Shock cord mounting

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bigone5500

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I would like to know what you guys have done in the past or still use on LPR types to mount the shock cord to the BT. I'm talking about the estes type mount that comes with their kits that you fold 2x and glue to the inside of the BT.

Is there a better way? I'm afraid that over time the mount may become weak and come loose. I use elmer's carpenter's wood glue in all my kits.
 
There's as many answers to this question, as there are people who'll give you advice . . .

Typically the first thing I personally do, before starting to build, is to get rid of the Estes elastic cord - from experience it's just too short. Then I can take one of two directions:

1) Depending on the model, I substitute the elastic with kevlar thread that is at least 3x the length of the body tube. I put a small hole into the top of the MMT. Then once the MMT assembly is glued and built, outside of the model, I tie the kevlar around the MMT, between the centering rings. I apply a small bead of epoxy on the knot, to ensure it holds. Once tied I then thread the kevlar up through the hole in the MMT and tie it to the NC. Glue the assembly in the BT, bringing the kevlar up through the top.

The second method is;

2) Do pretty much the same as No. 1, except I end the kevlar about 2-3", after it leaves the BT. Then I use a swivel and tie 1/8" or 1/4" elastic to the swivel (e.g. the kevlar is tied to the rocket at one end and the swivel at the other, the elastic is then tied to the other end of the swivel and then attaches to the NC.

Either way, just make sure that whatever method you use the combined length of kevlar and/or elastic is at least 3x the length of the body tube.
 
Originally posted by akpilot
There's as many answers to this question, as there are people who'll give you advice . . .

Typically the first thing I personally do, before starting to build, is to get rid of the Estes elastic cord - from experience it's just too short. Then I can take one of two directions:

1) Depending on the model, I substitute the elastic with kevlar thread that is at least 3x the length of the body tube. I put a small hole into the top of the MMT. Then once the MMT assembly is glued and built, outside of the model, I tie the kevlar around the MMT, between the centering rings. I apply a small bead of epoxy on the knot, to ensure it holds. Once tied I then thread the kevlar up through the hole in the MMT and tie it to the NC. Glue the assembly in the BT, bringing the kevlar up through the top.

The second method is;

2) Do pretty much the same as No. 1, except I end the kevlar about 2-3", after it leaves the BT. Then I use a swivel and tie 1/8" or 1/4" elastic to the swivel (e.g. the kevlar is tied to the rocket at one end and the swivel at the other, the elastic is then tied to the other end of the swivel and then attaches to the NC.

Either way, just make sure that whatever method you use the combined length of kevlar and/or elastic is at least 3x the length of the body tube.

What strength kevlar do you think is ok for most LPR projects?
 
120-150 lb. kevlar will suffice. 400 is nice tho, especially for MPR. In some instances, I run two strands of 120 lb, just cut it twice as long as you need it, but don't cut it into two pieces, just fold it in half, and use it like one piece.

Another way combo of kevlar and elastic, and my favorite, is totie a loop, and end the kevlar an inch or so below the break point (inside the body tube) of the rocket, then attach the elastic through this loop. this way, you greatly reduce the likiness of having any zippering, if an ejection charge fires too soon, or too late. Just my 2 cents
 
If the tube is large enough (BT-55 and up) I still use the tri-fold anchor. Quick, easy, reliable and replacable :)

When I use Kevlar, I like to attached it at a mid-point coupler when I have a connection between tubes. If I don't then I attach it to the engine mount.

If the model is minimum diameter I often attach the Kevlar on the outside of the body, usually along a fin root.

In most every case, when using Kevlar I also use a length of elastic cord.

General question: I have heard many people mention the use of Kevlar and in most EVERY case they say that the kevlar cord should be 3X the body tube length and I don't understand the reasoning...

Here's what I mean (two extreem cases - The Flea and the Corona):

The Flea has an 8.6" long tube (approx). This would then call for a 25.8" cord. The Corona has a 36" long tube so by this reasoning I should have a 108" cord. But it seems to me that with the smaller tube and the cone closer to the motor that I would want a longer cord on the shorter rocket...
 
I like to keep the kevlar below the top of the body tube..any longer is really not helping anything other than a zipper

a longer shock cord allows the nosecone to slow down before jerking so It probably has nothing to do with the length of the rocket but rather the weight of the cone and the force of the ejection charge. thats my thinking atleast.
 
Originally posted by jflis
General question: I have heard many people mention the use of Kevlar and in most EVERY case they say that the kevlar cord should be 3X the body tube length and I don't understand the reasoning...
Jim, for me, that falls into the category of "the rule denotes the extreme".

I agree with all the notes above regarding replacing shock cord with a 100-150lb kevlar tether. I have used both methods above (one LOOOOONG kevlar cord and a kevlar "tether" ending just past the BT and attaching an elastic cord from that point to the NC).

But the main point here, the one that is never addressed in kit instructions, is that the nose cone can and will snap back during ejection and damage the top of the BT. So we are all overly cautious and want the tension point to occur with the NC as far away from the BT as is reasonable on each rocket. Starting with 3x the length of the BT and working downward is safe method.

Another point is to remember to attach the parachute close to the nose cone so that when the chute opens and the NC is dangling, it is still not anywhere near the BT, no bouncing damage.

The Corona being 36" I would put a 24-36" shock cord and mount the parachute about 4" from the NC. Look at the artwork on the cover of the 36D Squared and the photo shows the parachute mounted in the cord center and the two halves of the rocket banging in to each other on descent. It cracked me up.
 
Originally posted by jflis
But it seems to me that with the smaller tube and the cone closer to the motor that I would want a longer cord on the shorter rocket...

Jim, first up, I'm not sure how much wisdom any of us can share with you - as you're than man! You've have far more experience in development and production, as well as knowledge than many of us here on the board . . .

But to answer your question, from my personal experience, I'd have to say up front, that overall, my reasoning for a longer shock cord isn't so much for "zipper" effect as it is for BT damage (if the NC was to hit).

Knock on wood, I've never had a zipper in any rocket, in my few years of launching. I simply make sure the ejection charge is appropriate for the rocket being launched. Simple as that.

As for damage to the BT. I WAS one of those that kept getting the "smile" using standard Estes/Quest lengths. Since reading some rocketry related material, and adopting the 3x method as a personal minimum, I've yet to get another "smile".
 
Would I be safe in Presuming the use of Kevlar cording is due to it's resistance to heat? Has anyone tried any other fibers and success?
 
First, let me say that I just HATE that stupid Estes parachute deployment blocker ..... I mean, shock cord anchor.
I see absolutely no reason to place an obstacle in the path of getting the recovery system out the front.
You can chalk it up to being one of my (many) quirks and foibles, but for internal attachments I like to use an anchor cord that reaches back to the MMT. The length of cord that reaches to the front of the BT presents only a tiny profile and avoids blocking the ejection of the parachute.

For cord materials, I know many of these guys like Kevlar, and it is indeed good stuff. It is more resistant to heat damage than a lot of other stuff, but it will still eventually burn. I use it and it works great, only problem is that it's seldom available locally and you will have to mail-order a supply. Check ebay, or some of our great TRF vendors.

I have also used nylon cord with great success. I coat the bottom half of it with white glue and work the glue into the fibers with my fingers, then tuck one end under the thrust ring. I pull it taut out the front end of the BT and drip some more glue onto the cord (from the front end of the BT) to hold it straight inside the tube. Just don't glue it right up to the front---leave room for the NC to seat. (I install Kevlar with the same method)

These cords do not replace the shock-absorbing material in my rockets. I tie a loop on the end of the tether cord and then tie on the rubber or elastic shock cord. You still need to attenuate those separation energies after the ejection charge pops, or else you will have snap-back if you are only using a tether cord.
 
Originally posted by JBeau
Look at the artwork on the cover of the 36D Squared and the photo shows the parachute mounted in the cord center and the two halves of the rocket banging in to each other on descent. It cracked me up.

But they sell more kits that way--
;)
 
the use of Kevlar cording is due to it's resistance to heat

Yes. Elastic will, over time, become britle and snap easily. Have not heard any other material being used.

I'm afraid that over time the mount may become weak and come loose.

No way! I have never had a problem with the tri-fold mounts (I use white elmers, but wood glue is probably better!) Has anybody had it come loose ever? My mounts are so flush, I highly doubt they ever resrict deployment. I do mount it before any glue inside folds have dried and hold it tightly againty the tube until it sets - smooth as butta!

I use 90#(? I thought) Kevlar from fliskits. The Kevlar extends ~ 6 inches from the front of the tube than tie 1/4 or 1/8" elastic ~ 2-3 times the body length. These are all LPR. Only problems I have, is it takes alot of time to stuff all the elastic! :p

Pat
 
I'm in the "no snagging!" camp. That tri-fold thing works OK, but it's caused me a lot of trouble over the years and I just won't use it anymore. I also prefer Kevlar(tm) to most everything, even on small rockets. I'll either backup the elastic with K. or not use elastic at all.

Typically, I will anchor the cord at the bottom of the motor mount (outside the rocket) threading it up through holes in the rings. If the cord gets worn or frayed, simply untie it, tie the new cord to the end of the old, pull the old cord out drawing the new through at the same time, voila. New cord. A dot of RC56 glue on the knot and at intervals around the loop on the mount tube will fix it there.

This isn't always practical, though. Rockets such as the Astron Sprint or Trident require something else.
 
I agree that the Kevlar TM is a good idea if the SC is mounted near the engine due to heat. But the question has to do with the elastic. The old kits by 'E', and I see some new kits as well, use the 'rubber band' type elastic, which tends to dry out & snap over time. The part-cotton type elastic which 'E' had used for a while seems to last rather well, but I recently noted that a kit with a peculiar cotton elastic had dried out, reducing its elastic quality. Has anyone else noticed this? That & the fact that kevlar cord does a wicked 'zipper' on conventional body tubes. 'Q' went to a part kevlar, part elastic cord, but has yet to deal with the zipper effect problem. Help is needed here!

I have rockets which have flown over the better part of 25, in some cases 30 years. So I get to see what age does to rockets... Never mind what it's doing to me!!!

Any comments on the shock cord itself would be appreciated. If the end of the Kevlar is inside the tube to prevent zipper, if the elastic needs replacing for age, you can't get to it. If it's long enough to change the elastic (I prefer the cotton-type), it will zipper. Could we 'pad' the Kevlar in the area of the body tube edge to try to distribute the 'impact' to prevent the zipper?
 
I also dislike the length of the shock cords that come in Estes kits, and I don't think the fold over papermount is too classy either. I have had good luck using a (I think) synthetic leather product I picked up at Michaels. It is basically a shoe lace material, and is pretty strong as well. It glues well with wood glue being as porous as it is. I drill a hole in the upper centering ring and thread the leather lace through and either tie a knot behind the hole (as big as possible without interfering with the mount fitting the body tube) or wrap it around the MMT and tieing it off. A dolop of glue makes me feel better also. If there isn't room for a knot it glues well right to the MMT (glue as much as possible for more surface area). This leather-like material really holds up well to the ejection heat also. I make my recovery harness about three times the rocket length, with one third being the stock elastic. Super glue glues the rubber to the leather very well also. I split the leather for a half inch or so with a hobby knife and sandwich the rubber in between. If I am really going big I will use "Plasti-Dip" and smooth out the transition. At the nose cone end I just tie a single overhand knot to secure the NC, then run a length of the leather out AT LEAST one and a half lengths of the nose cone. I then use a swivel and fold over the leather on itself and glue it. The 'chute ties to the other end of the swivel. I do this so during deployment the 'chute take a moment to unroll and everything is "getting situated" for the parachute to bloom. I think this helps keep the nose cone from flying through a deploying parachute which always bugs me. It will tangle in the shroud lines and mess up a picture perfect launch and recovery. The leather is fairly light, but I am sure I lose a bit of altitude because of it. That's OK with me, once I learned the highest flight possible is only cool if you get the rocket back. A slow takeoff, true flight, and beautiful parachute blossoms are more important to me. Just food for thought-

Oh, my bud had a rocket crash after the Kevlar cord ripped through his centering ring. It had two holes that you ran the cord through and tied off, if using Kevlar be sure to tie it TO something (like around the MMT).

To the gent unsure about how to replace the elastic on a combination kevlar and elastic shock cord where the transition is inside the body tube (to prevent zippering): I read a great tip about just feeding the cord through the bottom of the rocket (out the MMT) and repairing as required.

Rob
 
I use a modified version of the estes shock cord mount on many of my rockets that works quite well. I get thick cardboard cores from the large toilet paper rolls from at work and cut it to an appropiate size. It already has the curve needed to match a body tube. I then tie kevlar cord arround the card board and glue it in possition with a thin layer of gorilla glue. If needed you can coat the edges with extra glue. Works like a champ and you can use it on aleady assembled rockets.
 
I have used the tri-fold paper mount on almost all my models and never had it hinder a parachute coming out. After gluing and folding it, either I press it hard between finger and thumb, bending it a bit round the finger; or, like Huxley, I glue it into the body tube before it's set, pressing it hard against the inside of the body tube. Either way, the idea is to get it curved so that it ends up as near flush against the body tube as possible.

Elastic does suffer from ejection charges over time, although by using a tri-fold mount, you can have all the shock cord above the wadding so that it is protected. When I use kevlar (and sometimes that goes into a tri-fold paper mount too :)) I wrap some masking tape around where it will hit the body tube. This prevents zippering.
 
Originally posted by JBeau
Another point is to remember to attach the parachute close to the nose cone so that when the chute opens and the NC is dangling, it is still not anywhere near the BT, no bouncing damage.
Also, if the chute is attached close to, or even attached directly to, the nose cone, it's more certain to be pulled out of the body tube when the nose cone is ejected. Otherwise, particularly with a long shock cord, you could have the nose cone eject while the parachute stays in the body.
 
I think I need to come up with a second option to the estes paper mount. Today on its third flight the mount on my Bull Pup came lose. Its possible I didnt glue it in good enough but unlikely. More than likely it was an odd fluke.
 
I can't remember when i've put an Estes paper chute blocking mount in one of my models, I believe it was about 1972;)
Once I found Kevlar, and Stainless steel fishing leaders, and more recently multi-strand stainless nylon coated "beading" wire I completely got away from them.
I still use the combination Kevlar or Leader/elastic on standard and LMR's or just kevlar or beading wire/ kevlar on Micros.
Personally I believe the biggest change came when I discovered that moving the Kevlar line from the motor tube/centering ring joint to the forward ring outer edge/ airframe tube joint all but eliminated burning off the kevlar on all be minimum diameter models, that's the way I've installed shockcords ever since.
I don't have a photo of the motor mount but I do have a drawing of a T2+ to BT-5 mount showing this outer tube ring placement. Adding the Stainless leader or beading wire instaed of Kevlar in a length that just extends the crimped loop outside the body tube makes this mount last longer then the model Tying either Kevlar or elastic to the anchor loop. I've actually transfered a couple of these Stainless Shockcord anchors from one model to another after a motor mount burn thur or Cato. They seem to be holding up Extremely will;)
Hope this helps

PS edit:
To prevent zippering with either plain Kevlar or Stainless leaders, I place a piece of 1/2" wide masking tape about 1" long, folded & centered on the mount line, inside the tube about even with the end of the body tube and burnish it down. This anti-zipper tab moves with the shockcord anchor inside the body during packing, usually staying out of the way during deployment. I have had a couple of the lighter 50lb kevlar lines tangle with the tab a bit but usually not causing any deployment problem. Just takes a bit longer to untangle and repack back on the ground:)
 
Instead of drilling a hole in your Centering ring, file a small notch just a tad larger then the shockcord or anchor wire your using in the outer edge or the ring. feed the cord out the motor tube druing installation and the shock cord will be embedded in the motor mount adhesive as well. Doesn't show at all on the outside of the thinnest airframe body tubes. even tracing paper:)
When I'm using minimum diameter tubing as the model body I usually use a motor block and file a notch in the Ring or cut piece of spent motor casing to allow the kevlar to be tied around and thru the motor block. Below is a pic of a micro block with 50lb kevlar mount, this works great, but the nylon coated multi-strand beading wire works even better:D
Hope this helps.
 
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