Thermal and acceleration load analysis of new 122 mm rocket propellant grain

Abstract

In order to achieve very strong inter-ballistic requirements, a unusual free standing rocket motor propellant grain was designed, with variable channel diameter. Due to its very high length, it was expected for the grain to be exposed to dangerous combination of different loads during the design phase. Structural analysis of viscoelastic propellant grain is always very complex and it differs substantially from an elastic analysis, because the propellant mechanical properties depend on temperature and strain rate. The most complex case in the analysis occurs when multiple loads operate simultaneously. In this paper, the structural analysis has been done during the initial period of rocket flight, when the grain is under very fast load due to acceleration and at the same time under a slow load due to surface pressure, created as a result of temperature dilatation. In the absence of data on mechanical properties of the new propellant composition, the properties over the whole time-temperature range were estimated comparing by a similar composition that have been completely tested earlier. Using finite element method (ANSYS program) two different design solutions were examined and the more reliable one was adopted. Nomenclature T a-Time-temperature shift factor D-Cumulative damage E-Modulus, initial, tangent, elasticity R-Strain rate, t-Time ε , 0 ε-Strain, Initial strain, m ε , 0 m ε-Ultimate (Allowable) strain, Initial strain ξ-Reduced time η(t)-Propellant aging factor η E , η σ ,,η ε-Aging factor for modulus, stress, strain σ m-Tensile strength (Ultimate stress) σ m0 , σ m (t)-Initial strength, Time-dependent strength