Three HTPB propellants, varying in solids loading from 88% to 91%, were selected and developed for evaluation studies including aging. The 88%-solids propellant was verified to fit a previously developed aging model. The aging model has been further successfully applied to the measured mechanical-property aging data for the higher-solids propellants with excellent definition of behavior as well as HTPB propellants from other programs. The model has been used to predict long-time mechanical-property behavior from thermally accelerated tests and the results agree with 6 yr of measured aging data. The predicted failure properties can be used with motor requirements to determine service life for the grain of the motor system for a preselected margin of safety. Data are shown comparing the aging rate for the various composite propellants. Analysis of the aging results has provided a mathematical expression for the broad-spectrum aging behavior.