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Kevlar string as shock cord
#1
I vacillated on where to post this question.

Some older rocketeers are advocating the use of a kevlar strand or string or cord tied around a centering ring during assembly, and running out at least beyond the end of the body tube.  Then, they recommend, some other form of elastic shock cord from there up to the nosecone.

In almost all of my kits so far, there's only a rubber strip or elastic strap that runs from a tea-bag anchor up to the nosecone. Sad

Where does one find this Kevlar string or  cord, and does it only come in quantity?  Should I be looking in Hobby Lobby, Joanne Fabrics or solely on line for it?

I don't want to pay an arm and a leg for a large spool that would supply an entire grade school of cub scouts either.... Undecided
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#2
(10-28-2015, 09:32 PM)Kirk G Wrote: Where does one find this Kevlar string or  cord, and does it only come in quantity?  Should I be looking in Hobby Lobby, Joanne Fabrics or solely on line for it?

I don't want to pay an arm and a leg for a large spool that would supply an entire grade school of cub scouts either.... Undecided

Don't know what your arms and legs sell for but I paid about $15 a couple years ago for a spool of 600 feet of braided Kevlar cord from thekevlarstore.com. It'll last me a good while longer. The same site also sells, for about half that cost... less than a tenth that much length. (Currently 50 feet for $6.79 vs. 600 feet for $13.89.) 

In most cases I either tie/glue the Kevlar to the motor mount between the centering rings and run it through a notch or hole in the forward ring, or I make a removable Kevlar setup along the lines described by Chris Michielssen in the Apogee Peak of Flight newsletter a couple years back. Then usually I mark the Kevlar just short of the end of the tube and feed it back through the motor mount. I tie a braided elastic shock cord to the Kevlar at the mark and cut off the excess, then feed it forward into the body tube again. With this setup the Kevlar won't zipper the body tube — not that it'd be likely to even if it were to extend beyond the body, if there's a good long elastic shock cord which will limit the amount of force the shock cord system experiences.
Rich Holmes
Camillus, NY
Secretary / newsletter editor
Syracuse Rocket Club

http://richsrockets.wordpress.com
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#3
I can't think of a rocket vendor that doesn't sell Kevlar in a variety of widths and lengths.
John S.
NAR #96911
TRA #15253
MDRA
Level 1, 2014-Mar-15 -- Aerotech Sumo, H133BS
Level 2, 2014-Jun-21 -- Giant Leap Vertical Assault, J240RL
Level 3, 2016-03-12 -- MAC Performance Radial Flyer, M1101WH, 13,028 feet
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#4
(10-29-2015, 07:47 AM)Rich Holmes Wrote: In most cases I either tie/glue the Kevlar to the motor mount between the centering rings and run it through a notch or hole in the forward ring, or I make a removable Kevlar setup along the lines described by Chris Michielssen in the Apogee Peak of Flight newsletter a couple years back. Then usually I mark the Kevlar just short of the end of the tube and feed it back through the motor mount. I tie a braided elastic shock cord to the Kevlar at the mark and cut off the excess, then feed it forward into the body tube again. With this setup the Kevlar won't zipper the body tube — not that it'd be likely to even if it were to extend beyond the body, if there's a good long elastic shock cord which will limit the amount of force the shock cord system experiences.

Here's the link to the Replaceable Kevlar article in the Peak Of Flight:
https://www.apogeerockets.com/education/...ter338.pdf

I used to fish the Kevlar line to the front, mark it like you explain, then go out the back tie the loop and elastic on. 
Here's a tip from Sirius Rocketry, a much easier way to tie the right length and loop without running the line back and forth.
Set the engine mount  or block assembly along the outside of the body tube. Tie the loop before the end of the tube. 
The Kevlar doesn't go beyond the end of the body tube.


Attached Files Thumbnail(s)
   
Hans "Chris" Michielssen
Old/New NAR # 19086 SR
www.modelrocketbuilding.blogspot.com

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#5
(10-30-2015, 10:53 AM)hcmbanjo Wrote: I used to fish the Kevlar line to the front, mark it like you explain, then go out the back tie the loop and elastic on. 
Here's a tip from Sirius Rocketry, a much easier way to tie the right length and loop without running the line back and forth.
Set the engine mount  or block assembly along the outside of the body tube. Tie the loop before the end of the tube. 
The Kevlar doesn't go beyond the end of the body tube.

Well, you still need to feed the Kevlar (and the elastic, if it's tied on already) back through the motor mount when you go to glue it in, to keep from getting glue on the shock cord. But you're right that doing it this way reduces the number of times you need to feed it through.
Rich Holmes
Camillus, NY
Secretary / newsletter editor
Syracuse Rocket Club

http://richsrockets.wordpress.com
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#6
Balsa Machine Services will sell you what ever length and size you need. They also have great pricing and good selection on motors- hardware and misc. rocket stuff.
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#7
To buy kevlar cord try http://emmakites.com/
They sell many strengths, lengths, inexpensive and free shipping. It's all braided, not twisted.
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#8
Nice source python! ... bookmarked... Good prices on the 750-2000 lb stuff. For smaller size thread (6-225#) check out http://www.thethreadexchange.com - big spools available. I use the 64# size quite a bit for LPR.
Dave Cook 
NAR 21953 L2    TRA 1108
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