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Shock Cord Question (LPR/MPR)

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Sleepy_Steve

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Does anyone else use normal nylon string for shock cords? What is the general consensus on it?

From my perspective it's slightly thicker than Kevlar string, dirt cheap and available everywhere, has a reasonable test strength (130-160lbs), and tends to wick in glues and adhesives easily to make knots or attachments permanent.
Amazon link example of what I'm talking about

I feel like I've had good luck with it as I normally build my attachments 'estes style' ahead of an extra centering ring used as a baffle / bulkhead in the front 1/3rd of the fuselage, as opposed to the higher power methods that tie Kevlar cord around the motor mount. My guess is that with the motor mount tie off approach the main driver for using Kevlar is that you're trying to ensure it holds up to repeated exposure to motor ejection charges.
 
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Bkdoubleu

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Nylon is awesome burns easy, if I use it I use a nomex sheath. Kevlar is almost indestructible, smaller Kevlar can slice like a knife though (ejection whiplash). I never let cost be an issue, the best flight in the world is worthless to me if I only get 1/2 my vehicle back. How much to use? Stuff as much as you can fit in there then chop off 6". I once read on here "measure the tallest tree at the launch and add 10'. Priceless response."
 

Handeman

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I'm not sure if it is much of an issue with LPR/MPR, but I really don't like the idea of "use as much as will fit" for shock cords. You can get the cords too long, and being too long can cause damage to your rocket. That usually happens with dd setups which is why I'm feel you have a lot more leeway with LPR/MPR. I just disagree with the idea of making the cords longer to compensate for other design issues. One problem doesn't fix another, it just gives you two problems to balance and when things tilt one way or the other, and they always will, the problems come back to bite you.
 

Nytrunner

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I'm not sure if it is much of an issue with LPR/MPR, but I really don't like the idea of "use as much as will fit" for shock cords. You can get the cords too long, and being too long can cause damage to your rocket. That usually happens with dd setups which is why I'm feel you have a lot more leeway with LPR/MPR. I just disagree with the idea of making the cords longer to compensate for other design issues. One problem doesn't fix another, it just gives you two problems to balance and when things tilt one way or the other, and they always will, the problems come back to bite you.
Completely agree with you on the overkill shock cords. Until I can work out a good calculation template for the length (which will be annoying because deployment can have so many variables) I just stick with 3 times the length of the biggest PIECE its attached to.

For Lo/Mid, I'm fine sticking with the elastic that comes stock, then replacing with tubular nylon when they wear out.

Plus for nylon, you'll still want to protect it from the gasses (especially with a single line like you linked). They sell nomex sheaths, or you can do the ductape method and save a dime or 50.
 

Onebadhawk

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I'm not sure if it is much of an issue with LPR/MPR, but I really don't like the idea of "use as much as will fit" for shock cords. You can get the cords too long, and being too long can cause damage to your rocket. That usually happens with dd setups which is why I'm feel you have a lot more leeway with LPR/MPR. I just disagree with the idea of making the cords longer to compensate for other design issues. One problem doesn't fix another, it just gives you two problems to balance and when things tilt one way or the other, and they always will, the problems come back to bite you.
100% ...
You have a fault / problem with your set up
so instead of fixing it you make unreasonable adjustments to other
aspects of your set up to compensate for the problem...

Why not just correct the first problem...

This is how you wind up with a string of issues / faults / problems...

I sell 2 choices of Kevlar for small, light rockets...
It's light round Kevlar or 1/8" Tubular Kevlar...
They're not expensive, though certainly not as inexpensive as Nylon masons twine...

I just think you're better off using the right stuff in the first place...

Teddy
 
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