From someone who knows a lot about Aerodynamics, what is your opinion on 3 vs. 4 fins? Non-MD 4 finned rockets have qualitatively seemed to fly better for me. For MD's, if designed right they don't sim (by OR) to be any less efficient either. Adding the fourth fin allows the span of the four fins to be reduced to return to the same stability margin, which in turn seems to effectively negate the added drag of the fourth fin. Additionally, the lower span means the fin itself will be stiffer and less prone to flutter. The only real negative I see is that the four-fin design will weigh more. I would only see this mattering though for much larger (M,N,O) MD's where the optimal mass is effectively as light as you can make it.
I prefer 4 finned rockets, they appear to have better "flying qualities" characteristics than 3 finned rockets (less roll, less coning, etc., with combined headwind/crosswind). Higher order stability and control derivatives being considered to be added to RASAero II will likely confirm this,
But theoretically you can show that for the same stability margin, 3 fins will be lower drag. Although as you noted, 3 fins will have a longer fin span, and thus more likely to flutter.
One thing to note, I get nervous when the fin span (root to tip) gets down to the diameter. The fin span to diameter ratio should not be less than 1.0. Additionally, I'd make sure the fin span measured from the top of the fillet, divided by the diameter, is not less than 1.0.
RASAero II appropriately penalizes the stability contribution of these short span fins, although the impact of the fillet is not included.
So I'd start with 4 fins, a fin span from the fillet divided by the diameter with the ratio 1.25, then design the rest of the fin. A 45 deg leading edge sweep for Mach 2, a 60 deg leading edge sweep for Mach 3. And then size the fin up or down to get the 2.0 calibers stability margin at all Supersonic Mach numbers.
One additional note, just like when doing altitude prediction comparisons it is very important to measure the pre-flight weight of the rocket, an important piece of flight data for this rocket will be the measured lift-off CG, measured just prior to launch.
Chuck Rogers
Rogers Aeroscience
) the fillets, so they are finally acceptably aerodynamic. It's been sanded up to 400 grit, need to pickup some higher (or is it lower? lol) grit, finer sandpaper to get it perfectly smooth and a bit shinier.
How it goes into the rocket
When the harness becomes taught under the parachute
