¶ Design Criteria
- Provide enough lift to stabilise the rocket.
- Minimise drag.
- Survive aerodynamic forces and not break off.
- Maintain shape.
- Minimise fin fluttering.
- Survive thermal stresses.
¶ Failure Conditions
- Bending stresses from aerodynamic forces causing the material to yield or break.
- Excessive fin flutter (1)
- Shockwave forces (not an issue at subsonic speeds) (1)
- Thermal stresses (2)
¶ Shape
Some basic design guidelines:
- Span should be approximately 2x body diameter. (3)
- Root Chord should be approximately 2x body diameter. (3)
- Tip Chord should be approximately half root chord. (4)
Next, I went about looking to determine what was the optimal fin profile shape. I came across some research by Apogee Components (5) which compared several fin shapes and looked at how much drag they produced using CFD analysis. Each fin was mounted to a dummy rocket and was scaled 10x. The enlarged model was then exposed to an incoming flow of 100mph. The drag was recorded and is shown in the table below:
Table 1: Comparing CFD Drag Forces on Different Fin Shapes. Model scaled 10x and at 100mph (5)
|
Angle of Attack |
Elliptical |
Trapezoidal |
Rectangular |
Clipped Delta |
|---|---|---|---|---|
|
0° |
9.508 |
10.690 |
11.337 |
9.357 |
|
5° |
12.052 |
12.262 |
11.685 |
10.907 |
Figure 1: Sketches of the Fin Shapes being compared in CFD Analysis
¶ References
(1) Model Rocket Aerodynamics. Georgia Tech: Ramblin' Rocket Club. Available from: http://rocket.gtorg.gatech.edu/files/slides/Model_Rocket_Aerodynamics.pdf
(2) Andrew Ng. Fins and Fin Mount Failure Mode Analysis. Available from: https://docs.google.com/spreadsheets/d/1zov3Pugj2EFwyvIPqu53tUPwku7ew5FWaRten92Suy8/edit#gid=0
(3) Alvarez A, Grace G, Evan K, John O'Neill, Jackson Whitehouse. Design and Integration of a High-Powered Model Rocket-II. 2019. Available from: https://web.wpi.edu/Pubs/E-project/Available/E-project-032219-103507/unrestricted/Design_and_Integration_of_a_High-Powered_Model_Rocket-II.pdf
(4) Dr Nnorom Achara, Engr. Bongdap Nanbol Keza. Aerodynamic Characterisation of Rocket Fin Flutter Using Computational Fluid Dynamics (CFD). World Journal of Innovative Research. 2018; 5 (2): 30-34. Available from: https://media.neliti.com/media/publications/262463-aerodynamic-characterisation-of-rocket-f-a5fda0b0.pdf
(5) Tim Van Milligan. What is the best fin shape for a model rocket . Peak of Flight Newsletter. 2017; (442): Available from: https://apogeerockets.com/education/downloads/Newsletter442.pdf