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OK… So I’m finishing up the fin can portion of my LOC 4in AMRAAM build and I’ve run into a bit of a dilemma.

Will epoxying the top CR/recovery eyelet to the fin can by applying an epoxy to the inside and outside of the MMT(inside radius), then epoxying CR to the inside of the coupler going to be strong enough? Just the 3 points of attachment. I’ll attach the next section with screws. My mind says it should be ok or it’ll work but only for a limited # of launches. Possibly damage the attached section? but I’m wondering what are other peoples thoughts?

I appreciate the wisdom from more experienced Builders. :questions:


5E54C66D-5B31-44A2-A816-7137D13BD7AD.jpeg


Rob
 

bjphoenix

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If you use a U-bolt you connect to 2 locations on the centering ring. When I bolt through a centering ring I put a pretty big washer on the inside to distribute the load a little better.
 

cls

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by the time the eyebolt or u bolt rips out, the rest of the rocket is likely destroyed. epoxy the CR, good enough.

I prefer to use nylok nuts, instead of regular hex nuts with epoxy. I've seen that fail.
 
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If you use a U-bolt you connect to 2 locations on the centering ring. When I bolt through a centering ring I put a pretty big washer on the inside to distribute the load a little better.
by the time the eyebolt or u bolt rips out, the rest of the rocket is likely destroyed. epoxy the CR, good enough.

I prefer to use nylok nuts, instead of regular hex nuts with epoxy. I've seen that fail.

Thank you for the input. I’ll switch out the eyebolt for a u-bolt after the festivities..

Sorry my post my have been a bit confusing definitely love the u-bolt upgrade

But I was more concerned with the CR attachment. I can only epoxy the inner diameter(both sides) and only 1side of the outer diameter. Will this be strong enough to support about 4lbs conservative guess)throughout the recovery?

Thanks again everyone! 😁😁
 

cls

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But I was more concerned with the CR attachment. I can only epoxy the inner diameter(both sides) and only 1side of the outer diameter. Will this be strong enough to support about 4lbs conservative guess)throughout the recovery?

that's all I ever do, never an issue.
 

Handeman

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I used a u-bolt in a CR with a 54mm MMT into a cardboard coupler in the top of a scratch built "zipperless" fin can that had a glassed tube and 1/4" ply fins that were tip to tip glassed. About 5 lbs. It was mounted like you plan, filleted both sides along the MMT but only on the top face to the coupler. I retired the rocket after 50 flights and the fin can/u-bolt was never an issue. It absolutely needed a ball bearing swivel on the fin can because it would spin +100 RPM coming down under drogue.

DSCN0031.JPG


DSCN0033.JPG
 
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I used a u-bolt in a CR with a 54mm MMT into a cardboard coupler in the top of a scratch built "zipperless" fin can that had a glassed tube and 1/4" ply fins that were tip to tip glassed. About 5 lbs. It was mounted like you plan, filleted both sides along the MMT but only on the top face to the coupler. I retired the rocket after 50 flights and the fin can/u-bolt was never an issue. It absolutely needed a ball bearing swivel on the fin can because it would spin +100 RPM coming down under drogue.

View attachment 548953


View attachment 548955


Thanks Handeman. I guess it’s hard to explain…I wasn’t going to be able fillet on top the CR to any couple, as my CR rests on top of it. Just the outer diameter of the MMT. The diameter of the CR is the same as the coupler. I didn’t have a means to shave it down to fit inside the coupler. Did you sand yours down to fit the inner diameter of the coupler? Or did you find a specific CR of the same inner coupler diameter? Or maybe another method all together.

image.jpg

I did however, make a cut into the coupler and I filled the underside/inside of the coupler with epoxy, taped the hole shut and let it cure “upside” down to attach the the bottom to both the MMT and coupler. That was the main question I had for the forum. Is that type of attachment going to be reliable? I’m sure it was my explanation that fell short of my desired responses, but as this is all fun and games I went through with it anyways and come launch day I will find out for sure how well it holds up.

BCD709A0-0E5A-487C-B7A8-1ABA853BA8FB.jpeg
 

Handeman

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With that depth of epoxy inside on both the MMT and coupler, I think you will be fine. Just go easy on the ejection charges.

I built that in 2008/9 for my L2 cert in 4/2009. I don't remember exactly what I did (no build thread), but I believe I used my circle cutter to make the CRs. Then of course, sanded it to size.
circle cutter.jpg

A jig saw and sanding block would be more work, but doable also.
 
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With that depth of epoxy inside on both the MMT and coupler, I think you will be fine. Just go easy on the ejection charges.

I built that in 2008/9 for my L2 cert in 4/2009. I don't remember exactly what I did (no build thread), but I believe I used my circle cutter to make the CRs. Then of course, sanded it to size.
View attachment 549087

A jig saw and sanding block would be more work, but doable also.


Thanks for that tidbit of info Handeman! I’m not much a woodworker, most of my talents are in metal working, which are also pretty limited but, I’m not as familiar with woodworking tools.

Where do you purchase the 1/4 or 1/8 plywood that’s used? Also, isn’t it like aircraft plywood that’s typically used for rockets or is that even necessary?
 

Handeman

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Thanks for that tidbit of info Handeman! I’m not much a woodworker, most of my talents are in metal working, which are also pretty limited but, I’m not as familiar with woodworking tools.

Where do you purchase the 1/4 or 1/8 plywood that’s used? Also, isn’t it like aircraft plywood that’s typically used for rockets or is that even necessary?
Most 3mm to 12mm (1/8 to 1/2) ply I get from a Hobby/Craft store like Michael's.
You can order a lot of plywood like that through Lowes or Home Depot for store pickup.
Wicks Aircraft and Motorsports also sells aircraft grade ply in various sizes
 

jmasterj

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Thanks Handeman. I guess it’s hard to explain…I wasn’t going to be able fillet on top the CR to any couple, as my CR rests on top of it. Just the outer diameter of the MMT. The diameter of the CR is the same as the coupler. I didn’t have a means to shave it down to fit inside the coupler. Did you sand yours down to fit the inner diameter of the coupler? Or did you find a specific CR of the same inner coupler diameter? Or maybe another method all together.

View attachment 549061

I did however, make a cut into the coupler and I filled the underside/inside of the coupler with epoxy, taped the hole shut and let it cure “upside” down to attach the the bottom to both the MMT and coupler. That was the main question I had for the forum. Is that type of attachment going to be reliable? I’m sure it was my explanation that fell short of my desired responses, but as this is all fun and games I went through with it anyways and come launch day I will find out for sure how well it holds up.

View attachment 549063

I've done that same hole/epoxy backfill thing before, seems to work fine. I did make sure to prep the inside surfaces before assembling everything.
 
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I've done that same hole/epoxy backfill thing before, seems to work fine. I did make sure to prep the inside surfaces before assembling everything.

Okay that’s promising. thanks!… I did prep the insides as well prior to attaching/ epoxying the CR on. I had a feeling it will/would work but I wanted to see what more experienced builders thought.
 

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