Diagnostic investigation of flame spread mechanism in dual-thrust solid propellant rocket motors

Abstract

Numerical studies have been carried out to examine the flame spread mechanism and the chamber dynamics of high-performance dual-thrust solid propellant rocket motors during the starting transient period of operation. Using a three-dimensional unsteady, shear-stress transport k-ω turbulence model, detailed parametric studies have been carried out to examine the aerodynamic choking and the existence/non-existence of a fluid throat at the transition region during the startup transient of dual-thrust motors (DTMs). In the numerical study, a fully implicit finite volume scheme of the compressible, density based Navier-Stokes equation is employed. We 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 boundary layerdisplacement thickness. We also confirmed that in such motors the choked flow becomes unchoked flow during the flame spread period due to the mass injection from the wall as a result of the thinning of the boundary layer thickness. We have demonstrated that without altering the grain geometry one can alter the flame spread mechanism by altering the igniter jet turbulence intensity, igniter gas temperature and propellant conductivity. Additionally, the numerical results of inert simulators of dual-thrust motors are compared with that of the case with propellant injection for establishing the physical situations of the choked and unchoked flow conditions during the starting transient period of operation of dual-thrust motors with high-propellant loading density.