processable thermoplastic polyimide films by a solid-state foaming process with supercritical CO2. Gradient polyimide foam structures with tuned thermal protection properties are unique and not available until now. The advantage of this innovative approach is two folds: 1)Functionally graded, non-homogeneous polyimide foams that allows continuous variation in macroscopic mechanical and physical properties, to tune the thermal conductivity, specific heat, density, and ablative behavior. 2)The use of melt processable polyimides and the solid-state supercritical CO2 foaming process eliminates the needs of using high boiling solvents, and the costly, tedious solvent exchange process, which is required during the making of porous aerogels from solutions. Open-celled polyimide foams with large visible uniform pore sizes have been produced commercially, however, graded porous polyimide thin films with density gradient, open or close celled, are rare and pose more technical challenges, but can provide additional unique benefits.