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  #1  
Old 12-28-2009, 12:37 PM
GA_Bar GA_Bar is offline
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Question Estes Gemini/Titan GT-3 K-21 Fin can help

I have built a Gemini Titan K-21 clone. I made the fin can (first time I have ever attempted that) using clear PETG .015 thick for the can and .040 for the fins. The fin assembly fell apart right at engine burnout. My guess is the fin vibration tore it apart. Anyone have any hints or tips they can share for building a reliable fin can for the GT-3? What are the proper materials to use? Where can I get those materials?

Thanks in advance.

Bob
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  #2  
Old 12-28-2009, 01:00 PM
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BEC BEC is offline
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What did you use to glue the PETG? Almost nothing sticks to polyethylene....

The originals were made from butyrate plastic (no, I don't know where one can get it now) and clear butyrate dope (used on model airplane covering - or full scale airplane fabric coverings for that matter) was the adhesive. It (theoretically) partially dissolved the plastic to make the joints sort of welds, similarly to how "plastic model cement" softens the styrene most plastic models are made from and melts the parts together.

In practice getting the fins to stay stuck with the butyrate was problematical (or so I remember from when I built one in my youth).....

I'm sure there are others here who have some more useful advice .
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Old 12-28-2009, 06:18 PM
GA_Bar GA_Bar is offline
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Thanks BEC. I used CA to glue the parts. Actually the seam of the fin can held together and the fins (at least the ones I found) were still attached to a piece of the fin can. I'm hoping there is a better way or sturdier materials to make the fin can assembly
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Old 12-28-2009, 06:55 PM
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kurtschachner kurtschachner is offline
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Constructing a durable but attractive fin unit for this model is no doubt the most difficult aspect of its construction. When I cloned one back a hundred years ago (actually the late '90s) I used 0.020" and 0.040" polycarboate that I purchased from a local plastics supply house. I then had a machinist friend make me a BT-70 sized aluminum mandrel that I curled a strip of the polycarbonate to and secured with a piece of shim stock and two hose clamps. I heated the assembly in an oven to arouund 300 degrees F, (near the glass transition temperature of the polycarbonate). Once cooled it yielded a nice stress-free ring.

I then attached fins cut from the heavier stock using a solution of 10% polycarbonate in methylene chloride (placing one of the fins over the ring joint). There are commercial solvent bonding agents for polycarbonate however, you can get them at the plastics supply house. When I was done I had a nearly indestructable fin unit that looked pretty good on the model.

I still have the mandrel and I always make the offer to loan it out if anyone wants to borrow it. I'm out of the 0.020" sheet though so you're on your own for that. Below is a picture of a completed unit plus one I made the same way for the BT-60 sized Thor-Agena B. Also is a picture of the mandrel for BT-60, this one is made from stainless steel.

How did the construction go? Did you have any problems with the engine bells?
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Old 12-28-2009, 07:36 PM
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GregGleason GregGleason is offline
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Nicely done Mr. Schachner!

That is one of the best executed clear fin units I have ever seen.

I, for one, would be interested in a step-by-step process (with pictures ).

Greg
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Old 12-29-2009, 09:58 AM
GA_Bar GA_Bar is offline
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Wow, that's very impressive and way above my skill level. However, I may give it a try. I have .020 and .040 polycarbonate on the way. The mandrel may be a bit tougher to manufacture. Not sure if Mr. Gleason was joking or not but I would also be interested in step-by-step process if you are up to it.

As far as the construction, it went fairly well. The engine bells were a bit problematic and I made some mistakes but all in all a solid effort for a newbie. If I can get a solid fin can assembley I have enough material to give it another go.

Bob
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Old 12-29-2009, 10:53 AM
Rocketcrab Rocketcrab is offline
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Quote:
Originally Posted by GA_Bar
Wow, that's very impressive and way above my skill level. However, I may give it a try. I have .020 and .040 polycarbonate on the way. The mandrel may be a bit tougher to manufacture. Not sure if Mr. Gleason was joking or not but I would also be interested in step-by-step process if you are up to it.

As far as the construction, it went fairly well. The engine bells were a bit problematic and I made some mistakes but all in all a solid effort for a newbie. If I can get a solid fin can assembley I have enough material to give it another go.

Bob



I think you've hit on the most frustrating thing about cloning some of the golden oldies from the rocketry past. I've cloned both the GT-3 and the Estes Thor-Agena B, but they are both still hanger queens due to my inability to fabricate those darn fin cans!
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  #8  
Old 12-29-2009, 03:52 PM
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Kurt,

Where did you order the polycarbonate? It's expensive at Tower Hobbies.
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  #9  
Old 12-29-2009, 04:47 PM
GA_Bar GA_Bar is offline
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I ordered mine from Horizon Hobby (www.horizonhobby.com)
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  #10  
Old 12-29-2009, 04:54 PM
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Quote:
Originally Posted by GA_Bar
The mandrel may be a bit tougher to manufacture.

Bob


You can borrow mine. PM me with your mailing address and we can go from there.
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