How many flights does a nylon shock cord last vs Kevlar?

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Dave S.

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I have a few rockets that I built with the nylon shock cord that came with the kit instead of ordering a separate longer k blaring shock cord. The nylon shock cords are starting to look a little crummy after 10 flights or so.

The shock cord can’t be changed because one end of it is way down inside a small diameter rocket. I’m kind of stuck with them all the way they are.

Should I be worried about the nylon cord breaking at ejection anytime soon? Would it be best to just get another rocket and rebuild it with a longer Kevlar cord this time? Or is this nothing to worry about?
 
There's a bit "it depends" at play here. The more you protect the nylon from direct contact with ejection gasses, the longer it will last. UV also degrades nylon over time.

If it's starting to degrade enough that you're asking about it, I'd replace it proactively. Depending on the rocket, you may be able to install an eye bolt that will allow you to attach a removable harness without entirely tearing the airframe apart.

Cut a small hole in the tube below the top ring, then use a long drill bit and drill an appropriate sized hole. Reach a washer and nut through the hole in the side, then hold with a wrench while you tighten the eye bolt. Afterwards, patch the airframe.

-Kevin
 
I have no Kevlar cords at all, nylon in the heavier HPR rockets or good elastic in the lighter rockets. I use dog barf in every rocket (although messy) in front of every ejection charge, motor or canisters. For the rockets you are speaking of just pull the shock cord tight to side of BT add some dog barf and lay shock cord on top of dog barf. That or slide down a Nomex shock cord protector over the nylon (2 ft. or so) to the top bulkhead. Even with the protector you can also add some dog barf. They will last forever. My Big Brute has a 3/4 in. elastic shock cord with a 2ft. piece of Nomex protector and I use dog barf also. It has 40+ flights and the shock cord is holding up just fine.
Big Brute SC.JPGQuick links.JPG
 
I have zero nylon cords. I use nothing but Kevlar I just make it long.

2” to 6” diameter birds. 1/4” and 1/2” Kevlar.

I’ve had nylon burn through or get suspiciously brittle after a dozen plus flights, I’ve been flying the same set of Kevlar recovery harnesses now for over 4 years and dozens of flights with no issue
 
Might make sense to make the shock cord easy to replace. Nylon isn't all THAT expensive, and it does give. I'm curious what the advantage of flat straps is over round, braided cord. I guess 3 strand might be slightly more prone to tangling, but it's easy to splice.
 
I built a PML Callisto in 2003 that uses a thick 1" nylon strap between the MMT and the piston. The nylon shock cord above the piston is still like new. The nylon strap under it has a crusty surface from all the ejection charges, but it's still going strong after 200+ launches.

I think the type and quality of a nylon shock cord has a lot to do with how long it lasts.
 
I use both. I put a piece of heatshrink tubing over the shock cord were it rubs on the edge of the body tube. When I see damage, cut the old piece off and replace!
 
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