Article: 1909
By Robert Krausert, January 2010

Scott Barfield asked:
I have a bunch of high strength tubular nylon (basically climbing harness material) that is great, but it is probably considerably stronger than various other parts of my setup and it ends up weighing quite a lot and taking a lot of space. Any suggestions for lighter-weight materials (and sources for same)? Seems like a harnesss rated to several hundred pounds is overkill for a 6 lb rocket with proper design around deployment.

Denny Smith commented:
For smaller stuff like yours, I’ve been using flat nylon strap (non-tubular). I think the 1/2″ has a working strength of 1,000lbs. www.strapworks.com is one source I’ve used.

It’s about half the thickness/wt. of tubular, and packs really well.

Greg Clark commented:
Kevlar

Always Readly Rocketry commented:
For smaller stuff like yours, I’ve been using flat nylon strap (non-tubular). I think the 1/2″ has a working strength of 1,000lbs. www.strapworks.com is one source I’ve used.

It’s about half the thickness/wt. of tubular, and packs really well.

Marty Weiser commented:
I use 9/16″ tubular for most of my mid sized rockets. It is about 1/2 the bulk and weight of the 1″ and is generally rated to 1500 lbs. I also use some similar or slightly smaller Kevlar as the first part of the harness coming out of the body tube. It is less apt to cut on the airframe and is more heat resistant to the deployment charges. I have both tied (both overhand and figure 8 knots) and sewn the two pieces together. Sewn is smaller, but depends upon sewing skills and I can’t get the heavy Kevlar thread to run really well in my wife’s machine.

Greg Clark added:
http://www.rinconrocketry.com/shock_cord.htm

Andrew MacMillen commented:
I’ve used tubular Kevlar for over 10 years. 1/8″ for mid power & lo hi power, 1/4″ for L2-ish, and 3/8″ for L3. Nary a problem, and packs a heck of a lot smaller than TN.

Bob Yanecek commented:
I too am a fan of Kevlar though the zipper factor can be brutal. They make a flat pull tape for electricians that is light and really packs small but tends to fray and thus requires inspection and evaluation for replacement after each flight. The stuff can be had free if you find a friendly electrician as they tend to use the stuff once then discard.

  • I use 1/8″ tubular Kevlar for all my projects 29mm – 75mm.
  • I can get 20′ in 2″ of 38mm airframe.
  • Lots of z-folding + tape allows for shock absorption.

While tough, the stuff can snap if deployed above Mach (I’ve ‘proved’ that
twice now danGIT (very important to keep your fins on during boost ;-)).

I think I’ll go up to 1/4″ tubular Kevlar for my upcoming 4″ project.

Paul Bogdanich commented:
12 strand braided Technora is as heat resistant as kevlar, stronger with just slightly more elasticity. They make it in 1/16 inch increments from 1/8 inch. The problem is we don’t use in the rocket community yet so the vendors don’t carry it. At this point you have to get it from the manufacturers and that means you need to buy a roll. It compares very favorably to kevlar braided tube. For example 1/8″ 12 stranded braided technora has a breaking point greater than 1/4″ braided kevlar tube. Further the technora filaments don’t self weaken the strap by cutting the filamants next to them. I can’t remember the technical name for the problem but technora reduces it. I have rolls of 1/8 and 3/16 is anyone wants to see a small piece. On the down side its just one more example how all the best materials research is happening overseas anymore. Kevlar is about 25 years old now you know.