How do you select the right plywood for your rocket?

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I’m worried about this too. Reinforcing the trailing edge was the impetus for including a G10 plate in the core of the fin. That being said, I have also broken off knife-edge G10 fin tips during normal wear-and-tear.
ah good point (no pun intended). Obviously the most at risk portion will be the fin tip, which you could reinforce by not tapering the fin tip to a point. Basically, keep the airfoil shape at the tip. The drag increase would be minute. You could even put symmetric winglets (oh boy the college memories are coming back now lol) on the ends for added strength. That's going to increase drag slightly and affect the center or pressure slightly aft as the fins would be more efficient at producing "lift". Manufacturing is definitely harder tho.
 
It's easy to focus on flight stresses, but one should not forget transportation and landing damage. I had a Loc kit fly just fine on a big motor, but just happen to land with the fin edge on a rock and break it. My thought with the "J" motor was also the added rocket weight also means more force on landing.

There's sort of an interesting vicious cycle here: overbuilding makes rockets heavy which increases the forces on them. which tempts us to overbuild more...

I had this exact same experience. And to make it more interesting, I had a few flights on the rocket before I broke a fin. It just happened to land the wrong way, the wind caught the chute and pulled the rocket over and "SNAP" I lost a fin. So there's more to it than just surviving Mach, etc. The fin design and thickness as well as landing weight can also be a factor. Now I shy away from thin fins that extend beyond motor retainer. Unless it's a Goblin :)
 
I had this exact same experience. And to make it more interesting, I had a few flights on the rocket before I broke a fin. It just happened to land the wrong way, the wind caught the chute and pulled the rocket over and "SNAP" I lost a fin. So there's more to it than just surviving Mach, etc. The fin design and thickness as well as landing weight can also be a factor. Now I shy away from thin fins that extend beyond motor retainer. Unless it's a Goblin :)
I definitely prefer the trapezoidal fins that sit somewhat forward of the aft end of the rocket. Semicircles or semi-ovals are the most aerodynamic and least likely to break pieces off of, but a pain to cut and they aren't my favorite look in the world either ;)
 
For looks, for me, it's hard to beat a swept trapezoid with the pointy bottoms. For breakability it's hard to beat those too. One method for mitigation is to clip the points off parallel to the base of the rocket, even though still below it. The resulting five sided fin shape is similar to a Bertha fin, though Bertha's tip edges aren't quite parallel to her root edges.
 
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