Originally posted by ssthor
I am interested in any information regarding shock cord (flat elastic) packing to reduce the chances of a zipper. That is, what methods are out there to increase the cord tension during deployment (reduce wipping, thus zipping). I have found information on using tape or rubber bands , but nothing specific.
Much appreciated.
As others say, proper ejection timing is important. If the bird is going too fast and the chute tries to slow down behind it, "there's the rub". That's not always possible, nor is it the whole story.
Consider the physics of it.
Something can cut the tube because its pressure is placed on the tube in a small enough space that it is able to cut the material of the tube. Consider also that the tube must resist being spun (momentum) around by the force of the cord's pull in order to hold still while the cord cuts.
No method of packing can change this. You can beef up the end of the tube. Painting it with glue, most often CyA, can help.
As for changing matieral, that will only help if the new material has either (a) more give or (b) is wider. Both conditions make it less likely that the cord will cut the tube. Now, more give is not so good. Too much and it's too weak and can break.
I prevent zippers by doubling the cord. Rather than using a length of elastic, I use twice the length and double it into a loop. That lets twice as much material present itself against the tube, and that's half as likely to zipper.
Not good enough? Pull your shock cord out as far as it will go. Pull it around the edge of the tube, marking the cord where it meets to tube edge at all points around the edge. Take a couple inches of duct tape and place it on the cord centered on those marks. Place an identical piece of duct tape sticky-to-sticky with the first, over the cord. Now THAT'S wide. (Trim any edges hanging over that have adhesive showing.)
Still not enough? Start by marking as above, but use a couple inches of a smaller diameter tube instead. Wider, plus it has give of its own. It works, but it's difficult to construct once the bird's built.
Using tougher stuff is only less likely to break and actually more likely to zipper if it's the same width. Anything wider is better. Using tougher, wider stuff is best, but can get expensive.
I've had zippers from kevlar cord and single 1/8" and 1/4" elastic. I use only doubled 1/4" now (except on the smallest; they get doubled 1/8") and haven't had another. I do use kevlar anchors, but these don't go to the edge of the tube, the elastic connected to the anchor does.
