Shock Cord Question

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Feb 22, 2017
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I'm building my first serious mid-power rocket, a 2.6" Madcow DX3 and it seems to be an excellent kit. I do however have a question about the stock shock cord configuration. The setup with the kit is a length of kevlar cord (guessing around 1/8 inch) that would tie to another length of nylon cord (no elastic - jsut plain old while nylon). The kevlar portion would be knotted around the motor mount and passed through a notch in the upper mm retaining ring.

My question is why the two different types of cord? Is there an advantage to this over using kevlar for the entire length (with no knot in the middle)? I have enough 1/8 inch tubular kevlar on hand to swap it out. With the kit the total length of the shock cords is about 7 feet. I'd like to go a little bit longer than that.
Kevlar is more heat resistant than nylon but, has a reputation for slicing body tubes. the other problem is that Kevlar has very little stretch to if you want to use all Kevlar you should(IMO), tie a knot in it just before the end of the body tube to help spread out the load and increase the overall length to around 17' or so.
Thanks for the tip. With the kit, the kevlar section extends well past the body tube so it seems it's not to zipper-proof the thing. Would the nylon section be to give it a little more spring? I'm totally fine to keep this thing stock if that's the best configuration for the rocket...
When I do a Kevlar/nylon shock cord, I knot them together so that the knot is inside the body tube. This limits the chance for a zipper.
I would still try to keep the join below the top of the bt if possible or use some other anti-zipper method. a mini tennis ball should work.
I have reverted to using only tubular nylon for recovery harnesses. Kevlar is strong but snaps quite quickly at only 6-8% stretch typically. When I have used Kevlar I use waaaay overrated breaking strains, so that it is thicker and less likely to floss the tube.

I treat the nylon as consumable.

I usually make a loop and tie a bowline knot outside the tube. That way it gives twice the material to spread the forces and reduce the chance of a zipper.

I put a Nomex harness sleeve over the harness where it might be exposed to ejection charges.

This is only one opinion. The question you ask is a bit like a glue thread...

Make sure you use a reliable knot for joining two different materials together. There are ones that are particularly good for that.
The kevlar that comes with the smaller Madcow kits tends to be a pretty wide, loose braid, not much thinner than the nylon they provide, so I don't think the zipper worry is as bad as with thinner Kevlar. I've flown the stock cord on the 2.6" PAC-3 without any issue, but adding extra length is good insurance if you have the material for it.

My favorite shock cord material for those size rockets is the flat half-inch Kevlar strap that Onebadhawk offers - it's still not stretchy but the added flame/crud resistance and smaller packing size is nice compared to equivalent tubular nylon.
The reason they give you a short piece of Kevlar is so you don't have to protect the Tubular Nylon from the E charge..
Just arrange them so that all of the Tubular Nylon wind up behind the Nomex blanket and the only thing exposed to the E charge is the Kevlar..
If you already have enough 1/8" Tubular Kevlar that's great use that.. If you have any concerns about zippering the tube the harness isn't long enough.. You made mention of 7 ft.. That seems way too short to me.. If it comes out a bit too long no probs at all.. If it comes out a bit too short it'll make many different problems,, damaging the edge of the tube and shocking the attachment points.. You don't want to shock things when you get to start with electronics.. Don't forget,, this is a harness,, not a shock cord,, it should never take a shock load,, if it is, there is something fundamentally wrong with your set up,, too short of a harness or too energetic of a E charge..
One more..
If you want some extra zipper protection or just be extra gentle on the boosters edge so you don't damage it at all
then take a 6" piece of the "Tubular" Nylon and sleeve the Kevlar right where it exits the booster....

Short answer to the OP, cost. Kevlar is more resistant to the heat of the deployment charge, but nylon is cheaper. Also kevlar has no stretch, while nylon does have a little.
They give you just enough expensive heat resistant non-stretching kevlar to get you out of the fireball, then make you add cheaper, with a little shock forgiveness on the end.