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How the heck do you tie wide nylon shock cords?

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Justin

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I have a half inch-ish nylon shock cord I need to tie to a nose cone. How do you guys do that? For thinner cords I hear a bowline knot or PML's special knot. Mine is too thick for those, I tried.....:( Help...
 

Ryan S.

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overhand knot connected with quicklink Thats what I use with 1" Nylon
 

daveyfire

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I tie all my wide cords (TN, TK, flat nylon) and my narrow cords with double figure eights or figure eight follow throughs, as one does in climbing. If it's strong enough to hold me when I fall off a rock, it's strong enough for my rocket!

An excellent tutorial on the figure eight follow through is at http://www.apocalypse.org/pub/u/antony/tutorial/tutorial.htm . Attach to the rocket, not to you!

If you don't wanna do that, sew a loop in the end and attach with a quicklink. That works too!
 

daveyfire

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Originally posted by Ryan S.
overhand knot connected with quicklink Thats what I use with 1" Nylon
Do you mean a double overhand in a loop? Single overhands connected to an anchor tend to slip kinda easy... In an overhand, the rope also comes out at a funny angle to the knot, causing the rope to be stressed in unusual ways -- you lose a lot of strength.
 

Justin

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I did the double overhand to a quick link, then realized the loop on the nosecone was too fat to fit the quick link.:mad:

I'm gonna check out the rockclimbing knot Daveyfire suggested..
 

Elapid

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is a very strong knot that can be untied easily after heavy loading, which is critical in rock climbing.
unfortunately it uses up a lot of material and so it is relatively heavy.
since the knots for rockets don't need to be untied/tied into repeatedly, a barrel knot (fishing knot) might be a better choice. you'll probably have to cut the line to remove it though.
:D

in any case, most knots are stronger than the cord they attatch.
 

Justin

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The fishing knot (which seems similar to PML's special knot), or the figure 8 both would use up a bunch of material when the line is flat and half an inch wide. One of us must have a knot to tie the nylon when a quick link won't fit the nosecone loop....I could get a bigger quick link, but I already have a ton of harware in this bird...I'll keep digging. Let me know if you guys know something.

You can see the cord in this pic.
 

daveyfire

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A knot cannot be stronger than the material it is tying -- it will, however, <i>decrease</i> its strength. You're taking a bunch of little threads and bending them around sharp corners and tight bends, then pulling on them and stressing them into a tight squeeze. The rope is being brutally treated, and will fail much sooner than an untied piece of rope. That's why a figure eight or a fisherman's knot is a much better attachment than an overhand -- in the overhand, the rope comes out of the material at about a 150 degree angle, while on a figure eight, they come out parallel at 180 degrees. When the rope is stressed into a 180 degree line in the overhand, it experiences much more squeezing force on it.

For your attachment issues, tie a piece of narrow cord (Paracord, Tubular Kevlar) through two holes in the base of the nose cone and tie on to that with a quick link or a knot, or epoxy in an eyebolt. The preceeding solution is lighter, though!
 

Elapid

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are you only using intuition though?
please link me to test data!

if you examine failures in rope systems, the knot never breaks, the rope might break within a few inches of the knot (where the stress is highest), but i have never, ever heard of the rope breaking inside a knot.

i climbed seriously for over 8 years, if that makes any difference to you.

i found a couple links, looking at the data shows wide disparity in test results, possibly reflecting the importance of properly dressing the knot.

http://www.layhands.com/knots/Index.htm
http://www.rescue70.org/roperes.htm

while the knot may reduce the strength of the rope, the 2200kN rating of the nylon webbing i used to use is way above what a recovery system for anything lighter than a toyota might see. even with only 50% of the strength, most materials on the rocket will self-destruct before the knot breaks...
:D
 

Elapid

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doesn't seem to reflect real-world conditions
i'm not sure how fast his machine can pull, and a slow, steady tug is a lot different than a sudden jerk.
the ansi rope (at least that was the way they did it back in the day) test where they drop a 180lb (for rocketry, a lower mass might suffice) mass 20' on a 10' section of rope to shock load the system as might be experienced in a high-speed ejection situation might yield interesting data.
sewing kevlar to nylon webbing sure doesn't look like a good idea!
i'd bet sewing loops in each and using metal links would be far superior than trying to attatch two materials with different elasticity...

some great info!
thanks
 

Elapid

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no need for shouting or derision.
i apologize...
i was WRONG to post that a knot is stronger than the webbing, cord, rope used to tie it.

when i climbed, 10 years ago, i never had an equipment failure, though i didn't fall much. i was always the leader, and took several courses in advanced climbing and rescue techniques. it's been awhile, so i apologize for my faulty memory. i was still a pretty decent climber! ever do mama buttons in joshua tree? i got on that one expecting 5.8, but a hold had broken off and it was more like 5.10b. i made it, but i was confused at the difficulty of the moves... :)

i followed the link to the strength of materials site, i bookmarked it for further study. looks like there's a wealth of information there.

as far as webbing compared to dynamic rope, true enough, dynamic rope has a lot different characteristics than webbing.

i believe the 22kN figure has already been scientifically determined via the proper tests, so even with only 40% strength left after knot tying, the webbing should handle 8.8kN in a worst-case scenario (for this particular webbing), or about a ton...
the 1" nylon webbing can handle a falling nissan!
that's why we hang climbers from it.

i just feel that most rockets weigh a lot less than a human, so demand slightly less concern when overbuilding is concerned. iow, if it'll hold a falling human, it'll hold a falling rocket even without the strongest knot in the world.

that said, i followed a link from the hpr materials strength site to a kevlar attachment method that looks like it could be easily adapted to nylon tubular webbing ...
it appears to be roughly 2x stronger that a knot for kevlar, anyway.
 

Elapid

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were to show a diverse variety of knots that might be useful and for no other reason.
some of these might be helpful to the rocketeer.

and if you'd care to discuss modulus of elasticity, shear strength, or tensile strength, i can go there also.
;)
 

Justin

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Wow guys...It looks like I have a mountain climbers battle on my hands here.:D

I'm gonna check out those links. Thanks again. You guys know a lot about line and knots.

I went and got a bigger quick link and tied my origional knot. I'll post a pic in a little while. I considered the drilled hole with the parachute (or 550) cord, But I went ahead and did this. You guys will have to let me know what it's called. I think it's a double overhand?
 

Justin

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That is a cool site (your right though, evil pop ups). It is an overhand loop. I've used it all sorts of other stuff over the years, I think it outta work for this.
 

Steward

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In recent years I been sewing a loop at the end of the nylon... while setting a drop of CA between the fabric...

Then I simply loop the cord through the loop and snug it down... it's very easy and very smooth...

I've always thought that this was the best solution...
I've never had one come undone....

Thanks....
 

DynaSoar

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Originally posted by Justin
I have a half inch-ish nylon shock cord I need to tie to a nose cone. How do you guys do that? For thinner cords I hear a bowline knot or PML's special knot. Mine is too thick for those, I tried.....:( Help...
Haven't used any yet, but when I do I expect I'll use a slip buckle. You can take them off, put them on, and adjust them. I figure if people hang from the side of mountains from them, they're good enough for rockets.
 

Justin

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What is a slip buckle? A rock climbing gizmo I take it?
 

aksarben10

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For webbing I personally stay with the water knot. I find it easy to tie and untie and since I move my recovery system from rocket to rocket this is important to me. I do not use glue or epoxy on my knots as it can weaken the knot, no I do not have a link to that data.

While the figure 8 knot is has great holding power it is not as easy to untie but it works great on round cords. If you have to proper equipment to do sewn webbing that would be great as sewn is proven to be stonger then knots.

Just my opinions here.

Scott
 

Elapid

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just doesn't travel well over the internet...


tell me.
why do many HPR folks insist on NOT using some kind of stretchy line to limit the shock loading of the system?
 

DPatell

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It's easier to just use one long length of cord.

Personally, I prefer a very long cord. My Magnum has a 40' drogue cord and a 30' main cord. Lazily stretches out at deployment, no zippers, and if it gets stuck in a tree then the booster is always near the ground.


Deployment looks cooler with a long cord.
 

aksarben10

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Well elastic has a bad tendency of breaking and fatiguing with both heat and being overstretched.

What we need to do is use a shock absorbing system such as the Yates Screamers. Sewn runners that are bunched up so that they rip apeart as they go under pressure.

Here is the descriptions of the Yates Screamer.

Product Description:
Function: Shock absorbing sling designed to reduce peak loads in any climbing system!! Multi-functional energy absorbing device. Screamers will provide added protection in climbing and rescue situations. Screamers not only absorb energy directly, because of the stitch ripping effect, they also allow your rope to absorb more energy from the fall by increasing the time interval of the fall. Screamers can reduce shock loading in any rescue system. The standard "Screamer" can effectively reduce peak loads by 3-4 kN an any climbing or rescue system. YATES original Screamers were first developed over ten years ago, Yates has since developed numerous variations on the "Screamer" design with use in Aerospace, Military and explosive testing to decelerate various objects. Screamers were originally designed to be used on less then ideal climbing protection such as small nuts, old 1/4" bolts, snow anchors or marginal protection. We have determined that the most effective activation for a standard screamer to be >2kn(550lbf.) for climbing related situations. The new "SHORTY" Screamer offers the exact same properties as the standard Screamer in a more compact package unit. The new "ZIPPER" Screamer is designed for long falls with high fall factors. These type of falls occur usually within the first two or three protection placements that are run out. Ice climbing is a good example where the "ZIPPER" is exceptionally effective when used on the first few placements.
 

jetra2

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Originally posted by DPatell
Deployment looks cooler with a long cord.
Yeah, but it scares the heck outta me....



On this particular flight, I thought there had been a separation...I was wrong, though...LOTS AND LOTS of shock cord!

Jason
 

daveyfire

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Funny story about that...

When I was last flying rockets, some older rocketeers were out there who weren't exactly familiar with the new recovery technologies available (they used short lengths of elastic and tiny plastic or paper chutes... when they could get their rocket to fly straight...) and every time I flew, they always proclaimed a separation at apogee. Every time, the rocket was all attached and came in for a gentle landing after the dual deployment.

Long shock cords are fun :)
 

aksarben10

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Originally posted by jcrocket
Stretchy means breaky and no, I don't have a link for that. Stretchy also means they're energy -storing- devices.
Thats whats nice about the theory around the Screamer, no stored energy just a system to absorb it.

I agree that in a perfect world the parachute would come out at apogee, but many is the time that they do not and recovery systems should be built with that in mind. I have seen some pretty secure recovery systems ripped, torn and snapped by too much force on deployment. Long cords help dissipate that energy but adding another shock absorber along the lines of a Yates Screamer that dissipates, not storing, the engery could help.


Scott
 

DPatell

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Jason - that cord looks about right! I loooove long cords:)

An interesting story, though...

A guy in our club flew a big 7.5" rocket on a K650T 98mm motor. Deployed on time at 500' I think, the cord was really long and the chute took a while to unfurl. As the chute fully inflated, the tail end of the rocket touched down, it was really close;)
 

Elapid

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i even started a thread in recovery about 'em!
never took a fall on one, though.
i think they are an excellent idea for HPR, personally.

elastic or not, any cord that is below spec will snap.
i've been using some military spec 100% Cotton elastic webbing in one of my MPR models now.

i have a feeling that cotton will not degrade like your typical shock cord.

i wonder if there's a gizmo i could come up with to keep the nc from impacting the rocket after ejection...hmmm...
 

DynaSoar

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Originally posted by Justin
What is a slip buckle? A rock climbing gizmo I take it?
Yeah, though rock climbers use a different name. We used the same thing for our safety lines when we went into undeground fuel tanks in our "space suits". The line had to stay loose so you could move, unless someone was trying to haul your unconscious body back out because your suit leaked. It's essentially a metal bowline (knot), but has teeth that slip tighter and looser depending on whether you're pulling on the taut line, or pushing on the loop to open it. It pulls tighter easily, pulls open not at all without two hands.
 
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