moderated Re: Kevlar shock cord

Tom Augustyn
 

Any chance of adding diagrams/pictures?


-----Original Message-----
From: John Ulizzi <jaulizzi@...>
To: main <main@mtma.groups.io>
Sent: Mon, Sep 9, 2019 1:46 pm
Subject: Re: [MTMA] Kevlar shock cord

Great ideas !

Bob, you beat me to the punch in shock, and also degradation. Having thought about it since my original post, while I have yet to have a failure of the 100# with small rockets, some of which have been flown MANY times, I have had zippers, and I did intentionally ( although I had forgot) upgrade the bottom portion to the 250. It is easier to work with as well. The 100 is thin enough that if you are not careful you will cut yourself with it when you tighten the knot..

A few things I have learned:

1) I do something similar to Jeff’s mod of wrapping the kevlar around the engine hook, or in the case of screw on retainers, around the entire motor tube. I do this for strength more than anything else but Jeff I like your version that makes it replaceable. Mine has always been permanent.  

2) Re: zippering, shock absorption, etc:  Instead of a small piece of rc fuel tubing right at the end of the airframe, run a longer length of it with sufficient kevlar inside of it to allow it to stretch, making in in effect a snubber. This will prevent zippering and breakage due to shock. We sometimes use this exact item behind a dipsy diver when trolling for walleye for the same reason : to add some give in the system. Modern fishing line doesnt stretch like lines of yore, and if you build everything strong enough not to break, and there is no give, the fish “breaks”, and gets away. Counterproductive. Same thing when you blow the nose cone clean off and the swivel or the plastic retainer on the nose cone comes apart. As you can tell I am also a big fan of stealing technology from one hobby and applying it to another. If I remember I will steal one of these snubbers from my tackle box and throw it in my range box for the next launch I am able to come to... 

3) I am a big fan of snap swivels because they reduce twist and also let you change things out in a hurry. But be careful in your selection. I learned very quickly that the swivels I stole from my freshwater tackle were not strong enough, even the high quality ones, not even for model rockets. I’m not yet playing with rockets big enough to justify raiding my saltwater tackle, but what you want are good, durable coastlock style swivels, in strength to match the kevlar. I have not found the extra expense of ball bearings to be worthwile. But strength and quality
yes. I have seen swivels I have used to land 20 lb plus fish blown apart by a D engine.  

4) One other little trick... If you use a long length of kevlar in lieu of a shock cord, wrapping the kevlar in a figure 8 and then taping it together right at the intersection with one or two wraps of masking tape acts a very good shock dissipater as well...

Just my two, or, I guess my four cents worth...

Cheers,

John

 

On Sep 9, 2019, at 12:41 PM, Mike Nowak <MikeMNowak@...> wrote:

Sounds like a modification of the "lariat loop" which works well.

On Mon, Sep 9, 2019 at 12:25 PM Jeff Kodysz <jeffk813@...> wrote:
I’ve tried a method of making the Kevlar shock cord replaceable on model rockets. The method works well but is a little difficult to explain in an email. Basically, You poke a through-hole in each of the centering rings large enough to pass a carpet needle through. Then take your Kevlar and tie a small loop in one end, put the loop around the motor hook and using the needle, pass the free end from the rear up through the holes and into the body tube. 

You have to make the Kevlar long enough to reach from the very aftmost portion of the rocket all the way to its final desired length, but if it gets toasted and weak, you can easily pull it out and replace it. 

Jeff

On Sep 9, 2019, at 12:16 PM, Tom Augustyn via Groups.Io <taugy@...> wrote:

Great info, thanks!


-----Original Message-----
From: Bob Ferrante <robert.1.ferrante@...>
To: main <main@mtma.groups.io>
Sent: Mon, Sep 9, 2019 11:43 am
Subject: Re: [MTMA] Kevlar shock cord

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. 

Jeff

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...

John


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.
https://www.amazon.com/emma-kites-Braided-Creative-Projects/dp/B01850OSRW?ref_=Oct_RAsinC_Ajax_3473351_0&pf_rd_r=1VCH1S3KH6J3CC9RMP0Q&pf_rd_p=b86fab73-bcd1-5ac8-9680-864dd2068659&pf_rd_s=merchandised-search-6&pf_rd_t=101&pf_rd_i=3473351&pf_rd_m=ATVPDKIKX0DER


--
Jeff Kodysz
President
Mantua Township Missile Agency
NAR Section #606




--
Mike Nowak

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