Numerical simulation of fragment impacting solid rocket motors

For the initiation characteristics of solid rocket motors (SRMs) filled with high-energy solid propellant under fragment impact, the related theoretical critical criterion for shock initiation was established based on the critical energy criterion and equivalent analysis method. Afterward, numerical simulation of fragment impacting SRM was carried out by using the ANSYS/LS-DYNA software and the nonlinear finite element method. Based on the calculated pressure and reactivity in the high-energy solid propellant grain, the shock critical initiation velocity of SRM and its variation with different forms of fragments and impact conditions were determined. It is found that the numerical simulation results under typical conditions are in good agreement with the data calculated with the developed theoretical critical criterion for shock initiation of SRM. However, the shock initiation mechanism becomes more complex when the case thickness of SRM increases to more than 5 mm; thus, the applicability of the developed theoretical critical criterion reduces. Moreover, it is found that all the case thickness, fragment shape, material properties of the fragment, and impact attitude can significantly affect the shock critical initiation velocity of SRM, even the initiation position and time. First, the critical velocity increases linearly with the increase in case thickness, and the increment rate is faster beyond the thickness of 6 mm. Second, the shock critical initiation velocity induced by fragments with different shapes is as follows: spherical fragment > cubic fragment > cylindrical fragment, while the initiation capacity of different fragment materials is ranked as follows: tungsten alloy > 45 steel > 2024 aluminum. Third, the effects of impact attitude on the shock critical initiation velocity, position and time are complex, and these effects are also influenced by fragment shape. When the impact angle is less than 60°, there is a higher shock critical initiation velocity of SRM under inclined impact than that under positive impact. In addition, the critical velocity induced by cubic fragment is the highest under the combination of vertex impact and positive impact. The variation of the critical velocity under this condition is approximately consistent with that by impacting with cylindrical fragment. Furthermore, when the impact angle is greater than 60°, the shock critical initiation velocity of SRM is less obviously influenced by impact attitude and fragment shape. Meanwhile, the critical velocity decreases sharply under this condition, which indicates that it is easier for SRM to detonate.