#### threelapfancy

##### Well-Known Member
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

#### Handeman

##### Well-Known Member
TRF Supporter
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

##### Well-Known Member
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.

#### WillMarchant

##### Well-Known Member
TRF Supporter
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!

Last edited:

#### als57

##### Well-Known Member
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

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