One of the things I hope to try with my Level 3 certification rocket is the use of removable fins. I haven’t seen this done before, so this could be either really cool or heading for disaster. I’ll let you know when I find out. I’ve seen far too many people miss cert attempts because of broken fins. My Level 3 is by far the biggest and most ambitious project I’ve embarked on, and I don’t want to have to deal with busted fins. My thought is that if I break a fin, I can remove a fin retainer, pop out the busted fin(s), and slide in a new one. This should also be very helpful in the “must be capable of flying again” department in the certification process. As well as retaining my fins, this device will also have to retain my motor (which I value much more than the fins, by the way). This device, of course, is only one part of the fin retention mechanism. The rest is mounted in the motor mount and centering rings as a locking jigsaw mechanism.
My device incorporates an aft centering ring (1/4″ plywood) with three fin end tab slots. The fin end tabs are specially designed into my fins to meet this adapter. The slots are a closed geometry to hug the aft of the fins inward. A set of six screws (#6-32, zinc-plated steel, 1.25″) binds this special centering ring to a permanent aft centering ring (1/2″ plywood). This six-screw feature constrains the z-degree of freedom for the fins along with the forward part of the mechanism (not pictured) to prevent the fins from sliding.
Below is the reverse side of the retainer, more clearly featuring the fin end tab slot profile.
I’ve also incorporated my motor retention device into this–an Aero Pack 75mm flanged retainer. I’ve bolted this on with twelve #6-32 zinc-plated steel screws and steel tee nuts. The screw ends have been cut off at the tee nut surface due to insertion and removal difficulty into the pilot holes on the permanent aft centering ring.
I’ve test fit the fins and adapter and confirmed that this design works. There is currently a bit of a loose fit in one of the fins such that it flutters, though I think that may disappear with the addition of fiberglass plating on the fins and a slotted airframe. I had originally planned to resolve the issue with the addition of aluminum angle stock and binding posts, but the weight penalty seems too severe for my stability. I am currently exploring alternate options.