shock cord mount studies

Woody's Workshop

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I never took much confidence in the rubber band provided in the kits. My Mother's hobby was sewing, so she always had elastic in her sewing machine drawers. I stole it, of coarse, and used it for shock cords. I've never had a problem with the "Tea Bag" and elastic since 1970.
This is Rocket Science, but at it's lowest level in this instance. There is such a thing as "Over Thinking" something. Unless you have time to waste.
The last 25 years or so I've kept Kevlar on hand and always wrap it around the EM below the top centering ring, and attach elastic just below nose cone end, or dividing point with a swivel. Remember the longer the elastic, the less chance of a zipper and fatigue, which will yield a longer life span.
Advice and experience is plentiful here. You will have to weigh the responses that best suites you and give them a try. And choose which works best for your needs. My experience is from over 50 years dealing with mostly Estes, but some Quest, Centuri, Custom and some odd ball names that don't exist any longer. But it's all Low Power Black Powder, because I fly on my own.
 

BEC

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My current practice (as I've mentioned in other threads about this sort of thing) is to put 100 lb. Kevlar (a foot or so long) in a small tri-fold mount, then tie the supplied kit shock cord to the Kevlar. This is gIued in with Titebond most often.

I don't think I've had the Kevlar pull out of that arrangement more than once or twice in several hundred flights. I tried that with some fine stainless steel fishing leader (thinking about the wear-through issue I generally have with Kevlar after a couple of dozen flights). It pulled out on the first flight using it. So for now, back to Kevlar in the tri-fold. In recent builds I've put about 3/4 of an inch of 1/16 heat shrink tubing over the Kevlar where it rubs against the top of the body tube. I think I've had one wear through failure right next to the heat shrink since I've added that to builds.

I need to try the Stine style as it has the potential to be a little lower profile in the tube, which would be an advantage in smaller-than-BT-50 tubes where the tri-fold, even a small one with Kevlar in it, can be a problem, especially with a 'chute rather than a streamer.

For models big enough for baffles (for me, this is BT-55 and larger diameter and long enough to have room for a baffle) the kevlar gets attached to the baffle, of course, rather than with a paper mount. Yes, I know there are smaller baffles....and I really need to give those a try, too, I suppose.
 

BigMacDaddy

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lakeroadster

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That is an interesting solution but I would worry that it might crack the fillets when it is "shocked" (although obviously no experience so just a feeling).
Competition rocketry has used the technique of external shock chords, attached by just laying the shock chord at the fin root and then applying the glue fillet over it. It's been successful for decades.

This technique builds on that and is even stronger.

 
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BigMacDaddy

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Competition rocketry has used the technique of external shock chords, attached by just laying the shock chord at the fin root and then applying the glue fillet over it. It's been successful for decades.

This technique builds on that and is even stronger.

I guess I do not work with epoxy enough to fully appreciate its strength!!
 

hcmbanjo

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Good Article used the method ever since I read it. Here is one of the rockets.
Glad it's working for you.
You should check the condition and wear of the Kevlar after 10 flights.
I've had 150 lb. Kevlar in a LPR rocket deteriorate and break after 12 launches.
 

Alan15578

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I am old. I go back to the days when Estes instructed you to cut two slots in the body tube, tie the shock cord to the strap created. then pop the strap back into place and glue and fill. I use a variation of this for minimum diameter contest rockets.

First, for SD I attach a small plywood triangle to the outside at the recovery CG. I tie a leader to the triangle and this runs up the tube and around to the inside where it is tied to an elastic shock cord that is eventually attached to the streamer. The leader can be any convenient material, even Kevlar. With a balsa nose cone, I cut a small groove to accommodate the cord. With a plastic nose cone, I drill a small hole through it to accommodate the cord. Launch from a tower and if competition is fierce, use a piston as well.

For other events I make two pin holes inline on the body tube about 1/2 inch down from the lip and 1/4 inch apart. Using a needle threader or a piece of wire thread the leader through the holes and tie it off. You do not need a proper knot, a simple underhand loop will suffice. Secure and smooth it with a little glue inside and out. The leader can be inside or out. Inside is cleaner, but outside is easier, except as noted in the previous paragraph. PD is not very sensitive to weight and drag so I typically fly it with two launch lugs about 1/8 inch long, one in the fin fillet, and the other at the cord attachment. It is not difficult to redo this leader attachment, but it will likely outlive the rocket.
 

flyingeagle

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I've been retrofitting some old rockets so I end up studying or reinventing old concepts, trying to do them easier and better. My old rockets have deteriorated rubber shock cords plus I've lost nose cones, I've had nose cones with circular cuts and damaged body tubes. So I'm switching to kevlar. I wanted to try it with tri-fold mounts and found that there wasn't much strength so I did other experiments. All of these tests used Titebond II yellow wood glue.
First the basic Estes paper mount only provides one layer of paper to tear through as shown in the first picture. There is some strength there but not a lot.
View attachment 476417
I pondered ways to make this stronger and did some tests. I concluded that it needed more layers of material to tear through. To get something done quickly I doubled up the paper and figured out a different way of orienting the folds so that the cord would have to tear through 2 layers, which means 4 layers since I doubled up on the paper.
View attachment 476418
Later when I had more time I tested more ideas. I thought maybe cloth or fiberglass would be stronger so I tied a loop in the cord and ran the material through the loop per the photo below. I found that the kevlar cord would just zipper through the material. In this situation the cloth and the fiberglass would each zipper with surprisingly little strength.
View attachment 476419
View attachment 476420
The folded cloth has more strength because it can bunch up a little bit and the cord has to tear more fibers at one time. I don't know anything about materials that cloth is made out of, there are surely some types of cloth that are stronger than others. This red cloth was kind of a thin flannel material. From this experiment I decided that I needed some way to bunch up the fabric so it would all have to tear at one time. I think this could be done but it really increases the thickness of he mount. However the v-shaped piece of cloth has more strength than the 4-layer tri-fold mount and is easy to make. This all lead me to the last experiment which was to take a piece of heavier kevlar, fan out the threads and glue it on in a U-shaped configuration. This had the most strength, limited by how much contact area I could get between the fibers and glue and the cardboard. But not everybody has pieces of heavy kevlar laying around that they could use.
View attachment 476421
 

flyingeagle

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I found a much better way for a shock cord mount to the body tube. Sorry I have pictures but not sure how to post them. I sent this idea to Apagee Components and they wanted me to write a book. The pictures I sent them say a 1000 words. I created a kevlar mount with a loop just short of the body tube to prevent zippering and a way to replace a worn out braided shock cord. It consists of 100 lb card stock with 4 holes in the poster paper card stock with the Kevlar fed through “looped” through the holes and back in upon itself like a woven basket. The harder you pulled on the shock cord the tighter it become. This was glued in using Tite Bond ll like Estes tri fold mount but with it being flat against the body tube not taking up room for packing a parachute and cord. Once dried I coated over it with 30 minute epoxy. I gave it several tug tests when dried and nothing moved, no signs of tearing. Once I figure how to post pictures here I will send them
 

Back_at_it

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I found a much better way for a shock cord mount to the body tube. Sorry I have pictures but not sure how to post them. I sent this idea to Apagee Components and they wanted me to write a book. The pictures I sent them say a 1000 words. I created a kevlar mount with a loop just short of the body tube to prevent zippering and a way to replace a worn out braided shock cord. It consists of 100 lb card stock with 4 holes in the poster paper card stock with the Kevlar fed through “looped” through the holes and back in upon itself like a woven basket. The harder you pulled on the shock cord the tighter it become. This was glued in using Tite Bond ll like Estes tri fold mount but with it being flat against the body tube not taking up room for packing a parachute and cord. Once dried I coated over it with 30 minute epoxy. I gave it several tug tests when dried and nothing moved, no signs of tearing. Once I figure how to post pictures here I will send them


Something like this but with 4 holes in a square pattern?

shock cord.jpg
 

FredA

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I'd be cautious about using glue to secure Kevlar - that generally leads to a failure point in my experience.
 

techrat

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Why not create a kevlar loop threaded into a thin piece of cardstock? You can tie a shock cord, even an elastic one, to the kevlar loop. If you are worried about the kevlar cutting through the shock cord, thread in first, a piece of heat-shrink tubing, and then tie the shock cord to that. Glue the cardstock to the body tube and the other end of the shock cord (obviously) goes to the nosecone. Less chance of zippering with two kevlar braids against the body tube, and the glued in cardstock won't rip as easily as you are spreading the load.
 

SolarYellow

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I'd be cautious about using glue to secure Kevlar - that generally leads to a failure point in my experience.
Might not be right to hold the assembly to the BT, but I haven't seen people using adhesives that stay fully flexible/soft on the Kevlar.
rubber cement​
urethane​
silicone​
etc.​

Anyone use those and still have failure of the Kevlar at the edge of the glue?
 

MidOH

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Heck, I use epoxy and it's been fine. It's CA that seems like the killer.

But I only glue on LPR. But the kevlar I use is thick and oversized since I hate the thin stuff. It tangles and zippers.
 

bjphoenix

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Why not create a kevlar loop threaded into a thin piece of cardstock?
Depending on how the loop is threaded through the cardstock you can have zippering or tearing of the cardstock. Kevlar can zipper almost anything, look at my photos in the first post. I had good luck with the folded cloth because where the kevlar loops around the fold it can bunch up the cloth. If the kevlar wraps around material that is spread out it can tear it from one side to another, a zipper, because it only has to cur through one thread at a time. If the material can bunch up then it has to tear through a lot of threads at one time so it is stronger. If you can get the cloth strong enough then the next failure is the cloth tearing away from the body tube. I did one experiment with a larger piece of kevlar in a loop with threads splayed out to get more surface area for gluing. This was pretty strong, but requires that you have some of the heavier kevlar. I think you could do something similar by making the loop out of many pieces of small kevlar braided together.
For the past few kits I've built I used the folded cloth but I cut a piece of the bigger kevlar about 1.5" long and put it in the cloth before I folded it, so the small kevlar would have to cut through this piece too. I haven't done any tests on it to see if it behaves like I think it will.
Most of my builds these days are reasonably light BT60 rockets that don't get a lot of pull on the cord. On the end of the 6' cord at the nose cone and streamer I had a generic snap swivel from Walmart. In one instance the ejection charge blew the nose cone out pretty fast and when it reached the end of the cord it bent the wire of the snap swivel open, but the mount to the body tube wasn't affected. With parachutes I usually use a 10' cord and the nose cone isn't blown out as hard. I'm going to keep doing it this way until I see some problems.
 

tmhorvath

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There shouldn't be any zippering if the shock cord is long enough (3x the length of the rocket). For heavier kits, you can tie two perfection loop knots in the Kevlar that are spaced apart at least six inches. and attach a section of the elastic shock cord to those knots. The length of the elastic should be shorter than that of the space between the two knots. This creates a method of shock removal so the elastic slowly stretches out, but doesn't snap as it will stretch out to the length between the knots and the kevlar will now do its job. If it does snap due to damage or age, the rocket still stays intact and the elastic is easily replaced.
 

Donnager

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On retrofits where I've lost the connection to the MMT, I have taken a 1" piece of coupler or tube, split it so it is like a "C". Tie your shock cord to the coupler opposite the cut.

Trim the cut wider so the coupler will just slide into the body with the shock cord on it. Apply epoxy/glue, slide in the coupler (well past the nose or top section), and then glue down any tag ends.

I haven't had these pull out, yet. This tends to work better with a flat(ter) shock cord.
 
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