Kevlar - how small is too small?

[Pem Tech]

FWIW: I've had numerous successful launches of rockets that weighed more than a pound using single strand 100# Kevlar. More often than not keeping it simple is the best policy.

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Now that is rockin' cool! Almost 1960's cruise missile like.

 

[Joel]

For zipper protection, I use foam ear plugs over the Kevlar. Easy, works very well.

Hans.

This is a great idea and makes the most sense, rather that putting reinforcement items all around the top of the tube.
Might drill thru the center of the earplug and pull the Kevlar thru with a knitting hook and glue it in place with E6000.
May be just as easy to X-acto down the length of it to the middle, put the string in the slot and glue it down good.

I wonder if most of the zipper effect is being caused by the Kevlar being too short when the body is jerked away from the nose cone during ejection or when the parachute first fills with air?
Some place I read where the Kevlar length should be 3x the body tube.
I built a Quest Harpoon the other day and it used a combination of Kevlar, tied in series with some elastic stretch cord.

Cheers!

 

[Huxter]

I wonder if most of the zipper effect is being caused by the Kevlar being too short when the body is jerked away from the nose cone during ejection or when the parachute first fills with air?

That's a good question. I have always assumed the most stress is at parachute opens fully?

 

[4regt4]

I can pull 150# Kevlar through a foam ear plug with a large sewing needle. For larger cord, I slit it with an Xacto, then glue it over the cord using some foam safe glue. I experimented a bit with glues trying to find one that wouldn't turn the encapsulated Kevlar stiff and brittle. Wood glue works OK, but some of the Bob Smith foam glue seems more flexible.

BTW, I got a large bottle of ear plugs on Amazon for very cheap.

Hans.

 

[SolarYellow]

Might drill thru the center of the earplug and pull the Kevlar thru with a knitting hook and glue it in place with E6000.

I like that. Something to do with the E6000 I bought. It stays flexible when dried, which may be a big benefit relative to the weakening of aramid and any other woven/spun cord when saturated with stiff glue that concentrates tension on a smaller number of fibers.

 

[BABAR]

FWIW: I've had numerous successful launches of rockets that weighed more than a pound using single strand 100# Kevlar. More often than not keeping it simple is the best policy.

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Hammerhead approved!

That’s better than Gorilla Glue.

 

[BABAR]

I can pull 150# Kevlar through a foam ear plug with a large sewing needle. For larger cord, I slit it with an Xacto, then glue it over the cord using some foam safe glue. I experimented a bit with glues trying to find one that wouldn't turn the encapsulated Kevlar stiff and brittle. Wood glue works OK, but some of the Bob Smith foam glue seems more flexible.

Best of both worlds for thicker cords and foam earplugs. Not my idea, but I’ve adopted it and works well.

https://www.rocketryforum.com/threads/blaines-kevlar-anti-zipper-using-foam-ear-plug.154996/

 

[SolarYellow]

Why not just use the straw as a mandrel to get the Kevlar through the plug, then slide the plug off the straw onto some dries-flexible glue like E6000 once it's in its final position? Would work well for thicker Kevlar. For 100-lb, you can probably just use a heavy sewing needle to pull it through the plug.

 

[prfesser]

I bought 200 lb twisted Kevlar. Now I wish I hadn't. Braided line purchase in the near future.

 

[BABAR]

I bought 200 lb twisted Kevlar. Now I wish I hadn't. Braided line purchase in the near future.

Expound, please?

 

[BABAR]

Why not just use the straw as a mandrel to get the Kevlar through the plug, then slide the plug off the straw onto some dries-flexible glue like E6000 once it's in its final position? Would work well for thicker Kevlar. For 100-lb, you can probably just use a heavy sewing needle to pull it through the plug.

Many good ways to skin a cat.

As opposed to a cord to rocket or cord to nose cone attachment, both of which are subject to forces potentially multiple times the weight of the rocket, there isn’t much force pushing the foam plug forward or back along the cord. About the only glue I know you should NOT use is CA, as it makes Kevlar brittle.

I’ve used the straw technique and wood or even white glue without problems. It’s not “hell for stout”, certainly not the optimal recommended adherence agent for Kevlar, foam, and plastic, but for this purpose “good enough” is indeed “good enough.”

I do think the straw adds something, the bare cord I think even glued may have a tendency to “zipper” bare foam.

In both biology and mechanics, often “surface area” is a key factor. The greater the surface area of adhesion, the better the bond.

 

[prfesser]

Expound, please?

Twisted line doesn't necessarily stay twisted all that well. When pushed into the airframe and at other times, it opens up and shroud lines, ends of the cord, snap swivels get in the space.

Twisted line is probably fine for kites, where it's almost always in tension. Shoved into an airframe repeatedly, no.

 

[dakota196]

Well here it is. Clear coat needs to be done, but weather doesn’t permit. This is the braided 100# Kevlar with a piece of heat shrink for anti zipper protection. 3’ of Kevlar and 3’ of flat 1/4” black elastic shock cord.