locked Re: Kevlar shock cord

Bob Ferrante

One thing that must be considered with Kevlar is degradation.  100 pound line may work the first three flights but if the shock cord is close to the ejection charge then each time it is being exposed to the ejection charge it does degrade.  After a few flights your 100 pound line is now 75 pound line, and a few more 50 pound line or less. Also Kevlar will breakdown in UV light (sunlight). The strength is significantly reduced over time exposure to sunlight.  I passed on a bunch of Kevlar at NARAM this year because it had changed color on the reel from the UV breaking down the Kevlar. Several layers in had changed color because the vendor kept the reels on the back of his trailer and where constantly exposed to sunlight.

Funny thing is I purchased 150 pound and 500 pound from Emmakites right after NARAM.  I have had the web address in my favorites since NARAM-53.   www.emmakites.com

Experience has told many of us that it is better to overbuild the shock cord.

100 pound is good for small competition models and sport models up to B or C.  
250 pound is good for models up to E or F or egglofters.

Because Kevlar is so strong you may find it breaks parts on the model.  So a short section of elastic line run in parallel with the Kevlar to absorb some shock before the Kevlar line pulls tight with reduce the shock to the parts of the rocket.  This is great for egglofters and TARC teams to know.

Bob Ferrante

On Mon, Sep 9, 2019 at 11:14 AM Jeff Kodysz <jeffk813@...> wrote:
Yes, but Kevlar is excessively strong for its diameter. 100 pound test Kevlar is like carpet thread. 

On Sep 9, 2019, at 11:10 AM, Tom Augustyn via Groups.Io <taugy@...> wrote:

Okay but what about the actual size of the kevlar cord? Wouldn't heavier pound ratings have more braids and be thicker?

-----Original Message-----
From: Jeff Kodysz <jeffk813@...>
To: main <main@mtma.groups.io>
Sent: Mon, Sep 9, 2019 9:06 am
Subject: Re: [MTMA] Kevlar shock cord

Think about it this way... if it's rated to #100, you could attach a 99 pound weight to the body tube and pick the rocket up by the nosecone with the weight attached. The rocket itself would fail long before the kevlar would break.

The only thing to worry about is zippering, but if you make the cord long enough that's not a problem. You could also secure as small piece of silicone model aircraft fuel tubing to the cord where it touches the open end of the body tube. 


On Mon, Sep 9, 2019 at 9:02 AM Tom Augustyn via Groups.Io <taugy=aol.com@groups.io> wrote:
For all body tube sizes?

-----Original Message-----
From: John Ulizzi <jaulizzi@...>
To: main <main@mtma.groups.io>
Sent: Mon, Sep 9, 2019 8:49 am
Subject: Re: [MTMA] Kevlar shock cord

Just my two cents but I have used the 100# on model rockets and never had a failure of the kevlar. I have had the swivels pulled apart, which to me is an even greater testament to the strength of the 100#...

They are both very good...


On Sep 8, 2019, at 10:20 PM, Tom Augustyn via Groups.Io <taugy@...> wrote:

Thanks Wolf! I was actually looking for a source for this exact product!

-----Original Message-----
From: Wolfram v.Kiparski <astronwolf@...>
To: main <main@MTMA.groups.io>
Sent: Sun, Sep 8, 2019 7:51 pm
Subject: [MTMA] Kevlar shock cord

Here's an inexpensive source for kevlar shock cord.  I always replace the rubber shock cords that come in Estes kits with this stuff.  The 250 lb. test line is perfect for model rockets, packs well, and is easy to handle.

Jeff Kodysz
Mantua Township Missile Agency
NAR Section #606

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