Kevlar Shock Cord

[Rocketman35]

I did do a search on this but none of the threads answered my question.



I have some 150 pound single end strength kevlar and wanted to try using it instead of the rubber elastic cords.

Since only fly rockets up to "D" motors (for now) these rockets are all fairly small and light weight.

Going to attach to motor mount like standard quest rockets but was unsure of the length of kevlar i should use on a 2-3 foot long rocket (light weight)? 3 -4 times the length? more?
Would this zipper more easily?

TIA
Chuck

 

[rstaff3]

3-4 times its length is fine. Generally I use a heavier Kevlar for that sized rocket, but if its light you are probably OK. To prevent zippering, you can put a rectangle of tape (I use a thick cloth based tape) over the cord at the point that it touches the airframe when fully extended.

 

[Rocketman35]

150 pounds is not enough (or marginal)? granted the weight of the rocket(s) is unknown, but the only rockets i fly now are estes and quest rockets (bertha, black bryant, big betty, etc)

TIA
Chuck

 

[astronboy]

This weight sounds fine. I usually attach to a centering ring and wrap a little masking tape around the kevlar where it comes to the mouth of the BT. This prevents the Kevlar from cutting into the BT. I cut the kevlar so that it comes out about 4-6" from the end of the BT. I then knot a length of elastic 18"-24" long (1/8" wide for smaller models and 3/18" for larger) to the end of the kevlay to still get some 'spring' out of the whole thing.

Although I have had traditional 'rubber band' shock chords fail, I have never had a centering ring mounted, Kevlar/elastic chord failure.

 

[rstaff3]

Since I generally have heavier rockets I tend towards overkill on the lighter ones. I'm sure others will chime in but the 150lb is probably ok. I usually run it the kevlar twine thru the top CR and attach it with a small amount of 5-min epoxy. On retrofits, I usually tie it around a cardboard ring and epoxy this down in the BT, however I have tried many attachment methods

 

[rstaff3]

Originally posted by astronboy
....I cut the kevlar so that it comes out about 4-6" from the end of the BT. I then knot a length of elastic 18"-24" long (1/8" wide for smaller models and 3/18" for larger) to the end of the kevlay to still get some 'spring' out of the whole thing.

Although I have had traditional 'rubber band' shock chords fail, I have never had a centering ring mounted, Kevlar/elastic chord failure.

That's what I like to do also... same results

 

[arthur dent]

Quick question??..what is zipping??

 

[Micromeister]

I don't fly anything over 3.3lbs. but I do fly quite a few Large Upscales. I NEVER use any kevlar heavier than .032" dia 130lb thread size 34, it's NEVER failed on any model I own single to 5 D12 clusters. My rule of thumb is 4 times the length if I'm only using kevlar, or tie and overhand loop or bowline about 3" outside the end of the forward end of the bodytube for a Kevlar anchor/Elastic shockcord combination. If this set-up is used I tie 1/4" elastic 3 times the body length to the kevlar loop with two half hitches. Zipper stop is a 2" wide 3/4" making tape strip doubled over the kevlar centered on the line so strip creates a 1" x 3/4" rectangle about 1/2" or so inside the forward end of the bodytube. The only Zippered tube I've had was pulling a model out of a tree where the chute had wraped around the limb. the Shock cord held but it did zipper the tube.
Don't let the overbuilders convince you 150lb kevlar is to light. It is not, for that matter on single D powered model I most often use 70lb kevlar Stran braded fishing line. Never had a seperation or break.
For very inexpensive Kevlar lines check out www.thethreadexchange.com Great lines in a multitude of sizes and spool sizes for very little money.
Hope this helps.

 

[n3tjm]

Originally posted by arthur dent
Quick question??..what is zipping??

Zippering is when the shock cord rips through the side of the body tube from the nose down. Same effect as those "pull tab to open" on express mail boxes.... or the red string in a pack of gum.... or the clear strip on the celophane covering a CD jewel box.

 

[KenParker]

Originally posted by arthur dent
Quick question??..what is zipping??

"Zippering" is what happens when the rocket deploys the recovery device at high speed, and the parachute causes the shock cord to rip down through the side of the body tube. This rip often resembles a "zipper," hence the term. Short shock cords tend to zipper more often than longer shock cords. Thin shock cords, like the thread-type kevlar can also cause zippers. That's why a lot of people figure out a way to put a wide piece of something - tape, cloth, something - on thread based shock cords where they come in contact with the top edge of the body tube. Not a guarantee in preventing zippers, but it does help to spread the load over a wider area.

HTH

 

[rstaff3]

Originally posted by Micromister
.... Don't let the overbuilders.....

Hey! I resemble that remark

Looking back at the Pratt site, I have used the 100lb for light rockets, 250lb for mid to large modrocs, 500lb or braid for larger mid-power models. One day I may buy from a cheaper source....

 

[Rocketman35]

great replies guys!

Thank You!

 

[Micromeister]

Ah Ha dick!
what size lb test are you using in your micros
I'm using 28lb and 35lb test
OBTW the 8 or 10lb test makes wonderful Chute shroud lines, No more burning off the shrouds when you forget the wadding

 

[rstaff3]

micro, I use the pratt 100lb for the micros as its the lightest I have.

 

[Micromeister]

HOO,HOOO WEE 100lb test..Yeap yer a overbuilder alright!!!LOL next time we get together let me lighten you up a little, I've got a bunch of 50lb I'd be happy to share Will you be coming to the Christmas party meeting next month? Humm sounds like I know what to get you for Christmas

 

[rstaff3]

I'll be trying to make it

 

[Rocketman35]

ok went to the thread exchange but cant find out what thir numbers mean? what does this mean ? Kevlar Bonded - Size 346
is it twisted? coated? whats the Dia and the strength?

tia

 

[illini]

Micro, can you give us a quick summary of what strength *you* consider to be sufficient for a few types of modrocs? What would you use for a lightweight 13mm rocket? 18mm less than 4 ozs.? 24mm? etc. I was preparing to buy some spools of Kevlar and now I'm questioning my choices.


Rocketman35, where in the south suburbs are you? I was born and raised in Lansing, TF South class of '82.

 

[Rocketman35]

illini868891

Crete, went to Crete-Monee Class 87----We used to play you guys in football all the time

 

[Stymye]

Rocketman35,

SIZE,346


BREAK
APPROX,140 lbs


DIAMETER,.0255

3 twisted strands,slight wax coating,
I use it in all my mid power stuff

 

[Rocketman35]

thank you stymye

 

[Stymye]

here is a chart I put together
It can be used as a guideline.
thread exchange kevlar seems to run at smaller diameters.but the strength and weight seem to stay the same

 

[Micromeister]

Stymye's chart should be helpful. I think the numbers and sizes are consistent with what I've seen a both McMaster-Carr and The Thread Exchange.

I have an additional couple to add to the list:
Thread size 10, - .005"dia, Strength(lbs), 8-10lb
60, - .010" dia strenght(lbs), 28lb
" " 130, - .015" dia " , 50lb
" " 140, - .016" dia " , 70lb

There is also a 100lb material but I don't have the dia. or thread size.

8 to 10 lb 2 strand thread is great for 6 to 30" chutes, and streamer attachment lines.

28lb is my usual choice for micro maxx models. and most competition P/D and S/D "shock lines". 50lb is sometimes used of Micro-Maxx "larger models" 13mm bodies.

50lb and 70lb for most standard 1/2A to single D powered models.
90lb, 100lb and 130-40lbs kevlars are reserved for LMRs up to the 1500g,3.3lb weight limit.
Hope this helps.

 

[rstaff3]

Based on this chart, I wonder if what I am calling 100lb really has that strength. I just measured it to the best of my ability and is a lot smaller than 1/32 (.031). Probably closer to .015-.016. I guess 70lb and 100lb is not that far apart and my measurements are not that accruate.

 

[illini]

Originally posted by Micromister
50lb and 70lb for most standard 1/2A to single D powered models.
90lb, 100lb and 130-40lbs kevlars are reserved for LMRs up to the 1500g,3.3lb weight limit.
Hope this helps.

Great data. Thank you very much!

 

[Micromeister]

Originally posted by rstaff3
Based on this chart, I wonder if what I am calling 100lb really has that strength. I just measured it to the best of my ability and is a lot smaller than 1/32 (.031). Probably closer to .015-.016. I guess 70lb and 100lb is not that far apart and my measurements are not that accruate.

Dick:
I think it has more to do with how the line is fabricated than simply the dia.
Braided, Twisted braid, single strand, etc. I have some braided unwaxed material that is much smaller in dia. while still rated at 70lb test.

 

[Gus]

I'm in a quandry. I've just about used up the Pratt Kevlar micro- and mini-braid I bought a while back and need some new line. While I love the braided Pratt version, it is VERY expensive so I was looking at alternatives. I've read through this thread (so to speak) and am still confused.

The thread I see listed on the Thread Exchange, and on Ebay, all appears to be "twisted" instead of "braided". Does the, much cheaper, twisted type tend to unravel over time?

I've been using lengths 3-4 times rocket length, secured to motor mount, without any attached elastic. Is there really any significant advantage to using the dual system of kevlar plus elastic? Seems to me the added connection is just another possible "failure point" that can come apart or have the knot snag the shroud lines.

Coated versus uncoated, benefits, drawbacks?

Thanks,

Gus

 

[Micromeister]

Gus:
My personal preference, is for braided line over twsted. but that has more to do with limpness then strength. Braided lines usually start with smaller individual stands which make the finished line much less stiff. The first kevlar lines I used were Strene Fishing lines, which happened to be braided. Twisted lines seem to work just as well and last just as long as braided. For standard size models and LMR's I don't think it really makes a vast differece which type you use. For Micro models the braided material is easier to stuff in those tiny tubes.
Coated or uncoated. My understanding of these options are the uncoated accepts CA more readily...I can't say I've noticed a difference. I buy from both the thread exchange and McMaster-Carr, depending on where I can get the best price for the Size, lb test and length of spool I can get. I've never had a piece of Kevlar I didn't liked including lines from Quest, Pratt hobbies, Asp, I even had some from the Old before Tim Apogee components.. they have all performed well for me, some are just a little harder to get into the bodytube
Hope that helps a little.

 

[powderburner]

Originally posted by Gus
Is there really any significant advantage to using the dual system of kevlar plus elastic?

I think there is an advantage to the cord+elastic, but I recognize that there is also a bit more complexity too. The cord-only approach is indeed more simple and should be a bit more reliable. The cord-only approach is certainly a lot more 'packable' and requires less internal volume. But the cord-only approach does not offer any significant shock-absorption capability.

If your model is light, and your ejection always takes place at or near apogee, then you may not need any shock absorber in the recovery system. I guess that 90 percent of my reason for continuing to use shock cord is habit. I look at it as, it can't hurt anything, and it might come in real handy sometime.

No real science behind my preference to include elastic, no numbers or analysis, just the fact that those little plastic 'chutes don't take much of a tug to tear themselves up.

 

[Micromeister]

I agree with Powderburner. for models larger then micro-Maxx I was sort of under the impression the Kevlar/Elastic shock cord mount is the standard improved method over straight elastic or rubberband types.
Something to bare in mind is the amount of stretch in the elastic involved. over the years I've come to the conclusion 30" is the best elastic length to assure no nose cone impact damage the body. this is attached to 4 or 6 feet of kevlar depending on the size and length of the model. standared BT-5 to 60 models a little shorter BT-80 up a little longer. I trick is to find the length that will allow the nosecome to slow to a stop with little or no rebound toward the model body. depending on the size of the model I perfer 1/8" round, 1/8" oval or flat and LMR's on 1/4" flat elastic. a simple overhand single knot is all that's required to keep the smaller line/elastic jointed, while I like to from an overhand loop in heavier kevlar with 2 half hitches attaching the 1/4" flat elastic to the loop. Haven't had a seperation or burnoff at the knot or loop since changing to this system in 1980 I have burned off a few at the motor mount, so I switched the Kevlar location to the OUTSIDE centering ring/ bodytube joint. No additional burnoffs have been seen.