The issue isn't the knot coming untied, it's the knot itself. Kevlar and other aramid fibers don't like any kind of sharp bend - they weaken greatly due to stress concentration. When the military did studies in the '70s, they concluded that knots were not a suitable way to secure or terminate aramid ropes. They either potted, spliced, or sewed them. The bowline knot fares poorly with aramid cordage, and likely the figure 8 would as well.The Figure 8 stays tight even without a load on the line but can come apart easily. Used a lot in climbing…
Flyers have been using Kevlar successfully for years with rockets using a wide variety of knots and other methods. The big issue is understanding how a particular knot works with Kevlar. I used Kevlar for a good 10 years before I really got interested in how to take full advantage of what it offers while not falling prey to its weaknesses. It wasn’t until I suffered a couple of near misses with recovery harnesses made from Kevlar that I really started researching it. Like many others, I generally had such a huge safety margin in the strength rating that knots didn’t matter much. But once I started cutting it closer I learned the hard way.So despite what I've read, apparently Kevlar isn't useful for model rocketry unless you know how to splice or know a boat rigger. So how do you splice Kevlar to an elastic shock cord. Or am I missing something. Give up on knotting them together. Super glue. Oh wait, not supposed to glue Kevlar. I can barely tie a granny knot much less all these other convoluted knots that make my arthritic fingers hurt. What's a good, simple answer?
Fishline type notes similar to https://www.animatedknots.com/albright-knot work for joining dissimilar cords. Like a sheet-bend, with a lot more wraps. The kelvar will be much stronger than the elastic cord, so any loss of knot strength probably inconsequential.So despite what I've read, apparently Kevlar isn't useful for model rocketry unless you know how to splice or know a boat rigger. So how do you splice Kevlar to an elastic shock cord. Or am I missing something. Give up on knotting them together. Super glue. Oh wait, not supposed to glue Kevlar. I can barely tie a granny knot much less all these other convoluted knots that make my arthritic fingers hurt. What's a good, simple answer?
That looks like a great knot for that purpose. Wrapped knots seem to be very well suited for kevlar.Fishline type notes similar to https://www.animatedknots.com/albright-knot work for joining dissimilar cords. Like a sheet-bend, with a lot more wraps. The kelvar will be much stronger than the elastic cord, so any loss of knot strength probably inconsequential.
FYI - animatedknots.com is an awesome source of knot tying instructions, lots of instructions on splicing too.
View attachment 499818
You'd need to test your specific rope. If you are using kevlar kite cord, there are a few different weaves used. Some are woven a bit too solidly for me( does the cord feel hard) . Each weave will give different results. The rope diameter to knot minimum radius is your "weak point" Of course that's a generalisation....One question I have is ultimate strength, vs working strength. A typical small dia kelvar cord I use has a maximum breaking load limit of 550#, and a recommended working load limit of 110#. So a factor of safety of 5. A working load limit of 110# is plenty for a smaller rocket, and likely exceeds the strength of attachment to the motor mount, the 1.25-inch paper body tube, etc.
So heres the question - a knot in the cord clearly reduces the ultimate strength and thus the breaking load limit. But is that accounted for in the factor of safety used to arrive at the working load limit?
I believe it is, meaning that 110# is still a safe working load for the cord, even when knotted.
All important points. In different 'knot threads' I've linked to many different studies on knot strength, and typically they are either done for fishing or climbing. The same knot can perform very differently depending on the type of cord used. It's interesting that rjstone208's very first post is about being frustrated about what does and does not work with kevlar. It is confusing. And in spite of nearly all of us doing things we aren't supposed to do with kevlar, we've done just fine. As you point out, if your safety margin is high enough a bowline knot can work just fine. But I think most folks would be surprised at the loss of strength caused by that knot.If you read the Marlow ropes knot analysis it says after all the tests that the knot strength varies with the type and diameter of the rope.
Generalisations for knots are useful indicators but not necessarily the answer for a specific rope or cord.
Marlow ropes cover more specifically sheathed ropes. The outer sheath provides wear or sunlight protection for the inner structural element. The inner structural element may be simply parallel or very loosely woven strands or a fully woven interior structural core. So now you’re trying to get a knot in a rope system not just a single material. The outer sheath tends to allow the inner core to continue to slip even after the knot is tied. Sheathed parachute cord is an example of this most readily available.
We tend to use uncovered braided Kevlar. At least I do.
Another variable now it’s not covered. Many knots are based on fishing. They tend to use a monofilament which is slippery when wetted.
As Terry pointed out, there are a lot more variables at play here than the Marlow ropes manual or the fishing knots handbook. We need to remember why we’re using Kevlar in the first place. For me this is about it’s ability to survive a hot ejection charge And have sufficient strength to survive. As long as the knot you use has this allowed for, the one you use does not matter.
Personally I use the bowline. It has to date never failed me. I take into account the strength reduction and live with that decision.
The simple answer in all rocketry is test what you do and then do what you‘ve tested. Anything else is just an untested hypothesis.
That's really cool! Getting OT here but I'd love to see another post with details of your setup and process.I'm currently weaving my own kevlar cord. The weave has a spring to it. Something commercial cord doesn't. Saw a cord making machine on youtube. Couldn't resist it. Took me 2 days to make 7 metres so not cost effective and the neighbours are probably fed up with the noise.
You asked for it.....That's really cool! Getting OT here but I'd love to see another post with details of your setup and process.