1/8" Kevlar Snapped :/

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Lister
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1/8" Kevlar Snapped :/

Postby Lister » Sun Oct 25, 2015 11:39 am

I did a night flight last night with my darklite (darkstar lite with led's).. went well but when I recovered the section with the lights I found it lacking the VK Nosecone which contained the rrc3.. it was a 17ft shock cord so dont see it being caused by to much force durning seperation.. deployment was just after apogee so no extra force there.

Im currently searching a paddock for it but am curious if its common for Brand new 1/8" braided kevlar to snap on a1.3kg rocket... its hard enough to cut with sewing scissors so didnt imagine it snapping on the maiden flight.
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2015-10-25 12.24.21.jpg
Snapped shock cord
2015-10-25 12.24.21.jpg (78.54 KiB) Viewed 1206 times

drew
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Re: 1/8" Kevlar Snapped :/

Postby drew » Sun Oct 25, 2015 11:58 am

Hi Lister. Sorry to hear about the recovery harness failure. Where along the harness did the kevlar fail?
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Re: 1/8" Kevlar Snapped :/

Postby Lister » Sun Oct 25, 2015 12:48 pm

Wasnt the harness.. was the actual shock cord.. it snapped about 1ft from the nosecone just above where the chute was connected to.

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Re: 1/8" Kevlar Snapped :/

Postby kopius » Sun Oct 25, 2015 1:11 pm

Were you using motor ejection or a made up charge?
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Re: 1/8" Kevlar Snapped :/

Postby OverTheTop » Sun Oct 25, 2015 1:11 pm

Looks a bit stressed to the left of the knot in the pic too :shock:

Kevlar is strong but it goes to about 6-8% strain and then fails suddenly.

I use tubular nylon in my larger stuff and have never had a problem.

Consider using something with a bit of give in it, at least for part of the harness. You can z-fold and tape the harness to ease the deployment shock. Energy is absorbed as the tape rips. It is a bit more fiddly in a smaller rocket though.
Check here for a z-fold example: viewtopic.php?f=6&t=4612&hilit=nike+smoke&start=78
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Re: 1/8" Kevlar Snapped :/

Postby Lister » Sun Oct 25, 2015 1:29 pm

I always z fold and masking tape my shock cord to absorb some of the shock. Just about to go back out and resume the search.. will be using tubular shock cord and tracking from now on lol.

I just looked closely at the shock cord..your right, it looks like there is some fraying on the other side of the knot.. could it have been the knot that caused extra stress to not travel the full length of the cord leading to the snap?

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Re: 1/8" Kevlar Snapped :/

Postby SpaceManMat » Sun Oct 25, 2015 2:09 pm

Lister can you confirm that the small length with the frayed end is the failure point?

I'm thinking cut rather than strained.
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Re: 1/8" Kevlar Snapped :/

Postby Lister » Sun Oct 25, 2015 2:16 pm

Without having the rest of the shockcord thats still with the nose I would say yes that was the failure point.

I did think that but there is nothing that could have cut it.. the nosecone was 1ft away from point of failure, and there are no sharp edges anywhere on the nose or the airframe that could cut it nose has a removeable cap and wingnut and the harness on the nosecone is 1/8 kevlar aswell so its snag free.. I blame the russians

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Re: 1/8

Postby Lister » Sun Oct 25, 2015 2:19 pm

kopius wrote:Were you using motor ejection or a made up charge?


Motor eject.

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Re: 1/8" Kevlar Snapped :/

Postby drew » Sun Oct 25, 2015 2:43 pm

SpaceManMat wrote:I'm thinking cut rather than strained.


That's my thought as well. I'm not sure if you beveled the fins but sharp edges made out of FWFG or CF are exceptionally efficient in cutting braided kevlar. Have you looked at the fins and the corners on the airframe to see if there's any "paint rash" for lack of a better term that might indicate a cut?
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Re: 1/8" Kevlar Snapped :/

Postby Lister » Sun Oct 25, 2015 3:29 pm

Just checked every mm of the parts I have.. no damage at all to fins or the corners of the airframe.. infact theres no damage i can see anywhere and no kevlar fibres where they shouldnt be.

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Re: 1/8" Kevlar Snapped :/

Postby SpaceManMat » Sun Oct 25, 2015 3:38 pm

Is that a swivel I can see in the picture? Perhaps it was caught in it, looks like the right sort of length.
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Re: 1/8" Kevlar Snapped :/

Postby Lister » Sun Oct 25, 2015 3:46 pm

Indeed it is.. i thought of that before flight. So again all snagable edges were removed or wrapped in tape.. and there are no marks on the tape and no fibres in any of the swivel. It seems strange that it would happen so close to the knot.. and there was no epoxy or anything on the shock cord that could have made it brittle..

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Re: 1/8" Kevlar Snapped :/

Postby High Impulse » Sun Oct 25, 2015 5:43 pm

Well i know it is unlikely but perhaps it was a bad batch of kevlar?

If you have any left you could do a tensile test of sorts to see if the other rope fails prematurely? Just tie the rope in a similar manner around a balcony and drop a weight from the other end which will approximate the force in the member during ejection.

If it is a blatant manufacturing fault that would pick it up, otherwise without the nosecone there's no real way of telling what caused failure by the looks of things...
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Re: 1/8" Kevlar Snapped :/

Postby Lister » Mon Oct 26, 2015 8:06 am

Good idea Stewart.. Think I will test with the rest of the shock cord and use tubular nylon or kevlar strap for anything over 1kg.. not willing to loose more parts, at least I got my motor case back :) might get a mate to come gold detecting in that paddock as the nose has some all thread that should show up on a detector.


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