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Kevlar vs. Nylon shock cords

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LampertRocketry

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Anyone care to share their opinion(s) on using kevlar versus tubular nylon shockcords on larger projects?

I am building a 100 pound Blossom V-2 and was initially inclined to use 1.125" wide Kevlar strapping (rated at 13,500 Lbs). Giant Leap calls it "Titan Cord" - 35 feet with loops $50.

It has been suggested to me that I should be thinking about using tubular nylon with a Nomex or Kevlar protector, because the tubular nylon has some 'give' and will absorb some of the shock of parachute deployment.

Thoughts? Experiences? I sure like the sounds of 13,500 pound test line! On the other hand... that wee bit of stretch in the tubular nylon sure sounds good. On the other hand, Kevlar is a whole lot stronger than tubular nylon. On the other hand, tubular nylon is less abrasive than kevlar.

What sayest thou?

Murray
 

kgholloway

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My personal preference is for tubular Nylon(TN). Tubular Nylon has a lot more "stretch" than tubular Kevlar(TK) resulting in reduced shock loading at parachute deployment. On the other hand tubular Kevlar will not degrade as much as tubular Nylon from the heat of the ejection charges.Personally I've used TN with a Kevlar "protector" before with excellent results.

On my next 4" rocket I will be trying something new. I'll take a length of oversize tubular Kevlar about a foot longer than my parachute compartment and sew a loop in one end to attach to my shock cord mount. The other end will be sewn into a loop around a welded steel ring. This end will be passed through a slit in the center of a Nomex or Kevlar parachute protector and the protector will then be sewn to the tubular Kevlar. A length of tubular Nylon will form the main shock cord and will be attached to the TK with a standard "quick link". This will give me the best of both worlds, Kevlar used where high heat will be encountered and Nylon for the rest of the harness to reduce shocks to the airframe.

Ken Holloway
 

GL-P

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I prefer tubular nylon because it isn't so abrasive on the airframe. The first time I've used kevlar was just for the length of the rocket body then I attached tubular nylon. The rocket zippered (a little of a late delay) but I think nylon wouldn't have zippered the tube. I suggest (but haven't bothered to use) a kevlar or nomex cord protector that's an inch or two short of the edge of the airframe. That way the protector protects the nylon but it doesn't cut into the airframe.
 

kgholloway

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That's very interesting. Could you post details as to the diameter of the airframe, size of the Kevlar, and size and length of the tubular Nylon shock cord?

Ken Holloway
 

GL-P

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Airframe: 3 inch
kevlar:1/2inch
Nylon length:18ft
nylon:1/2 inch

The delay was 1 second late. I almost didn't get my cert because of the dang 4inch zipper.
 

als57

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Originally posted by LampertRocketry
Anyone care to share their opinion(s) on using kevlar versus tubular nylon shockcords on larger projects?

I am building a 100 pound Blossom V-2 and was initially inclined to use 1.125" wide Kevlar strapping (rated at 13,500 Lbs). Giant Leap calls it "Titan Cord" - 35 feet with loops $50.


What sayest thou?

Murray
Murray:

I favor the nylon myself for the stretch. I have used a combination on medium sized rockets. Basically kevlar to the Rocketman shield and TN out the other side. The Rocketman shield has a independant ring on each side.


Have you asked Lloyd? He probably has some useful input.

Al
BRS VP
 

BlueNinja

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I prefer Kevlar, it's stronger and more heat resistant. If you use enough (+/- 5 yards) in your rocket, it should have slowed down enough form ejection that when it gets enough tension to cut the tube, it's probably out all the way unless something is catching it inside the airframe, so it's not gonna do anything to your rocket.

IMHO.




Blue
 

Ryan S.

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I dont think Tubular Nylon really has any give at all. Kevlar is awesome, it will last longer, it is just expensive
 

Stymye

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don't tell a rock climber that! Nylon does indeed have a certain amount of give and adds some cushion twords breaking a fall. I don't think kevlar has the same properties and in this case could possibly be added to the "overkill" category (next to steel cable)

I'm not in any way a high power specialist but it appears to be a better choice if you think about it.
 

LampertRocketry

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Thanks everyone for your input and comments. From the postings and private emails and consultations that I have had, it would seem that the consensus amongst those who have built large and heavy rocket ships is..... tubular nylon with a kevlar or nomex protector. The only significant reason given is that the nylon has a stretch factor and the kevlar does not..... meaning that for a given instantaneous applied stress, the nylon will actually be stronger than the kevlar for a given size. I am going to try and come up with some empirical data to see if I can confirm that consensus and will post same if and when I do find it on my L3 project website, which should be up and running shortly.

It will be located at www.lampert.ca/Rocketry.html and click the link when it is up and running for Level 3 Project.

Thanks again.

Murray
 

daveyfire

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A little late, but...

When Kurt Gugisberg flew Nibbles for the first time at LDRS XX, some of the airstarts lit weird (mostly K1275Rs had nozzle burn-through problems... these were of the first run). The rocket ended up deploying really late over the top (side, really) of a ballistic arc. It all held together, but when we got to the landing site, we found something interesting. Kurt had used TN for his shock cord, with TK taped along the side of the TN as a backup. The TK had snapped cleanly near the body tube, but the TN was still intact. Granted, the Kevlar did take a lot of the deployment stress off the nylon, but the Nylon still stretched enough to deal with the stresses AND break the Kevlar.

Nylon's stretchi-ness was also illustrated during the recovery of The Cow after its first flight at MDRA. According to the launch report, they tried to pull the rocket by its TN recovery harness back through the trees. The nylon stretched out, the rocket got stuck, and it pulled the truck they were using to tow the rocket out backwards in 4WD! Try it for yourself... wrap some TN around a tree and pull on it as hard as you can... definitely some elasticity there.

Food for thought. I personally use nylon whenever I can. The only place I use Kevlar is in minimum diameter/minimum length rockets where there is little room for flame protection For a rocket with a large internal volume (like the V2), I would use nylon with Nomex protectors.
 

wwattles

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I posed this same question to Andy Woerner (Polecat Aerospace) a couple months ago, and he was very emphatic that Nylon was far preferable, even to woven tubular kevlar. He said that the give in the nylon was the main reason for his opinions.

If you are afraid of zippers, you can either go with a zipper-proof design (do a search for that on here) or use an anti-zipper device (I think one company calls them "Fireballs").

WW
 

eebert

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I used to use kevlar for the reasons other people have given, but now I've switched exclusively to tubular nylon. The reason I switched is something that hasn't been mentioned yet:

Tubular nylon tends to degrade gracefully. After enough flights it gradually gets frayed, you can see it getting frayed, and when it gets bad enough you replace it. Kevlar, on the other hand, tends to look fine visually, right up until the day it fails catastrophically and you loose your rocket. This is speaking from experience. I've since heard it from several other rocketeers.

'daveyfire's experience is not necessarily wear on the kevlar, but probably related. While kevlar is theoretically stronger, nylon's stretchiness probably makes it stronger in real-world applications. In other words if you put kevlar and nylon on a bench-test rig and gradually increase the load, kevlar may take more load before it snaps. But in a deployment situation, since since the kevlar doesn't stretch, all the load is applied at once in one huge impulse and the kevlar snaps. But since the nylon stretches, the load is gradually applied over a longer time, so the peak impulse is less and the nylon survives.

Which is also why you don't get zippers with nylon. Same load, much smaller impulse.
 

wwattles

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Originally posted by eebert

Which is also why you don't get zippers with nylon. Same load, much smaller impulse.
You don't? I'll have to tell that to the guys I've seen get zippers with nylon. Saw a drag-separation result in a 8-inch zipper down a quik-crete tube, despite using a 3/4-inch tubular nylon shock cord.

I think the elasticity of the nylon needs to be weighed against the flash/abrasion resistance of the kevlar. Using a sleeve over the nylon pretty much negates the flash/abrasion damage, and a anti-zipper device protects against a zipper of any kind.

Almost anything is preventable, if you have the technology/space available. (Said he, just before getting run over by a guy named Murphy driving a bus!):rolleyes:

WW
 

eebert

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OK, which is why you don't get _as many_ zippers with tubular nylon. :)

-- Erik
 
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