Multi-scaled Physics-based Models of Environmental Effects on Propellant Aging

ABSTRACT: After being cast, a solid rocket motor (SRM) may potentially be stored for tens of years before it is fired. During this time, both strategic and tactical motors are subjected to constant changes in humidity and temperature that can lead to premature aging of the propellant, liner and insulation materials. Firing motors that have aged beyond their known safety thresholds can be disastrous. Hence, an accurate method for predicting the effects of environmental changes on the aging, and service lifetime, of these SRMs is needed. In this effort, we will develop a multi-scale, physics-based model which combines computational molecular-level chemistry and physics simulations with microconstitutive theory to predict aging in solid propellants. In addition, a database of material properties, diffusion constants, kinetic rates and Arrhenius parameters will be created. In Phase I, we will focus on the effects of humidity loading on the HTPB/AP binder/oxidizer interface. In Phase II, our models will be expanded to include other aging phenomena such as oxidative crosslinking in the HTPB polymer and dewetting along the HTPB/AP interface. As part of this effort, the calculated diffusion constants and kinetic rates will be input into microconstitutive models and validated against empirical and independent model predictions. BENEFIT: The Phase I test system will concentrate on calculating improved kinetic parameters for processes occurring at, or near, the HTPB/AP interface and, hence, will begin to immediately provide the physics needed to improve the current description of this element of the solid propellant microstructure over a wide variety of aging, loading, and temperature boundary conditions. Given that our current knowledge of how aging occurs at this interface is derived from a limited set of measurements made over a narrow set of boundary conditions, the simulation capability and improved material properties and kinetic database created in this effort calculations are anticipated to show immediate benefit to our ability to determine more accurate service lifetimes for SRMs. Combining the molecular level computational chemistry/physics simulations with microconstitutive theory will help establish a complete set of boundary conditions, and hence, allow for an accurate prediction of propellant aging from the initial seconds after curing to sometime decades later.