Quantcast

Kevlar vs Nomex

The Rocketry Forum

Help Support The Rocketry Forum:

DynaSoar

Well-Known Member
Joined
Mar 15, 2004
Messages
3,007
Reaction score
0
Regarding kevlar shock cords:

I learned at the CATO launch that while nomex is heat resistant, kevlar is less so. On one bird the kevlar shock cord burned through and I ended up with a tumble recovery body and chute recovery nose (it was the upscale Rock-A-Chute, which otherwise survived without a scratch). Another burned pretty good, but not quite through.

The lesson is that kevlar is fine, but it should not be strung directly foreward from the opening of the MMT. From now on, I'll be stringing it between the upper centering ring and the body, or else through a hole drilled in the upper ring somewhere mid-ring, rather than between the upper ring and MMT tube.
 

flying_silverad

Well-Known Member
Joined
Jul 23, 2002
Messages
3,141
Reaction score
1
Of course there are many factors affecting the life span of kevlar cord in the recovery area but what I see as the biggest mistake is the use thinner or cheaper kevlar.

For instance, you can purchase "Hem Bob" kevlar cord on line, mostly I've seen it on Ebay, and it's quite flat. The thinnest part being as little as .020. It's fairly curly and burns easily.

I use and sell .035 and .030 "cord" which is round and consistent in thickness.

I learned at the CATO launch that while nomex is heat resistant, kevlar is less so.
I think actual burn temps are the ther way around. I think I read nomex is about 600 deg where Kevlar was around 800 deg. Besides, don't forget the tensile strength of kevlar vs nomex.

All of my kits have it as part of the recovery section with the exception of the Mike IX where it is placed in the chute bulkhead.

I think you'll find that most quest kits have .025 cord.

You can also coat the first inch or so (forward of the motor mount) with glue to help prolong life. (Thanks for the tip Doug!)
 

DynaSoar

Well-Known Member
Joined
Mar 15, 2004
Messages
3,007
Reaction score
0
Originally posted by flying_silverad
Of course there are many factors affecting the life span of kevlar cord in the recovery area but what I see as the biggest mistake is the use thinner or cheaper kevlar.

I'd used a twisted pair of the thickest I had, 240 pound.

The glue idea is good. One small gob of epoxy, drag the portion of the cord that'll be exposed through the gob, and hang it to dry. No reason for it to be flexible along its entire length. I *still* won't route it past the MMT opening if at all possible. In fact, next bird I build, I'll try attaching to the MMT, running up the side, and epoxying it against the tube for a ways.
 

brianc

Well-Known Member
Joined
Jan 18, 2009
Messages
3,220
Reaction score
1
Here's a pic of the remains of my recent E30 CATO. There was a 10"
fireball that burned for several seconds out the front of this MMT.

The kevlar is charred, but otherwise undamaged.
 

DynaSoar

Well-Known Member
Joined
Mar 15, 2004
Messages
3,007
Reaction score
0
Originally posted by brianc
Here's a pic of the remains of my recent E30 CATO. There was a 10"
fireball that burned for several seconds out the front of this MMT.

The kevlar is charred, but otherwise undamaged.
Then the possibility exists that I bought something that wasn't as advertised.

Time for some testing.
 

flying_silverad

Well-Known Member
Joined
Jul 23, 2002
Messages
3,141
Reaction score
1
My thoughts exactly.

Then the possibility exists that I bought something that wasn't as advertised.

Send me a PM with your address and I'll send you a free sample of .035. You can at least compare the two.
 

DPatell

Well-Known Member
Joined
Aug 5, 2002
Messages
1,419
Reaction score
0
Giant Leap did a comparison of chute protectors. They compared Kevlar and Nomex's resistance to flame. Take a look...it appears that Kevlar is better at flame protection.

Kevlar vs. Nomex
 

Elapid

Well-Known Member
Joined
May 2, 2009
Messages
1,734
Reaction score
0
Thermal: NOMEX(R) is inherently flame resistant, which means that it does not melt but start to decompose only at temperatures above 370°C.

KEVLAR has the main characteristic does not melt and support combustion under normal environmental condition and carbonizes at about 800 degree F (427 degree C). It is suitable on sewing application where short-term flame or heat resistance is required. In commonly for seaming operation on protective clothing, used by fireman welder’s gloves, police and military etc.

KEVLAR is an aromatic polyamide fibre sometimes referred to in military specification as “para-aramid”. It has a high tensile strength according to size ratio {gram/denier}, much higher than the tenacity of polyester, flex & nylon.

KEVLAR has the main characteristic does not melt and support combustion under normal environmental condition and carbonizes at about 800 degree F (427 degree C). It is suitable on sewing application where short-term flame or heat resistance is required. In commonly for seaming operation on protective clothing, used by fireman welder’s gloves, police and military etc.

KEVLAR sewing thread has a low elongation and high stress modulus contributing to excellent needle loop formation. It can reduce thread breaks and skipped stitches on many hard-to-penetrate fabrics because of its high tensile strength and low elongation. Thread lubricant is applied to the thread to enhance the heat-resistant feature and allow it to be sewn with minimum machine thread tension.

Kevlar
Although Kevlar's heat resistance is not as great as that of Nomex, Kevlar's strength is superior to all other fibers. In fact, Kevlar is five times stronger than steel by weight. It has become the thread choice in those special high stress, volatile applications that require extreme strength and heat protection.

http://www.johnhowardcompany.com/nomex_Kevlar_Thread.html
 

DynaSoar

Well-Known Member
Joined
Mar 15, 2004
Messages
3,007
Reaction score
0
Originally posted by DPatell
Giant Leap did a comparison of chute protectors. They compared Kevlar and Nomex's resistance to flame. Take a look...it appears that Kevlar is better at flame protection.

For woven cloth like chute protectors, I suspect the tightness of the weave would be an important factor, in addition to the weight of the cloth.

And same with the cords, probably. Some of the cord I got is very tightly twisted, and some is so loose I have to tie knots in it to work with it. Even then it untwists into hair-like fibers.
 

flying_silverad

Well-Known Member
Joined
Jul 23, 2002
Messages
3,141
Reaction score
1
Originally posted by DynaSoar
.....And same with the cords, probably. Some of the cord I got is very tightly twisted, and some is so loose I have to tie knots in it to work with it. Even then it untwists into hair-like fibers.

That's the difference between "bonded" and "Non-Bonded"
Bonded keeps the stuff from fraying. Some feel that the bonded (Waxy coating) cord is more difficult to adhere to anything though I have not found this the case.
 
Top