A Simplified Coupled Structural-Flowfield Analysis of Solid Rocket Motors Ignition Transient

Ignition transient of a solid rocket motor can be characterized by strong unsteady phenomena such as waves propagation and pressure oscillations inside the combustion chamber. Depending on their frequencies and their amplitude, these oscillations can generate undesirable effects on the launcher, such as thrust fluctuations and transient loads on structures and-or payload equipments. This paper wants to present a simplified flow field/structural model based on a quasi-1D unsteady model of the ignition transient internal ballistics (SPIT) coupled with a simplified structural model, able to account for the radial dynamics of the grain and SRM casing, with the assumption of a standard linear elastic behavior of the structure. The parametric analysis performed with the coupled internal ballistics/structural model allows to show and evaluate some effects in the chamber pressurization rate, when reducing the elastic modulus of the grain propellant towards small values. Concerning the dynamics aspects of the fluid/structural coupled system, instead, a small but clear coupling between the acoustic flow field phenomena and structural dynamics is possible especially, as expected, when both fundamental oscillatory phenomena fall in the same range of frequency. The results and the parametric analysis with the fluid-structural model developed are shown for two solid rocket motors of the new European launcher VEGA: P80FW, first solid stage and Zefiro 9 old version of the third solid stage.