Kevlar LIne question please

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Flash

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I've built some 17 to 20 oz rockets out of the light 3" tubing from Mercury or Balsa Machine shop and ordered the 200lb Kevlar line from them, but it looked on the small size, I then ordered some 500lb test Kevlar line from emmakites, it is gray in color, it looks no larger than the balsa machine shop stuff.

Can I trust the AVG STRENGTH of the Emmakites line, 500lbs? In other words, shouldn't it work for the rockets I mentioned above? It is braided with no center and looks to be just inside 2mm thick.

Thank you!
James
 

LithosphereRocketry

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Even on the 200lb you're looking at a 160 G or so shock load before it breaks... Even if Emmakites is wrong, you have plenty of safety margin.

Keep in mind that I've never used Kevlar, I'm just looking at the numbers - but 160 G sounds kinda ridiculous.
 
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OverTheTop

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When using Kevlar on my smaller rockets I use the thicker 1/8" braided stuff. I don't need the strength but it helps stop the cord flossing the airframe. I hate zippers.
 

dcullen

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When using Kevlar on my smaller rockets I use the thicker 1/8" braided stuff. I don't need the strength but it helps stop the cord flossing the airframe. I hate zippers.
Same here, I like the stuff Emma sells as 500 lb. to 1000 lb braided. Don't need that kind of strength for MP but it doesn't cut through body tubes and seems to be a lot less likely to tangle into a knot.

There doesn't seem to be any real standard for how this stuff is rated/labelled. One place calls it 380 lb and another rates it as 500 lb. And it looks exactly the same. When it's listed, I look at the diameter. And Emmakites is such a value at 20% of the price of most other suppliers.

For instance, Apogees #300 is 2.5 mm while EmmaKites #750 is 2.0 mm. But the Apogee kevlar seems to be a looser weave while the Emmakites is tighter and stiffer.
 
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CORZERO

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It's my understanding that the advantage of Kevlar is flame and heat resistance. If neither of those are a concern, I believe harness material and strength should come second to conditions, delay time and speed calculations for a given event for any flying level. Nylon is probably fine for most flights. Rock climbers have trusted the stuff for a while now. I think you have bigger problems if you're ripping apart 200lb test Kevlar cord. Simultaneously, you could applaud yourself for constructing such robust attachment points should the Kevlar cord/tape fail before they do.

I like to consider the amount of speed that is scrubbed as the vehicle components separate. I have yet to experience any adverse effects of using cord lengths far greater than the discussed norm in this forum and the only limiting factor for me is fit in the airframe, as the weight of Kevlar pull tape/cord is negligible for anything in which you'd consider using it. I've even used smaller 1/8th inch stuff (Estes Black Brant, D-Region, etc.) along with Kevlar thread for really small LPR (Viking, Yankee, etc.) with no problems, as I use extended lengths of it. Although resistant, Kevlar is not fire-proof, and I have burned through tape. So include some method of replacement or replaceable protective method in your design.
 
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rharshberger

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It's my understanding that the advantage of Kevlar is flame and heat resistance. If neither of those are a concern, I believe harness material and strength should come second to conditions, delay time and speed calculations for a given event for any flying level. Nylon is probably fine for most flights. Rock climbers have trusted the stuff for a while now. I think you have bigger problems if you're ripping apart 200lb test Kevlar cord. Simultaneously, you could applaud yourself for constructing such robust attachment points should the Kevlar cord/tape fail before they do.

I like to consider the amount of speed that is scrubbed as the vehicle components separate. I have yet to experience any adverse effects of using cord lengths far greater than the discussed norm in this forum and the only limiting factor for me is fit in the airframe, as the weight of Kevlar pull tape/cord is negligible for anything in which you'd consider using it. I've even used smaller 1/8th inch stuff (Estes Black Brant, D-Region, etc.) along with Kevlar thread for really small LPR (Viking, Yankee, etc.) with no problems, as I use extended lengths of it. Although resistant, Kevlar is not fire-proof, and I have burned through tape. So include some method of replacement or replaceable protective method in your design.
For many of my MPR rockets I use a kevlar harness from the MMT to just below the airframe top and then a 550 paracord harness from there on, that way I get both the heat resistance for the lower section and the stretch of the nylon on the upper section, the knot between the two helps to prevent zippers (I have been know to add a pad of masking tape to the knots to widen the area).
 

Zeus-cat

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You need to protect your Kevlar cord from flame. I had a competition rocket where I tied the Kevlar to the thrust ring of the motor mount. Two flights on A10 motors and it was toast. I pulled on it gently and it snapped on the charred bit.
 

dhbarr

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Heatshrink the preferred solution?
 

mwtoelle

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You need to protect your Kevlar cord from flame. I had a competition rocket where I tied the Kevlar to the thrust ring of the motor mount. Two flights on A10 motors and it was toast. I pulled on it gently and it snapped on the charred bit.
I've heard of using stainless steel fishing leaders leaders for the lower portion of the shock cord. A Nomex sleeve with the Kevlar threaded through it should provide some fire resistance as well.
 

DavidMcCann

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If you're going to pop 200# kevlar.... the rest of the rocket is likely long, long gone ;)
 
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