Numerical studies have been carried out to examine the pre-ignition chamber dynamics of dual-thrust solid rocket motors. Using a three-dimensional unsteady, second-order-implicit, shear-stress transport k-ω turbulence model, detailed parametric studies have been carried out to examine conclusively aerodynamic choking and the existence of a fluid throat at the transition region during the startup transient of dual-thrust motors. In the numerical study, a fully implicit finite volume scheme of the compressible, pressure based Navier-Stokes equations is employed. It is confirmed that, at the subsonic inflow conditions, there is a possibility of the occurrence of internal flow choking in dual-thrust motors with large length-to-diameter ratio (L/d ＞ 44) due to the formation of a fluid throat at the beginning of the transition region induced by area blockage caused by boundarylayer-displacement thickness. The internal flow choking results in the formation of shock waves inside the dual-thrust motor. The shock waves and the new turbulence level altered the location of the reattachment point and also enhanced the heat flux to the propellant surface, which obviously will lead to undesirable startup transient due to erosive/transient burn rate enhancement. More numerical results of inert simulators of dual-thrust motors with horizontal and flip-horizontal positions are presented with tangible explanations in this paper for establishing the internal flow choking in dual-thrust solid rocket motors with narrow upstream port.
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