Non-destructive method to follow ageing of solid propellant in rocket motors

In all case-bonded composite rocket propellant (CRP) types and in all case-bonded elastomerbonded high explosive charges (HEC) so-named bondline stresses occur when the temperature is unequal to the so-named stress free temperature. The stresses or the forces, which act as tensile force on the bondline when temperature is decreased, can cause de-bonding from the case. Further on, the occurring tangential or ring stresses inside the material can lead to cracks inside the material, especially when the strain capability is not sufficient or decreases with ageing. Therefore a method was developed to monitor such stresses. The method enables a prediction tool for critical loads with respect to failure of the material. Thismethod works nondestructively, means the quantities were determined without destruction or firing of the munitions, here rockets and high explosive warheads. In a first step, built-in stress sensors are used to follow the development of stress values during ageing and thermal cycling. The material is a typical composite rocket propellant with elastomeric HTPB-IPDI binder and AP plus Al as solid fillers. The experimental setup consisting of several small rocket motors equipped with stress sensors is explained. Results of the measurements are presented and discussed. Again to say any other elastomeric binder with any other solid filler load are composite materials which suffer from the mechanisms outlined