Shock cord knotting

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dr wogz

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A thread for knotty things:

It seems there's a lot of threads about shock cords, lengths, types, anchoring, etc.. But nothing about knotting or securing at either end.

Do we have preferred knots for:

Tubular nylon? (I usually go "trilene" or a bow line knot. Duncan Loop maybe?!)
Kevlar cord? (Again, trilene or bowline, with a hitch to secure the tag end)
Woven / flat nylon? (what prompted me to ask, I've done a few different ways..)
Woven / flat Kevlar?

Would you use different knots for different dia or cords? flat straps? Loads?
Do you add something to the knot to prevent undoing? like a shot of glue? (I've seen some people flood a knot with CA or epoxy..)

Any protection at the anchor point down in the tube? I've done 'heat shrink tubing' on the lower end on a few kits, to secure the knot and lock it on the eye bolt..

thoughts? comments? discussion?
 
Actually for me what I tie is a slip knot. On most LPR and MPR depending on the size/diameter of the cord itself I'll use heat shrink around the knot after it's attached. Gives it a clean look. On larger diameter cords I'll go with a sewn loop at the end and use a quick to swivel or just a quick link.
 
I used the bowline until just recently when I switched to the figure 8. I reason if the figure 8 is safe enough for climbers than it is good enough for my rockets. I found the draw backs of the figure 8 are 1) it takes a lot of length to tie and 2) it is difficult to size the loop when tying. I frequently have to re-tie the knot to get the loop dia to my liking.
 
Figure 8 it is easy to tie and you can untie it after loading. It works well with flat or tubular webbing.
 
I used the bowline until just recently when I switched to the figure 8. I reason if the figure 8 is safe enough for climbers than it is good enough for my rockets. I found the draw backs of the figure 8 are 1) it takes a lot of length to tie and 2) it is difficult to size the loop when tying. I frequently have to re-tie the knot to get the loop dia to my liking.

This is why I use it...the figure 8 follow-through though, to be clear.
 
A bowline, with the knot and end of rope wrapped in adhesive heat shrink tubing is my go to.
 
Grog's knot app is great. Most useful one I use is for loops mid segment called an Alpine butterfly loop.
 
For tubular nylon, I've just been using the sheepshead or slip knot from the Apogee videos. I check the knots before and after each flight and haven't seen any loosening. (plus I put ductape in areas exposed to ejection charges)
 
Quest has a good method of tying shock cords to Kevlar string for their LPR rockets. The same method could be used for MPR.
So far I have never had the knot fail on any rocket in my possession using this method:

Shock%2BCord%2BKnot%2BA.jpg
 
Shockcords regardless of line diameter connect to the NC with either 3-Half Hitches or a simple bowline knot. Attaching the recovery Chute or Streamer for LPR & MPR models add a locking Snap swivel connect to a mid-shockcord Butterfly knot.

Generally our Shockcords consist of 70-130lb Braided Kelar tied around the motor mount with 1/8" Oval Elastic or Flat 1/4" elastic for larger MPR Models.
 
I like slip knots and overhand loops. I will take the extra cord and tie a half-hitch and wrap whatever is left with tape.
 
Most knots will work. Sometimes you are concerned about how easy it is to untie after loading (bowline is good here). My usual concern is loss of cord strength from the knot. All ropes/cords with knots are weaker than one with no knot. Here are some common estimates of strength of ropes with various knots:

No knot 100%
Bowline 70-75%
Figure 8 75-80%
Overhand 60-65%
 
Found it!

PML has a "tubular nylon" attachment recommendation. it's kinda like a 'Trilene' knot..

https://publicmissiles.com/images/tyingtubularnylon.pdf

That is a basic clinch knot. Well known in fishing. It does work well for TN. The Trilene knot is more complicated and not worth the effort, imo.

I use a bowline for most attachments just in case I ever need to untie it. Which I usually don't.....
 
For round stuff (thread or cord) I usually use the Improved Clinch knot (which is, I think, a lot like the Trilene knot).
For flat and tubular webbing, I use the Water Knot -- I learned it as a climber and I've been told it's one of the stronger ones out there. It's primary use is to tie the ends of two slings together, but I adapt it to make a loop - tie the overhand knot, pass the end through whatever you want the loop to go through (e.g. an eyebolt) , then "follow" the knot you tied with the loose end. Knock on kevlar ;) but I've never had anything come loose.

https://www.animatedknots.com/waterknot/#ScrollPoint
 
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