Weird fin break

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Well-Known Member
Jan 18, 2009
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I've always said that if you stick around this hobby long enough, you'll always see something new. Yesterday I got the chance to take advantage of the beautiful weather here in SE PA - light winds, mid-60's temps - to fly a few models. One of them was my SEMROC Mark, which had never flown before. Even though the breeze was light, I wanted to keep the alititude down, so I flew it on a 13mm A3-4 using an adapter.

The boost was great, but between burnout and ejection, I saw it go squirrelly. When I recovered the model, I saw why - one of the fins had failed across the grain. Not at the root, not with the grain, but against the grain at about the mid-fin area. It did not break upon landing, because the fin piece was no where in sight. I wouldn't have thought that the stresses caused by an A3-4 would do that. :confused2: I'll try to post a pic later.
That is very weird! Sounds like a flutter break or there was damage to the fin stock. Was this a new rocket?

I'm new to SE PA, where do you fly at? I'm in the Ridley Park / Springfield area.
Have to go along with the bad fin stock. :( Mostly on a A3-4.
The only time I've had a fin break like that was in the car on the way home from a launch.... :blush:

Don't have much else to offer, maybe weak balsa, or the fin got damaged during transport/handling. Sometimes when the young kids are on recovery duty the rockets take a beating... ;)
The fin of the Astron Mark has a 2.125" root edge, a 1.125" span and a 1.0" sweep. Fin flutter would definitely not be an issue with is design, even if you flew it on a D24! The design is quite durable and can take a lot of abuse, so I don't think that a crack suffered during mishandling would be too likely either, unless someone used it in a tug-of-war match. It's the type of rocket that you can just toss into a box with a bunch of other loose rockets without worrying about damage to it (except for some scuffed paint maybe). My guess would be that the fin already had a scratch, light gouge or crack in it when it was glued on. It could have been something that was too light or faint to really stand out and be noticeable, which is why you didn't pick up on it during assembly. A couple of questions: did the fin show any separation of the root edge bond with the body tube? Also, what glue did you use to bond the fins onto the tube, and did you give the fin/tube joint fillets?

I don't have the Semroc Mark xKit (I have a Mark that I scratch-built) but I would guess that the fins were laser-cut in a sheet of balsa in the kit. When I want to separate laser-cut fins from their sheet, I don't press them out by hand; instead I place the sheet down on my cutting mat and use my hobby knife to cut the little stems that are still holding the fin to the sheet. I started doing it that way early on, after cracking a couple of fins when I tried to simply press them out with my thumbs. I'm not trying to imply that this is what happened in your case (I didn't watch you build it, after all); I just wanted to mention it as a tip.