Tufts SEDS Rocketry Team
Tufts SEDS Rocketry Team
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HODGE - International Rocketry Engineering Competition 2025
HODGE - International Rocketry Engineering Competition 2025
During the 2024-2025 year, I served as the Mechanical Engineering Lead for our competition rocket, HODGE, the Highly-Optimized Data-Gathering Explorer. The primary focus of our project was to improve upon our work from the previous year, streamlining and optimizing all aspects of design and the fabrication process. The scope for the subteam includes design, testing, manufacturing, and analysis of all airframe and internal mechanical components. We use 3D Computer-Aided Design (CAD), and Finite Element Analysis (FEA) software to model and simulate rocket components, allowing us to calculate expected failure criteria and simulate critical loads. Having learned a lot about high-strength composites with CARM last year, I aimed to push the boundaries of our design by heavily optimizing our fins for aerodynamic stability and precise factor of safety. Using carbon fiber cores due to their stiffness and weight, I reduced our fin thickness from 1/8” to 1/16”, lowering the vehicle’s weight and drag therefore increasing performance. To ensure that these changes were viable without risk of fin flutter, I leaned into characterizing our tip-to-tip layups and conducted testing and simulation to support design decisions. Additionally, I led the architecture design for HODGE's layout, including payload assembly and load path, avionics bay layout and accessibility hatches, and various bulkheads throughout the system. Since I was away in the Spring semester interning at Blue Origin, the majority of my contributions as mechanical lead lie in the simulation and analysis supporting optimizations across the launch vehicle.
Optimization of thrust plate in Solidworks FEA
In order to analyze fin flutter, I developed a robust flutter velocity calculator based on NACA Technical Paper 4197.
Dennis Martin's NACA Fin Flutter Velocity Equation
Professor John Bennett's correction for DN ≠ 39.3
Lekhnitskii's (1981) Anisotropic Shear Modulus
The above fin flutter velocity (denoted as Vf) is the point at which violent induced oscillations from the airflow can cause catastrophic failure in the fins. GE is the shear modulus of the fin material, a is the speed of sound at the altitude above median sea level (MSL) of the maximum rocket velocity, A is the dimensionless fin aspect ratio (twice the height divided by the area), t is thickness, c is root chord length, λ is the dimensionless taper ratio (tip chord length divided by root chord length), p is pressure at the altitude (MSL) of maximum velocity, and p0 is air pressure at sea level. Because the constant of 39.3 (sometimes referred to as the “Denominator Constant,” or DN) in Martin’s equation derives from the assumed
forward-to-aft symmetry of the fin, I dug deeper and found a substitution to correct for fin sweep, per Professor John
Bennett’s derivation. Here, cx is the axial distance from the forward end of the fin to the fin center of mass and k is the adiabatic constant of air. The 0.25 represents the offset of the center of mass from the quarter-chord, which is the airfoil’s center of pressure; the distance between this and the fin’s center of mass affects the aerodynamic forces that catalyze fin flutter oscillations. Extra care was given this year to characterize fin flutter through a detailed analysis involving calculations of flutter velocity across various thicknesses and layup configurations of composite materials. This was supported in large part by Instron testing which was conducted to determine the factor of safety associated with different thicknesses and layup configurations of composite materials. Samples were cut in both lengthwise and crosswise orientations to evaluate material strength through the shear modulus, a key parameter in the fin flutter equation from NACA. To estimate the shear modulus, we used the following relationship derived by Lekhnitskii where Ex and Ey are the Young’s moduli in the x and y directions, and vxy is Poisson’s ratio. The results of material testing were used to determine the minimum fin core thickness and number of layups to maintain the 1.5 factor of safety while reducing the weight of the vehicle.
Another major feature of HODGE’s aft airframe is a second set of removable fins, placed behind the larger fixed fins. There are two significant reasons for this design decision. First and foremost, these fins will be designed to break in case of hard impact, allowing the booster section to quickly and easily be reused after a wider range of landing scenarios. Second, these aft fins can be swapped with alternate fins of different geometries, intended to modify our projected altitude or stability. This lets us adapt to any changes during launch day conditions, such as non-ideal windy conditions, or errors in simulation that affect performance.
One concern for the second set of fins was the boundary layer behavior. As air travels along the first set of fins, a boundary layer begins to grow. In the gap between the first and second set of fins, there is a flow separation, essentially creating a vacuum. To address this, I ran a COMSOL study to identify three criteria - drag forces due to variable fin geometry, stresses on the material, and velocity profile for each design. The geometry used in this study is a simplified section of the rocket airframe containing just the fins. Three versions of the booster section geometry were created: one with the standard aft fins, one with shorter fins designed to raise the apogee, and one with taller fins designed to lower the maximum altitude reached. After each simulation was completed, the max stress in the fins was identified as well as the drag forces over the fin surface using a surface integral condition. For the drag force, the viscous and pressure-driven forces were also compared. Results revealed that pressure-driven drag played more of a factor than viscous drag which varied significantly with fin design, while stresses remained safely below material limits.
A summary of our simulation and analysis work can be found in the HODGE Preliminary Design Review, seen below.
HODGE Preliminary Design Review.pptx
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