Choosing the right size of kevlar

I am currently working on two FG rockets, a 3" Punisher and a 4" Der Red Max. I have quite a bit of 1/4" tubular kevlar (from Wildman) that I'd like to use for the harness, but in most build threads for rockets of this size I see 3/8". Is there a rule of thumb that I can use to estimate what thickness of kevlar will be appropriate for a given rocket?

 

The diameter or width of the product does not necessarily correspond to load rating. When choosing harnesses for my L3 project I worked with Teddy at OneBadHawk to make the best choices.

 

1/4in tubular kevlar will be fine. I use Teddy's 11/32 tubular kevlar in my 50-75 lb birds. It has a break strength of 4000lbs. His 3/16 should be good for rockets under 20lbs. You can use 3/8 but it's overkill.

When sizing, look at the total weight of the components and the expected force you want to sustain. I normally base my decision on sizing the harness to support at least the force of the heaviest piece with a 75 G acceleration. For a 3in bird, I suspect your heaviest piece is gonna be no more that 10 lbs. which means a 750 lbs shock load at 75g. Size you harness to support that or greater and your good. If you have that type of acceleration at deployment chances are good something went wrong and you have other things to worry about.

 

1/4 tubular kevlar should have pull rating of 3000lbs. Should be fine for a 3" rocket.

I have been asked during RI before if 1/4 tubular kevlar is big/strong enough on a 3" rocket for the drogue recovery harness and wasn't asked if the 9/16 tubular nylon was heavy enough for the main recovery harness on the same rocket. By rated strength the kevlar should be stronger than the nylon. On a 3" rocket, I'm comfortable with either.

Greg

 

Thanks. That's exactly what I was looking for.

 

Yes,, there's a rule..
The single most important rule in harnesses...
It has to be long enough..
It shouldn't be taking a shock load..
If it is there is something fundamentally wrong with your set up,,
( either to short of a harness or too energetic of a separation charge )...
As long as the harness is long enough,,
it doesn't have to be killer strong at all....

So is my harness strong enough isn't just a function of the weight of the rocket and the strength of the harness,,
it's also very much how long is the harness....

Every mill measures physical size and weight rating differently..
I've seen 1/4" a bit bigger then my 1/8" super small light Tubular Kevlar,,
and I've seen 1/4" really big heavy strong material...
I'm pretty sure if I remember correctly that Tim L's was really strong heavy stuff...

Make sure it's long enough,, no probs at all...

Teddy

Ohh,,
And thank you all very much for the kind words...lol..

 

Listen to Teddy... he is THE man.

 

Ha,, lol..
Thank you so much Cris..
You're the electronics man Cris,, lol..

Thank you all so much for your support..
I only wish I could make you understand how much it means to me...

Thank you..

Teddy

 

I'm using the exact same 1/4" Kevlar for my 3" Punisher and my 4" Fiberglass DRM as well. But for bigger and heavier rockets, definitely trust one of Teddy's harnesses. I have quite a few, in all different sizes, lengths and materials. Unbeatable quality!