Problem- HPR fin root/trailing edge cracking, G10 , multiple examples. Help?

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Rob702Martinez

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Having this reoccurring theme and not sure where to go from here. I have 5 HPR that all have the same crack. It seems like it occurs in flight.

Fins are all kit supplied G10, G12.
RocketPoxy, kevlar for bonding.
Internal fillets.
Rounded or routed leading edges.

Pics below.

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That fillet crack propagation from the aft end of the fin is what I’ve seen on some of my rockets with fin flutter. A stiffer fin (stiffer material or thicker g10) and/or larger fillets can prevent that.
It could also be that your fins sweep back and the impact of landing is causing this. Same solution.
 
Make sure that you properly bond prep. Sand wherever the epoxy will reside and then clean thoroughly with alcohol. Do not touch the area after prepping. Even with that, my Dynacom Scorpion developed such a crack when one fin landed firmly on a rock. The fin had a large ding in it and the root of the fin developed a crack. I ground the fillet away around the crack, bond prepped the area, then re-applied the damaged fillet. Flew last month on a 76-3500 L700 to mach without issue.

I think that it's due to the flexibility of the G10. The fin flexes when it takes a shot but the epoxy does not. The hard and brittle epoxy cracks as a result.
 
Thanks for the replies. I would imagine this is fin flutter OR from landings... I can say for sure that the 2 Blue rockets did that in flight, as it spiraled half up. The others are suspect on the landings maybe. Had you not mentioned it I would have never considered it. Also bonding and composites all done by the book.

Just short of making my own plate (materials on hand) or Laminating (wrap and vacuum) the existing fins with carbon/Kevlar, then using them as usual?
 
Either of those should work fine. I don’t know that they even need vacuum bagging. Just a wide strip on each side and a slightly narrower strip, overlapping the bottom third or so of the fin and an inch of the body tube should be sufficient.
 
Having this reoccurring theme and not sure where to go from here. I have 5 HPR that all have the same crack. It seems like it occurs in flight.

Fins are all kit supplied G10, G12.
RocketPoxy, kevlar for bonding.
Internal fillets.
Rounded or routed leading edges.

I agree with the previous replies questioning possible fin flutter and/or landing impact on a fin.
A few questions:
What kind of velocity do your rockets see?
What are your descent rates under the main chute? Any chance you're touching down faster than ~20 ft/sec?
What size radius are fillets? (for each tube diameter)
 
Kinda like a full tip to tip, but just on the root area?

That said, if the problem is fin flutter here is what I would do.

Large filets with Kevlar pulp or chopped carbon fiber. Then a series of layers similar to a tip to tip. Start with a layer that is smaller than the fin then another larger layer, and then a full sized tip to tip.


Sent from my iPhone using Rocketry Forum
 
It would be nice to get the specs on each rocket/fin combo and what motor you flew when first noticing the crack. Learned early on how the G10 fin thickness typically supplied with kits today are far below the advertised motor range when it comes to flutter.

From the Ether...
 
My 4" bird has the same problem from landings. I've flown her six times, and two of the six cracked a fin root on the trailing edge. The first time not so bad... Landed under main chute. The last time I flew it, pretty bad... I had a piece of tape holding the Kevlar blanket burrito'd around the main chute... Forgot to remove the tape. But I was careful enough to not wrap it, and literally just put enough tape to let it rip off under descent (I made sure I didn't even push the tape into the blanket...). Well, guess what? Tape stayed put. Main popped, but no chute... Just a burrito coming down.

From what I can gather, the upper section "staked" into the ground vertically, then the motor section was still horizontal, came down right on top of the upper section -- The damage indicates that the upper section collided with the motor section pretty aggressively. I don't even think I'll bother fixing this one. If I do, I'll just cut off the body tube above the motor tube (assuming there's enough space) and splice a new body tube over it. We'll see.

I should take pics sometime.

-Mike
 
This might be a stupid question, but here goes; does anyone ever see (hear) fin flutter with tube fins?
 
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