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TheAviator

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I was discussing with a friend at NARAM-51 the prudence of using proper knots on rocket recovery systems, and I figure sharing this could prevent some needless losses. From reading and experience, I know that a knot can weaken a rope/shock cord by upwards of 60% of its original breaking strength. Listed below are knots I have found that have a rather high efficiency (some as low as 20% strength loss) and have experience using in my rockets. For some of the more difficult knots I have listed easier alternatives that have slightly less efficiency.

If anyone has any suggestions/better ideas, please let me know.

Standing Ends of a shock line/shroud line
Standard: Two-half hitches, multiple overhand knots. Efficiency: 50%. Easy to tie, may slip, may be hard to untie.

Improved (Option 1): Figure-Eight Loop, follow-through method. Efficiency: 80%. This knot is used in climbing and does not slip. May be difficult to untie if heavily loaded.

Improved (Option 2): Double Bowline. Efficiency: 75%. Also used in climbing and does not slip. Easier to untie than the figure-eight loop if heavily loaded, but not quite as efficient. Harder to tie than a figure-eight loop.

Mid-line attachment point (parachute, tracking smoke charge, etc)
Standard: Overhand on a bight. Efficiency: <50%. Easy to tie, hard to untie after loaded.

Improved: Figure-Eight Loop on a bight. See above for discussion.

Best: Butterfly knot. Efficiency: 75%. While theoretically less efficient than a figure-eight loop, the Butterfly can take the three-way (both standing ends and the bight) loading better than a figure-eight loop, which is designed to take only a two-way load (loop and standing end). Can be easily shifted up and down the line or untied. Very difficult to master.
 

Micromeister

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This graphic may help learning to tie the butterfly: as mentioned it's by far the best chioce for attaching things mid-line.
and 40 standard BSA knots for your tying pleasure:)

ButterFly Knot_4pic tutorial_07-20-05.jpg


knots-a1_BSA 40 knots_05-04.jpg
 

RCBrust

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You named the two that I use, and I've used them on everything up 16 pound rockets with 1" tubular nylon. I use the figure-8 knot for a loop at the end of a line and the butterfly (think it's actually called alpine butterfly) for a loop in the middle of a line.

The alpine butterfly is pretty easy to tie if you know the right technique of looping it around the palm of your hand. Here's a good animation of it. The one thing you want to do with any knot in tubular nylon is to tie it loosely and then twist the line until it lays nice with little or no twists. Then tighten the knot.

Randy
 

judo

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I've found that butterflys are tough to tie on flat nylon.

And the sheepshank. I can tie one, but for what is it used?
 
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mkadams001

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I've found that butterflys are tough to tie on flat nylon.

And the sheepshank. I can tie one, but for what is it used?
The Sheepshank is used to bypass a weak part of a line.

The Figure Eight is a great knot. I will have to try the Butterfly knot.

Thanks for starting this thread, I think it is a good idea to learn about proper knot tying.
 

Daedalus

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I have been using the figure of eight, re-threaded figure of eight and alpine butterfly for all MPR and HPR for quite some time now. Excellent knots and can be undone reasonably easily when required.

Remember with the figure of eight knots to tie one or two stop knots (just overhand knots in the tail around the live line) behind the main knot to help ensure the knot doesn't become loose. Also remember to check the knots frequently and re-tie them if they need it.
 

Zack Lau

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How about building in some strain relief or clever ways of absorbing the shock that don't add much weight? For example, taping the cord and letting the cord break the tape to dissipate some unwanted energy.
 

Micromeister

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How about building in some strain relief or clever ways of absorbing the shock that don't add much weight? For example, taping the cord and letting the cord break the tape to dissipate some unwanted energy.
An excellent way to do this is with a Chain knot, it's used extensively with rescue & Lifesaving gear, throw buoys, rings and bags. Allowing a couple hundred feet of line to be stowed coiled very neatly in a very small amount of space. We had a rescue launcher that shot a Chain knot coiled bag of 1000 yards of light weight (about 1/8"dia) nylon line using a 12ga shell. Sorry I don't have a photo of this particular knot. If I get the chance I'll pull out some line and chain knot it for a photo or two.
I've used this method for years with various size Nylon lines onboard boats, life guard stands and in Scouting without any problem. haven't used it in model rockets but have used it on braided Kevlar shocklines in micros a few times....worked well.

Edit: Found a photo, in with Kite flying knots of all things. Also used alot now in lumberjack and timber/tree rigging.

knots-e__ Chain knot (starts with slipknot on right)_08-09.jpg
 
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MarkII

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I've found that butterflys are tough to tie on flat nylon.
From what I have read, the Water Knot is the preferred knot for use with flat material such as flat nylon.

(For some reason I suddenly can't get Animated Knots By Grog to display properly. I see the text, but only get placeholders for the graphics...)

The one knot that I use more than any other is the Anchor Bend. I use it to attach the shock cord to the screw eye in the base of the nose cone, and to connect the shock cord to the shock cord leader (usually Kevlar). I do the latter with the help of a split ring, or with a welded ring in larger rockets. I attach both the Kevlar and the shock cord to the ring with anchor bends. I usually finish the knot by tying the tag end off with two half-hitches. To tie a Kevlar shock cord leader around the motor mount, I use a Constrictor Knot. In rocketry, I almost always intend my knots to be permanent, so the ability to untie them is irrelevant to me. I often put a little bit of epoxy on my knots as insurance that they will not come undone.

MarkII
 

mkadams001

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From what I have read, the Water Knot is the preferred knot for use with flat material such as flat nylon.


MarkII
The Water Knot (I have always called it a Ring Bend) is nearly impossible to untie without a hammer and fid. I use it to make runners for rock climbing with tubular nylon webbing.

I would not recommend using epoxy or other type of glue on a knot. If I were concerned about a knot coming undone I would use a different knot or, lash or sew the bitter end to the standing end.
 

MarkII

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I would not recommend using epoxy or other type of glue on a knot.
What is the reasoning for that?

If I were concerned about a knot coming undone I would use a different knot or, lash or sew the bitter end to the standing end.
Well, yeah, I could do that too. I should explain that I'm not tying knots in climbing or rescue ropes or in firehose thick TN or Kevlar that will be used for recovery harnesses in a Level 3 rocket. Most of my knot-tying is done in 100 lb. Kevlar microbraid (about 0.035" diameter) and either 1/4" wide flat elastic, 3/16" wide oval elastic, or round elastic cord (mini-bungee - 0.063" to 0.094" diameter). I also tie knots in 15 to 25 lb. twisted Kevlar thread (diameter comparable to human hair - I'm not kidding) for use in micro rockets. I have read up some on knots (and I'm a former Boy Scout), so I am aware that knots tend to weaken the line, some knots worse than others. Also, ever since becoming a BAR five years ago, I have come across a warning every two weeks or so about the dangers of putting CA on Kevlar, so I don't do that. Whipping and lashing the bitter end to the standing end would be be possible (but difficult to do) with the 1/32" diameter microbraid, but I am at a loss for how I would do that after tying the knot in the 15 lb. Kevlar thread. Any suggestions?

MarkII
 
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MarkII

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One technique that I use to secure the bitter end after tying my knot in flat elastic is to bond it to the standing end with Beacon 3-In-1 Advanced Craft Glue. Yes, it holds really well; I haven't seen any bonds let go yet in the three years that I have been using it. But the real beauty of it is that the elastic retains all of its stretch after the adhesive has cured. The bonded area stretches right along with the rest of the elastic, and springs back to its original length when the elastic is relaxed, but the adhesive bond never gives out. Forget Magic Mend-It; this is the stuff to use! The wide bonding surfaces of the flat elastic probably help the glue hold so tenaciously; I haven't tried it with round cords yet. I bought the glue at Michael's Crafts.

I get the best bonds with it using the venerable "double-glue" method. I put a fair amount of it on each bonding surface (because the elastic soaks it up like a sponge), clamp the two parts of the elastic together between my fingers, wait about 30 seconds, and then let go. The two parts come apart, which is facilitated by the fact that they are usually lightly bonded to my fingers :rolleyes: , and after I peel them off (which at this point is easy, because the partially-cured glue resembles rubber cement), I allow the glue to cure for 2-5 minutes. Then I apply fresh glue to each surface and clamp them together again between my fingers. I hold the two parts securely together like that for at least 5 minutes, and then carefully unclamp the now-bonded joint. I monitor it for another few minutes, and if any of it starts to come apart, I re-clamp for another couple of minutes. Usually after that I can leave it alone until it fully cures, which will take a couple of hours. I do not put any stress on the joint until 24 hours have passed. The liquid matrix that holds the glue has a volatile component in it, and once that evaporates, only a very thin layer of cured glue remains.

Sometimes I cover the knot with a little bit of Beacon 3-In-One instead of epoxy. Usually the reason that I apply a little epoxy in the first place is because I often have some mixed up that is left over from other construction steps, so it is readily available, and I figure that a little bit on the knot won't hurt.

MarkII
 

Shade

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And the sheepshank. I can tie one, but for what is it used?
The Sheepshank is used to bypass a weak part of a line.
And used to shorten a peice of line in a pinch.

This knot was often used on sailing ship for both reasons, and when there
was not time to rig a new line, I have used it to by pass a damaged line
during a sail boat race (it was on one of the jib sheets) then when we tacked
I had time to swap out the damaged line for a new one.

Typically if you are using a Sheepshank, you are in hurry to get some thing
done and do not have time for a proper repair.

... lash or sew the bitter end to the standing end.
Now you are sounding like a sailor... :D

About epoxying knots, a post from Handeman explains it best; http://www.rocketryforum.com/showpost.php?p=42055&postcount=56
Good post about the fiber and glueing knots. One thing to keep in mind is
different fibers stretch different amounts. Nylon will stretch up to 50% of is
length without parting (breaking) or getting damaged. Poly propylene, will
stretch 20-30% IIRC. Sisal or Hemp rope 10%, Kevlar very little, The more
the fiber stretch the more damaging the glue will have on the fibers. The
less the fibers stretch the less the damaging effect will be. Nylon because of
it ability to stretch is almost exclusively used in mooring lines.

I always use a dab of CA on my Kevlar thread leaders. 400# test Kevlar will
rip the innerds out of a rocket long before it parts. Also some knots work
better with some fibers than other the Bowline is an ideal knot for Nylon.
 

MarkII

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Now you are sounding like a sailor... :D
ARRRRRGGGGHHHH! :boat:

Good post about the fiber and glueing knots. One thing to keep in mind is different fibers stretch different amounts. Nylon will stretch up to 50% of islength without parting (breaking) or getting damaged. Poly propylene, will stretch 20-30% IIRC. Sisal or Hemp rope 10%, Kevlar very little, The more the fiber stretch the more damaging the glue will have on the fibers. The less the fibers stretch the less the damaging effect will be. Nylon because of it ability to stretch is almost exclusively used in mooring lines.
So that fact that I put a little epoxy on knots that I have made in Kevlar thread and Kevlar microbraid means...?

I always use a dab of CA on my Kevlar thread leaders. 400# test Kevlar will rip the innerds out of a rocket long before it parts. Also some knots work better with some fibers than other the Bowline is an ideal knot for Nylon.
Really? And you don't find that the CA "reacts with" the Kevlar (somehow - I've never read any explanation of what actually occurs during this reaction) and "weakens" it "significantly?" (How much "significantly" actually means is never specified.) :confused:

MarkII
 

MarkII

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Since you are using very light line maybe check out some fishing knots since they are not designed to be untied.
:cheers:
That's a good idea; I'll look at them again. AFAIK, boat anchors aren't mean to be untied from their lines, either, and so the Anchor Bend (my most-used knot) works well for me. I have checked out many fishing knots, and I will occasionally use one. The knots that are used to attach a fish hook to a line are kind of awkward to tie when the task is to attach a nose cone to a shock cord. Also, many fishing knots are designed to be used with monofilament line, and using them with braided line is specifically not recommended.

The reason that I use a split ring to connect my shock cord to my shock cord leader (tying the cord and the leader to the ring with anchor bends) is that if I need to replace the vulnerable and meltable shock cord, I can just cut it off of the split ring and then tie on a new one, without affecting the leader in any way. If I was to omit the ring and just tie the cord directly to the leader (but using, say, a Surgeon's Join or a Nail Knot), then any time that I would need to replace the cord, I would need to cut it off of the leader below the knot. This would keep shortening the leader until it got to the point where I would no longer be able to reach it inside the tube.

I have tied 1/4" flat elastic to the split ring and to screw eyes at the base of nose cones using anchor bends and a couple of half hitches, with nothing else done to secure them, and had them hold securely and never loosen. The reason is the elastic. It gets stretched when the knot is tightened down, and the tension keeps the knot from getting worked apart.

MarkII
 

Shade

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Really? And you don't find that the CA "reacts with" the Kevlar (somehow - I've never read any explanation of what actually occurs during this reaction) and "weakens" it "significantly?" (How much "significantly" actually means is never specified.) :confused:
Kevlar is fairly inert. Other than it known lack of UV stability. Kevlar is has a polyamide structure but due to the aromatic units in the polymer chain the fibers are insoluble in most every solvent save sulfuric acid. A property that make manufacturing in expensive and a general PITA.

CA glues cure (polymerize) via an SN2 reaction, initiated by some form of a nucleophile, in most cases, water or actually the hydroxide ion present in the water, including water vapor in air.

So one due to the lack of solubility of the cyanacrylic monomer in the Kevlar polymer then the lack of any reaction sites on the Kevlar polyamide stucture. There will be very little interaction between the two.

And even though Handeman is correct in his post, I believe that with the very low elasticity of the Kevlar fiber, the presence of any adhesive/glue/epoxy is minimized with Kevlar.

IMHO.
 

RangerStl

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Ever drip CA on your pants or shirt?

Every time I have done that (and it's been a few times) the brittle glue will start to crack and split as the cloth flexes. Because the clothing fibers are encased in this glue, they split right apart also and I end up with holes and cracks in my shirt.

Then there is the other method mentioned about all the flex being at one point. A cord of many small fibers will see the individual fibers of the cord slip past each other axially a bit as you bend it. This makes sure that as you load the bent cord, the fibers running the inner radius of the bend shift allowing them to pick up load. When you glue a knot, any angular load put on the cord is carried bu only the fibers on the outside of the radius. You will more easily overload the capacity of this far fewer number of fibers and they will begin to break, thus the "fraying" you see.

N
 

Micromeister

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Ever drip CA on your pants or shirt?

Every time I have done that (and it's been a few times) the brittle glue will start to crack and split as the cloth flexes. Because the clothing fibers are encased in this glue, they split right apart also and I end up with holes and cracks in my shirt.

Then there is the other method mentioned about all the flex being at one point. A cord of many small fibers will see the individual fibers of the cord slip past each other axially a bit as you bend it. This makes sure that as you load the bent cord, the fibers running the inner radius of the bend shift allowing them to pick up load. When you glue a knot, any angular load put on the cord is carried bu only the fibers on the outside of the radius. You will more easily overload the capacity of this far fewer number of fibers and they will begin to break, thus the "fraying" you see.

N
Exactly: it works on the fibers in Kevlar just like it does in Sisal, Stretch has very little to do with the process. Glues and epoxies simple should never be used to secure a knot in any line. May take some time longer but the fraying will end in failure of the line regardless of chemistry.
 

jorpet

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I have been using bowlines in the kevlar thread for my LPR parachutes for a bit. They seem to hold very well, don't loosen when I need them to stay tied, etc.

I have been considering a surgeon's knot for the kevlar to elastic transition in the shock cord. Have also been thinking i would just attach the kevlar to the nose cone and put a shock chord of elastic between a point in the kevlar and the nose cone (like a snubber on a mooring line). That way the kevlar will always take the full load, but the elastic would help take out some of the shock.
 

MarkII

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I have been using bowlines in the kevlar thread for my LPR parachutes for a bit. They seem to hold very well, don't loosen when I need them to stay tied, etc.
Not sure how you mean. Where do you tie the bowlines?

MarkII
 

MarkII

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Ever drip CA on your pants or shirt?
Ever accidentally drop CA down your pants or shirt? :eyepop: :y:

I can see your point about that process developing if I had encased the knot and some of the standing end inside a big gob of epoxy or had saturated them with CA, but that isn't what I did. Re-read my original post about this. The strands of the cord within the knot aren't supposed to flex, at least not in the knots that I use. They stay tightly bound and don't move, so there is no flexing action that can cause fraying. Look, I never suggested that this ought to be a routine practice; I just mentioned in passing that it was something that I have done. I do realize that it is an unnecessary step, and as I mentioned, I haven't done it on every knot that I have tied. BTW, none of my rockets have ever had a knot failure or a cord separation. But ten years from now, I will attempt to get back to you and give you an update on how they have held up, provided that I haven't lost all of them by then. ;)

I usually lose rockets when they drift off into the woods. Premature fraying of my shock cord system is usually the least of my worries. Not many of my rockets stick around long enough for that.

MarkII
 

mkadams001

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Quote:
Originally Posted by mkadams001
... lash or sew the bitter end to the standing end.
Now you are sounding like a sailor...
Well, I was a boatswain mate in the Coast Guard at search and rescue stations. After that I spent several years rock climbing. Then, back to the ocean in Hawaii racing outrigger canoes. So, through all this you do get some skills using various ropes and knots.

Learning knots is really a lot of fun and you only need to know a few knots to get you through most tasks and impress your friends.:p

I read someone's web page and they suggested using white glue to secure a knot. If you wanted to glue a knot I think this would be better than epoxy since white glue is a bit flexible. That said, considering the types of knots and the types of line we use in rockets I would find it unlikely that the knot would come undone. If you tighten down a figure eight knot it won't come undone on its own.:)

Quote:
Originally Posted by mkadams001
The Sheepshank is used to bypass a weak part of a line.
And used to shorten a peice of line in a pinch.

This knot was often used on sailing ship for both reasons, and when there
was not time to rig a new line, I have used it to by pass a damaged line
during a sail boat race (it was on one of the jib sheets) then when we tacked
I had time to swap out the damaged line for a new one.

Typically if you are using a Sheepshank, you are in hurry to get some thing
done and do not have time for a proper repair.
I have only had to use a Sheepshank once but it is nice to know it when you need it.:cheers:
 

TheAviator

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I have only had to use a Sheepshank once but it is nice to know it when you need it.:cheers:
What would you use a sheepshank for that you couldn't use a butterfly for? All you do is put the weakened or unwanted part of the line in the loop of the butterfly. I find the butterfly vastly more secure than the sheepshank anyways.
 

Micromeister

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What would you use a sheepshank for that you couldn't use a butterfly for? All you do is put the weakened or unwanted part of the line in the loop of the butterfly. I find the butterfly vastly more secure than the sheepshank anyways.
The sheepshank is really a line shorting knot intended for use on lines that remain or stay under some load. It's used often on ships and in pioneering projects where one would not want to cut a length of line but want to shorten the overall length. A butterfly just would not take up that kind of slack. To bypass a weak spot in a line a butterfly I guess would be OK in an emergency, but I'd rather cut out any such weak portion and splice the line or use a more permanent joining knot depending on what the line is used for.

A more stable form would be the Knotted sheepshank sometimes referred to as the Man-o-war Knot.
 

mkadams001

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Wow, who knew that the sheepshank could cause such a discussion.

You have to pick and choose what knot works for your situation. The sheepshank needs to be loaded to work, the butterfly does not need to be loaded. However, after loading, the butterfly may be very difficult to untie while the sheepshank will fall apart.

In climbing, you could use a sheepshank and cut the unloaded part of the knot, rappel to the ground and when your weight is off the line you could retrieve most of the rope. I know that it has been done but, no way I would do that.
 

TheAviator

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In climbing, you could use a sheepshank and cut the unloaded part of the knot, rappel to the ground and when your weight is off the line you could retrieve most of the rope. I know that it has been done but, no way I would do that.
I'm good at knots, but there's no way I'd trust my life to a knot like that! Just too unstable for my tastes.
 

MarkII

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Yeah, that's just knuts! :roll:

MarkII
 

judo

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Wow, who knew that the sheepshank could cause such a discussion.
Yeah, I just thought they were fun to tie.
 
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