Kevlar Shock Cord

[joeliny]

Hi all,

BAR here, with a question about Kevlar for Shock Cord leader. I had a bad experience with an elastic cord the other day even though I used plenty of wadding it still burned and separated.

I did read about the charges being real hot these days and I found out the hard way.

Anyway got some 150# Kevlar and retrofitted my Xray with it LOC style with a loop at the top to tie the elastic to.

I am wondering though just how well the Kevlar holds up to the ejection charges. I am planning to go with a Nomex blanket in place of the wadding but am concerned about the Kevlar that will be below the blanket at the attachment point of the engine ring. Are there any other precautions I need to take or will this stuff really work?

Thanks for any help on this,

Joe

[CPMcGraw]

Hi all,

BAR here, with a question about Kevlar for Shock Cord leader...got some 150# Kevlar and retrofitted my Xray with it LOC style with a loop at the top to tie the elastic to...just how well the Kevlar holds up to the ejection charges?

Joe, Welcome!

I have also read some of the replies over on OldRockets (this forum is sort of a "companion list" to that group, most of the same "usual suspects" are on the line-up here...), and have only one suggestion: Don't worry about it melting before you lose the model while flying. The odds are in favor of typical flight loss due to fly-away, or parachute thermal, over actually seeing the Kevlar melt and fail...

[severenz]

I've finally got my hands on various gauges of kevlar thread and am ready to attach the shock cords to my newest models. I'm wondering what the best way to do this is and hoping you all could share your expertise.

It sounds like some elastic is still necessary on the end of the shock cord that attaches to the nose cone. Is this required? What about lengths? Do I simply mimic the lengths that were included in the kit?

Thanks

[A Fish Named Wallyum]

I've finally got my hands on various gauges of kevlar thread and am ready to attach the shock cords to my newest models. I'm wondering what the best way to do this is and hoping you all could share your expertise.

It sounds like some elastic is still necessary on the end of the shock cord that attaches to the nose cone. Is this required? What about lengths? Do I simply mimic the lengths that were included in the kit?

Thanks

The best way I've found is to tie it around the motor tube behind the forward centering ring before you put the motor mount in the rocket. You might need to cut a notch in the centering ring, but the glue should cover this.
My QModeling Rogue uses no elastic at all, just a nice, long piece of Kevlar. Works great so far.

[snuggles]

I like Semroc's way of attachment, tie the Kevlar around the motor hook by the engine(oops, MOTOR ) block, pinch it under the forward CR, then run it up to the end of the BT.
Tie a loop where it protrudes. VOILA
Mark T :D

[CPMcGraw]

I like Semroc's way of attachment, tie the Kevlar around the motor hook by the engine(oops, MOTOR ) block, pinch it under the forward CR, then run it up to the end of the BT.
Tie a loop where it protrudes. VOILA
Mark T :D

I prefer Bill's technique of cutting the notch for the thread to pass through. A very simple way to do this is to get some sanding "twigs" (thin flexible sticks of foam with sandpaper faces along two sides). They're about 1/8" wide, and you can sand a flat notch in the inside diameters for the thread very quickly. They're also great for filing a flat notch for the motor / engine hook to lay into...

The advantage to this is it keeps the tubes from being deformed where the thread gets trapped between it and the ring edges...

[severenz]

For future models, I'll definitely be using the "tie around the front centering ring" technique; I believe this is also referred to as the LOC style, no?

I suppose for my already-built models, I'll have to use a modified Estes mount where the cord is woven through a few holes in the card-stock before folding up and gluing.

Thanks for the quick responses and help.

[CPMcGraw]

For future models, I'll definitely be using the "tie around the front centering ring" technique; I believe this is also referred to as the LOC style, no?

I suppose for my already-built models, I'll have to use a modified Estes mount where the cord is woven through a few holes in the card-stock before folding up and gluing.

Thanks for the quick responses and help.

Try this:

Take a short length of BT the same size as your motor tube, say, about 1/2" long. Glue a thick centering ring at each end, one ring with a notch cut into it. Tie the Kevlar around it like we're describing and run the thread through that notch. Secure the thread with glue. When this assembly has dried, insert it down from the top of the model until it touches the motor tube, then drop and dribble some thin CA to hold everything in position. You've basically just extended the motor tube and added the Kevlar mount, so the exhaust gasses should pass cleanly through the center. The additional weight should be minimal, larger motors should not have any problem lifting it.

[Gus]

Kevlar can be attached with an Estes style mount if retrofitting. It will hold up fine. You don't need to weave the kevlar through the paper, the standard trifold with white glue will hold up fine.

You do not need to use elastic if the piece of kevlar is long enough (3 -4 body tube lengths).

I've put kevlar in a ton of rockets and only had it burn through once. My impression was that the piece of kevlar caught a burning chunk of ejecta which eventually scorched the kevlar in two.

The braided kevlar sold by Pratt Hobbies is pricey but way nicer than twisted kevlar since it doesn't unravel with repeated use.