The Fin Can Assembly includes the fins, fin can, and a thermal barrier layer between the motor casing and fin can.
- The Assembly shall be designed in such a way that the static stability of the rocket remains greater than or equal to 2.0 calibers throughout the entire flight.
- The Assembly shall withstand thermal heating due to streamline stagnation and skin friction.
- The Assembly shall withstand lifting loads proportional to dynamic pressure and fin area.
Stagnation Temperature
The stagnation temperature on the root side of the leading edge of the fins is given by:
formulize T_stag = T_static + 1/(2*c_p) * (v_2 - v_1)^2
where v_2 - v_1 = v_2 * cos(λ)
where c_p = 1004.5 for air and Lambda is the sweep angle of the fins
Assuming the rocket reaches a target of 30K ft AGL:
T_static = 59F - .00356(F/ft) * h_{max_v} (ft)
Therefore,
T_stag = (59F - .00356(F/ft) * h_{max_v} (ft)) + (4.9776*10^{-4}) * (v_2 * cos(λ))^2
v_2 = Mach 2 = 2*343 (m/s) = 686 (m/s)
T_stag = (59F - .00356(F/ft) * h_{max_v} (ft)) + (4.9776*10^{-4}) * (686 (m/s) * cos(λ))^2
Note that the first half of the sum is in degrees F whereas the second half of the sum will be in degrees K.
Dynamic Pressure
Dynamic pressure (q) represents the aerodynamic pressure exerted on a vehicle in motion through a fluid (air). It is defined by the equation:
q = (1/2)*ρ*v^2
where:
ρ = density of air
v = vehicle velocity
As a rocket ascends through the atmosphere, its velocity increases, and the air density decreases. The point at which these two factors result in the greatest aerodynamic load is known as Max Q—the maximum dynamic pressure. This moment represents the peak mechanical stress experienced by the rocket due to aerodynamic forces.
Structural integrity of the rocket must be maximized to withstand the peak load at Max Q.
Dynamic Stability
Dynamic Stability is how systems respond to disruptions over time. Defined by:
d^2x/dt^2+2𝞯⍵(n)(dx/dt)+⍵(n)^2x=0
In which:
x: displacement
𝞯: damping ratio = (c/c_c): actual damping/ critical damping
⍵(n): natural frequency