Breakable loop in Kevlar shock cord?

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

ActingLikeAKid

Well-Known Member
Joined
Nov 16, 2015
Messages
1,134
Reaction score
16
Wondering if anyone's ever played around with this, it's something that I saw in some rock climbing and other applications where you need to account for the possibility of a sudden and catastrophic yank on a rope... (I can't for the life of me remember the name of this, or I'd just google it and paste in an image).... so I'll try to describe it. I was thinking about how sometimes there's a hard deployment - too much BP in the ejection charge, or whatever...

So the idea is you make a loop in your Kevlar cord, and stitch the bottom of the loop together, but with just a little thread - enough so that it will break at, say, 25lbs. You do, say, 5 of these. In an event which would otherwise tear the nose-cone off, instead, some of that energy is dissipated breaking the threads.

Is this making sense? Has anyone tried this? Is it worthwhile?

I'm sure the name for the technique will come to me moments after making this post. Or at 2am or something ;)
 
Most people use masking tape around loops for something similar, though it's harder to get the right amount of tape. An easier solution is to use a section of nylon instead of kevlar.

Kevlar shock cord is not without its problems, even for normal deployment forces.
 
masking tape, rubber bands, large igniter straws, around z-folds in the shock cord(even nylon).
Rex
 
Gene at Fruity Chutes made one for my L3 project. It's 1/2" Kevlar two yards long and I think it was 5 yards long before he sewed it up. I can post a picture later if you'd like.
 
I was wrong. It's 7 yards long with 2 yards folded and sewn.

ImageUploadedByRocketry Forum1463625084.721740.jpg
 

Attachments

  • ImageUploadedByRocketry Forum1463625021.522848.jpg
    ImageUploadedByRocketry Forum1463625021.522848.jpg
    71.7 KB · Views: 165
I used the taped loop idea on my last two dual deploy flights and it worked great. I used to have problems popping the nose off at apogee and had issues trying to get shear pins to work as the rocket was heavy duty cardboard. The cardboard ripped during my ground tests. The loops have worked well for me.
 
Anyone doing "high" work such as in a bucket truck for example
must have a 5 point harness on and have a " Shock Arresting Lanyard " attached to the 5 point harness he is wearing and a hard point on the bucket.....
It ain't the fall that gets ya,, it's the sudden stop,, lol...
I'm sure you'll find pics if you Google the term...
There are more then 1 "approved" method of construction. ..
Some have stitching that let's go in segments, ,
some have elastic in them I think because they kind
of krinkle up when there's no tension on them....

Teddy
 
I've made the kind that krinkle up using elastic inside of tubular nylon. The only drawback I found was that they don't fold up in the BT very well.
 
There was another post recently about "crocheting" your cord (chain knot); this will absorb some energy, but not as much as tape or the other ideas I would imagine.
 
Crocheting is mainly to provide a method of reducing the bulk in a manner that allows knot/tangle free deployment. Won't absorb much, if any, shock at all.
 
Crocheting is mainly to provide a method of reducing the bulk in a manner that allows knot/tangle free deployment. Won't absorb much, if any, shock at all.

True on HPR. Crocheted Kevlar serves both purposes for the LPR ultralight stuff quite effectively.
 
I use Z-folds and masking tape on almost all my laundry, and it seems to work. You could probably do the same sacrificial thing without sewing by tying two points of your kevlar cord (say 5 feet apart) together with 3-4 feet of stretchy nylon/elastic, but I've not had any kevlar failures yet with just the z-folds.
 
I was wrong. It's 7 yards long with 2 yards folded and sewn.

View attachment 291404

Was this on your Punisher? If so what does it look like now? Not to add salt your wounds, but I'm assuming the OP is asking for a problem in deployment - high speed or popping the chute at burnout. Your cord from fruity looks exactly like what he is wanting.

In order for this to work, I would think that you would have to take the possibility of the cord getting cut across the lip of the booster tube - or at least take this in consideration during the build - like rounding the lip of the tube or pool noodle or the like.

I really like the idea as another possible fail safe! I don't like the idea of figuring out how much force to make the loops breakable. Too light and takes a few shock cords to figure it out and too strong may defeat the purpose. Can get expensive to test the breakaway of the threading :)....oh Teddy....
 
Was this on your Punisher? If so what does it look like now? Not to add salt your wounds, but I'm assuming the OP is asking for a problem in deployment - high speed or popping the chute at burnout. Your cord from fruity looks exactly like what he is wanting.

In order for this to work, I would think that you would have to take the possibility of the cord getting cut across the lip of the booster tube - or at least take this in consideration during the build - like rounding the lip of the tube or pool noodle or the like.

I really like the idea as another possible fail safe! I don't like the idea of figuring out how much force to make the loops breakable. Too light and takes a few shock cords to figure it out and too strong may defeat the purpose. Can get expensive to test the breakaway of the threading :)....oh Teddy....

Yeah,, I hear ya Nick,,
I think getting the breakaway tension consistent would be a challenge...

Teddy
 
In a weird/funny twist, there was an article about something similar to this in the latest edition of Sport Rocketry. The idea they seem to be using is retrofitting a rocket so that it has a Kevlar shock cord of length X and an elastic shock cord of length 0.5x. In the article, they attach the elastic deep inside the rocket and at the swivel. I like that idea, but it seemed excessive; I had a new idea. You use whatever existing method you normally would to build your rocket; the kevlar from the body tube and the nose cone are both attached to a swivel; a parachute is also attached to the swivel.
You put a knot (or tie in a ring or something. You just need a "lump") in the kevlar between the BT and the swivel, about 2 feet away from the swivel.
You tie a piece of elastic between the swivel and the kevlar cord. Its length should be slightly more than half the distance from the swivel to the knot.

On ejection and chute deployment, the impulse of ejection is spread out over the time it takes the elastic to expand. Instead of (relatively) weak elastic attaching everything, you have the strength of kevlar. But instead of a massive yank on the system at deployment, everything is softened a little. This should improve rocket durability. Best part? Cost is tiny: a piece of elastic strap is like $2 and you can retrofit any MPR/HPR rocket with this in minutes.

I used an advanced graphics program to come up with a 3D render of this working. ;)
(green is kevlar, red is elastic)
rocketdiagram.jpg
 
It seems to me that after awhile, all that masking tape would leave residue on the cord. Plus, you have to go back and pick off the pieces that remain stuck. I always use ponytail hair bands, and can't see the advantage of using tape.
 
In a weird/funny twist, there was an article about something similar to this in the latest edition of Sport Rocketry. The idea they seem to be using is retrofitting a rocket so that it has a Kevlar shock cord of length X and an elastic shock cord of length 0.5x. In the article, they attach the elastic deep inside the rocket and at the swivel. I like that idea, but it seemed excessive; I had a new idea. You use whatever existing method you normally would to build your rocket; the kevlar from the body tube and the nose cone are both attached to a swivel; a parachute is also attached to the swivel.
You put a knot (or tie in a ring or something. You just need a "lump") in the kevlar between the BT and the swivel, about 2 feet away from the swivel.
You tie a piece of elastic between the swivel and the kevlar cord. Its length should be slightly more than half the distance from the swivel to the knot.

On ejection and chute deployment, the impulse of ejection is spread out over the time it takes the elastic to expand. Instead of (relatively) weak elastic attaching everything, you have the strength of kevlar. But instead of a massive yank on the system at deployment, everything is softened a little. This should improve rocket durability. Best part? Cost is tiny: a piece of elastic strap is like $2 and you can retrofit any MPR/HPR rocket with this in minutes.

I used an advanced graphics program to come up with a 3D render of this working. ;)
(green is kevlar, red is elastic)
View attachment 291932

Mooring snubber. Whoever decides to sell Rocket Snubbers, send me a couple :)
 
Yes it was. IIRC they take about 400 lb. to start popping. Both zipper cords were completely unzipped. The deployment stretched out a 1000lb rated swivel and bent a 1500lb rated pin about 45 degrees. These cords are mainly to help with a fast deployment (weathercocked horizontal deployment), but mine deployed at ~650 mph. You'd need a lot of these to soak up that much energy.

I have a loop of 9/16" 4000 lb rated tubular Kevlar glued to the motor mount. It survived and tore a 2.5" x 1" zipper into the booster.



Was this on your Punisher? If so what does it look like now? Not to add salt your wounds, but I'm assuming the OP is asking for a problem in deployment - high speed or popping the chute at burnout. Your cord from fruity looks exactly like what he is wanting.

In order for this to work, I would think that you would have to take the possibility of the cord getting cut across the lip of the booster tube - or at least take this in consideration during the build - like rounding the lip of the tube or pool noodle or the like.

I really like the idea as another possible fail safe! I don't like the idea of figuring out how much force to make the loops breakable. Too light and takes a few shock cords to figure it out and too strong may defeat the purpose. Can get expensive to test the breakaway of the threading :)....oh Teddy....
 
A shiner like that would be hard to size. The amount of energy it would remove would likely be tiny compared to the total and the snap would still be significant at the end of the stretch
 
In a weird/funny twist, there was an article about something similar to this in the latest edition of Sport Rocketry. The idea they seem to be using is retrofitting a rocket so that it has a Kevlar shock cord of length X and an elastic shock cord of length 0.5x. In the article, they attach the elastic deep inside the rocket and at the swivel. I like that idea, but it seemed excessive; I had a new idea. You use whatever existing method you normally would to build your rocket; the kevlar from the body tube and the nose cone are both attached to a swivel; a parachute is also attached to the swivel.
You put a knot (or tie in a ring or something. You just need a "lump") in the kevlar between the BT and the swivel, about 2 feet away from the swivel.
You tie a piece of elastic between the swivel and the kevlar cord. Its length should be slightly more than half the distance from the swivel to the knot.

On ejection and chute deployment, the impulse of ejection is spread out over the time it takes the elastic to expand. Instead of (relatively) weak elastic attaching everything, you have the strength of kevlar. But instead of a massive yank on the system at deployment, everything is softened a little. This should improve rocket durability. Best part? Cost is tiny: a piece of elastic strap is like $2 and you can retrofit any MPR/HPR rocket with this in minutes.

I used an advanced graphics program to come up with a 3D render of this working. ;)
(green is kevlar, red is elastic)
View attachment 291932

I have been going over something like this in my head for some time now. Would like to know if it actually works. I've got some 1/8" bungee that would probably work.

The chain/crochet method with a few rubber bands on the loops would probably work about like rubber bands on z-folds. Just a PITA getting the rubber bands all the way down the shock on the loops (for the way I am seeing it that is).
 
If I were going to do this I'd probably not want the bungee in the middle for fear of tangles. One inside a nomex cord protector could make sense, and another near the nosecone if deemed necessary.
 
The problem with bungees is that they frequently result in rocket pieces slapping back together. Breakable loops and tape work because there's no restorative force. I always thought Velcro loops would do a great job of absorbing energy. The two main problems there are the bulk and that the hook side of Velcro can really mess up nylon shroud lines and shock cord.


[emoji1010] Steve Shannon [emoji1010]
 
Bungee and harness enclosed in a sleeve sounds like a good idea. Full length bungees are known for "slap back", but a shorter bungee would have less stored energy I would think.
 
Bungee and harness enclosed in a sleeve sounds like a good idea. Full length bungees are known for "slap back", but a shorter bungee would have less stored energy I would think.

Going back to the snubber idea I'd think winding the two before covering w/ Nomex might be the right thing. Essentially the bungee would function as a floppy spring.
 
Back
Top