Lots of good advice and specifics in this thread. There are a few rules to live by, axioms if you will, that if followed will result in consistent successes…
- Kevlar does abrade and hence weaken, so inspect it. If it goes through a bulkhead make ensure it can be replaced.
- Never use glue on a Kevlar between then attachment point and the load, it makes it brittle and prone to breaking. This has happened to enough rockets that the occurrence seems more than anecdotal.
- Knots generally weaken Kevlar. Stitch it instead. It does take some time, but IMO its sort of fun and affords you some attachment options you would not have otherwise.
- Harness length is the great redeemer nothing more reliably reduces shock load than the longest harness you can get into the rocket.
So 2... what type of "glue"; thats a pretty broad statement. I'm no chemical engineer, but to make the material brittle it would be reasonable that the "glue" would need to impact the chemistry of the material. Certainly gluing kevlar to a tube, which is common technique seen in plenty of kits and builds, does create a potential weak point or at least a point where abrasion and potential weakness could arise. Got any studies on what type of "glue" makes Kevlar, or similar materials brittle?
Oh point 3. Its not just Kevlar, its pretty much any rope that is knotted is weaker; different knots have different strength ratings. But where are the studies that show that sewing is superior to knots? After all the forces are now acting on smaller, and potentially weaker threads, and there is not nearly the number of them combined together as in the base material. There is probably a good pattern for sewing too that provides the most strength with this technique. Got any studies, papers, etc. on this?
Thanks
I'm confident "studies" have been done. I have seen some informal test and as I have already mentioned this has happened to enough rockets that the occurrence seems more than anecdotal. BTW, you are making some assumptions. Do some research, talk with others, and watch the video, it discuses a common pattern and how the number of stitches is determined.
As long as the rocket meets the NAR and Tripoli standards there should not be any issues; but as always the RSO controls the field within the NAR/Tripoli guidelines as appropriate.You are not allowed to fly at section and prefecture launches because rocket construction and configuration are not considered sound.
You betcha, and they are going to rely on accepted engineering practices, common sense, and their own experience to determine whether the guidelines are being met as appropriate.As long as the rocket meets the NAR and Tripoli standards there should not be any issues; but as always the RSO controls the field within the NAR/Tripoli guidelines as appropriate.
Me too! The hand stitching I have seen and performed myself was done using Kevlar thread and a thread count whose aggregate strength exceed the rating of the Kevlar harness it was used in. As a practical matter this is probably unnecessary, but I suspect the reasoning is simply for the harness to fail before the stitching can possibly.Aside from standards and testing...I will say that learning a little bit about sewing taught me a lot about just how strong stitches can actually be. ... Stitches themselves are interesting from a materials standpoint. They weave between the other fibers and put a lot of thread behind a tiny joint. Even just straight stitch two pieces of fabric together with a running stitch. It's not going to be super strong but you might be surprised how well it actually will hold, because it looks like it'll hold nothing at all.
So 2... what type of "glue"; thats a pretty broad statement. I'm no chemical engineer, but to make the material brittle it would be reasonable that the "glue" would need to impact the chemistry of the material. Certainly gluing kevlar to a tube, which is common technique seen in plenty of kits and builds, does create a potential weak point or at least a point where abrasion and potential weakness could arise. Got any studies on what type of "glue" makes Kevlar, or similar materials brittle?
Oh point 3. Its not just Kevlar, its pretty much any rope that is knotted is weaker; different knots have different strength ratings. But where are the studies that show that sewing is superior to knots? After all the forces are now acting on smaller, and potentially weaker threads, and there is not nearly the number of them combined together as in the base material. There is probably a good pattern for sewing too that provides the most strength with this technique. Got any studies, papers, etc. on this?
Thanks
My experience (and I am a chemical engineer) is that there is very little apparent chemical degradation when you attempt to use glue for a knot "stopper". The glue tends to make a hard point that gives the braided kevlar a fulcrum to separate from.
Think about bending a piece of wire back and forth until it breaks. If you have a hard point to flex against, then you should expect the material to degrade in repeated bending, very close to the hard points. A falling rocket will stress this connection in most every direction and make the joint difficult to maintain.
Enter your email address to join: