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Kevlar how to?

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jf_reinhard

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I'm also just getting back into rocketry after a LONG absence and I saw a few posts where people talked about using kevlar for shock cords instead of elastic. Did I get that right?

Anyway, where can I get the type of Kevlar they're talking about and how do I use it?

Also, what's the best way for securing a shock cord to a tube?

Thanks,
Jim
 

mach7

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Hi Jim,

Kevlar string is used as an anchor for the shock cord. Kind of like a leader. Most people/kits call for it to be tied around the motor mount or motor block before it is glued in place. Tie it around the motor tube, add a few drops of glue to hold it. I use CA, but what you are using to build the rocket will work fine. Then pass the cord through the tube out the top before gluing the mount.

Make sure you have enough Kevlar string to you can tie a regular shock cord on to the end. I like to have about 3 inches extra hanging out.

You can get Kevlar string from many rocket vendors. I usually get mine from Fliskits, but have had good luck with Semroc and BMS. I know there are others also.
 

El Cheapo

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depending on the diameter of the Body tube, take some masking tape and double it just at he very end of the BT. You can usually slideit don. This ill keep the kevlar from zippering the Body Tube.

I'm juste micromeister will hav about a thousand pictures to demonstraight this technique.
 

MarkII

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Tie the Kevlar around the motor tube, and then pass it through a hole in the top centering ring or through a small notch cut into the edge of the ring. Make it just long enough to reach the top of the tube with none extending outside of the tube. Then tie your shock cord to it. Kevlar is very abrasive and has absolutely no stretch, so if any extends beyond the top of the tube, it can really mess up the edge when the recovery system deploys. In order to keep glue off of it when you go to glue the motor mount in, take your Kevlar/shock cord combination and pass it down through the top of motor tube and out the aft end before you insert the mount into the rocket. After you have bonded in the motor mount, you can push the cord back out the front of the motor tube and up out the top of the airframe.

The idea with using a Kevlar "leash" for your shock cord is that you can anchor the cord deep down in the tube, below the parachute. Because Kevlar won't burn, you can use it for the first section of your shock cord system - the part that is always in the airframe - and anchor it down near the top of the motor or around the motor mount itself without worrying that the ejection charge will burn through it. The traditional paper mount requires anchoring the shock cord up near the top of the tube so that it is as far away from the motor's ejection charge as possible. But with that arrangement, the parachute and most of the shock cord is below the anchoring point prior to deployment, and it all has to get past the anchor on its way out of the tube at deployment time. This can sometimes cause a bit of a traffic jam, resulting on occasion with an incomplete deployment or even a failed deployment. By anchoring the shock cord low, the anchoring point and most of the cord is below the recovery device prior to deployment, and there is nothing above the recovery device to get in the way at deployment time (except the nose cone). The recovery system can simply extend out of the rocket at deployment in a straight line, without having to make any U-turns.

High power rocket fliers usually don't have the same issues with where to anchor the cord, because most of their designs allow for much more customization of the location and method of deployment of the recovery system. But Kevlar (usually in a tubular weave) is still a popular choice with them more for its strength than for its resistance to high heat, although that is usually an important consideration as well.

Kevlar line comes in a variety of forms and is usually identified by its test strength or breaking strength. Some amazingly strong cord can be found in Kevlar line that is quite small in diameter. Common forms of the cord are Kevlar thread (very fine strands twisted tightly together, just like sewing thread), Kevlar string (similar to thread, but larger in diameter), Kevlar twine (two or more "strings" of material - each composed of tightly twisted individual strands - that are then twisted together along with a bonding agent), braided Kevlar cord (fine strings of Kevlar are incorporated into a complex braid, producing a very strong cord) and tubular Kevlar (the strands are braided together in an open tube format, which allows for larger diameters of cord with nearly the same strength as solid braided cord but with less weight, and which may also give the cord a slight amount of stretch).

Kevlar cord is hard to cut; you will need a strong and sharp pair of scissors or shears or even wire cutters to cut it. You will almost never find it at your local hardware store or home improvement center, or even at many hobby shops. Fortunately, it is readily available online.

MarkII
 

johnnwwa

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I'm also just getting back into rocketry after a LONG absence and I saw a few posts where people talked about using kevlar for shock cords instead of elastic. Did I get that right?

Anyway, where can I get the type of Kevlar they're talking about and how do I use it?

Also, what's the best way for securing a shock cord to a tube?

Thanks,
Jim
Jim
This is where I buy kevlar, Size 346 Bonded Kevlar
http://www.thethreadexchange.com/mi...GY&Store_Code=TTE&Category_Code=kevlar-thread

I have used it injust about all my rockets to date since becomming a BAR in 2003.

John
 

Micromeister

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Kevlar/elastic, or Stainless steel leader/Kevlar/elastic Shockcords have been in use a good while now.

Just about anything replacing that nasty "teabag" stuck inside the forward end of our models is really a PLUS, removing the obstruction allow free'er movement of our recovery device and wadding. Kevlar does have one potential problem which is zippering the body tube if not fitted with an anti-zipper masking tape tab or something similar.

Long ago I started using these methods on all my builds from minimum dia Micro models to BT-101 Upscale Clustered mid power. below are a couple pic pages of some of the Kevlar materials available. We can no longer get the Stern Kevlar fishing line but there are many other sources. McMaster-Carr has several sizes, The Thread Exchange is an Excellent source, and very light Kevlar thread spools are available from Edmund Scientific #3034863. that makes Excellent Chute & streamer Shroud lines..etc.
Kevlar has become one of those "Need it" materials for most building benches:)
Hope this helps

Kevlar lines-a-sm_28lb to 138lb (92dpi)_06-05.jpg


Comp Chutes-a-sm_25mil, KevlarThread& tapestrips_08-27-06.jpg


451e8-sm_30in MB-Shockline 7pic pg_10-06-06.jpg


Shock-Cord Mount-h1_Retro mount 8pic pg1of2_09-28-06.jpg


Shock-Cord Mount-h2_Retro mount 6pic pg2of2_09-28-06.jpg
 

Micromeister

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To expand just a little on the anti zipper tab I'll add the following pic. and few more versions of Kevlar/Stainless/Elastic shockcord anchors...All are related depending on your application:)
The motor mount drawing is for micro's but exactly the same method is used for just about any Motor Mount/centering ring combination.
One of the most important small changes i've made is to make sure to bring the Shockline to a small notch in the "Outer edge" of the forward centering ring so it is between the models airframe and ring. This puts the Shockcord anchor (Kevlar or Stainless Steel) in the Glue Fillet when the Motor mount is installed and moves the shock cord as far away from the Ejection charge Heat as possible. It's important to remember that Kevlar is Heat Resistant, not Flame Proof.
Hope these help

Beading wire Shockcord MMT-f_5pic page(128dpi)_07-08-06.jpg
 
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MarkII

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According to what I have read, Kevlar will not actually burn; instead, above a certain threshold of heat it starts to degrade, but it never ignites. That temperature threshold is very high, though, much higher than any temperature that it will ever see inside your rocket's body tube or recovery section. (The rocket's entire airframe would be immolated before it ever reached Kevlar's temperature limits.)

Here are three more good sources for Kevlar cord:

Pratt Hobbies (Scroll down the page.) I really like Doug's tubular Kevlar microbraid and I use it extensively in my builds.

Commonwealth Displays - lots of good stuff here, at great prices.

Apogee Components - a very knowledgeable vendor who gives a lot to this hobby.

All three of them are rocketry-specific vendors who sell the varieties of Kevlar cord that work well for sport rocketry.

Hope this helps.

MarkII
 

Micromeister

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According to what I have read, Kevlar will not actually burn; instead, above a certain threshold of heat it starts to degrade, but it never ignites. That temperature threshold is very high, though, much higher than any temperature that it will ever see inside your rocket's body tube or recovery section. (The rocket's entire airframe would be immolated before it ever reached Kevlar's temperature limits.)


Not exactly Mark: While Kevlar doesn't burn it does weaken with exposure to the high heat generated at ejections. Couple This Degrading with the stress shock at ejection eventually we do have shockcord failures.
This is exactly way we want to move our Kevlar as far as possible from the heat and/or use another product such as a Stainless steel Anchor to improve longevity of the system.
This is also a great example of where Braided/tubular Kevlar lines can outlast Twisted lines due in part to the difference in strand area exposed to the specific heatings of each flight.

Experience has shown Kevlar fails less then 1" above the attachment point in average diameter models. I wish I had a nickel for all the Shockline replacements i've documented like this. Seems the larger the airframe body, which moves the Kevlar the furthest away from the heat source, the longer the Kevlar anchor point lasts. Minimum diameter "Thurst Ring" Shockcord mounts tend to have the shortest lifespan but I still prefer this Method over the "Teabag" Estes mount handsdown.
On Average most Kevlar/elastic Shockcord mounts hold up well; Some are as old as the model, others have required replacement after 20-25 flights. This number isn't set in stone either. Minimum diameter models don't usually fair as well.

Kevlar only shocklines which I use on every Micro model, have a slightly lower longevity as there is no stretch to reduce the shock as the nosecone hits the end of the line. While most hold up 10-15 flights easily some minimum diameter micros have snapped the 30-50lb kevlar lines on the 2nd or 3rd flight. Lengthening the line helps but with limited space this is sometimes not an option.

All of this is way I mentioned the option of Stainless Steel Shockline Anchors. They are not subject to degradation by heat and can be made long enough to place the Kevlar or Kevlar elastic attached lines outside the heat generating area inside the model.
Stainless multi-Strand "Beading wire" works Great for Micros. Stainless Fishing leaders or small diameter Avaition cable for Standard Model Rockets and larger birds.
While I'm not suggesting every model be fitting with a stainless shockcord anchor, I've decided to use this option on those models I plan to fly alot or are in some way special to me.
 
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MarkII

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...
Experience has shown Kevlar fails less then 1" above the attachment point in average diameter models. I wish I had a nickel for all the Shockline replacements i've documented like this. Seems the larger the airframe body, which moves the Kevlar the furthest away from the heat source, the longer the Kevlar anchor point lasts. Minimum diameter "Thurst Ring" Shockcord mounts tend to have the shortest lifespan but I still prefer this Method over the "Teabag" Estes mount handsdown.
On Average most Kevlar/elastic Shockcord mounts hold up well; Some are as old as the model, others have required replacement after 20-25 flights. This number isn't set in stone either. Minimum diameter models don't usually fair as well.
I have never had a Kevlar shock cord leash burn through on any of my models. Not even a hint of degradation, and I attach all of them to the motor mount. BTW, 20-25 flights is a LOT of flights for one rocket; it will be quite awhile before most of mine see even half that many. With the number of launch opportunities that I get during the year, it would take me a decade or more to log that many flights on all of my existing rockets, assuming that I didn't lose any and assuming that I never built another one. But I am building new ones all the time. The interval between flights for each one of my rockets grows longer with every new one that I build. It will take any one of them a very long time to accumulate 25 flights, so even if what you are saying is true, then the Kevlar line in my rockets is, for all practical purposes, immortal. ;) :D

Kevlar only shocklines which I use on every Micro model, have a slightly lower longevity as there is no stretch to reduce the shock as the nosecone hits the end of the line. While most hold up 10-15 flights easily some minimum diameter micros have snapped the 30-50lb kevlar lines on the 2nd or 3rd flight. Lengthening the line helps but with limited space this is sometimes not an option.
I have had no problems there, either. And these rockets do tend to get more flights on average than my larger ones.

Stainless multi-Strand "Beading wire" works Great for Micros.
Yeah, I'm familiar with the stuff:

http://groups.yahoo.com/group/MicroMaxRockets/message/2127 (scroll down to the third large paragraph)
http://groups.yahoo.com/group/MicroMaxRockets/message/2188
http://groups.yahoo.com/group/MicroMaxRockets/message/2190
http://groups.yahoo.com/group/MicroMaxRockets/message/2201
http://groups.yahoo.com/group/MicroMaxRockets/message/2243
http://groups.yahoo.com/group/MicroMaxRockets/message/2553
http://groups.yahoo.com/group/MicroMaxRockets/message/2554
http://groups.yahoo.com/group/MicroMaxRockets/message/2557

While I'm not suggesting every model be fitting with a stainless shockcord anchor, I've decided to use this option on those models I plan to fly alot or are in some way special to me.
Stainless steel leader is good for shock cord anchors, too. (Titanium is even better. ;) ) I have only used it a couple of times myself, but I agree that it is great material to use for this purpose, and it is likely to be very durable. I believe that it was what NCR used for their "gorilla" mounts, wasn't it? If you find a vendor that sells fly-tying supplies, it will almost certainly have small spools of leader material for sale, so that you are not limited to buying pre-made leaders and adapting them to your rocket. The Cabela's web store is a good source for it, too. I wonder if fluoropolymer ("fluorocarbon") leader material would work for lighter models? They are very tough for their size and, like all fluoropolymers, are tolerant of high heat. It might be worth checking out sometime.

MarkII
 

Micromeister

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Just didn't want folks getting the wrong idea about good Ole Kevlar. It's great stuff, I love it....been using it as long as it's been around. but It's not flame even "Heat"proof. Dupont calls it Heat resistant, It does char, degrade and break in our partucular ejection charge/Shock filled environment applications.
That you've been blessed with not having had a problem to date is very good news but it's doesn't change the fact that Kevlar isn't permanent.
For those of us who do fly alot, or have only a model or two in the fleet.. We well still need to pay attention to and over time replace some and have replacement methods handy. A couple really good examples of this need were after the first flights of my stock built Tiny Triskelion and micro TOG which snapped the suppled kevlar shockline on the Tog and BURNT-off (Very nice Charred ends on both sides of the break) line in the Triskelion. A good while ago I had to replace the Pratt microbraid 100lb +/- Kevlar line in an Upscale 3 D12 clustered Ram-Jet on it's 6th flight. These three models are the ones I can recall with the Shortest Kevlar lifespans....most as mentioned earlier last much longer.

I like Titanium lines as well; It offers a pertty good mass savings and are generally is tad thinner in dia for a give break stength, but way more expensive.
Since this thread is in the beginners education forum I didn't think it necessary to list every possible anchor material other then to note the additional option of shockcord anchors to augment our standard everyday Elastic or Kevlar/Elastic shockcords. As folks progress they'll find other materials as their experience and budgets allow.
 
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RangerStl

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I have never had a Kevlar shock cord leash burn through on any of my models. Not even a hint of degradation, and I attach all of them to the motor mount. BTW, 20-25 flights is a LOT of flights for one rocket; it will be quite awhile before most of mine see even half that many. With the number of launch opportunities that I get during the year, it would take me a decade or more to log that many flights on all of my existing rockets, assuming that I didn't lose any and assuming that I never built another one. But I am building new ones all the time. The interval between flights for each one of my rockets grows longer with every new one that I build. It will take any one of them a very long time to accumulate 25 flights, so even if what you are saying is true, then the Kevlar line in my rockets is, for all practical purposes, immortal. ;) :D
Mine just get run over by cars... :cyclops:

http://www.rocketryforum.com/showpost.php?p=28831&postcount=21

N
 

Micromeister

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Mine are usually meet their end being taken by the Rocket eating TREES!!!!
 
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