Small sugar-propellant rocket motors, often called R-Candy motors, provide an accessible platform for studying propulsion and nozzle design. Their relatively simple construction makes them suitable for educational and experimental rocketry, but predicting how combustion pressure, temperature, and exhaust flow interact inside a small motor can still require considerable trial and error. In particular, the nozzle must accelerate the combustion gases while maintaining stable chamber pressure, manageable thermal loads, and consistent thrust.
Researchers at the Instituto Politécnico Nacional combined a systems-based design process with Computational Fluid Dynamics (CFD) to connect simulation, physical measurements, and design refinement. A preliminary motor and convergent-divergent nozzle were developed in SolidWorks and transferred to FEATool Multiphysics, where the internal flow and thermal fields were simulated. The FEATool results were then compared with static-firing measurements and fed back into the design process. The study found that the CFD model reproduced the main observed propulsion behavior and provided flow and temperature information that could be used to improve the nozzle geometry and simulation setup.
The motor consisted of an igniter, combustion chamber, and convergent-divergent nozzle designed around a potassium nitrate-sorbitol propellant. The preliminary components were modeled in SolidWorks and the geometry was transferred into FEATool Multiphysics for fluid dynamics analysis. With the easy-to-use GUI of FEATool, the researchers prepared the computational geometry, generated the mesh, specified the governing equations and boundary conditions, solved the model, and evaluated the resulting pressure, temperature, velocity, and streamline fields. The internal gases were modeled as a compressible ideal gas, with the Navier-Stokes equations coupled to energy transport. The simulations also used a k-omega SST turbulence model, and different mesh refinements were tested to check that the predicted thermofluid fields were not strongly dependent on mesh resolution.
The FEATool simulations showed the expected main behavior of a convergent-divergent rocket nozzle. Pressure decreased from the combustion chamber through the nozzle, while the flow accelerated from the converging section into a supersonic regime in the divergent region. The calculated temperature field revealed strong thermal gradients around the throat, highlighting a region where material selection and thermal management were particularly important. Streamline and velocity results also allowed the researchers to examine local recirculation, flow acceleration, and shock-related behavior that would be difficult to characterize from external thrust measurements alone. During the numerical work, nonphysical negative pressure values were identified as numerical artifacts. The model was subsequently refined through changes to the local mesh, outlet boundary conditions, and pressure treatment until the spurious behavior was removed.
Rather than treating each flow calculation as a final result, the researchers used the FEATool output as feedback for successive design decisions. Pressure-gradient behavior near the nozzle neck led to a reduction of the throat diameter from 6.80 mm to 6.57 mm, improving pressure uniformity and stabilizing the flow before the divergent section. Localized thermal loading identified in the simulations also motivated a change to the nozzle convergence angle to redistribute heat more effectively. Mesh density, inflow conditions, combustion parameters, and boundary conditions were likewise adjusted between simulation cycles. The resulting designs were evaluated using pressure uniformity, outlet temperature homogeneity, and exhaust velocity consistency as quality indicators, connecting the CFD solution fields directly to the wider design and optimization process.
Experimental testing provided an independent check on the numerical model. Six static firings were performed, with two of the most stable tests selected as reference cases for detailed CFD comparison. For these cases, the reported FEATool predictions closely followed the measured chamber pressure and thrust, while burn time, total impulse, and specific impulse also showed small differences between simulation and experiment. This agreement supported the use of the computed flow fields for evaluating and refining the nozzle rather than relying entirely on empirical adjustments. The model nevertheless remained an engineering approximation: combustion was simplified, effects such as fuel-grain regression and nozzle erosion were not fully represented, external atmospheric variations were not dynamically modeled, and the available experimental dataset was limited.
In this work, FEATool Multiphysics provided a fully integrated CFD environment connecting CAD geometry, meshing, equation and boundary-condition definition, solution, post-processing, and iterative comparison with experiments. This allowed simulated pressure, temperature, velocity, and streamline fields to become practical inputs to nozzle refinement rather than isolated numerical results. For readers developing related models, the FEATool documentation includes the Supersonic Turbulent Flow Past a Prism tutorial for compressible high-speed flow and shocks, and the Heat Exchanger tutorial for coupled fluid-flow and heat-transfer modeling. These provide useful starting points for related CFD workflows, although they do not reproduce the R-Candy motor model.
References
- Alejandro Pisil-Carmona, Emilio-Noe Jimenez-Navarro, Diego-Alfredo Padilla-Pérez, Jhonatan-Fernando Eulopa-Hernandez, Pablo-Alejandro Arizpe-Carreon, and Carlos Couder-Castañeda. Systemic CFD Framework for Performance Optimization of R-Candy Propulsion Systems, Applied Sciences, 16, 1592, 2026, doi: 10.3390/app16031592.