Performance Analysis of an Infinite Array Linear Clusterd Plug Nozzle

Abstract

I N the challenge of realizing a new generation of space launchers, either single or two stage to orbit, an important role is played by the performance of the engine expansion system working in a varying pressure environment. In this framework a great interest has been devoted to the linear plug nozzle, which has been the subject of several studies in the last decade [1–6]. The plug nozzle is an external-expansion nozzle that yields self-adaptation of the exhaust jet to varying ambient pressure ratios, in a certain range of the launcher trajectory. This self-adapting capability allows high nozzle expansion ratios while avoiding the risks of flow separation that would exist in equivalent bell nozzles. The plug nozzle is made of a primary internal expansion nozzle, which is a conventional supersonic nozzle, and an external-expansion ramp, referred to as the plug surface. Most of the different engineering solutions proposed for plug nozzles have the following common feature: the primary expansion is made through a cluster of bell nozzles (or modules) exhausting onto a common linear plug surface [7–9]. The primary nozzle partitioning allows easier manufacturing, lower thermal loads, easier cooling and higher thrust vector capability. However, clustering causes additional performance losses due to three-dimensional flow inside the modules and to the interaction of jets exhausting from adjacent modules. For these reasons the three-dimensional features have to be studied in depth to better predict the engine performance and the expected mechanical and thermal loads for nominal operating conditions both at sea level and altitude and for differentially throttled modules as well. In fact, the thrust vectoring could be achieved by differential throttling of modules and, when thrust requirement is reduced in the final part of the ascent, some of the modules could be intentionally shut down. To this goal, the present paper studies by numerical simulation, the three-dimensional flowfield generated by themodules on a reference linear plug surface. The attention is focused on the effects of the three-dimensional flow features that take place when two different kind of modules are considered: the first module is obtained by dividing the reference two-dimensional primary nozzle by vertical walls and the second one is a full three-dimensional round-to-square nozzle. The performance analysis of these different module configurations allows weighing separately the role of clustering (i.e., just divide the primary nozzle into modules with infinitely thin flat walls) and the role of module design. A further subject of this study is the analysis of the effects produced by the shut down of a module of the cluster, for both module configurations. The analysis of the different configurations is made by comparing the thrust losses with the reference two-dimensional solution.