Characterization of Swirl-Driven Hybrid Rocket Engines

Abstract

Although hybrid rockets offer several potential benefits in terms of safety and performance, their use has mostly been limited to lab-scale motors and academic settings. Foremost among the factors contributing to this situation are the various drawbacks associated with the slow regression rates of the solid fuel that make hybrids less attractive for larger-scale launch vehicles. As a result, increasing the regression rate in hybrid rockets is probably the most active research area in the field of hybrid rocketry today. As new configurations and fuels are explored and tested, it is imperative that a physical understanding of the basic mechanisms behind these new designs is also properly developed. Thus, the creation or alteration of analytical models for regression rates in hybrid rockets is as important now as it ever has been and constitutes a critical but currently neglected component of the development cycle for novel hybrid concepts. After an introduction to hybrid rockets and their unique advantages in the first chapter, the next two chapters of this dissertation are devoted to literature reviews covering previous efforts to model regression rates in classical (axial-flow) hybrids as well as swirl-driven engines. A new, reduced-order model for predicting fuel regression in swirl-driven hybrids as a function of engine and injection geometry, propellant properties, and oxidizer mass flow is then presented and validated in the fourth chapter.

In addition to accurately modeling regression rates, understanding the internal flowfields of novel, swirl-driven engine concepts also produces valuable insight into their performance. In fact, modeling regression rates and understanding internal flowfields are typically co-requisite due to the nature of hybrid rocket combustion processes. Solutions for the internal velocity fields in two novel engines are described in the latter part of this work. The first is a detailed, axisymmetric solution, complete with viscous corrections, for incompressible flow within a bidirectional vortex engine such as the Vortex Injection Hybrid Rocket Engine developed by Orbital Technologies Corporation. This analysis identifies the parameters that define the velocity field and uncovers a new form of the vortex Reynolds number, which controls the extent of the forced core region and wall boundary layers. The second solution describes, for the very first time, an untested concept dubbed the quadrupole vortex engine and is rudimentary by comparison. Nonetheless, the two-dimensional, potential-flow solution for the quadrupole vortex set permits an initial investigation into the hydrodynamic stability of such a configuration. With solutions for the velocity fields of these novel engines in hand, the regression rate model developed in the first half of the work is applied to both configurations in order to quantify their performance advantages.