Analytical and empirical studies were conducted to define and investigate problems associated with restartable solid propellant rocket nozzles. Thermal analysis covered the complete temperature history from startup through cooldown of the duty cycle. In addition to conventional convection, conduction, and radiation, the effects of the pyrolysis gases of charring ablators were included. The structural analysis included the investigation of the thermo-elastic behavior of cylindrical and washer inserts, the existence of plastic zones, the effects of contact pressures in stacked washer inserts, and the effects of property degradation of tungsten due to grain growth. The materials investigation included tungsten-carbon reactions, tungsten grain growth and the resulting effects on properties, and the high temperature c- direction compressive deformation of pyrolytic graphite. Rocket nozzle firings were used for verification of analyses and further problem definition. It was found that the strength of tungsten is degraded by 50 or more with very slight grain growth. Large permanent c-direction compressive deformations are attainable in pyrolytic graphite above 2600 C with stresses of 7000 psi in 30 sec. Ablation of char-forming insulators provides the principal mode of cooldown by the pyrolysis process and by heat transfer to the resulting gases as they leave the nozzle. Radiation was found to be an order of magnitude lower in significance.