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tri-fold mount strong enough for my rocket? Also need help identifying shock cord

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maherakhtar

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I'm building a 6.5 foot tall 3" diameter rocket which will weigh 1kg after everything is built. I'm thinking of using this shock cord (http://imgur.com/rGKafAH), it feels like cloth and looks like its woven, but its quite stretchy and about as wide as my fingers. I attached a 1.5kg weight to it and let it drop and it held fine, but I was wondering if anyone can let me know what kinda material this is.

Is a tri-fold mount strong enough? Is there anything I can do to increase the strength of a standard tri-fold mount? A regular motor mount might be too impractical considering how tall this rocket will be. Are there any alternative mounts I could use that doesn't require feeding the shock cord all the way through the body of the rocket?
 

Steve Shannon

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I've seen deceleration from chute deployment at 30 gees or higher. To test for that you would need to attach a 30kg weight to that cord and let it drop. Dropping a 1.5 kg weight to test a shock cord for a 1kg rocket only tests it to 1.5 gees.
As far as mounting your shock cord, there are lots of ways to mount it. A U-bolt in a centering ring is my favorite. Gluing the cord to the side of the MMT is another.
 

cerving

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Just about as important as your shock cord attachment is the length of your shock cord. Long is good. You don't want the ejection charge to bottom out the shock cord, because that puts a lot of extra stress on the shock cord mounts on both ends. For a rocket of your size, you'll need at least 20'. Since the rocket is so light for its size, you can probably get away with 1/8" braided Kevlar.
 

maherakhtar

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I dropped it out of a balcony to kinda simulate my 1kg rocket, but point taken. Is there any way that I can mount the shock cord without involving the motor mount?

Cerving, good point about the length. Still looking for suggestions that don't involve the motor mount :) thanks everyone!
 

rstaff3

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It appears to be a cloth based elastic. (Putting on my Kevlar underwear.) I have a BT-101 Harpoon that is about that weight and uses a long section of something similar. I used a Kevlar leader attached to the top CR. I personally would not use a trifold mount for that thick of a material. I had an extra nomex cord protector that I slip on and the rocket comes apart for repairs.
 

dr wogz

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There is the 'LOC' method, where the end of eth shock cord is epoxied to eh inside wall of the BT.. Splay out the end of the shock cord & liberally soak in epoxy along with a small portion of the cord itself. Apply to the appropriate location inside the BT. Add more epoxy & squish / run it thru so you have a good glob nicely encapsulating the frayed end. Cover with tape, paper, or something similar to hold it all in place. Let cure. LOC actually have you make a knot in the end of the shock cord. I prefer to fray the end, less of a 'build up' inside.


Some rules of thumb:
Shock cord should be at least 2x the rocket length. 3x or longer is best..
Shock cord should be rated for 150x total rocket weight.
 

Cabernut

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There is the 'LOC' method, ....
I used this method on my L1 rocket with 1500# kevlar. Held up well even after being dragged through the dirt for 20 min. Next time I'll try and flatten the bump a bit more though.



 

cerving

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I've used it too, with a 1/8" Kevlar loop, a little below the nose cone shoulder. It works fine with light rockets, it will work with small fiberglass rockets too if you rough up the fiberglass real good and use a strong epoxy (like Rocketpoxy).
 

ActingLikeAKid

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I would go with something stronger than the elastic; there are a couple of reasons:
(and take all of this with the grains of salt that I've really been into this hobby for less than a year, but these are what popped out at me)

1. In addition to the G-forces at ejection, a 6-foot 1kg rocket is a pretty big thing. As it descends, the wind can and will tug on the body, making it pull/twist/spin.
2. When those forces disappear, the elastic is going to tend to want to snap back with a lot of energy. So your NC blasts out with a 30g ejection force, the elastic expands, and maybe 80 or 90% of that force is dissipated, but even if that NC/upper section comes back with 2g of force, that means that a 1 oz NC is going to smack the rocket like a 2 oz blunt object. At the very least, I'd use Kevlar inside most of the body tube - that cloth can get burnt with repeated ejection charges, and then you're stuck replacing the shock cord (which is no fun). Kevlar is fireproof and relatively cheap. https://www.wildmanrocketry.com/ProductDetail.aspx?product=2922 would be the good sort of overkill ;) A double-length of this would probably be plenty: https://www.amazon.com/dp/B00ABV7FW4/?tag=skimlinks_replacement-20
3. I'm not a fan of the fold-over-paper shock cord mounts on smaller rockets because they can reduce the available area for a chute. If you really want to do a tri-fold mount, what I would do is glue the first section with JB-Weld, let it cure, fold it, glue it with JB-weld, let it cure, then use a heaping gob of JB-Weld to attach it to the inside of your body tube. JB Weld is good because it's relatively resistant to high temperatures. (I'm wondering why you're avoiding motor mount solutions ... is it already glued in? If so, okay, we can work with that! ;) I think that, given the rocket's length, some sort of surface-mount shock cord should be OK. The key will be to spread the load out over as much as possible of the body tube. (presuming the BT is cardboard, and on a 6 foot 2lb rocket....I'm thinking it's cardboard.) Open to suggestions from others on here, but you might try something like this: Make a "U" out of heavy kevlar cord (like the Wildman stuff I mentioned earlier) and use a healthy gob of epoxy to attach each end to the inside of the rocket tube, below where the chute will go (so if the rocket is pointing up, the U is upside-down). That should give you a solid anchor.

Oh, one more thing. If you ARE using cardboard, I'm going to go back and change my mind about a previous thing: You do want at least some of your cord to be elastic; kevlar will zipper that body tube like a knife through butter. What might work is something like this:
Kevlar anchor -> Kevlar cord to JUST below the top of the body tube -> two-foot-long loop of elastic ->Kevlar to the nose cone

Then tie a loop in the middle of the elastic and attach your parachute to that.

Good luck!
 
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