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Old 06-09-2018, 11:22 AM
luke strawwalker's Avatar
luke strawwalker luke strawwalker is offline
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Join Date: Dec 2007
Location: Needville and Shiner, TX
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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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