Cold Flow Investigation of a Stepped Helix Grain Geometry Configuration in a Solid Propellant Hybrid Rocket Motor

The goal of this research is to conduct cold flow simulations of the stepped helix geometry to study the fundamental fluid behavior as the flow traverses through a stepped helical port. The stepped helix configuration is a combination of individual solid fuel “pucks” stacked in the axial direction. The fuel pucks port positions are offset approximating a smooth helix model design. In 2023, Glaser et al. [1,2], achieved a 245% increase in regression with flow rates of 330 g/s and thrust levels of 800 N by retrofitting a stepped helix fuel grain design into an existing motor. The cold flow simulations of the stepped helix were developed using CONVERGE [12], a commercial, general-purpose, computational fluid dynamics (CFD) solver. As expected with any stepped geometry, the flow patterns indicated regions of recirculation in the vicinity of the steps. The confined flow-space and unsymmetrical step-in did not allow for a solid recirculation region expected in a step-constriction. Similarly, the step-out indicated a region of low pressure without a well-defined recirculation region. The flow also showed a reduced axial velocity component in the post step region, or after the step-in. This is indicative of a stronger radial component at the cost of the axial flow. As the flow progresses further through the chamber, we see lesser coherent structures due to increased turbulence. The step-in region showed no votrtical recirculation, whereas step-out indicated faster recovery. Towards the end of the model the flow has significantly higher turbulence, shown by even lesser coherance in structures at step-in, or step-out. This overall flow trend can be attributed to higher inertial effects. Despite the increased turbulent effects of the stepped helix geometry the magnitude of the axial velocity at the surface of the chamber remained relatively constant. This could influence a uniform regression throughout the chamber.