Anyone have any experiences they'd like to share regarding the use of Kevlar thread?

I have not yet used anything but the old method of shock cord mounting. I have, on occasion, had a shock cord tear into the body tube upon ejection. Maybe there was an improper delay...maybe the cord was too short...

The thing is this:
That thread is as thin as....well...a thread. Won't it cut right through the body tube when there's the least bit of "side load" put on it?

I use kevlar cord as a shock cord anchor. I tie it to an elastic cord (a really long one) using a barrel swivel. The kevlar cord stays inside the body tube, so it never has a chance to cut through anything.

The longer the cord, the less likely the 'Zipper' effect.

My most recent 'cure' for the problem was in my 'Deuce-bash', where I used Jim's BT60 baffle which comes with a healthy piece of Kevlar. Upon installation and before attaching to the screw-eye at the nosecone (with a snap-swivel, of course!), I used a piece of Shoe Lace (even an old piece will work ) and used contact cement to hold it at the right position with the (in my case 2" length) centered with the end of the body tube. Although the Lace is only 1/4 inch or so wide, it distributes the force along the BT edge enough to prevent tear-through.

Give it a try next time out. You should be pleased with the results. Let us know of any further refinements to this technique as well!

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Glen A.
N.A.R. #26298 L1

I only use a length of Kevlar long enough to reach the end of the BT but not pass it. Then attach an elastic cord to it. Zippering can be caused from two things (ok...there may be more but I think these are the prevalent causes)....

1. The delay is too short. The recovery device deploys, creating drag for the nose cone, but the rocket is continuing on to apogee. As a result, the shock cord tears into the end of the body tube.

2. The delay is too long. The recovery device deploys after apogee while the rocket is on a downward plunge to earth. The recovery device slows but the rocket continues down and the shock cord tears into the end of the body tube.

Too short of a shock cord won't zipper if the recovery device is deployed close to apogee, when the rocket is at it's slowest. But it will cause a rebound that results in the nose cone or even the body tube becoming damaged, assuming the shock cord isn't ripped from the body tube or the nose cone isn't ripped from the shock cord.

Actually, the cord is a way of keeping the rocket coming down in one piece to aid in the recovery. It doesn't need to be elastic if it's long enough to prevent a sudden "shock". General rule is one and a half times the length of the rocket. I use two times myself.

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Don
NAR 53455
"Carpe Diem"

A thread about thread...

Not much to add. I use kevlar that comes in kits from Semroc and Quest. Otherwise, I just use elastic in my other rockets. I don't have zipper problems with any of them. It boils down to proper motor selection and a nice long cord. I like to roll my chutes up kinda tight on higher flying birds so that it will take them a few seconds to open during decent. It acts almost like a drogue/main. That gets them back a little closer to the pad, and doesn't yank the heck out of the rocket at ejection. I don't do that on low and slow models, though. (Oh, and it doesn't work with the Quest chutes because they don't like to open no matter how they are packed.)

Don and TB said a mouthful when it comes to zippering. It's almost always improper delay selection that causes zippering. To late or too early, and the heavier the rocket, the better chance you have at zippering. But, there are ways to help prevent it.

The advice already given is right on. The only thing I do different is to let my Kevlar extend well past the end of the body tube, followed by the elastic, but it's your choice. I use as much elastic as I think I can get away with, this usually ends up being two or three times the length of the rocket. My moto is, "there's no such thing as a shock cord that's too long!" I also coat the inside of the top end of the body tube with thin CA, about an inch or so. Sand smooth to ensure proper nose cone fit. I've also heard suggestions putting a piece of tape or cloth on the kevlar or shock cord where it meets the end of the body tube, although I've never done that myself. I've heard a comment or two like, "Shock cord long enough?" >snicker<
I just smile back. I've never had a zipper....ever.

Veracity, you may be interested in my use of kevlar tape as an anti-zipper shock cord mount. Check my post in this thread over at The Rocketry Forum.

http://www.rocketryforum.com/showthread.php?t=40460

I've been using a method I found in a thread at TRF (don't have a link to the thread but some searching turned it up while I getting started a few months ago). Basically you put the kevlar thread through a fairly short piece of launch lug and then loop it through a second time to hold it in place where the thread would normally hit the top of the body tube. Tape it in place. I've also used just tape and it tends to slide around. With this method it's sturdier and doesn't slide. No problems with the rockets I've done this with although it gets a little tight on BT-20 or smaller rocket bodies (especially if your using the shock cord mount method).

Space_bus, that sounds like it would work really well and your experience suggests that it does. There's a good picture of that method in the TRF thread I mentioned above. I'll have to try it sometime.

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Mike Fields

[QUOTE= I also coat the inside of the top end of the body tube with thin CA, about an inch or so. Sand smooth to ensure proper nose cone fit. I've also heard suggestions putting a piece of tape or cloth on the kevlar or shock cord where it meets the end of the body tube, although I've never done that myself. I've heard a comment or two like, "Shock cord long enough?" >snicker<
I just smile back. I've never had a zipper....ever.[/QUOTE]

Forgive me, but what is CA?

I like the idea a lot.