Numerical Assessment of the Backward Facing Steps Nozzle

Abstract

The backward facing steps nozzle (BFSN) is a flow adjustable exit area nozzle for large rocket engines. It consists of two parts, the first is a base nozzle with small area ratio and the second part is a nozzle extension with surface consists of backward facing steps. The number of steps and their heights are carefully chosen to produce controlled flow separation at steps edges that adjust the nozzle exit area at all altitudes (pressure ratios). The BFSN performance parameters are assessed in terms of thrust and side loads against the dual-bell nozzle (DBN) with the same pressure ratios and cross sectional areas. The DBN is a two-mode flow adjustable exit area nozzle for low and high altitude. Three-dimensional turbulent flow solutions are obtained for the BFSN indicating that the flow is axi-symmetric and does not generate significant side loads. Further confirmation of the axi-symmetric flow is obtained by comparing the three-dimensional flow with the two-dimensional axi-symmetric solutions. The comparison of the thrust generated over the PR range from 50 to 1500 shows that BFSN generates more uniform and higher thrust than the DBN in the intermediate pressure ratios. At PR 1500 (high altitude), the BFSN thrust is 0.28% less than the DBN. All numerical solutions are obtained using the Fluent code.