Shock Cord Packing?

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ssthor

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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.

:cool:
 

Karl

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I think the largest reason why rockets suffer 'zippers is because the delay is either too short or too long. If a rocket deploys to early , the speed of the rocket & ejection combined makes the shockcord run accross the edge of the BT causing it to zipper. Or if the delay is too long , the speed of the rocket coming in balistic causes the shock-cord to come out of the BT rapidly & cut into the bodytube.
So if you can , find out which delay is best for your rocket and hopefull this shoudn't happen

Hope this helps
Karl
 

llickteig1

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Getting the right delay is the ideal solution, but there are many uncontrollable factors that prevent other than optimal delay sometimes.

The best way to avoid the problems you mentioned with elastic shock cords is to stop using them in your rockets.

On new builds, don't use them. On existing rockets, get out your scissors and cut them off. Go to www.pratthobbies.com and order 20' of Tubular Kevlar MicroBraid for $5 or order custom lengths if you wish. Keep a supply on hand for whenever you build.

Now, instead of using 12" or 18" like the elastic shock cord you're replacing, use at least double the length of that elastic. On even small (Alpha sized) model rockets I start with about 3'. If you have a heavier model, use more. D model like StormCaster or Super Big Bertha get about 5 or 6 feet of the stuff.

You can use the same mounting method as described in the kit instructions, and oh joy, the Kevlar is easier to work with than elastic. I don't use glue on the knots, although YMMV on that.

Packing the kevlar is easy. I takes up by far less room than elastic.

If/when you start building mid-power, you can graduate up to the larger sizes also available from Pratt Hobbies.

HTH, --Lance.
 

DynaSoar

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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.

:cool:
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.
 

Stymye

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ssthor,
I gather that your referring mainly to low and midpowered rockets.
what causes a zipper is basically that the body tube is outrunning the nosecone after ejection,
so a long enough shock cord is one of the easiest and best ways to help prevent it.
keep in mind it's not a cure all,as the others have mentioned, the proper delay is very important, but it will go a long way to preventing a zipper.

good question !
 

ssthor

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Yes, that is correct. My question is geared toward the mid-power genre with elastic cords of 3/8 and above. Probably should have cleared that up earlier.

:cool:
 

DynaSoar

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Originally posted by ssthor
Yes, that is correct. My question is geared toward the mid-power genre with elastic cords of 3/8 and above. Probably should have cleared that up earlier.

:cool:
That's what I'm talking too. I might go to 3/8" on a 38mm designed for H and above, but for G and below, 2 x 1/4" = 1/2".
 

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