Shock Cord

[AKPilot]

Yesterday, I had a NC/body tube separate at apogee on one of my Maxi Alpha 3s. It was my fault really, I upgraded it to an E and didn't take into consideration the greater thrust. Consequently the stock Estes shock cord just snapped. The nose cone floated down quite nicely, but the body tube can screaming in. Easily enough fixed, though.

But, this leads me to a question. I've searched the forum on how people have improved on their cords. While I've read various ways of doing it, I'm under the impression that a shock cord should roughly be 3x-4x the length of the body tube, give or take. I've also seen that people tend to anchor them to the a centering ring and use kevlar-to-an-elastic transition.

Now here's my question, is there any sort of kevlar/elastic combination out there, for LPR & MPR, that has an elastic cord wound by kevlar - thereby having both properties, the fireproofing and elasticity? If so where do you get it from, and what's a preferred size, based on the weight of the rocket (e.g. < 8oz., 9-16 ozs, >1lb, etc.).

Thanks

 

[11bravo]

is there any sort of kevlar/elastic combination out there

Yes. Scroll WAY down.

Think its for pretty big rockets though.

My suggestion would be to just use enough kevlar thread that you don't need any elastic.

Greg

 

[Stefan_Jones]

Something a lot of people skip when it comes to shock cords:

Barrel swivels.

I usually put one in between a kevlar anchor cord and elastic shock cord.

They all but eliminate the annoying "wind up" effect.

 

[AKPilot]

Something I've asked, but never understood . . .

If people are using kevlar-to-elastic, aren't they still BOTH still siting on top of the wadding, and charge?

Thinking logically, if I put the wadding in first, and then put in my kevlar shock cord, swivel, elastic, and chute. Then my elastic is still exposed to the exhaust. It's just sitting on top of the kevlar.

Hence my reason to find an elastic wound in kevlar. 11Bravo found some stuff from Giant Leap, but it's for HPR.

 

[Mad Rocketeer]

It's a matter of degree (and of degrees, F or C).

If the shock cord attaches to a centering ring or the motor mount, the bottom (aft) end of the cord is going to be under the wadding at the moment of ejection and will therefore be exposed to more heat than the cord that's piled on top of the wadding. As the ejection gasses push everything out of the tube, the shock cord will be straightened out and the wadding (and the hot gasses following it) will pass along its length. By the time the wadding passes the end of the Kevlar and gets to the elastic, it should have cleared (or be clearing) the end of the tube, allowing the hot gasses to disperse, greatly lessening the potential for heat damage to the ramaining cord.

If the shock cord attaches farther up the tube, then the wadding may be partially or completely below it at the point of ejection, but everything else ramains the same.

The longer and wider the tube and the farther up the shock cord is mounted, the less the potential of heat damage to the cord becomes.

Using more heat resistant and tougher materials in the positions exposed to the most heat makes sense.

 

[BAR_Daddy]

I like attaching kevlar to the motor mount and stopping it just short of the top of the body tube, with a loop on the end.

I then attach some elastic that I purchased from the sewing department at Walmart. About the length of the body tube.

To that I attach more kevlar, about twice the length of the body tube.

Since I have started doing this, I do not get zippers, or the dreaded "Estes bounce-back".
I don't seem to have any problem with scorching either.

Even after a dozen or so flights.

Just my three cents (inflation you know).

 

[Fore Check]

Originally posted by BAR_Daddy
I like attaching kevlar to the motor mount and stopping it just short of the top of the body tube, with a loop on the end.

I then attach some elastic that I purchased from the sewing department at Walmart. About the length of the body tube.

To that I attach more kevlar, about twice the length of the body tube.

Since I have started doing this, I do not get zippers, or the dreaded "Estes bounce-back".
I don't seem to have any problem with scorching either.

Even after a dozen or so flights.

Just my three cents (inflation you know).

I'm curious why you bother with the length of kevlar between the elastic and the nose cone?

It seems to me that elastic is far, far cheaper than kevlar and you could complete your shock cord without the added kevlar leader by simply using a longer length of elastic.