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Estes Ascender + G125 = broken fin

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dhkaiser

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Ascender Broken Fin.jpgAscender-fin.jpgAscender-fin2.JPG

We launched the Ascender today with a G125, less the payload bay as I had a BRB900 in the nosecone. The launch looked good however one of the fins broke off in flight. I had paid extra attention to constructing this rocket as my old Ascender's fin came off with a G80. Added additional epoxy fillets on the outside of the fins.

The altimeter 3 graph plainly shows the action began about 1300ft. A photo of the descent also shows the broken fin.

At the last Indiana Rocketry Club launch I witnessed an Ascender launch with a G80 and all went fine. Either the owner was better at assembly or there was a defect in the fin. Thoughts?
 

Cabernut

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Fin flutter I bet. What was max velocity and what are the fin dimentions? I can see what I get in AeroFinSim
 

neil_w

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The first picture is interesting. Maybe I'm misinterpreting it, but it looks like the top half of the fin was not securely attached to the BT. Maybe it started to peel away, and then the whole thing snapped off?

On the other hand, I just checked the instructions for that kit and it doesn't look like they even tell you to glue the fin to the BT, only the tab into the TTW slot (and they don't mention fillets either).

Is there epoxy left on the BT, or did it stay with the fin? It also seems possible that the epoxy pulled off the finish of the body tube, not really having been adhered structurally to the BT itself.

But I'm just wild guessing on a single picture. The damage looks like an upside version of one of my Elliptic II's fins, which peeled off starting from the rear on landing; the fillets and everything came with the fin, and pulled up the glassine layer.
 

ksaves2

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Fin flutter. Good luck with Aero Fin Sim as you likely won't find a material to simulate the fin material on that rocket. Are they hollow plastic? The easy answer is don't use a G125 or build another rocket with solid plywood fins with internal and external
fillets. Kurt
 

dhkaiser

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The fins are two piece plastic cemented together with CA after being sanded, then fastened to the MM through the BT with Epoxy which was also used to attach to the outside of the BT. Only the center part of the fins go through the BT to the MM. The epoxy stayed on the BT mostly including the leading edge on the BT.
 

Rex R

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have seen this happen before, quite likely the fin got 'plastic' fatigue after a number of flights.
Rex
 

cerving

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I had my PSII Nike Smoke's fins come apart on the first flight, with an H180. Those 2-piece plastic fins just don't like too much thrust... or for too long. AT SU G40 or a CTI 3G H longburn would be fine, I'm sure.
 

shreadvector

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I had my PSII Nike Smoke's fins come apart on the first flight, with an H180. Those 2-piece plastic fins just don't like too much thrust... or for too long. AT SU G40 or a CTI 3G H longburn would be fine, I'm sure.
Maybe, maybe not. The fins do not care about thrust - they react to airspeed and aerodynamic forces. A high thrust motor will get the rocket moving fast and if it reaches a speed where fin flutter occurs then the fins will fail as the flutter bends them past the breaking point. If you have a low thrust motor it could flutter if it is a long burning high total impulse motor since it will go faster and faster and eventually get to the flutter speed.

Less total impulse and select motors that will not result in speeds that can destroy the fins or rocket.

FYI I've seen a fin come off and the rocket went unstable at high speed - it was either a G80 or a G78 in one of the PSII E2X kits.
 

JumpJet

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My guess would be the air got between the left and right half of the fin at the leading edge. When this happens the fin halves are simple pushed and then ripped apart. you could add a piece of clear tape over the leading edge to prevent this from happening.

I had my PSII Nike Smoke's fins come apart on the first flight, with an H180.
Not surprised at all by this since the model doesn't list this size motor for flight. It to will have the same problem with the two fin halves splitting apart.


John Boren
 

midpower_madness

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Same thing happened with my rocket - I was using the same motor on the same model of rocket. The fins flew apart at roughly the same altitude as well!
 

JumpJet

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Just a thought. If you are going to push any of the Estes rockets that have the two piece fins you might want to apply a layer of clear tape over the entire leading edge. It's possible the air is getting into the seam along the leading edge. If this happens I can see the two halves simply being pushed apart with fin failure being the result.


John Boren
 

GregGleason

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The fins are two piece plastic cemented together with CA after being sanded, then fastened to the MM through the BT with Epoxy which was also used to attach to the outside of the BT. Only the center part of the fins go through the BT to the MM. The epoxy stayed on the BT mostly including the leading edge on the BT.
IMHO, CA is a marginal adhesive in this application. CA does not perform well in shear loads, which is the stress those fins are being subjected. Epoxy would be a better choice in this instance, as it seemed to at least hold onto the fin tabs. That said, there is a speed limit for these type fins. As suggested by JumpJet, taping the LE's might help, or as ksaves2 has suggested switching to a more robust material such as plywood.

Greg
 

Charles_McG

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I'm working on a PSII based Nike Apache, so this is timely info. I had planned to push the stack quite high at some point, so I'm keen on hearing about failures.

I can sim an H180. Do we have any more reports?
 
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