Numerical and Experimental Investigation of a Passive Mixing Device in a Flight-Scale Hybrid Rocket Engine

Hybrid rocket engines (HREs) retain continued interest across academic, commercial, and student-led areas due to their inherent safety, simplicity, and throttling capability. By decoupling the oxidiser and fuel states, HREs are able reduce many risks and complexities of liquid and solid propulsion systems. However, they remain limited by low regression rates and inefficient mixing between the fuel and oxidiser. HRE combustion is characterised by a thin diffusion flame, constrained by the turbulent boundary layer, leading to incomplete energy release and reduced characteristic velocity efficiency (ηc*). This paper presents a novel investigation into overcoming these challenges through the integration of a passive mixing device, a perforated plate located in the post-combustion chamber designed to increase residence time, turbulence, and improve combustion efficiency. The research is centred on the USYD Rocketry Team’s flight-scale HRE, Pardalote Mk II, which employs acrylonitrile butadiene styrene (ABS) as the solid fuel and nitrous oxide (N₂O) as the oxidiser. Under the baseline configuration, the engine exhibits predicted ηc* values near 77%, confirming under-utilisation of propellant energy. The motivation of this study is to introduce a solution that is simple and manufacturable, providing a practical alternative to complex multi-port grains or swirl injectors. Unlike single-hole diaphragms explored in prior literature, this work introduces a new mixing-plate geometry with an eight-spoke wagon wheel design, a 24 mm central restriction, 12.7 mm thickness, and perforations sized to total 2.0, 2.5, and 3.0 times the nozzle throat area. Numerical Methodology and Results Computational Fluid Dynamics (CFD) simulations were conducted in Ansys Fluent using a two-dimensional axisymmetric geometry including the pre-combustion chamber, fuel grain, mixing device, and post-combustion chamber. To reduce complexity, ABS pyrolysis was approximated as gaseous ethylene injection, with N₂O defined at the oxidiser inlet. Reynolds-Averaged Navier Stokes (RANS) with a k–ε turbulence model and the Eddy Dissipation Model (EDM) was applied. Steady-state conditions were assumed with oxidiser and fuel mass flow rates of 0.997 and 0.0997 kg/s respectively and chamber pressure of 20.39 bar. Four cases were considered including a baseline configuration along with three mixing-plate configurations with varying perforation areas. Simulations resolved the chamber temperature, velocity, and species mass fraction fields and CO₂ mass fraction at the outlet was used to estimate ηc*. The simulations consistently showed that the mixing device disrupts the oxidiser core, forcing radial flow and generating vortices along the post-combustion chamber walls. These vortices increased turbulence and enabled cross-stream mixing. Compared to the baseline, mixing-device case simulations exhibited higher CO₂ fractions downstream, more uniform temperatures, and reduced unburnt N₂O. Quantitatively, the smallest perforation area (2× throat) yielded the highest ηc* (15% improvement) but also the greatest upstream pressure rise, while the largest (3.0× throat) achieved a smaller efficiency gain (8%) with less upstream pressure increase. Experimental Hot-Fire Campaign To validate the CFD results, two hot-fire tests were conducted on the Pardalote Mk II engine at the USYD Rocketry Team’s facility. Thrust was measured via load cell, with chamber and tank pressures recorded by high-frequency transducers and plume dynamics captured by high-speed video. The baseline configuration produced a peak chamber pressure of 20.39 bar and ηc* of 77%, consistent with predictions from the team’s in-house simulator Candlebark. In contrast, the configuration with a mixing device (3× throat area) produced 24.75 bar peak chamber pressure and ηc* of 86% under nearly identical upstream conditions, a 9% improvement confirming the device’s effectiveness. The ceramic-coated steel plate maintained performance through the burn, with a structural failure only at the end, providing useful insights into material optimisation for future work. Cold-Flow Testing (Planned) As additional hot-fire testing could not be accommodated, a complementary cold-flow nitrogen campaign is planned. This will characterise loss coefficients of the remaining devices across a range of mass flow rates and measure pressure drop, which will then be scaled to hot-fire conditions to predict ηc* improvements. At the time of writing, the campaign has not yet been conducted and results will be included in the final paper. Discussion The combined CFD and hot-fire results show that passive mixing devices provide a simple, robust means of enhancing hybrid rocket performance. Incorporating a mixing plate improved ηc* from 77% to 86% under identical conditions, validating CFD predictions that increased turbulence and residence time enable more complete combustion. While only one perforation area was experimentally assessed, results confirm the potential of optimising plate-to-throat area ratios to balance efficiency gains with acceptable pressure rise, with a new material solution/design required. Conclusions and Future Work This research demonstrates that passive mixing devices in the post-combustion chamber of a flight-scale HRE can deliver noticeable efficiency improvements. Novel CFD simulations and a new geometry confirmed the device’s positive effect on turbulence and mixing, while hot-fire testing provided experimental validation. Planned cold-flow testing will extend the work by providing the first direct quantification of loss coefficients for such devices, strengthening the predictive design framework. Future efforts should expand hot-fire campaigns across a broader range of tank and chamber conditions and refine the material choices to withstand extreme thermal and mechanical loads present in the combustion chamber. These findings represent a practical advancement for the hybrid propulsion community, offering insights to student teams and small-launch companies seeking improved performance without complex injector or grain redesigns.