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All Things Shock Cord Design for High Power

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emlang93

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I want to start a new thread on shock cords because I have not seen a thread that answers all the questions to the point. Particularly length seems to have a lot of variation in the answers I get. I have heard 2x all the way up to 5x the rocket length. For size I have seen people say rate to 75G. which for me means a 30lb 4'' rocket can safely use 1/8'' tubular kevlar (Which I have done successfully). Some have said that 1/8'' is okay then others say no less than 1/4'' kevlar on a rocket that size. I have built a lot of rockets and I always feel uneasy about the shock cord length and size, so let's start a discussion and agree on a standard.
So here are the questions.


  1. How long should a shock cord be for a high power rocket? 3''? 4''? Component weight considerations? How to know if the parts slow enough in separation?
  2. Kevlar cord size and pull strength? Is, 75G x attached component mass, sufficient? Should there be a buffer of say 15%? (If the cord is the right length do you really need 75G strength?)
  3. Flame protection on Kevlar? Should you use flame protectors for kevlar? (I don't and haven't had any issues. I just change the harness when it looks rough)
  4. Where should you place the swivel for the chute?

I added 3 and 4 so if others come alone I hope it can answer some of these questions too. Feel free to elaborate on things.
If there is another thread with all of these questions answered, or a 1 and 2, please let me know.

Thanks for the help!
 

blackjack2564

Crazy Jim's Gone Banana's
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3in. fiberglass rocket. [any]

Recovery 61 through 68 http://www.rocketryforum.com/showth...-quot-Stock-quot-3IN-DARKSTAR-CJ-Step-By-Step covers most of your Q's

Shock cord:
http://www.rocketryforum.com/showth...x-quot-build-starts-now!!&p=454920#post454920

This is sticky at top of high power.
Flame protection is in there, but I don't use swivels, can't help with that one.

By the way your logic needs to incorporate how thin cord is, 1/8 may be strong enough for 4in 30lb rocket..[which it is not if things go wrong] but it will cut through an airframe like a knife & zipper. Need to encase it in thicker shroud to distribute load.

I have snapped 1/8 cord like string on 5lb rockets with aggressive charges & obtuse firings.
 

cbrarick

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3in. fiberglass rocket. [any]
I have snapped 1/8 cord like string on 5lb rockets with aggressive charges & obtuse firings.
Obtuse? Are you calling that rocket obtuse? That's gonna cost you three months in the hole, or am I being obtuse??
 

blackjack2564

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Obtuse? Are you calling that rocket obtuse? That's gonna cost you three months in the hole, or am I being obtuse??
Yeah.... since ya asked,
it's about as obtuse:
as a moose
in a chartreuse.
......caboose!#@$%#! Hi Rick.
 

emlang93

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Good information and detailed build Jim! Thanks for the links. I guess I probably got lucky for that flight of my L3 with the 1/8 kevlar. I do have an excessively long shock cord on it though. After your input I immediately bought 18 yards of 1/4 to replace the 1/8 and I will move that to my 3''.
A trick of mine is I wrap the shock cord up in small bundles and wrap tape around those. Then I tape all the bundles togther. Tis seems to really slow the shock of the kevlar down a lot. For my knots I like to cover the note and extra cord in adhesive backed, high shrink, heat shrink. This protects from the gasses and keeps things tight togther. Best is that it can always be cut off with a little bit of work.

I am thinking about stitching in some elastic in the shock cord to slow it down but the kevlar will still take all the load. Will see if I have time to do that before BALLS... probably not.
 

K'Tesh

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I have a method of making installation inspection of shock cords easier... I call it the "U Loop" method.

 

Handeman

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I want to start a new thread on shock cords because I have not seen a thread that answers all the questions to the point. Particularly length seems to have a lot of variation in the answers I get. I have heard 2x all the way up to 5x the rocket length. For size I have seen people say rate to 75G. which for me means a 30lb 4'' rocket can safely use 1/8'' tubular kevlar (Which I have done successfully). Some have said that 1/8'' is okay then others say no less than 1/4'' kevlar on a rocket that size. I have built a lot of rockets and I always feel uneasy about the shock cord length and size, so let's start a discussion and agree on a standard.
So here are the questions.


  1. How long should a shock cord be for a high power rocket? 3''? 4''? Component weight considerations? How to know if the parts slow enough in separation?
  2. Kevlar cord size and pull strength? Is, 75G x attached component mass, sufficient? Should there be a buffer of say 15%? (If the cord is the right length do you really need 75G strength?)
  3. Flame protection on Kevlar? Should you use flame protectors for kevlar? (I don't and haven't had any issues. I just change the harness when it looks rough)
  4. Where should you place the swivel for the chute?

I added 3 and 4 so if others come alone I hope it can answer some of these questions too. Feel free to elaborate on things.
If there is another thread with all of these questions answered, or a 1 and 2, please let me know.

Thanks for the help!
These are all good questions, but I seriously doubt you will get anything close to a consensus answer. I'll give you what I think, but I really doubt many would agree.

* How long should a shock cord be for a high power rocket? 3''? 4''? Component weight considerations? How to know if the parts slow enough in separation?
I think this depends on too many factors to have a valid answer. I use about 2X the length of the rocket for main and drogue on DD rockets. I also make sure I don't overdo the drogue charge. Just enough to reliably open the rocket and let the drogue get into the airflow. The drogue provides enough drag to pull all of the shock cord out if it didn't deploy. You don't need to stretch out the cord on an apogee charge. As for the main, the same thing, enough charge to get the main into the air flow so it opens.

* Kevlar cord size and pull strength? Is, 75G x attached component mass, sufficient? Should there be a buffer of say 15%? (If the cord is the right length do you really need 75G strength?)
This is a matter of preference. Again, I'm talking DD and not motor eject. With the right apogee ejection charge, apogee to main deploy flight profile, and correctly sized main deploy charge, you shouldn't need much more than about 5x - 10x the mass of the components. The question becomes, how much over design do you want to compensate for anomalous events and incorrect charge sizes and poor recovery design. That is all risk mitigation.

* Flame protection on Kevlar? Should you use flame protectors for kevlar? (I don't and haven't had any issues. I just change the harness when it looks rough)
Again, I think this is personal preference. I have protection on my nylon cords, but nothing on the Kevlar. Again, I use primarily DD. Most of my motor eject, I hold the rocket up by the shock cord and fill the BT with dog barf until the motor is covered, then drop the cord in and fly. YMMV

* Where should you place the swivel for the chute?
I don't use swivels on chutes. I don't put them on with motor eject because they open right away and the chute doesn't don't twist up. The fin can my spin, but it needs the swivel, not the chute. On DD rockets, the swivel goes on the fin can, not the chute. The fin can is what spins and twists up the cords, not the chute. I'm seeing a theme here.....

As I said, these are my opinions and I doubt you will find much agreement on any of the questions..

Good Luck.
 

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