Overview
Design of a fin can requires considering many factors including, but not limited to aerodynamic, structural, and thermal considerations, while operating under material and manufacturing limits. This article will specifically work under the assumption that you are designing a composite, minimum diameter fin can. Using a requirements based model, the necessary parameters of a fin can can be ascertained. The 'flow' of requirements will be similar to the following:
- Stability (Mass, Moments, Flight Profile)
- Thermal (Flight Profile)
- Aerodynamic (Altitude Target)
- Structural (Flight Profile)
There are also dynamic considerations. The vehicle should have an overall low pitch-roll couple to avoid 'coning' failures. The natural frequency all structural members should either be well damped or far from any frequencies excited in flight.
Stability
Main Article: Stability
The primary function of a fin can is to impart corrective forces to the vehicle if it's angle of attack is non-zero. If your vehicle architecture features an active stabilization feature, fins may be eliminated entirely. Fin geometry is the primary method of controlling a fins impact on stability. RasAero2 and OpenRocket are the primary comprehensive methods of simulating vehicle stability. For subsonic cases, or as an academic exercise, the Barrowman Equations can be solved to get an idea of a fins contribution to center of pressure location.
Thermal
Main Article: Fin Thermal Analysis
There are several concerns and constraints when doing the thermal design on a fin can. The primary sources of heating are aeroheating during ascent and conductive heating from the motor burn. The primary constraint is the resin system used in the construction of the fin can.
Aerodynamic
Main Article: Fin Aerodynamics
All Flight Regimes
Chord is the primary driver of profile drag
Subsonic
Skin friction? Induced drag?
Supersonic
Shock impingement (tribal knowledge or nonsense?)
Flow turning
Structural
Main Article: Fin Mechanical Design
The team does not currently possess nuanced analytic methods for fin can design. The included method is a conservative first estimate.
References:
Black Brant III Fin Design Paper - Internal Use Only
Minimizing Pitch-Roll Coupling - Internal Use Only