Experimental and Computational Investigation of a Dual-Bell Nozzle

The proliferation of earth orbital missions enabled by nanosatellites, of which CubeSats are the most common, has focused increased attention on low-cost, mobile launch systems. Advanced nozzle design can address the demand for higher booster performance, and has resulted in the investigation of alternate nozzle geometry. A dual-bell nozzle represents a novel approach to this problem by utilizing two theoretically ideal design points over the ascent trajectory instead of just one. The primary focus for this project was to investigate the performance of a representative dual-bell nozzle both computationally and experimentally. First, a dual-bell nozzle was designed which could be used on a representative, nanosatellite launch system with design altitudes of 3 km and 17 km. The nozzle contours were modified to facilitate machining, and incorporated in a supersonic wind tunnel. The dual-bell nozzle fabricated for testing in the wind tunnel had a “first contour” expansion ratio of 1:9.85 corresponding to an ideal design pressure ratio of 134. The downstream, “second contour” had an expansion ratio of 1:19.7 corresponding to an ideal design pressure ratio of 374. The conventional nozzle fabricated for testing in the wind tunnel had an expansion ratio of 1:11.3 corresponding to an ideal design pressure ratio of 102. Imaging of flow structures was achieved using a schlieren optical system. In addition, the computational fluid dynamics program ANSYS Fluent was used to model both a hypothetical, full-scale nozzle as well as the scaled version tested in the wind tunnel. Simulations were performed for full-scale, dual-bell nozzles operating at pressure ratios (chamber to ambient) of 20, 50, 100, 150 and 1000. These results were then compared to a similar conventional nozzle for each case. Preliminary schlieren results indicate that the scale wind tunnel dual-bell contours show flow structures consistent with the Fluent results.