Shock Cord Elastic vs. Kevlar

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Patternflyer

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All,
I'm sure this has been an ongoing debate for years, but simply put, for those of you that prefer using Kevlar, do you replace the elastic rubber cords with Kevlar on 100% of your rockets, or just those that are more prone to burning through? Obviously, doing it from the beginning of the build is a lot easier than retrofitting it after building, or even worse, after the rubber cord burns up..........

I'm building an Estes Spirit right now and am considering cutting out the pre-installed rubber cord and running Kevlar through the fin can before I glue everything up. Again, my question is if there is a significant number of folks that run Kevlar in ALL rockets if at all possible? Thoughts?

Thanks in advance,
 

shawn_rocket

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For my all LPR, I run 100% kevlar from the motor tube. I create a loop at the top of the kevlar just below the top of the tube. I then attach 1/4" braided elastic band to the loop/nosecone. When possible, I also use a kevlar loop in the nosecone as well to attach the elastic band to. I haven't been back as a BAR long enough to let you know how long it lasts, but from the posts I've read here, it seems to be a long time (much longer than the fancy rubber bands). And I mostly use 100lb kevlar line. On smaller ones, I'll use 50-70lb line.
 

cbwho

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Since BAR, I made some with kevlar and got burn thru. Perhaps it is too thin? But I think it's flawed to have the cord run to the motor tube imho. Not enough distance to cool gases perhaps? Estes tri-mount works fine imho. Also you can easily glue in replacement cords as the rocket ages.

On my baffled rocket, I find that it slightly bulges the seams below the baffle and the rocket interior is more filthy. I now don't bother with baffles either.
 

neil_w

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I do all of mine like @shawn_rocket above. If I ever have a Kevlar break, and the rocket survives, I will probably repair with a teabag mount (no good way for me to replace the Kevlar.)

One nice thing about the Kevlar + elastic approach: it's easy to replace the elastic, which I've had to do on a few occasions.
 

Spitfire222

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On a RTF rocket with a pre-installed shock cord like the Spirit, I would just leave it and use it as it. For other kits, I run kevlar on mostly just the larger ones (i.e. C, D and up rockets). For the small, minimum diameter rockets, I just use the trifold mount. I replace the rubber shock cord with longer, elastic ones, and I have three different width elastic cord to choose from depending on the size of the model.
 

Back_at_it

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I do the same as Shawn. Kevlar that runs up and stops about 1/4" from the top of the tube then Elastic from there. Where I attach the Kevlar differs from rocket to rocket depending on if I install a baffle or the length of the rocket. If I'm running a baffle I install a screw eye to the top plate and connect the Kevlar to that.

As for protecting the Kevlar. I use 200# minimum and protect the bottom few inches with heat shrink tubing. I have never had a Kevlar line burn through or fail.
 

Pem Tech

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Well, I haven't used an elastic shock cord (SC) on any of my rockets since returning to the Hobby in the early 2000's. This is for the same reason all our kits come with 5 feet of #440 KEVLAR thread. I've only had one burn-through and that was with a D12 CATO, all the rest are still flying with their original SC. Using 60 inches of KEVLAR gives plenty of length to avoid stress on the recovery system and negates the chance of "snap-back". The only hitch is to be sure sure to wrap the SC with tape at the point it contacts the BT. Over years I've neglected to do so and have the razor thin zippers to prove it.
Not to continue striking the deceased Equus caballus, but if you have sufficient KEVLAR SC why use elastic at all? They dry out, crumble, and break in very short order. Not to mention the taste and texture, I mean *blerg*, they are nasty. Never cook with them.
 

Back_at_it

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Not to continue striking the deceased Equus caballus, but if you have sufficient KEVLAR SC why use elastic at all? They dry out, crumble, and break in very short order. Not to mention the taste and texture, I mean *blerg*, they are nasty. Never cook with them.
I use it to absorb the snap when the parachute opens so I don't end up with zippers. If you have sufficient length of elastic the body sees very little force when the chute opens. But you are right that elastic does dry out and will need to be replaced at some point. I've not had to do that yet except for the couple of times I didn't use enough wadding in rockets without a baffle but I'm sure it will happen.
 

Huxter

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And cost - elastic is much cheaper. I also do all of mine like Shawn! I love my 500# kevlar. I do not want to run that kevlar out 2+ times the length of the rocket. Elastic is cheap at wally world.

And kevlar is more of a strain on the recovery system too. I attach to the engine tube or coupler, baffle...

I tie a knot in elastic by nose cone and attach parachute to it. This will not put as much strain on the nose cone mount. No (fishing) hardware anywhere - just knots!

protostar02.jpg
 

Patternflyer

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Thank you everyone for your replies. Moving forward with new rockets, I plan on incorporating Kevlar from the can, baffle, or motor mount.
Now regarding my existing rockets, even ARFs, a little creativity is required to retrofit, but I’m slowly going through them one at a time. The important thing is that I no longer have rubber elastic running down the tube and taking direct heat and eventual failure.
 

Back_at_it

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For the rockets that you are retrofitting. There’s nothing wrong with using kevlar attached to the trifold mount inside the body tube. It works exceptionally well. Use a piece of Kevlar for the permanent mounting inside the tube then attach elastic for the shock cord

I’ve done this for a number of retrofits of my older rockets
 

mikewrt

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For the rockets that you are retrofitting. There’s nothing wrong with using kevlar attached to the trifold mount inside the body tube. It works exceptionally well. Use a piece of Kevlar for the permanent mounting inside the tube then attach elastic for the shock cord

I’ve done this for a number of retrofits of my older rockets
Like this: https://www.rocketryforum.com/threads/big-bertha-memories.160213/post-2015901

See post 8 and 37

I do most of my new builds and rebuilds this way.
 

BABAR

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What I really don’t like about the elastic is not so much the elastic, it’s the trifold/teabag mount. I prefer my rocket body interior to be as smooth as the proverbial baby’s behind, so chutes shock cord, and wadding (yeah, I haven’t invested in dog barf yet, I still use the Estes TP that comes with the bulk motor packs) all slide out easily. For some rockets that can be a tight fit, and having a three fold of paper on one side doesn’t help.
 

Back_at_it

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I prefer my rocket body interior to be as smooth as the proverbial baby’s behind said:
Agreed. The nice thing about Kevlar is it’s much thinner than the rubber or the elastic so it makes for an almost flat install against the interior wall. Once dry, a finger full of wood glue laid over it makes it smooth
 

Old School Doug

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I started using Kevlar shortly after returning to the hobby. I've never had a burn through problem but my first several attempts caused some tube zippering. At that point I decided to add a length of elastic to the Kevlar. The problem with zippering stopped but I'd occasionally get a parachute shroud line hang up on the knot. So after noodling around with possible solutions I came up with method I now use exclusively. The 100# Kevlar shock "leader" and the 1/8" flat elastic are joined with a swivel, surrounded by a 1" piece of 3/16" heat shrink tubing. Since using this technique I haven't had any zippering or shroud line hangups. As a side benefit, if I'm using a JLCR the exposed portion of the swivel is a great tether attachment point.

RCVR1.jpg

RCVR2.jpg
 

afadeev

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I'm sure this has been an ongoing debate for years, but simply put, for those of you that prefer using Kevlar, do you replace the elastic rubber cords with Kevlar on 100% of your rockets, or just those that are more prone to burning through?
Yes, 100% replace rubber with Kevlar.
The answer to the other part of the question is also yes, since 100% of elastic and rubber cords are prone to fire damage and premature failure.

Elastic cord goes straight into the trash can, right after I open a new kit bag. Then a Kevlar length of at least 3x the completed airframe gets cut from the spool.
I am yet to encounter a single incident of zippering, or have any other reason to not 100% rely on braided Kevlar strings. Then again, I also routinely CA impregnate the forward 1-2" of the airframe that mates with the nose cone shoulder. This also serves to minimize paper tube diameter variation with humidity, ejection singing of the tube, and reduces water damage if the airframe comes in contact with a stream.

I'm building an Estes Spirit right now and am considering cutting out the pre-installed rubber cord and running Kevlar through the fin can before I glue everything up. Again, my question is if there is a significant number of folks that run Kevlar in ALL rockets if at all possible?
After one too many elastic failures, I now use Kevlar cord in 100% of my low-power rockets.
150# braided Kevlar for low power.
700# braided for mid-power.
HP get Kevlar harnesses (flat or tubular, depending on the weight of the airframe).



YMMV
 
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rklapp

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I've had one or two times when the Kevlar cord broke near the MM while using dog barf. I think it clogged at the inside end of the tube causing the cord to burn through.
 

K'Tesh

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I'm gonna drop this here... I'm on my phone, and don't have time or energy enough to retype this...

 

cbwho

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I've had one or two times when the Kevlar cord broke near the MM while using dog barf. I think it clogged at the inside end of the tube causing the cord to burn through.
I had the same thing. Kevlar in a trifold mount works well. I always have a section of elastic in however and that's due to trees. I've had to fish a rocket out of a tree many times and the elastic breaks freeing the rocket. Then I simply knot in new elastic and launch the same day. The key is only have a short section of kevlar for easy retying of the elastic. A long section of kevlar will wrap around the tree limb.
 
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rklapp

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I had the same thing. Kevlar in a trifold mount works well. I always have a section of elastic in however and that's due to trees. I've had to fish a rocket out of a tree many times and the elastic breaks freeing the rocket. Then I simply knot in new elastic and launch the same day. The key is only have a short section of kevlar for easy retying of the elastic. A long section of kevlar will wrap around the tree limb.
It’s only happened with dog barf so only use it on BT-80 tubes.

I have an Exocet stuck at the apex of a tree above a knot of branches. It has about 3ft of elastic. I built it too well. We tugged on that knot of branches for an hour but the elastic won’t snap. We’ll try again next week to see if the elastic has weakened.
 

AfterBurners

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I started using Kevlar shortly after returning to the hobby. I've never had a burn through problem but my first several attempts caused some tube zippering. At that point I decided to add a length of elastic to the Kevlar. The problem with zippering stopped but I'd occasionally get a parachute shroud line hang up on the knot. So after noodling around with possible solutions I came up with method I now use exclusively. The 100# Kevlar shock "leader" and the 1/8" flat elastic are joined with a swivel, surrounded by a 1" piece of 3/16" heat shrink tubing. Since using this technique I haven't had any zippering or shroud line hangups. As a side benefit, if I'm using a JLCR the exposed portion of the swivel is a great tether attachment point.

View attachment 447605
View attachment 447606
I like your setup using Kevlar as an anchor and elastic as the main cord. Easy to change out if necessary and the elastic provides some "give" and stress relief
 

hermanjc

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I'm gonna drop this here... I'm on my phone, and don't have time or energy enough to retype this...

I agree with your post here and I have too recovered multiple tree ornaments by returning after some small amount of time to collect both the body of the rockets and the nose cones thanks to deterioration and excessive twisting/spinning breaking the elastic. I am going to start adding the kevlar mounting to my rockets, however, as a main attachment/leader, while then using the rubber from the body to the nose to still provide that retrieval opportunity in the event of unintended forest decoration.

I have also recovered a couple lost rockets using a baseball on a string, but have since added an arrow/tube setup similar with the fishing reel (for tree use only). Fingers crossed I never have to use it, but that it works well if I do.
 

Long_Gone

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I agree with your post here and I have too recovered multiple tree ornaments by returning after some small amount of time to collect both the body of the rockets and the nose cones thanks to deterioration and excessive twisting/spinning breaking the elastic. I am going to start adding the kevlar mounting to my rockets, however, as a main attachment/leader, while then using the rubber from the body to the nose to still provide that retrieval opportunity in the event of unintended forest decoration.

I have also recovered a couple lost rockets using a baseball on a string, but have since added an arrow/tube setup similar with the fishing reel (for tree use only). Fingers crossed I never have to use it, but that it works well if I do.
I use the dowel inside of a pvc pipe with the dowel tied to fishing line and a reel, and have had to use it to retrieve Big Daddy 40' in a tree.
 
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