Minimizing Hydraulic Losses in Additively-Manufactured Impinging Liquid Rocket Injectors

Hydraulic losses across a liquid rocket injector are undesirable, and are largely dependent on injector flowpath design. The expanded design freedom of additive manufacturing (AM) enables flowpath tapering which mitigates the inertial losses generally associated with sharp features within conventionally-manufactured injector geometries. However, the increased surface roughness inherent to AM parts can introduce additional frictional viscous losses, potentially offsetting the hydraulic benefits of the AM geometric optimization. Here, computational fluid dynamics simulations informed an iterative design to minimize hydraulic losses in a canonical conventionally-manufactured impinging rocket injector. Preliminary, smooth-wall steady CFD models exhibited how AM-enabled flowpath tapering, particularly applied to the orifice inlet, can improve injector discharge coefficient by significantly reducing inertial viscous losses, roughly +10% for the injectors modeled herein. Further computational analyses that account for surface roughness effects were performed to predict hydraulic performance, and a brief experimental validation effort was conducted. Several promising AM tapered injector designs were down-selected for printing that ranged in orifice diameter and flowpath surface roughness. Chemical and abrasive honing methods were further evaluated to reduce surface roughness. Injector discharge coefficients were experimentally determined via cold-flow testing over a range of injector pressure drop, showing good agreement with computational results. Both computational and experimental results suggest that the benefits of optimized flowpath tapering can outweigh the downsides of AM surface roughness beyond some characteristic minimum orifice diameter, which was found to be approximately 0.020" in this study. Post-print honing was also found to provide significant mitigation of viscous friction losses associated with AM.