Here's how Quest shows to tie the Kevlar to an elastic shock cord. We've all tied shock cords on this way.
I was concerned that the tight knot might weaken the tightly cinched elastic.
For years I've tied the elastic on like this. There is no knotted elastic where the Kevlar makes contact. The tension at ejection will be spread out more.
An overhand loop knot was tied in the lower Kevlar line.
The only way I could attach the elastic was to tie a square knot. The end of the elastic ended up out and away, not side by side like on the Kevlar below it.
I know I'm probably not the first person to tie the Kevlar and elastic shock cord like this. In the next post I'll show how to tie the knot.
This cinch knot spreads out the tension even more and has a neater look with both lines side by side.
This is one of those things that I should have figured out years ago. Sometimes a simple solution can slip right past you.
That is so obvious... in retrospect. I shall adopt this approach immediately.
ReplyDeleteHi Neil,
DeleteYeah, I feel a little stupid not seeing this before!
Having stared at it a bit more I realize I shall benefit from your forthcoming post on how to do it. I guess you need to do this while the other end of the elastic is still free? I have a hard time visualizing knot-tying in my head....
DeleteIt'll all be clear in tomorrows post. Both the elastic and Kevlar loops are tied before the two are joined. It took a few tries before I got it right, but it's pretty easy.
DeleteFew of my LPR rockets use both Kevlar and elastic. Those that do, I have always followed the manufacturer's instructions (although, I would never tie one like the Quest instructions). I will be interested to see how those knots are tied. It looks similar but different to the double fisherman's knot I use on my MPR rockets to connect the 1/4" Kevlar leader to whatever else I am using for the shock cord.
ReplyDeleteHi Metalhead,
DeleteI like the double fisherman's knot too. I looked at the nail knot but thought it used too much of the elastic length to tie it. The connection I used in this post is simply a better variation of what I used to do.
I have used this approach for several years, actually. You tie each loop separately, then pass the un-tied end of the Kevlar through the elastic loop, then through the Kevlar loop. Pull it tight, working the Kevlar knot back through its own loop. And Bob, as they say, is your uncle.
ReplyDeleteYou can do the same in reverse as well, if the Kevlar is anchored already but the elastic end is free. When doing replaceable Kevlar, you'll generally want to do it the first way.
I have a few rockets where I attach the nose cone the same way, i.e. by tying a loop in the end of the elastic, slipping the whole loop through the eye of the cone, then bringing the elastic over the tip of the cone and pulling it tight.
Hi Chris,
DeleteI knew I wouldn't be the only one to figure it out!
I tied on the elastic to the Kevlar already attached to the engine mount.
I've been using the Duncan Uni-Knot to tie the shock cord to the nose cone eye.