Characterization of the startup and pressure blowdown processes in rocket nozzles

This work is focused on the one-dimensional equations that prescribe the functional modes of a converging-diverging nozzle operating under a range of chamber pressures. Specifically, our study aims at characterizing the flow regimes that are likely to develop inside a Laval nozzle during the blowdown process that takes place at mission's end. Blowdown transients can lead to undesirable sideloads in the nozzle due to flow asymmetries and shock transitions, shock excursions, flow separation, and the formation of recirculatory zones. By representing the flowfield with one-dimensional equations, a direct analytical solution is obtained for the key pressure ratios that control the evolving flow character: supersonic with external shocks, supersonic with optimal expansion, supersonic with internal shocks, or subsonic throughout. These delimiting pressure ratios are determined here using novel asymptotic expansions that enable us to bracket the flow regimes that are particularly susceptible to sideload excursions. The flow attributes of successive flow regimes and their corresponding shock transitions are subsequently explained in view of the pressure evolution that accompanies chamber blowdown. We close with a discussion of experimental observations that suggest the possibility of spin generation during tail-off in the upper stage of a sounding rocket in which slag accumulation is reported.