Shock cord advice...

[threelapfancy]

Hello all. I am building a 2" 38mm 5 foot tall all fiberglass rocket. This will be dual deploy. Rocket has a similar deisgn to a Wildman Jr or PR Little Dog.
I would like to use Kevlar shock cord for the rocket but I am trying to decide between 1/8" or 1/4". Any input?
Thanks
Steve

 

[WillMarchant]

https://www.tulsarocketry.org/rot.htm recommends that it be at least 50 times as strong as the static weight of the rocket.

This thread https://www.rocketryforum.com/showthread.php?p=73418 has lots of commentary.
Best wishes,
Will

 

[Handeman]

I have a scratch build DD rocket that is about that same size. It weighs about 3.5 lbs w/o the motor. I use 1/8" tubular Kevlar and the only issue I've had was the Kevlar got hooked under a rail guide while falling drogue-less and ripped it off at deployment.

 

[Commonwealth.Net]

FYI: We stock Braided Tubular Kevlar in 200, 750, 2000, and 4000 pound test.
The 50 X rule is stupid.
The formula to use is F=MA from physics class. So unless your rocket will pull 50 G's that is major over kill. Also keep in mind our Kevlar is rated at a 13 seconds at the rated breaking point. This means the 200 pound test can hold a 200 + poind load for 13 seconds before breaking.

The only advantage to a higher test is the width to reduce the ZIPPER effect. I would use our 750 pound test, the 200 lb test will hold up
but is is so thin it could lead to a zipper if you do not fold your chute properly.

For more info call the store Saturday 12-8pm

 

[WillMarchant]

The 50 X rule is stupid.
The formula to use is F=MA from physics class. So unless your rocket will pull 50 G's that is major over kill.

In order to use F=MA you must have a number for A. You can calculate the separation velocity and do whatever calculations you want to bleed velocity until you reach the end of the recovery harness. Then A is calculated from the elasticity of the recovery harness material and the remaining separation velocity. What is the stretch for your recovery harness material?

At https://www.rocketmaterials.org/datastore/cord/ gives some values that seem to indicate about 7% stretch. Quite a bit actually!

 

[als57]

The 750lb test stuff Commenwealths sells is about the same stuff as Wildman supplied with the Wildman Jr kits. You can also get it thru Performance Hobbies. Pratt Hobbies sells a small tubular kevlar.


Al