Humidity induced burning rate degradation of an iron oxide catalyzed ammonium perchlorate/HTPB composite propellant

Burn rate degradation of ammonium perchlorate based solid propellants can occur when moisture diffuses in and out of the material while exposed to fluctuating ambient humidity conditions. For high burn rate propellants with significant mass fractions of ammonium perchlorate particles less than 10 μm, small changes in particle diameter can significantly alter the total oxidizer surface area resulting in propellant burning rate reductions. Ammonium perchlorate propellant samples are aged at various relative humidity and constant temperature. The samples are subsequently dried to equal moisture content and examined by SEM and optical microscopy. The propellant samples are combusted in a closed combustion bomb to measure the burning rate of the aged samples. The results show a clear correlation of the burning rate degradation and the level of humidity exposure. Evidence indicates that the degradation is a result of ammonium perchlorate crystal size growth and surface morphology changes reducing the available surface area. The changes are shown to be correlated with the moisture content of the aging environment.