Analysis of In-Flight Behavior of Truncated Plug Nozzles

Plug nozzles are usually designed to yield adaptation in a certain range of chamber/ambient pressure ratios, under the assumption of exhaust in a quiescent ambient. Nevertheless, the actual in-flight operations of plug nozzles take place in a flowing air-stream that can affect the performance, yielding a displacement from the ideal case. To understand the flow behavior in such conditions and to evaluate the displacement from the ideal nozzle performance, an investigation is carried out by a validated numerical tool based on the solution of turbulent Navier-Stokes equations and on shock-fitting. A sample rocket plug nozzle is analyzed parametrically to evaluate the effect of varying Mach number at constant pressure, and the effect of varying pressure ratio at constant Mach number. The results indicate a possible dramatic decrease of nozzle performance in the transonic region if the slipstream effect is neglected in the design. They provide also useful indications on how the Mach number and the shroud shape can affect the value of ambient pressure where transition from open to closed wake takes place.