Analysis of In-Flight Behavior of Truncated Plug Nozzles

Plug nozzles are usually designed to achieve altitude adaptation in a wide range of chamber/ambient pressure ratios. In actual in-flight operations of plug nozzles, this phenomenon must take place in a flowing airstream, which can affect the performance. To understand the flow behavior in such conditions and to evaluate the departure from the ideal nozzle performance, an investigation is carried out with 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 that the interaction with a flowing airstream reduces the pressure of the external air, as seen from the nozzle exhaust jet, yielding a reduction of the performance. In particular, a dramatic decrease of nozzle performance may take place in the transonic region if the slipstream effect is neglected in the design. The results also provide useful indications on how the Mach number and the shroud shape can affect the value of ambient pressure where the transition from open to closed wake takes place.