Gas density effects on dual-bell transition behavior

Abstract

The concept of dual-bell nozzle, first proposed by Foster and Cowles in 1949 [1] has received renewed attention in recent years due to its one-step altitude adaptation capability [1-6]. This concept uses two shortened nozzles combined into one with a bump or inflection point between them, as shown in Fig. 1. During ascent it functions first at the lower area-ratio with controlled flow separation occurring at the inflection point, Fig. 1 (a). While the lower area-ratio helps to achieve high sea-level thrust, a controlled and symmetrical flow separation helps avoid dangerous side-loads. As the altitude increases and the gases expand further, the flow undergoes a transition process during which the flow jumps downstream and attaches itself close to the nozzle exit, with the flow filling the full nozzle exit section thereby utilizing the full geometrical area-ratio, Fig. 1 (b). Because of the higher area-ratios that are achievable through this design, a higher vacuum performance is feasible. Despite the losses associated with this design (such as aspiration drag in low altitude mode, non-optimum contour in high altitude mode, etc. [6]), the dual-bell nozzle shows better overall performance than a single bell nozzle of similar area-rati