Shock cord for heavy rockets

[mars]

I am working on a pretty heavy rocket (~80/90lb dry) and am researching the best kind of shock cord to use. For a previous project of a similar size, I used 1in tubular nylon, but I would like to save mass and volume this time around.

I'm currently looking into Kevlar lines (like this 3.5mm, 2000lb line from EmmaKites) as well as Spectra/Dyneema (like this 5/32in, 4000lb line from Go2Marine). On paper, their low mass/high strength is awesome, but I'm a bit scared because I've never heard of them being used on projects this size before. Does anybody have experience using these or other small diameter, high strength shock cord on big rockets?

I am planning on using a stretch of thicker webbing in the coupling sections to avoid zippering from thin lines.

 

[dhbarr]

Avoid the Spectra/Dyneema. It's not particularly heat resistant and almost anything will abrade it pretty heavily.

Check out onebadhawk Kevlar harnesses, great stuff.

 

[rharshberger]

Honestly get with Teddy Chernok from One Bad Hawk, he can recommend/design/make the right harness for your project and for less than a mistake will cost. The materials you listed are to narrow and will cut through an airframe like a saw in the event of an off nominal deployment aka "zipper". The 1" TN you use before or a wide kevlar of at least 5k breaking strength would be my recommendation, but Teddy is the man.

 

[cbrarick]

Spectra is cool stuff, much stronger then steel and lighter then almost anything. Unfortunately it's not happy with heat and many solvents.
And they're right, it's so narrow it's almost guaranteed to zipper a rocket.

I do have 50' 1/4 spectra that I put inside of some 3/8 tubular kevlar with loops on either end. If's my self rescue rope for when I'm a firefighter. As long as I have a tool, or some sort of other anchor point, I can bail out of any window, 5 stories or less....

 

[Titan II]

Tubular Kevlar™ shock cord [Large: 1/2” 5500 lbs Test].

https://giantleaprocketry.com/products/components_recovery.aspx#Shockcords

If you have the room, this is something to consider.

 

[Worsaer]

You'll hear again and again that Teddy at OneBadHawk.com is the go-to guy. He understands the differences in materials and will help you make a good choice. He also stands behind his work.
I used his Kevlar harnesses in an L3 project in 2016 and could not be more pleased.

 

[RocketFeller]

We also used a OneBadHawk for our big rocket project.

For our upscale (12.75") Dragonfly we ended up going with a 55'x1" tubular nylon Y-harness, at Teddy's recommendation. The advantages of the nylon are more stretch and less chance of zipper, as mentioned previously.

It seems like the weight/volume difference would be fairly inconsequential in a rocket that size, so unless your recovery compartment is super-tight, I would go with a nylon harness.


Sent from my iPhone using Rocketry Forum

 

[mars]

Thanks for all of the recommendations, everybody! It's much appreciated--I'll be checking out OneBadHawk right away.

For this project, I'm not too worried about zippering because both coupling sections that it will be attached to are aluminum, but abrasion could be a real problem (thank you, dhbarr for the heads up on that!). I agree that the poor elasticity of spectra/dyneema/kevlar is also problematic. Has anybody ever had a recovery harness made from multiple sections of different materials? (i.e. get the best of both worlds: elasticity of tubular nylon, strength and low volume of thin kevlar?). I do not think I have enough room in my recovery bay to use 1in tubular nylon/kevlar alone.

 

[blackjack2564]

What size/kit rocket is it?