Revisiting an older thread. There are a lot of knot (knotty?) threads here, but this particular one kind of fits with my way of thinking about all this. In particular, the suggestions here are all good, and got me to experiment with the various knots. Let me emphasize that I'm talking about small, light LPR. You guys launching telephone pole size HPR are using harnesses or rope - as opposed to string - that require better technique than I need. Here's what I've come up with:
Criteria:
Easy to tie, not too bulky. In particular, I'm concerned with knots that will loosen over time, as I recently had a knot in a shock cord come undone, resulting in a crash. When not under tension, Kevlar seems to "relax" and the knot loosens. That's what has lead me to research this and experiment. Lots of discussion here and in other threads about knots reducing the overall strength of the cord, but I'm compensating by using (what I think) is a Kevlar line that is considerably bigger than needed. Easy to tie is a must. One of my experiments was to join the ends of a simple loop together with the Fisherman's knot. It was a very small loop, and it took me over a half hour to tie it, as the 60# Kevlar seems to have a mind of it's own and doesn't want to cooperate. At one point, I had 3 very small flat jaw alligator clips and/or self closing tweezers holding the partially completed knot together while I attempted to complete it. Then I realized that I forgot to include the snap swivel in the loop, and had to start over. Enough. I'm knot going to use that not. (See what I just did there? Oh, never mind...)
As for splicing using the "finger trap" method, that would be all but impossible for me to accomplish in the very small diameter lines I'm using, given my relative lack of dexterity and poor eyesight. So knots are a must for me.
Here's what I'm using and why:
1. Small Kevlar loop that is used to "gather" the shroud lines - see Peak of Flight #368 (
https://www.apogeerockets.com/education/downloads/Newsletter368.pdf) for what I'm talking about. I'm using a simple offset overhand bend to join the ends of the loop together.
https://en.wikipedia.org/wiki/Offset_overhand_bend It's not the best knot by far, however any attempts by me to use something better just simply won't go together with my fat, very clumsy fingers given the short length of the loop. I finish it up with a bit of wood glue, and a very short length (~1/4") of heat shrink on the ends to (hopefully) prevent it from coming apart.
2. For just about everything else, I'm using a Figure 8 knot. I tried the Kreh, but it doesn't finish up for me like in the pictures. Instead of a nice wrap, I get a messy ball. It's either the string I'm using or bad technique on my part. I tried bowline and surgeon's loop. Again, working on small line, especially if the length is short, simply didn't work for me. Some of the knots I tried ended up being too bulky. I'm finding the Figure 8 easy to tie, and "reverse threading" to make a loop around something (think snap swivel) is surprisingly easy. And it's not too bulky. A dab of wood glue, and a bit of heat shrink to hold the end down finishes it.
3. Rubber or elastic: I'm using 350# Kevlar inside the rocket, which I hope will be permanent. It's too awkward for me to replace. Typically I'm anchoring it to a baffle. I put a Figure 8 loop in the end of this, to which I attach rubber or elastic. Then at the end of the elastic, I'm using several feet of 150# Kevlar which goes to the chute (on a loop 18" or so from the nosecone) and to the nosecone - again with a Figure 8 loop on each end of the Kevlar. I don't attempt to knot the elastic and Kevlar together, as the size difference I think makes most bend type knots ineffective. Tying 1/4" wide rubber using a "decent" knot is difficult. So I'm using what is suggested in the Estes' instruction: a simple overhand knot, followed up with another. I'd never do this with any normal cord, but the rubber is "grippy" and the knot seems to resist coming undone. A spot of wood glue, and tape the loose end so it doesn't catch on anything. Too afraid to try heat shrink to hold the loose end on rubber! I've yet to have this come undone, at least not before the rubber/elastic needs to be replaced anyway. And replacement is easy, as it just attaches to a Kevlar loop on each end.
4. Loop mid cord in Kevlar to attach the chute using a snap swivel: I tried both the Alpine butterfly and a simple overhand loop. For this application, I don't think it matters. In theory, it could loosen, but it can't come undone given that the loop is trapped by the snap swivel. The Alpine probably has less effect on the overall strength of the cord though.
Hans.
