Kevlar - how small is too small?

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

dakota196

Well-Known Member
Joined
Jul 16, 2010
Messages
228
Reaction score
152
Location
Oak Harbor, WA
I am downscaling a GL Talon 2 to 24mm, and I have some Kevlar cord. It is 100# and I think to myself, is this too small? This is my first time using Kevlar, so it looks too thin. I’m at the point of finishing the motor mount, but need to attach the cord. But if it is too small, then I need to reorder more. I want to use Kevlar and elastic shock cord on a 15” chute. Any thoughts?
 
I think 100# kevlar looks thin too. It also has great zippering ability - being that small. To avoid zippers, I use much heavier Kevlar on my LPR. 300 - 400# is the smallest I'll go. It's ok to be over-engineered - it adds negligible weight. This rocket will wear out before the kevlar!
 
I think as far as strength goes, you should be OK - that seems like plenty of strength, the main concerns off the top of my head are having it's anchor point preferably not in the direct blast of the ejection and either use of nomex or wadding or even better - an ejection baffle taking the brunt of the ejection blast for longevity at the anchor point.

The other concern is the "zipper" tendency at the top of the body tube. I put shrink wrap tubing over the small section where the kevlar is going to contact the top edge of the body tube and I also like to put 2-3 layers of of copy paper rings applied with yellow glue wrapped around the top of the outside of the body tube to prevent zippering and be more impact resistant to nose-cone-backlash from a boing elastic band (which I now use much longer connections of either elastic, woven nylon line or kevlar to reduce that problem.

Hope that's of some use -PTH
 
When I first started using Kevlar, I bought some rather thin string. I'm sure it's strong enough, but it's not real practical to use as a shock cord just because of it's size. For example, I had some get "sawed off" after a few launches as it abraded on the top of a CA reinforced body tube. And another rocket had 60# burn through from ejection charges. So for very small rockets, I don't go smaller than 150#, and usually I use at least 350#.

Hans.
 
I’ve had thin Kevlar burn through right near the top of the motor, depending on how it’s anchored. So I covered the entire length inside the body tube with small diameter heat shrink. I also put two layers at the lip of the body tube to protect both the Kevlar and the tube.

I also found early on that twisted Kevlar (like thread) and braided Kevlar rated for the same strength are not the same for our use – 100# twisted was not nearly as robust as the same rating in braided.


Tony.
 
I bought some of the emmakites cord on ebay.

Back to the OP's question- I've been using thin kevlar, 100# or 150#, recently and it has worked on rockets up to and including my Big Daddy. If you intend to anchor it near the motor mount then I suggest some form of strengthening or protecting it. I attach mine near the front end of the body tube similar to how we did it a long time ago. (I should buy some steel leader line and attach that to the motor mounts.) That thin kevlar does look small, but as long as it hasn't been deteriorated by wear or heat in reality I think it has more strength than any other component in the system- such as the loop on the nose cone, the connection to the body tube, the various snap swivels, etc.

I was using 10' of thin kevlar on all of my rockets but I made up a 6' length to use with a streamer. I launched my ESAM one time with the 6' cord and a streamer- the ejection charge blew the nose cone off at high speed and it reached the end of the 6' cord still going pretty fast because it didn't have much drag to slow it down. When it hit the end of the cord it straightened out the snap swivel. I launched on a B6-4 which doesn't get much altitude on an ESAM so both pieces were easy to recover. I ordered some heaver snap swivels that have the safety hooks to keep them from straightening out. Well now the failure would have to happen at the next weakest spot.
 
Suggestions on where to get Kevlar cord in "bulk."
As others have suggested, "Emmas Kites". But I buy it on Amazon. I have a lifetime supply of 150# that I use for everything including wrapping Christmas presents. Bigger stuff costs more. However I'm a long ways away from using my small spools of 350#, 550#, and larger sizes.

Hans.
 
Put a shock cord protector over it. IIRC Apogee sells them. Others too. I make my own. I take a strip of Nomex cloth and sew it into a tube and slide it over the Kevlar. At first I was making them for every rocket. That got old fast. Now I just put one on the shock cord of the rocket that is going to fly and not just hang from the ceiling. I like the ear plug idea. I use long shock cords to avoid zippering and try to get the delay just right. I typically use 20' or more on a large rocket. It is usually 1500lb stuff. In a LPR I use 500lb. MPR 1000lb. The working load limit of Kevlar is only 20% of the rated strength. I don't think you can use too much length. I don't recommend elastic because of the recoil.
 
I've had a few failures using 100# Kevlar on LPR and MPR.

Pretty much every failure was due to bad techniques on my part (too short of a shock chord, incorrect attachments, etc).

That being said, 300# braided Kevlar is a better choice if there's enough room in the airframe for it. It's much more forgiving.
 
Last edited:
100# is fine strength wise. There is typically nothing in a LPR that would be stronger than the cord. The problem I have is with burn through of the cord. Being thin it will only take so much before it eventually fails. At the very least, cover the first few inches of in some type of shield. Heat shrink tubing works great for this. I personally don't use anything less than 250# and will use 500-700# on rockets that have the room. I don't want to go in there and have to replace it.
 
BT 60 and 4’ of triple braided 100# Kevlar and 3 or 4’ of elastic shock cord. I am putting it in a downscale GL Talon 2.
 

Attachments

  • 82273E14-3CB4-469B-8E14-B73F825EA570.jpeg
    82273E14-3CB4-469B-8E14-B73F825EA570.jpeg
    757 KB · Views: 0
Back
Top