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Quick Link Sizing?

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jmmome

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65 pound rocket. two 120" main parachutes with a heavy duty swivel attached to each, which will be deployed at apogee. 50 feet of 2000# kevlar shock cord, and each chute will have a six foot bridle with heavy duty swivels on each end.

When attaching the bridle to the chute, and when attaching the bridle to the nosecone, what size quick link would be suitable? I was thinking of using 1760 pound quick links.

Thanks!
 

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rharshberger

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3250lbs working load limit will provide for a 50G event, quicklinks are iirc rated about 2:1 (WLL x 2 is breaking strength) forchardware store steel and stainless steel types. I rate all of my recovery gear for at least a 50G event as its an easy threshold to reach if things go sideways (both literally and figuratively).
 

Handeman

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If you're on TRA facebook check out this post https://www.facebook.com/groups/cjscjn/?post_id=10159994751924046
I'll add a screen shot of that post below.
1760# seem kind of lite fr the job. Double up 1760# is better. flip one over so they pull in different ways (?)

View attachment 426831

Granted smaller then your 1760#..but **** happens..

Tony
You can't see from the pic, but if the threads on the end of the quick link aren't stripped and some still stuck in the nut end, and they don't look like they are, that link was NOT screwed closed, no matter what the poster says.

I had a near miss when I forgot to close a quick link. Fortunately it didn't open that far and things stayed together.
 

Rocketjunkie

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Quick links are rated with a 4:1 safety factor. However, in the 65 lb. rocket I would use 1/4" quick links. I also don't like Kevlar shock cords, much prefer nylon. Nylon has about 10% elongation at break, Kevlar 3%. For equal size cords, the energy to break is about the same but the peak loads on the attachments are 3 times higher with Kevlar.
 

rharshberger

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Quick links are rated with a 4:1 safety factor. However, in the 65 lb. rocket I would use 1/4" quick links. I also don't like Kevlar shock cords, much prefer nylon. Nylon has about 10% elongation at break, Kevlar 3%. For equal size cords, the energy to break is about the same but the peak loads on the attachments are 3 times higher with Kevlar.
I will disagree with Quicklinks being rated 4:1 as 4:1 and 5:1 are generally lifting grade hardware, the best listing I have seen for hardware/trailer type QL's is 2.5:1, hopefully I can get a chance to question my ironworker/rigger neighbor tomortow to confirm my statement. FYI many states do not allow the use of QL's on safety chains. A rated QL should have a grade marking on them, everyone I have seen has no cert or grade markings.
 

KenC

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You can't see from the pic, but if the threads on the end of the quick link aren't stripped and some still stuck in the nut end, and they don't look like they are, that link was NOT screwed closed, no matter what the poster says.

I had a near miss when I forgot to close a quick link. Fortunately it didn't open that far and things stayed together.
As unlikely as it may seem, the FB quick link could have been screwed shut. I had a very similar incident that opened a 1/4" quick link broke a 2500# Kevlar shock cord and elongated the eyes on a 1500" swivel (second photo).

My incident's root cause was simultaneous firing of both primary and backup ejection charges at apogee 8.5g of FFFFg on a 7.5" 43 lbs rocket.

Fortunately the quick link was recovered still wedged in the main body tube to allow inspection. I know this quick link was screwed shut and tightened with pliers according to my pre-launch check list. The threads on both sides of the quick link still work although they are a little worse for the wear.

As a result I have switched to D shackles so the threads are not the structural component resisting the forces



20190716_132346.jpg


Note the elongated eyes on the left swivel versus the new swivel on the right

20190715_130328.jpg
 

Theory

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I rate all of my recovery gear for at least a 50G event as its an easy threshold to reach if things go sideways (both literally and figuratively).
X2

I have used the "50 G" bench mark when designing all of my recovery systems.

note that often the weak link in this will be the plywood centering ring used to anchor the main harness. if you are going that route that is...

no reason to use massive kevlar and huge links if your anchor is a "welded" eye bolt bolt through a piece of 1/8" ply
 

JoePfeiffer

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If you're using 2000lb shock cord, there's not much point in having quick links rated substantially stronger than that. Whatever analysis led you to that shock cord also tells you what your minimum quick link should be; use the next size up from there.
 
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