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  #11  
Old 06-06-2018, 01:17 PM
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BEC BEC is offline
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Guilty as charged.
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  #12  
Old 06-06-2018, 01:55 PM
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Originally Posted by BEC
Guilty as charged.

I claim "unindicted co-conspiritor."
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  #13  
Old 06-06-2018, 02:41 PM
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Originally Posted by BEC
No, I haven't seem them use it. I have been using Kevlar with the tri-fold with good results - usually putting a foot or two of Kevlar in the tri-fold and then tying the kit-supplied shock cord to that.


Yeah me too... Like this method.

Kevlar around the motor tube works, but subjects the Kevlar to a lot of damage that adds up over time... The stuff ain't bulletproof ya know... LOL

Yeah it IS bulletproof, but over time the clay ejection charge particles, hot gas, and burning BP particles, plus remaining burning delay flames (hibachi effect) does a number on it and hastens replacement requirement... At which point you'll probly end up with a tri fold anyway...

Estes SERIOUSLY needs to ditch the stupid rubber band junk though... I've had that stuff fail the first flight, so I just throw that crap away when I open the kit and substitute good sewing elastic, which works fine...

Later! OL J R
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  #14  
Old 06-06-2018, 04:33 PM
BARGeezer BARGeezer is offline
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Quote:
Originally Posted by luke strawwalker

Estes SERIOUSLY needs to ditch the stupid rubber band junk though... I've had that stuff fail the first flight, so I just throw that crap away when I open the kit and substitute good sewing elastic, which works fine.


Even sewing elastic can fail. My Rocketarium Jayhawk had its' 1/8" elastic shock cord snap on its' first flight. There is a lot of nose weight on that model. May have been too much for it to handle. Replaced with 1/4" elastic. And you know how elastic can stretch out over time, right? Just look at my BVDs. On second thought, don't do that. Laters.
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  #15  
Old 06-06-2018, 05:11 PM
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Default Yay for tri-fold!

However,

Back in '12 during NARAM the Brighton doinked due to either drag separation or short delay.

The elastic snapped at the shock cord mount and both parts fluttered back to Earth on their sides. No damage other than a shredded 'chute!

John Dyer of Red River Rocketry taught me a neat trick - the LOC Precision shock cord mount.
It's soo simple and works! 1 loop of heavy kevlar taped inside the body and then the tape/knot
is painted over with epoxy. Replace elastic as needed.

Bob
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  #16  
Old 06-06-2018, 05:36 PM
BARGeezer BARGeezer is offline
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Quote:
Originally Posted by Blushingmule
However,

Back in '12 during NARAM the Brighton doinked due to either drag separation or short delay.

The elastic snapped at the shock cord mount and both parts fluttered back to Earth on their sides. No damage other than a shredded 'chute!

John Dyer of Red River Rocketry taught me a neat trick - the LOC Precision shock cord mount.
It's soo simple and works! 1 loop of heavy kevlar taped inside the body and then the tape/knot
is painted over with epoxy. Replace elastic as needed.

Bob



The newer LOC kits I've built have a length of nylon instead of kevlar. A knot tied on each end of the loop, taped to the body tube, and top coated with epoxy. Very durable and it works. Might be overkill for LPR birds. Plus schoolkids using epoxy might get messy real fast.
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  #17  
Old 06-06-2018, 05:50 PM
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Quote:
Originally Posted by BARGeezer
The newer LOC kits I've built have a length of nylon instead of kevlar. A knot tied on each end of the loop, taped to the body tube, and top coated with epoxy. Very durable and it works. Might be overkill for LPR birds. Plus schoolkids using epoxy might get messy real fast.


It is overkill for LPR, was thinking of the repair at the field. I didn't use the baffle in the kit and used a traditional shock cord mount.

Bob
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  #18  
Old 06-06-2018, 06:40 PM
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Quote:
Originally Posted by Jerry Irvine
Wow. Geeky question. Well researched answer. Good read. Anybody that posts to this thread is obsessed with model rocketry.


You misspelled forum.
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  #19  
Old 06-07-2018, 09:46 AM
stefanj stefanj is offline
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I most often, for lower power models, use a tri-fold and sewing elastic.

Kevlar to elastic, using a barrel swivel, for larger models.
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  #20  
Old 06-09-2018, 12:22 PM
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luke strawwalker luke strawwalker is offline
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Quote:
Originally Posted by BARGeezer
Even sewing elastic can fail. My Rocketarium Jayhawk had its' 1/8" elastic shock cord snap on its' first flight. There is a lot of nose weight on that model. May have been too much for it to handle. Replaced with 1/4" elastic. And you know how elastic can stretch out over time, right? Just look at my BVDs. On second thought, don't do that. Laters.


I'd never use 1/8 elastic on anything bigger than a BT-20 size rocket... I always use 1/4 inch elastic or larger.

Yes, elastic can break, but it's FAR stronger than the stupid rubber band crap Estes puts in their kits... That stuff seems to have two main failure points that I've identified-- 1) if the kit sits in a display or window where the rubber is exposed to solar UV, it can EASILY break on the first flight... same way tires get weather-checked and dry-rot from solar UV exposure, the rubber bands get exceedingly weak from solar UV exposure. 2) the rubber band material gets repeatedly exposed to the hot gases and sulfur compounds of the rocket motor ejection charge, which tends to "vulcanize" the rubber over time (no, it's not "true vulcanization", but the effects are much the same). The rubber gets harder and more brittle over time and after a few flights it tends to dry out and break, particularly down in the tube near the shock cord mount where the exposure to heat and sulfurous gases is the greatest...

The rubber in elastic is *somewhat* protected by the cloth structure of the elastic band covering it, which helps to minimize damage. Additionally, elastic usually has SEVERAL rubber cords inside it, which multiple means redundancy. Plus, the cloth covering that connects all the cords also gives some additional support and redundancy.

I typically double or triple the length of the 'stock' shock cord when I replace it as well, because Estes shock cords are NOTORIOUSLY short, which also contributes to failures, as well as "snap back" smiley-faces in balsa nosecones and stuff from forceful collisions between the nosecone and the tube edge due to snap-back of excessively short shock cords. 2-3X longer shock cords are usually not a problem to load, except in very small rockets (which I don't particularly like anyway).

That's been my experience, anyway. 1/4 inch elastic shock cords have lasted the life of the rocket for me, so that's what I use... Even kits (like Dr. Zooch) that use kevlar shock cords, I usually will add a length of elastic to double the length, because kevlar has ZERO elasticity and when it comes to the end it comes up HARD... elastic takes some of the "slam" out of that hit and also softens the impact of chute opening... The main thing is, the elastic has to be long enough to spread that shock out over a great enough length that it doesn't "bottom out" and snap, which is when the elastic stretches to the point that the cloth cannot stretch any more, and then it rips and snaps. Doubling the length of the existing kevlar cord in such kits is a MINIMUM I'd recommend when adding elastic-- if I can, I usually DOUBLE that...

Later! OL J R
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