Shock pattern in the plume of rocket nozzles - needs for design consideration

Summary

For ideal nozzles, basically two different types of shock structures in the plume may appear for overexpanded flow conditions, a regular shock reflection or a Mach reflection at the nozzle centreline. Especially for rocket propulsion, other nozzle types besides the ideal nozzles are often used, including simple conical, thrust-optimized or parabolic contoured nozzles. Depending on the contour type, another shock structure may appear: the so-called cap-shock pattern. The exact knowledge of the plume pattern is of importance for mastering the engine operation featuring uncontrolled flow separation inside the nozzle, appearing during engine start-up and shut-down operation. As consequence of uncontrolled flow separation, lateral loads may be induced. The sideload character strongly depends on the nozzle design, and is a key dimensional load for the nozzle’s mechanical structure. It is shown especially for the VULCAIN and VULCAIN 2 nozzle, how specific shock pattern evolve during transients, and how - by the nozzle design - undesired flow phenomena can be avoided.