I asked a question about joining two different types of shock cord earlier, and received several great replies in response. Here's another 'knotty' problem:
I launched my Black Brant VB yesterday, and at ejection, the shock cord knot completely separated from the D-ring on the top of the piston -- just came untied. Now, I had originally tied the regulation, PML recommended knot, and used the regulation, PML recommended dot of epoxy on the end to keep it from coming undone, but I didn't use the regulation, common sense method of re-checking the knot before flight (which lesson I've hopefully learned).
Anyway, is there a 'better' rocket-tieing knot for putting a single shock cord line to a D-ring? Should one attempt to glue it, or (k)not?
Knot knowing what the "PML Regulation Knot" is, I can't answer that half. I usually use either a bowline or an improved fisherman's knot for such things.
Yes, I usually glue them, but NEVER use epoxy or CA. They can attack or damage the material. White/yellow glue is what I'd recommend. Thin it so it can soak into the knot, and rub it in with your finger tips.
Bob Kaplow NAR # 18L >>> To reply, there's no internet on Mars (yet)!
I don't trust using glue on my knots to hold them in place, I always tape everything tight. This makes everything smooth and less likely to hang up on a shroud line, protects the knot from ejection gases and it's easy to inspect the condition of the tape after each flight.
My personal preference in the situation you describe are "follow through" knots. In this case you might loosely tie an overhand knot about 12" from the end of the cord, thread it through the D ring, and then thread the free end back through the knot (follow the twists and turns of the knot) and pull it tight. (for all I know this may be the PML standard... I don't do many kits )
If that knot seems too weak, a "figure 8 follow through" will certainly do the trick. Once again, a loose figure 8 knot on the single line, through the D ring, and thread the loose end back through the knot and pull it tight. If you adequately "set" the knot in shock cord, I would venture it would never come untied, short of a Gordian solution.
These knots end up the same as either knot "on a bight" but allow you to tie them using a ring-type anchor point. As far as adhesives of any sort, I say "Nay!" Too much chance that the chemicals in the adhesives will affect the material. Also, putting glue on a properly tied knot is like putting a padlock on a bank vault. If you're really concerned you might whip the tail to the mainline, but as a rule of thumb, if you leave enough tail on the knot it won't pull through under any load. "Enough" usually means a tail long enough to tie a stopper knot, whether you tie the knot or not.
If you are good at knots you can also use a bowline, which is a very strong knot, but has the drawback of usually being tied badly, causing loss of strength as a result. While the bowline is still taught, it is rarely used in any serious attachment situation... too many failure modes.
There are scads of pages out there showing the various knots. IMO the best approach to knots is to learn a few basic knots, get so you can tie them blindfolded (literally!), then use them. The Figure 8, Overhand, Butterfly and Double Fishermans knots will fill the bill in virtually all situations, and are all very strong knots, easy to tie. Also, these knots have been thoroughly analyzed for strength and failure mode, and it is easy to get numbers for engineered solutions.
One more thought - a bowline will stay tight only if you tie it properly and tighten it properly. To tie it properly, the working end must "go around the tree" around the standing end (the end going away from the D ring), NOT around either side of the loop. It is very common for people to tie a bowline incorrectly by going around one side of the loop. That knot looks just like a bowline, but does not handle the stress on the standing end of the rope the same way as a proper bowline, and can come loose with stress rather than tighten with stress.
The proper way to tighten a bowline is to hold the working (free) end together with the line it is next to in one hand, and the standing end in the other hand, and pull. If you pull on the working end and the other side of the loop, the knot will not tighten properly and will come loose easily. If you tighten it REALLY incorrectly, you can even pull part of the loop through, and the bowline will turn into a completely different knot that will slip (a proper bowline will never slip; that is its main purpose).
The reason it does is it is stiffer at the glue/line interface and the lever forms stresses on the cord material there. The "softer" yellow glue does not have as strong of lever action (materials test!!) and the main goal is to keep the knot from unraveling and even white glue would do that.
Please pardon the tech and GLUE post. It is highly out of character for me. I apologize.
All this talk aboaut types of knots makes me glad I saved this link when found on the last discussion of knots. Not only describes the knots, it will show you - in stop motion animation - how to tie them.
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