METHODOLOGY FOR EXPERIMENTAL EVALUATION OF TACTICAL MISSILE SOLID ROCKET MOTOR SERVICE LIFE USING THERMAL ANALYSIS

The minimum service life for a typical solid rocket motor (SRM) is ten years. SRMs are subjected to static and dynamic loads from environmental conditions and due to transport from storage locations to launch sites. A tactical missile SRM with 84% volumetric loading of non-metallized AP/HTPB (low smoke) propellant, operating between -40°C and +70°C, is studied in this paper. The structural integrity analysis revealed that the internal port of the grain was subjected to 32% strain during motor operation due to thermal loads and an internal pressure of 11 MPa. The reduction in maximum % elongation under tensile loading was monitored after accelerated ageing of the specimens at 65°C, 60°C, and 55°C for different time periods. The activation energy of the ageing process was estimated as 84.3 kJ/mol using the Arrhenius equation. The shelf life of the tactical missile SRM during which the maximum % elongation would be above 32% was estimated to be about 12.7 years. The propellant also had a residual strain of 6.3% in the port and 2% at the propellant-liner interface due to the initial curing process. In addition, SRMs are carried onboard combat aircraft for endurance and combat sorties during training exercises at frequent intervals. Temperature fluctuations in the propellant near the propellant-liner interface during these sorties could lead to degradation in properties, which would influence the service life. Based on the isothermal ageing studies and FEA results, a methodology is proposed to estimate the service life of SRMs using laboratory experiments while accounting for pre-strain and the thermal cycles corresponding to endurance and combat sorties that an SRM may undergo before launch.