Shock cord design questions.

I know that these questions have probably been covered in a number of other threads, but my searching efforts did not turn up much.

Here's the two basic questions for all of you veteran rocket builders:
1) What ratio of Kevlar to elastic do you use for low power and mid power rockets?
2) What guidelines do you use for selecting the size (width) of the elastic and the size (lbs) of the Kevlar? (e.g, for a BT55 D-powered rocket, use 200lb Kelvar [or 150lb, er whatever]).

When I first started building again as a BAR a few years ago, I was using the old Estes all-elastic with a tri-fold paper mount. Soon, I went to using the Quest style mount with Kelvar mounted to the motor mount. I know that many people recommend shock cords that are three times the length of the body tube.

However, I am still getting damage after ejection when the nose cone or upper section slingshots back to the rocket and knocks it sensless. The worst example was my brand new Semroc upscale Laser X, where the rebound did a shocking eek: maybe that's why they're called shock cords) amount of damage to one of the fins. Most of my D powered rockets are getting smacked, and even some of my 18mm rockets are getting dinged.

Some people have recommended going with a cord that is mostly Kevlar with only a short segment of elastic. Others say to use mostly elastic, but make the cord amazingly long.

Thoughts?
Thanks.

I use 70-90lb kevlar for all my LPR and LMR model rockets. Rarely do I use or need anything heavier then 130lb kevlar on even the largest of my LMR's like the 3X BT-80 size Orbital transport teather that hold the main body and 36" hemi nylon chute.

My general ratio is 4 or 5 feet of kevlar (that included the kevlar inside the tube to the motor mount) with 3 to 4 feet of elastic. in micros 42" to 60" of 50lb kevlar alone.
Hope this helps.

Where do you get kevlar anyways? Is that something that can be found in a hardware store or do I have to order it online?

-Dave

I use a 90 lb. kevlar loop epoxied to the side of the body tube, much like a LOC/Precision shock cord anchor. I anchor it about 6" down from the top of the body tube, and I extend the loop about 3" out of the top of the body tube. I do that so that I can easily inspect and replace the nylon/elastic shock cord if I see any severe scorching, or other signs of imminent failure. Depending on the rocket, I usually use about 20"+ of 3/8" elastic shock cord.

I would put as much length of shock cord as the rocket/body tube can store. It's real cheap at Walmart, it's not going to add a lot of weight, and the more you have, the less chance of snap back damage to the tube.

Originally posted by DAllen
Where do you get kevlar anyways? Is that something that can be found in a hardware store or do I have to order it online?

-Dave

Click to expand...

https://www.fliskits.com/

https://www.thrustaero.com/

Both sites have kevlar at a decent price.

If you have a lot of rockets and are going to switch over both shockcords and shroud lines, you may want to buy a small spool.

Here's a link that might help:
https://www.thethreadexchange.com/miva/merchant.mv?page=TTE/PROD/KEV092/KEV092NATL02B

Originally posted by DAllen
Where do you get kevlar anyways? Is that something that can be found in a hardware store or do I have to order it online?

-Dave

Click to expand...

Dave:
I used to buy Braided 70lb kevlar fishing line by Stren but haven't been able to fine that material for the last couple years.
Many of our cottage industry Kit manufactures carry a limited supply of kevlar lines, Pratt Hobbies, FlisKits, Asp, totally tubular to name just a few.
But by far the best way to buy Kevlar is from one of the many on-line Thread companies. the Thread exchange is one, I also get some from McMaster-Carr. Like these shown below.

I always leave the Kevlar 2 or 3 inches in the body tube. I guess it doesnt make a difference but I was thinking it would be better for the elastic to hit the end of the tube rather than the Kevlar. Wouldn't this lessen the chance of a zipper? I dont really know I have never had a zipper or an Estes dent (knock on wood). With that being said though...I dont launch my rockets as much as most folks
Oh yeah and nearly all my shock cords are 3x the BT length. give or take.

Easiest way to prevent the kevlar from zippering is to add a fold over piece of 1/2" masking tape centered on the kevlar about 1/2" inside the end of the tube. it stays with the kevlar and folds down into the body along with the rest of the line. I've Never had a zippered body tube caused by the Kevlar line. mainly because I choose my delays based on the LOwt of the model and secondly due to this little tape strip

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Originally posted by Micromeister
Easiest way to prevent the kevlar from zippering is to add a fold over piece of 1/2" masking tape centered on the kevlar about 1/2" inside the end of the tube. it stays with the kevlar and folds down into the body along with the rest of the line.

Click to expand...

I do something similar. I run about 2 - 3 inches of masking tape on the kevlar. This also prevents the kevlar from fraying and eventualy severing from friction against the body tube. I put enough tape on the length of the cord, so that if you take the thread and circle it around the rim of the BT, it is protected.

I also prefer to terminate the kevlar below the body tube opening
any longer is just wasting kevlar.

Y'know, this reminds me...

I asked our elder rocket designer (over 30 years of Estes!) why we don't use Kevlar. His reply is that our body tubes are very thin wall and when we DID use Kevlar, it tended to rip the body tube like a banana. What I take from this is make the Kevlar not extend past the top of the body tube.

That makes sense to me Rogue and that is exactly how I was thinking but....after reading Micros method it makes a bit more sense to me too (if you use Kevlar). For this reason, if ever the elastic shock cord should break or need replacing for some reason it sure would be easier if the connection was outside the BT. I could replace mine as it is but it would be a pain. If ever mine should break I would just go to the trifold mount. But like I said I just dont launch them 20,30, or 40 times or even more. Matter of fact most of my rockets have less than 5 flights and some only 1. So the odds are in my favor anyway. Best thing to do is experiment see what fits your style of flying. There are definately plenty of ways to try.

Originally posted by Roguepink

I asked our elder rocket designer (over 30 years of Estes!) why we don't use Kevlar. His reply is that our body tubes are very thin wall and when we DID use Kevlar, it tended to rip the body tube like a banana.

Click to expand...

The extra step of "zipper proofing" a kevlar-based design (of which there are a lot of easy methods) is a lot easier than dealing with failed rubber-band type shock mounts.

Originally posted by Ed Wagoner
I always leave the Kevlar 2 or 3 inches in the body tube. I guess it doesnt make a difference but I was thinking it would be better for the elastic to hit the end of the tube rather than the Kevlar. Wouldn't this lessen the chance of a zipper? Oh yeah and nearly all my shock cords are 3x the BT length. give or take.

Click to expand...

I'm hardly an expert, but that's how I do mine, too.

Johnny

I either have my kevlar either anchored to the motor mount, or use a LOC loop. Either way, I have the loop right at the body tube opening. I also use as much elastic as the body tube will hold, with a minimum of about 5 feet. One thing I'm going to start trying, is to attach the chute at the end of the shock cord, with the nose down a bit from the chute. This should help with the spinning during recovery, and might help with body/nose collisions. I've yet to try it, so I can't say.

Originally posted by stymye
I also prefer to terminate the kevlar below the body tube opening

Click to expand...

Same here. Kevlar from the mmt, then a ribbon thru the opening, then back to kevlar to the nosecone. The ribbon spreads the load around a wider arc of the opening to minimize the knife effect.



This is a 2.6" rocket with a 1" wide nylon ribbon. It's folded then sewn. A knot would take too much space (usually). For smaller rockets, I use narrower ribbon. Avoid polyester - it melts too easily. (First hand experience.)

Doug

Flyin:
I took a pic of the tape tab anti-zipper I'll post as soon as I get it out of the camera

Kevlar ribbon is an option, if you don't mind the sewing and expense.

1-1/2" of 1/2" masking tape is about a Cheap and easy a way as I've ever seen to prevent body tube zippering.
I've yet to see any zippering at all of any model or Large Model Rocket I've ever flown using this method.

Heres that Photo of the Anti-Zipper device that I've been using for goodness knows how long...to date I've yet to have a bodytube zipper from any ejection micro to 5 D12 cluster LMR's.
I'm sure it's not the most Hi-Tech method, but it's Very inexpensive, Super Easy to install, permanent, and MOST important...Has a LONG proven history of reliability
Hope this helps.

Thanks for the pic Micro.

Sorry about the delay in getting back to everyone, but I wanted to thank everyone for their input on this thread. I guess that there is no clear consensus on the ratio of elastic-to-kevlar, but it is clear that the cords need to be *really* long. I was making my cords longer than stock Estes lengths, but it appears that I need to make them longer still.

So far, I have not had a problem with zippering... just problems with dings. I try Micromeister's taping solution on most LRP models.

Thanks,
DeWain