Influence of starting chamber dynamics on nozzle flow choking time and the liftoff time of dual-thrust rockets

Abstract

In this paper comprehensive studies on chamber dynamics have been carried out to examine the intrinsic flow physics pertaining to the nozzle flow choking time and the liftoff time of dual-thrust solid propellant rockets. Diagnostic investigations have been carried out using a three-dimensional unsteady, double precision, density based, shear-stress transport k-ω turbulence model. This model solves standard k-ω turbulence equations with shear flow corrections using a coupled second-order-implicit unsteady formulation. In the numerical study, a fully implicit finite volume scheme of the compressible, Reynoldsaveraged, Navier-Stokes equations is employed. Numerical studies reveal that narrow port and long flow development ahead of the divergent location of dual-thrust motors (DTMs) are likely to have multiple flow choking and spread rate enhancement. As a result the transition location of the DTM will act like a second throat and/or the upstream port will perform as a second igniter, which will certainly delay significantly the flow choking time of the rocket nozzle due to the delay in establishing the choked flow condition leading to delay in the full development of thrust for takeoff. The internal flow choking can lead to the formation of shock waves inside the DTM. The shock waves and the connected new turbulence features will alter the flamespread and chamber dynamics and will indeed alter the nozzle flow choking time and the liftoff time of dual-thrust rockets with high length to diameter ratio. We conjectured from the parametric analytical studies that the altered variations of the igniter jet impingement angle, igniter jet turbulence level, time and location of the first ignition, flame spread characteristics, flow development history, the overall chamber dynamics and the thermoviscoelastic response of the grain are having the bearing on the starting transient and the nozzle flow choking time for establishing the required full thrust for the rocket liftoff. We concluded that the higher igniter jet turbulence intensity will cause delayed nozzle flow choking warranting delayed liftoff time of dual-thrust solid propellant rockets.