Shock Cord Mounting (LOC)

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Hospital_Rocket

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Does anyone here actually use thet tape - string - epoxy technique specified by LOC to mount their shock cords? When you consider the quality of a LOC kit, it seems that part is so cheezy....

A
 

Rocketmaniac

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I have built two LOC kits and have NOT used the matterials that LOC provided for the Shock Cord Mounting. In both kits, I mounted my shock cord to the forward most centering ring.

I am interested to see if anyone does actually follow their instructions.........
 

n3tjm

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It works great. I never had one fail... I have LOC kits I have lost count how many times I have flown them, and the shock cord mount still holds :).

One word of caution... make sure you use epoxy.... if you use something else.... you be surprised what does not bond to paper very well ;).
 

Ryan S.

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I have used it, and it does work very very well. Which is pretty suprising
 

Stymye

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I used it on my graduator, no problems still holding strong
 

solrules

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Many people do not seem to trust the LOC type shock cord attachment (maybe because it is pretty much upscaled estes method). Cheap, dirty, unrefined, but this mounting method works very well. I have never seen one fail. The epoxy wicks itself into the body tube, making a very strong joint, not to mention the shock cord is easy to replace. In reality, the underpant elastic band is the weakest link in LOC kits, not the mount.
 

eugenefl

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Originally posted by stymye
I used it on my graduator, no problems still holding strong
Agreed, same rocket, same great results. I've flown my Graduator many times without a problem.

On the downside, I've used this method on a scratchbuilt rocket before and it didn't work as good. Reason - paper mailing tube. I had an early ejection and the mount tore away along with a layer or two of inner paper lining. Luckily the rocket "rolled" and recovered horizontally without a scratch.
 

DynaSoar

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Originally posted by Hospital_Rocket
Does anyone here actually use thet tape - string - epoxy technique specified by LOC to mount their shock cords? When you consider the quality of a LOC kit, it seems that part is so cheezy....

A
I used it on both Weasel and Vulcanite. I'll let you know Saturday night if there were any problems.

One the Rock-A-Chute Mk III, I used that method to reattach the burned through kevlar. I've flown it several times since with no problems. I may go pre-patch some others this way that might have problems with the kevlar.

Compared to more exotic methods it may seem cheezy, but if it works, it works. If it's good enough, better doesn't matter.
 

Hospital_Rocket

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Originally posted by DynaSoar
I used it on both Weasel...
That's what I am building and am finding it hard to make sure I have enough epoxy on the mount as the tube is so small, Ditto for making sure it is far enough back to not interfere with the coupler.

Compared to more exotic methods it may seem cheezy, but if it works, it works. If it's good enough, better doesn't matter. ...
Ahhhh...value engineering!
 

DynaSoar

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Originally posted by Hospital_Rocket
That's what I am building and am finding it hard to make sure I have enough epoxy on the mount as the tube is so small, Ditto for making sure it is far enough back to not interfere with the coupler.



Ahhhh...value engineering!
If the string is submerged, it's enough. More is wasted. Got a penlight?

Worked fine, by the way, on both. Both had far shorter delays than they could have and so must have got quite a rip at ejection. I checked them both. No sign of strain.
 

lets fly high

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Originally posted by Rocketmaniac
I have built two LOC kits and have NOT used the matterials that LOC provided for the Shock Cord Mounting. In both kits, I mounted my shock cord to the forward most centering ring.

I am interested to see if anyone does actually follow their instructions.........
I have two loc kits and never had a problem
 

Missileman

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I only have 1 LOC rocket, the Weasel. But I did use their method and have had no problem at all.
Hospital, I used 2 coats of epoxy to get a smooth finish so my chute had nothing to snag on.
 

Stymye

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I also glued a strip of duct tape over the knot to make the buldge smoother
 

Mad Rocketeer

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I bought a Graduator some weeks ago and am considering what will be needed to ensure that it will fly on H motors. The LOC web site mentions D through H motors for the Graduator, but the packaging lists D through G.

I figure I'll need to put filets on all the joints (internal as well as external) for the through-the-tube-to-the-motor-mount fins. That's a step up from the instructions, which call for putting on both centering rings, then installing the MMT, then gluing in the fins (if I recall correctly). [That wouldn't let me do any internal fillets.] I'll put the rear CR on after the fins so I can do internal fillets.

I also had questions about the shock cord mounting, as specified, most of which are answered here. Have all of you who have reported success here been using the supplied material (nylon, I think) or have you been swapping in kevlar? Have you been thickening your epoxy? Have you been doing more than just sanding the tube rough at the mounting point? (E.g. have you been adding pinholes for deeper epoxy penetration, etc.?)

Does a Graduator need to have its body tube or fins strengthened to handle H power? I had planned to do something with the fins to beef them up a bit, but I don't know what it needs or what will help plywood. Will tissue strengthen plywood appreciably? Soaked-in CA? Coating with finishing epoxy alone? Do I need to use fiberglass?

I want to build the Graduator strong enough to handle H power (so I can eventually certify for Level 1 with it), but light enough to fly on the specified D motor. I want a tough bird that can handle most hard landings, but I don't want a cast iron beast that survives a nose dive from 3000 feet into concrete (and punches a hole in whatever it lands on) (and can't launch safely on less than an F or a G).

Do I need epoxy fillers? What kinds for which joints/fillets? Would it be OK to go off-directions and use wood glue for the butt joints and epoxy for the fillets, rather than all epoxy, in order to save weight?

Expert advice is hereby solicited. Thanks to all. :D
 

Stymye

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For a L1 build,
probably Build the Graduator with epoxy, epoxy all the fin joints ,like you said, with the rear ring off,
on the external fillets you can add microballoons to the mix and it will sand nicely, or you can just use straight epoxy(or add a thickener) and use a favorite technique to smooth them out
a cracked joint may or may not kill the cert .
It can be done with yellow glue but you would want to pay closer attention to tolerances , butt the fins to the rings, generally be a little more critical about things.

I would forget the D motor If you want a L1 rocket
the rocket can be built to fly on that wide range of motors but it would have to be light enough for a D and strong enough for an H and noseweight might be an issue from D < H motors

the tube and fins are plenty strong for an H motor no glassing is really needed, keep in mind that it will still be pretty light for an H motor... If you like the Graduator than you would probably really like the Loc IV for level 1(a great deal for a L1 capable rocket)
it uses the same materials and technique you can build it heavier and might keep the cert flight out of the clouds... heh

If you don't feel good about the loc type shock cord mount,you could attach the shock cord to the motor mount top ring if you haven't started yet

also try some searches on L1, Graduator,Fillets,Epoxy,ect...

good luck with it!
 

JoJo

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i trust you guys when you say it'll work if done correctly, but i guess i did it incorrectly on my weasel since it seperated and i wasnt able to find either half ( landed in the woods at NARAM) so when my onyx comes im gonna attach to the CR
 

Mad Rocketeer

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Andy: Thanks for the good feedback.

Sounds like all epoxy will be what I use. I have some Bob Smith 30-minute stuff. Will that be adequate? Do I need the 20-minute finishing epoxy for the smoothest outer fillets?

I haven’t done anything at all with the model yet, other than unpack it and give the parts a cursory inspection. It looks nice.

I’ve never done epoxy anything on a rocket before, so I don’t have a favorite technique for epoxy fillets yet. I’ve read a great deal, and lots of techniques sound good. I’ve seen epoxy clay used, but that sounds heavier than necessary. Microballoons sound good. What proportions of microbaloons to epoxy works well for fillets? I don’t want cracks, but it’s good to be sandable, so there’s got to be a good level to use. I’ve heard of using finishing epoxy and letting it run down a gentle slope, smoothing it with a spoon or a gloved finger dipped in alcohol. I’ve seen the technique of using the fine glass fibers to make a really strong (but heavy) initial fillet, covered by a very smooth finishing epoxy fillet, but might that be too heavy? I've also seen epoxy (possibly filled) as an initial fillet, with Fill-N-Finish applied over it, sanded smooth, dripped with water thin CA, and sanded again. Chime in, folks, with your favorite methods. I want strength enough for an H but lightness enough for a D (if reasonably possible) or an E in any case.

Sounds like most of those posting on this thread who have used the standard LOC shock cord mounting have liked it and found it good enough. Sounds like attaching Kevlar to the front centering ring is considered a stronger technique in general but probably not necessary here. Is Kevlar usually good enough here, or is a steel leader better? I’ve seen some posts saying that Kevlar isn’t really all that fireproof after all and that it is best attached to the outer part of the CR, rather than the inner part, to keep it a bit farther from the flames. The standard LOC mount would do that too, but that’s no help if it breaks. I like hooking to the CR because it leaves the tube less obstructed as well. Guess I should pick up some Kevlar. What weight do I need?

I might want to arrange for adjustable nose weight to help with balance across the engine size range (whatever that ends up being). I’d want to be able to leave it light enough for my smaller engines to lift and to be able to add weight forward to help keep the stability correct for the larger motors (as well as keeping the rocket in sight on an H). The Graduator isn’t a payloader though. I don’t want to just add enough nose weight for an H and glue it in, because it would then be too heavy for some motors and perhaps overstable on others. Any ideas here?

Good to know that the fins and tube are good enough for an H without extra reinforcement. Would tissue be a good idea for making the fins smooth, or should I use Fill-N-Finish, or regular balsa filler and sanding, or . . . ?
 

Mad Rocketeer

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Any further advice from anyone, based on my two sets of questions above? The more I know and the more opinion I've sifted through before I start gluing, the better.

Thanks.
 

bobkrech

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The LOC method works well, sometimes too well.

Build a Vaugh Brothers Javelin 2.6 a few years back, and put it in a tree 35 ft. above the ground. Got the rocket retrieving pole and snagged the shock cord, and put a 400 pound load on the rocket (myself and someone else pulling). Thats equivalent to a 250+ G load! Bent the tree over 90 degrees, but nothing broke. Stared at the rocket up in the tree all summer.

'Nough said.

Bob Krech
 

lalligood

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Originally posted by Mad Rocketeer
Sounds like all epoxy will be what I use. I have some Bob Smith 30-minute stuff. Will that be adequate? Do I need the 20-minute finishing epoxy for the smoothest outer fillets?
Bob Smith 30-min epoxy should be more than adequate.

Save the 20-min finishing epoxy for toughening up (& smoothing out) your balsa nose cones and/or for your fiberglassing projects. You don't want to use it for the attachment of any components!

HTH,
 

lalligood

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I would use (& have used) the LOC attachment method for MPRs/HPRs that have a tube diameter that is too small fo me to get my hand inside down to the forwrd centering ring. Otherwise, what would you/I do if the shock cord broke/burned/needed to be replaced?!? Might as well make it easy on yourself...

HTH,
 

DynaSoar

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Originally posted by Mad Rocketeer
Any further advice from anyone, based on my two sets of questions above? The more I know and the more opinion I've sifted through before I start gluing, the better.

Thanks.
Slower epoxy is stronger than fast epoxy
Epoxy is stronger than wood glue*.
Both are stronger than wood and paper IF you use them right.

For the shock cord mount as LOC instructs, things would break in the order of:
1. anchor cord and/or elastic
2. body tube
3. glue

For fin-to-body, the surface area covered by adhesive is the strength. Fillets are for aerodynamics, sure, but more to increase the amount of surface area that is covered by glue on both fin and body. You want stronger joints, increase the surface area that's glued.

Some people use a strip of fiberglass epoxied to the joint surface and that gets covered with the fillet. That'd definitely add strength to both the primary joint and for the fillet to grab.

On my Mach buster, I used a strip of the same stock as the fin to make a root brace, same width and cut square, glued along each side of the fin root. That increased the fin-to-body contact area by a factor of 3. After gluing the fin to the tube, I put fillets between fin and the brace and between the brace and the body. Then I put a fillets over all until the brace was submerged and the fillets smoothly concave. The 1/8" basswood fins will shred before the glue lets loose (the braces CAN'T shred, being submerged in epoxy). And this is surface mount, on a 29mm minimum diameter bird. I designed the bird, and its components, to handle the kick of an Ellis Mountain H275.

I used epoxy because the amount of wood glue needed would have meant lots of bubbles forming and so lots of little craters to fill later to get a smooth fillet. I used, and only ever use**, 5 minute epoxy, 1200 lbs/sq. in. lap shear strength.

This bird just did an aerodynamic terminal velocity lawn dart and stopped dead in 6 inches. I don't know how many Gs it was, but I felt it hit 50 yards away. The fin joints didn't even notice.

*Actually, there are a couple wood glues stronger than the weakest epoxies. Again, IF you use them right. They've got instructions.

** Not entirely true. I use JB Weld for high temperature exposure joints, like a Slimline retainer-to-motor tube. And knowing it would be held in place under the epoxy fillets, I only used CyA to tack the motor retainer onto this bird; details will be in a scratch-built review on EMRR. Also, my 24mm cluster test bird Sandman (see Descon entry) uses only wood glue. Fin-to-body contact area is increased by gluing the fins into the joint between two body tubes. Next test 3 x E30; 120 n/s in 1.2 sec. I'm betting on the wood glue.

PS: Lalligood's comment "what would you/I do if the shock cord broke/burned/needed to be replaced?!? Might as well make it easy on yourself..." makes real good sense to me. I used MMT anchored kevlar on a scratch built. It burned through. I fixed it using the LOC anchor method. It's still flying, no more burning.
 

Mad Rocketeer

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Bob Krech:
Maybe you should have tried one of those pruning blades that can be extended on a pole and moved in a sawing motion either by pulling a rope or by moving the pole. You might have been able to cut the shock cord. That might or might not have freed the rocket though, depending on what was snagged.

Lance:
Thanks for the good advice on finishing epoxy and shock cord attachment. I've thought about the arm-thicker-than-the-tube problem and have no great answer. I might use a longer MMT and a third CR to make a baffle, a la Bob Kaplow. That would give me a bulkhead I could attach a Kevlar line to, but it would probably still be too deep in the tube. I'll probably just use the LOC method.

Is the LOC-provided (nylon?) line good enough, or should I use Kevlar instead?

The LOC instructions would end up with some masking tape under the epoxy that holds the loop (anchor cord) to the inside of the tube. Is that a problem, or should I tack it in with a smidgen of CA in a couple of spots instead? Will the epoxy soak through the masking tape and into the tube? Do I need to make a few pinholes in it to help? Etc.

Should I add any reinforcement to the epoxy here? If so, what? I'm thinking that if I do I should add a straight epoxy layer first, to get good penetration into the tube.

DynaSoar:
Lots of good feedback. Thanks!

I might add fiberglass strips under the inside fillets. Seems like to do them outside would require either very narrow strips or very wide fillets in order to avoid having a bump at the edges of the strips. If I do inside and outside fillets, do I need the fiberglass strips for an LOC/Precision Graduator?

I thought about adding small wooden fin root braces, but may not, due to weight and COG considerations. I want this to be light enough to fly on a D if possible and an E in any case, yet strong enought to do a Level 1 cert on an H. I like the idea of having a wide range of motor choices.

I've never been able to get thick fillets with wood glue. I can put on a zillion layers, and it still comes out thin. They're strong enough, but they never come out to the shape and curve that I want. Putting on a thick glob all at once woudn't be as strong and would be prone to run, so I've never tried it.

Tell me more about JB Weld. I've seen it in the stores, and it has claims like welding tractor engines back together rather than relacing them, etc. How heavy is it? How strong? What applications should it be used for?

I'd considered using aliphatic wood glue, at least for the fin to MMT joints because I know that heat will weaken epoxy. I doubt I could justify the cost of the specialty high temp (500 degrees Farenheit) epoxy, so near burnout and on through ejection the fin to MMT joint is likely to be soft. Would JB Weld be good there? Would it throw the COG backward enough to require adding nose weight? Would wood glue for this joint and epoxy for the rest make sense?

Thanks all for the help and advice! :D


Steve
 

DynaSoar

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Originally posted by Mad Rocketeer
Lots of good feedback. Thanks!

I might add fiberglass strips under the inside fillets. Seems like to do them outside would require either very narrow strips or very wide fillets in order to avoid having a bump at the edges of the strips. If I do inside and outside fillets, do I need the fiberglass strips for an LOC/Precision Graduator?

I thought about adding small wooden fin root braces, but may not, due to weight and COG considerations. I want this to be light enough to fly on a D if possible and an E in any case, yet strong enought to do a Level 1 cert on an H. I like the idea of having a wide range of motor choices.

I've never been able to get thick fillets with wood glue. I can put on a zillion layers, and it still comes out thin. They're strong enough, but they never come out to the shape and curve that I want. Putting on a thick glob all at once woudn't be as strong and would be prone to run, so I've never tried it.

Tell me more about JB Weld. I've seen it in the stores, and it has claims like welding tractor engines back together rather than relacing them, etc. How heavy is it? How strong? What applications should it be used for?

I'd considered using aliphatic wood glue, at least for the fin to MMT joints because I know that heat will weaken epoxy. I doubt I could justify the cost of the specialty high temp (500 degrees Farenheit) epoxy, so near burnout and on through ejection the fin to MMT joint is likely to be soft. Would JB Weld be good there? Would it throw the COG backward enough to require adding nose weight? Would wood glue for this joint and epoxy for the rest make sense?

Thanks all for the help and advice! :D


Steve
JB Weld is heavy and a little hard to work with. But it's only for special applications, high heat exposure. It'll do what they say, but you wouldn't want to build a whole bird with it. Anything touching a motor tube could make use of it.

Heat will weaken wood glue too. Recall, they tell us we can heat a crooked fin over a light bulb until the glue get soft and straighten it. I had one of my cluster airframes warp 10 degrees after sitting on a hot car hood for an hour.

Adding fin bracing can be done lots of ways. People mix stuff with the epoxy but that strengthens all the epoxy everywhere the same. A strip of glass mat on before the fillet will give a great deal of area for epoxy to grab. It's not the epoxy that's strong, it's the bond the epoxy makes. Way way back when it was common to use a patch of gauze on fin joint on modrocs (Astron Scout?) for the same reason. If you have access inside the fin can and can get enough strengthened fillets in there, that should do most of the job -- the fin won't come loose. But will it shear if you push it too hard? That's what the microballons mixed with epoxy, or a thin strip at the joint, is for.

I don't have the inside of a fin can to work with, and am just building as strong as I can while resorting to as little high strength materials as possible. I expect to have more success than failures,, but I'll learn from both and improve it

Not something you can easily do with an expensive kit. I don't either. All my test bed birds are scratch and as cheap as possible, because I build several of each.
 

Mad Rocketeer

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How often does epoxy (or wood glue) end up failing due to heat? Makes me wonder if some of the stripped fins that occur might be getting cooked off rather than blown or fluttered off. I would think that fins that strip right near the pad would be lost due to acceleration, and fins lost near apogee would be due to flutter and/or heat.

I've never used JB Weld. Is it hard to use due to gooiness, preparation issues, adhesion qualities, other? Does it soak in, or is it more of a surface grabber? Does it adhere to a wide range of substances or more limited? Etc. Hmmm. I guess I could just go and read the package. :)

Back on LOC shock cord mounts: I'd think that attaching the cord up near the nose end of things would protect it from the worst heat. Kind of goes full circle, doesn't it? First, we use the paper mounts for light rockets (which work better than they seem like they should), then we do Kevlar mounted to the MMT or the front centering ring, then we come almost back to the folded paper strip, just doing it better.

Should I be using Kevlar to replace the nylon provided in the kit?
 

DynaSoar

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Originally posted by Mad Rocketeer
How often does epoxy (or wood glue) end up failing due to heat? Makes me wonder if some of the stripped fins that occur might be getting cooked off rather than blown or fluttered off. I would think that fins that strip right near the pad would be lost due to acceleration, and fins lost near apogee would be due to flutter and/or heat.

I've never used JB Weld. Is it hard to use due to gooiness, preparation issues, adhesion qualities, other? Does it soak in, or is it more of a surface grabber? Does it adhere to a wide range of substances or more limited? Etc. Hmmm. I guess I could just go and read the package. :)

Back on LOC shock cord mounts: I'd think that attaching the cord up near the nose end of things would protect it from the worst heat. Kind of goes full circle, doesn't it? First, we use the paper mounts for light rockets (which work better than they seem like they should), then we do Kevlar mounted to the MMT or the front centering ring, then we come almost back to the folded paper strip, just doing it better.

Should I be using Kevlar to replace the nylon provided in the kit?
That's more or less the point... the fins that come off will be pulled off by airlfow and/or flutter, but when and why they fail may well be due to repeated heat exposure weakening the epoxy. I don't know whether wood glue would accumulate weakening. Epoxy does.

Even when it doesn't happen (cue "but I've flown my SouthPark Oobajoob eleventy times") I'd bet the epoxy IS weakening. It may take a while, and maybe the fins won't come off that Oobajoob while they still have it, but the epoxy is weakening.

JB is hard to use, yes, due to the gooey factor. Definitely not for cosmetic fillets. Inside the can, you can gob stuff on, and who's going to see it? It'll grab most anything quite well. One of their selling points is the fact that you could glue a tractor's cracked engine block with it (and I think someone has).

If your anchor is close enough to the top of the motor, kevlar would be a good idea. On the Vulcanite and Weasel, it's far enough from the motor that it doesn't matter. Mine haven't shown the slightest darkening.
 

Mad Rocketeer

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I'd never realized that epoxy will weaken over time like that. Bummer that it would accumulate invisible damage over time and then maybe just give way some day. Do amendments, such as strengthening fibers (Kevlar pulp, chopped carbon. glass) or fillers (microballoons - possibly an insulator?) make any difference?

I'll probably use JB Weld for my MMT-touching bonds. How much heavier than epoxy is it? Can it be spread, smeared, or mashed into a thin enough film for the fin-to-MMT butt joints and MMT-to-CR joints, and can it be formed into relatively thin but strong fillet-type reinforcements for the corresponding fillet locations?

The Graduator has a diameter of 2.63" and a length of 39.5". It has a 29 mm motor mount with an adapter for 24 mm motors. I'm not sure how long the MMT is, and the length of the nose and nose shoulder have to be subtracted form the overall length, but is sounds to me like the shock cord anchor loop will be out of harms way, for the most part.
 

DynaSoar

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Originally posted by Mad Rocketeer
I'd never realized that epoxy will weaken over time like that. Bummer that it would accumulate invisible damage over time and then maybe just give way some day. Do amendments, such as strengthening fibers (Kevlar pulp, chopped carbon. glass) or fillers (microballoons - possibly an insulator?) make any difference?

I'll probably use JB Weld for my MMT-touching bonds. How much heavier than epoxy is it? Can it be spread, smeared, or mashed into a thin enough film for the fin-to-MMT butt joints and MMT-to-CR joints, and can it be formed into relatively thin but strong fillet-type reinforcements for the corresponding fillet locations?

The Graduator has a diameter of 2.63" and a length of 39.5". It has a 29 mm motor mount with an adapter for 24 mm motors. I'm not sure how long the MMT is, and the length of the nose and nose shoulder have to be subtracted form the overall length, but is sounds to me like the shock cord anchor loop will be out of harms way, for the most part.
I've no idea if additives would help.

It's heavier but not by a lot. You can use it for all those things you mention. Just don't expect to be able to smooth it out without a lot of work. But like I said, it's inside the can, so who's going to see it?

If you glue the anchor in where you can reach it (even if that requires some small tools), that does sound far enough away.
 
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