How should I design My Nylon and Kevlar shock cord?

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LilFajita

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For my schools rocketry program I have been assigned to the recovery team and we decided on using a combination of braided kevlar and nylon shock cord together to minimize stress on the shock cords, is this even a good idea or should I only do nylon or kevlar? If I end up doing both, what are yalls' suggestions for the length difference between the two cords, I can't find any info online about how much nylon shock cord stretches and Im at a loss for what cord I should longer. For context my rocket is using a L550 and we're breaking the sound barrier this year, it is 4 inches wide and 5 feet tall.
 
Is this for high school or college?

Is that L550 a Cesaroni hybrid motor?

Can you provide any additional design info? For instance an OpenRocket or RockSim design file?

How about the rocket's overall weight, ready to launch ?

Is it a traditional dual deploy setup, with two sections and an avionics bay?

Etc. . . . .
 
Is this for high school or college?
why? does not change the (good) Q?
Is that L550 a Cesaroni hybrid motor?
+1 that is important.
Can you provide any additional design info? For instance an OpenRocket or RockSim design file?
+1
How about the rocket's overall weight, ready to launch ?
+1
Is it a traditional dual deploy setup, with two sections and an avionics bay?
+1 very important info.
 
Mixing Kevlar and Nylon is fine. Lots of us do it all the time.

Usually the Kevlar is short, and for areas not protected from heat of deployment charges. Then a long length of Nylon where it is rolled and/or wrapped up in a protector of some kind.

2 issues.

1) While Kevlar is great at high heat, it can still be damaged. (By heat or abrasion, etc.) Check it after each flight; and if possible have a way to replace it if needed. (Glued directly to the motor mount is easy and usually works "for a while", but its not replaceable.)

2) Joint between Kevlar and Nylon needs to be thought about, and design reviewed. Knots decrease a cords strength by 30-70%. So a poor choice of knots, and 1000# cord is only good to 300#. If you use braided cord, and someone does a properly built eye splice, the cord can retain 90-100% of it's strength.

Mike
 
Mixing Kevlar and Nylon is fine. Lots of us do it all the time.

Usually the Kevlar is short, and for areas not protected from heat of deployment charges. Then a long length of Nylon where it is rolled and/or wrapped up in a protector of some kind.

2 issues.

1) While Kevlar is great at high heat, it can still be damaged. (By heat or abrasion, etc.) Check it after each flight; and if possible have a way to replace it if needed. (Glued directly to the motor mount is easy and usually works "for a while", but its not replaceable.)

2) Joint between Kevlar and Nylon needs to be thought about, and design reviewed. Knots decrease a cords strength by 30-70%. So a poor choice of knots, and 1000# cord is only good to 300#. If you use braided cord, and someone does a properly built eye splice, the cord can retain 90-100% of it's strength.

Mike

You may want to offer them some knot suggestions - you know that question is coming . . . .
 
I'll add that on traditional dual deploy with motor backup, the drogue section on some of mine have 3 sections. Kevlar-Nylon-Kevlar. The Kevlar at the AV Bay is for the heat from the altimeter fired ejection charge. The the main length of Nylon wrapped in a Kevlar blanket for protection, provides shock absorbing. Then Kevlar at the motor for heat of motor "backup" ejection charge.
 
I can't find any info online about how much nylon shock cord stretches and Im at a loss for what cord I should longer.

Try searching this site for information, the subject does come up for discussion.

When packing for flight, some flyers bundle their shock cord in sections, then wrap those bundles in a layer or two of masking tape or electrical tape. Some of the energy is dissipated as the cord cuts through the tape wraps.

As a start, look at the “similar threads” at the bottom of this page.
 
Some I've done is flake out the shock cord and tie each end with fishing line, threading the line though each flake. Then tieing a knot to secure the fishing line in place.

So during recovery deployment as the shock cord travels out each flake will break the tension of the fishing line, there by absorbing the kinetic engergy of deployment.

It is infinitely variable by the choice of weight of line chosen.

Yes it is time consuming process, but it does absorb energy of the deployment charge of recovery system.
 
it all really depends on how big/ heavy your rockets is.
maybe just some masking tape around a folded shock cord.
Maybe heavy enough or fast enough to need electrical tape or several wraps of tape and several bundles..
https://www.rocketryforum.com/threads/im-shocked.187830/

or heavy enough and fast enough to use zip ties in an engineered bundled system.
z1.jpg
z2.jpg
z3.jpg
z.jpg

It's all easy enough to get some tape/zip ties, shock cord and play around and pull on them.
z4.jpg

Tony
 
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Since the OP hasn't replied in 5 days, I'm not sure they are actually taking any of the great advice given.

If this team is like many I've seen, they are a group of L0 members trying to build a large L2 or L3 rocket. They have little to no practical experience in rocketry and may not even understand the advice given.

Hopefully they are gathering the suggestions provided and also have a L2 or L3 mentor that can help them.
 
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