Shock cord alternatives to kevlar?

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14bis

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Hello everyone.


I'm building a rocket that will have around 10 lb at apogee (~3300 ft, with horizontal velocity of around 33 ft/s)
My problem is: I don't live in the US and I can't seem to find the 1500# shock cord here (chosen from previous experience).(https://www.apogeerockets.com/Build...ar_Cord_1500?zenid=gp72kqr2aq7mq8f2mib89b7214).


What I used to do for single event rockets was to use nylon cord (the same as suspension lines) braid it and cover it with rtv silicone. It works, however it is ugly, demands a lot of time and it's a cheap solution that can cause expensive problems.


So I want to use kevlar, I've used it in the past (in the US), and I found this one (site in Portuguese, but you can see it's image: https://produto.mercadolivre.com.br...amida-40-m-25mm-assist-hook-corda-de-arco-_JM). Specs: 2.5 mm or 3 mm, 1200 lbs.


I calculated my max loading as follow: 10 lb x 20 g = 200 lbs, so I have a good safety margin, but I don't have any experience to how this thin (I think) shock cord would handle the 'shock'.

So if anyone could shed some experience using similar cords or different materials I could use, I'd appreciate it. I read about using nylon straps too, and tubular nylon too. This thread has helped me a bit (https://www.rocketryforum.com/showthread.php?48638-Calculating-Shock-Cord-Size-and-Lengths), but I'd like to try to expand on the material type.

I'll use ejection cannisters and around 3g of 4F BP. Fire is a concern, that's why I'm not sure about using nylon.

Thanks for the help!
 
Tubular nylon works very well. It has more stretch than Kevlar so it doesn't have to be as long.
I'm using 5/8 tubular nylon in my ~12lb rocket and expect it to be overkill.

As far as flame protection, is Nomex (fire retardant cloth) readily available to order in your region? It can be made into cord sleeves or chute protectors.
Also, duct tape can serve as inexpensive protection for nylon cords. I fold a strip around the first foot (.3 meter) of my nylon and elastic cords .
 
Tubular nylon works very well.

+1. But if you use it make sure you use protective sleeves and wrap well in the burrito. In my experience nylon seems to get superficial scorch marks from the BP more easily than kevlar. Here's a shot of 5m of 1.3cm width tubular nylon doing its thing on a 3kg rocket last weekend. As you can see the portions nearest the AV charge (2g 4F BP) and the backup motor eject charge (2g again) are protected by nomex sleeves. The dark blue sleeve is homemade. Good luck!

Screen Shot 2018-05-10 at 23.29.37.png
 
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Thanks for the response.

Do you have a post here with more info about your rocket?

As for nomex, I bought it while in the US, so I'll use it (the orange one from apogee), but I had success using cotton waste inside the bp cannister and regular clothing as chute protector.

I like the duct tape idea.
 
+1. But if you use it make sure you use protective sleeves and wrap well in the burrito.

So, the shock cord is with the parachute inside the burrito? Or should I wrap the chute in one burrito and the shock cord in another one?

Great pic by the way :D
 
Last edited by a moderator:
So, the shock cord is with the parachute inside the burrito? Or should I wrap the chute in one burrito and the shock cord in another one?

Great pic by the way :D

I z-fold as much of the 'naked' shock cord inside the burrito as possible. The rest I ensure are covered with the sleeves.
 
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