MidOH
Well-Known Member
My shock cords get epoxied to a coupler now. That coupler has a baffle that also acts as a laundry shelf.
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).I just stumbled upon this post by @AeroTech and wanted to add it to this thread.
For shock cord mounting on a minimal diameter rocket, he runs the Kevlar thread under a fin fillet.
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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.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.
Here's an article I wrote up about Replaceable Kevlar Shock Cords for the Apogee Peak Of Flight newsletter.
Hard to believe it goes back to 2013!
https://www.apogeerockets.com/education/downloads/Newsletter338.pdf
Glad it's working for you.Good Article used the method ever since I read it. Here is one of the rockets.
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.
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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.
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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.
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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.
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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
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.I'd be cautious about using glue to secure Kevlar - that generally leads to a failure point in my experience.
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.Why not create a kevlar loop threaded into a thin piece of cardstock?
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