Remarks on plug cluster nozzles

Plug and plug cluster nozzles are said to have superior performance as compared to conventional nozzles. A preliminary comparison of different nozzles at an identical area ratio shows that at the design point the plug cluster nozzle performs slightly worse than a conventional or an extendible bell nozzle. However, for certain applications, like atmospheric flight or the realization of high area ratios at short nozzle lengths the advantages of the plug cluster nozzle come to play. The performance gain due to the external expansion along the plug contour is of the same order of magnitude as the losses inherent to the plug cluster principle. Thus a detailed performance modeling and an adaptation of the module shape and the plug contour are required. Any plug cluster nozzle performance model must include the effects of the merging stream tubes emanating at the module exits as well as a reliable determination of the pressure acting on the basc area of the truncated plug. The main objective of the study underlying this paper is to provide a reliable performance model to allow a comparison of various plug cluster configurations as a function of the design parameters without having to rely on complex CFD-tools.