Subbing kevlar thread for elastic in trifold shock mount

[Zeus-cat]

Has anyone tried substituting kevlar thread for elastic in the traditional tri-fold shock cord mount? That is the only thing I have left to do on my Fliskit's ACME Spitfire.

I am asking this as I launched a few rockets recently and after returning home I noticed that one of the shock cords was only hanging on by a single string. I have kevlar thread running up from the motor mount and tied to a swivel snap. Replacing the elastic on this rocket willl be quite simple as I simply can disconnect the snap tied to the elastic and attach a new piece. A piece of elastic glued into a shock mount can't be replaced so easily.

It's too late to tie off to the motor mount, so I need to do something from the nose cone end of the rocket. Does anyone have any experience and tips gluing kevlar thread in a tri-fold mount? Or is there another mounting method you would recommend?

 

[sj_h1]

Never tried it, but my concern is that the kevlar thread doesn't have as much surface area as a shock cord and may pull loose. May you could thread it several times through the folds and knot it. Just a suggestion.:2:

 

[jadebox]

For retrofitting Kevlar shock cords into rockets, you can cut a couple of small slits in the body tube. Bend the section between the slits in a little and feed the Kevlar through the tab it forms on the inside of the rocket. Tie a knot around the tab to secure the Kevelar. You can add a drop of glue to the outside between the slits to also secure the thread.

-- Roger

 

[gpoehlein]

Easiest way I know is to use a Centuri style mount. Take a piece of cardstock or index card about 1-1/2" long by 3/4" wide and punch (or cut with your xACTO) three holes along its length. Thread the kevlar into the hole on one end, back through the middle and back through the third. Tie the loose end to the exposed part of the kevlar to secure it. Then, glue this (double glue joint for strength) inside the tube like you would a tri-fold. It will be very secure and won't pull out like it might in a tri-fold. I use this method for securing kevlar shock cords to my competition model nose cones all the time - once that glue sets, it won't come out.

 

[Micromeister]

I don't use either the Estes "Teabag" or Centuri's old tape lock. but Do retro fit Kevlar/elastic or Stainless leader/Kevlar/elastic shock line in Micro, Model and Mid power model.

Over time I've found it's usually the minimum diameter models that sometimes have problems with Kevlar Shockcord anchors. Generally I use a centering ring or sanded bodytube coupling as the "anchor" for my retro-fits.
If the model is minimum diameter to the motors I'll often switch from Kevlar to Stainless aircraft cable or Fishing leaders for the first 4 to 6 inches. tying on the Kevlar/elastic above the high heat area. On most other body sizes (BT-55 and up) the Leader isn't all that necessary. it is however very important to keep the Kevlar as far away for the ejection charge coming from the end of the motor as possible. By this I mean moving the attachment from the motor mount/inside centering ring to the outside edge inside the airframe of the mount or if retro-fitting using a Coupling or centering ring with the standing end of the shock line sandwiched between the inside outer body tube wall and the outside edge of the Coupling or ring.

I file a slot in the outside of the couple or ring to just pass the thickiness of the Kevlar or leader thickiness so the line doesn't show up as a bulge on the outside of the model. tie the kevlar in place with the knot laying along the very forward edge of the coupling or ring. using a dowel, tube or other long impliment spread epoxy or adhesive of your choice (elmers works just fine) down inside the tube as far was you are going to push the new mount. Try to get a good coating all the way around the tube. insert the new mount coupling and slide it into place, seating it with a dowel or other "Pusher" that is long enough to reach the glue seat of your anchor. after the glue has setup these new anchor point gererally out last the model, while being far enough out of the way as to not hamper chute deployment.
Hope this helps.

 

[Multi-Stage]

When I retro-fit kevlar for shock cord's I use epoxy and glue my shock cord's where you would the tri-fold at, I make sure I do a good job of making it very unobtrusive as not to impede the chute as it's deployed. I have had no problem with this technique. Since becoming a BAR I've only used kevlar as a shock cord 2x the length of my rocket's with no zipper's. I've used both the motor mount style and the above way of mounting the kevlar shock cord with great sucess.

 

[Zeus-cat]

Thanks for all the responses; one of them will do the trick for sure.

I'm leaning towards the centering ring method suggested by Micromeister as I have plenty of BT-50 centering rings lying around.

 

[rstaff3]

For new models, I attach the Kevlar to a centering ring. Or on mid dia I use and notched piece of a spent motor. For retrofitting I often run the cord thru 2 slits in the BT next to a lug if possible. I then fillet over it with epoxy and touch paint.

 

[luke strawwalker]

The last couple Zooch kits I've built have used that method-- a trifold with kevlar embedded in it.

What I've been doing is, first, tying a simple overhand knot in each end of the kevlar to keep it from unraveling (except the very end of course) and then putting both those ends under the first flap of the trifold, about 1/8 to 1/4 inch apart, apply plenty of glue, fold it over, apply more glue and work it into the kevlar, then fold it over the second time, and clamp it up with clothespins or hemostats to dry. Once the glue dries the thing is pretty well impregnable.

That said, the last one, I tied a surgeon's knot in the end of the kevlar with a sliding loop (Lasso style) in the end, and slipped it over the motor mount, and cut a small notch in the centering ring to pass the cord through, cinched the loop up tight around the motor mount when it was dry, put a dab of glue to "lock" the loop up tight against the motor tube, fed the kevlar line down through the motor mount, applied the glue, and installed the motor mount. Running the kevlar down through the top of the motor tube will keep it out of the way while applying glue and installing the motor mount. Once everything's dry, simply drop the line back through the motor mount to the inside of the main body tube.

I usually just tie an overhand knot loop in the end of the kevlar and tie the elastic shock cord onto that. Be sure you apply a dab of glue to lock the knot, as kevlar doesn't hold knots well by itself.

Good luck! OL JR

Good luck! OL JR

 

[traveler]

I was just looking at frame D2e in the thumbnails posted by Micromeister.

What assurances are there that the two half hitch knot tightened and trimmed with no glue is secure?

 

[asm109]

For Micromeister the assurance comes from 30 plus years of Scouting

The two half hitches are formed in the elastic. The elastic is a high friction material so knots do not tend to release. Two half hitches will slide and pull tighter the harder you pull on the line.

The overhand knot the micromeister shows is a great way to put a loop in a low friction string like kevlar. They don't slip either.

I cringe every time I see people put glue on a rope. The glue wicks into the fibers and forms a hard plug. The fibers then have a stress concetration and they will begin to break and fray at the edge of the glue. The knot won't slip but the rope will break either way you lose connection.

 

[Micromeister]

Exactly:
A properly tied knot does not slip or come untied unless it is purposely UNTIED. The Choice of the proper knot for the job comes with a little research and experience. Those Shown in the photos have worked well for me over 3 or 4 decades. I can't remember the last time I had a knot fail, I've lost a few Elastic shock cords to burn-thurs but they have alway been much higher up well away for the knots.

To go one further the asm109, more then 30 some year in both Scouting and Rocketry LOL!!! Sometime if I've used a little too much slack when tying I'll through another half hitch in the elastic just to take up the excess but it's not necessary. When you trim off excess in either line or elastic leave about the wide of the knot beyond the knot. This ensures any stretch will not allow one of the hitches to untie.

PLEASE FOLKS! If you lean nothing else for this thread I'd sure like it to be.. NEVER add glue to any knot, string, rope or line.... it WILL cause the line to fail just at the boundry where the glue meets the line. Dried glue acts like adding shards of glass to the fibres of the line. Flexing and bending at this boundry joint break down and shears the fibres over just a little time.
Hope this helps.

 

[traveler]

Thanks for the replies asm109 and Micromeister.

You are absolutely right about the extra strain that occurs where glue terminates on an elastic cord if glue were to be used. I'll pass on any glue at that location. But I'm also looking to make the attachment more bulletproof than an unsecured two halfhitches. If it can, it will. And it will when you're not looking. So I want to make it so it absolutely cannot. It's the mountain climber in me that's looking for the extra security of some kind of retainer knot or backup knot or something. I don't want to rely on only the friction.

I'm also considering a few stitches or wraps of kevlar thread through and/or around the tail end of the elastic to secure the tail end back to the load end (didn't they do something similar to the standing rigging on old square rigger sailing ships?).

I want to be sure my camera payload will be returning safely. And I'll be doing this twice, once on the rocket for its deployment system and again on the nose section for its deployment system.

So what do you think of a few stitches of kevlar thread through the flat tail end and the knots, with a firm wrapping of the kevlar cord around the knots turning the flat elastic cord into a short tube between the attachment point on the rocket body and the end of the knot section? I have seen metal Snap Rivits put through the loop end of a flat shock cord on a factory rocket kit for security purposes I suppose, but there's not enough room on the 1/4" elastic shock cord which I'm discussing here for a rivit. I should add that the current project is to retrofit an Oracle for a two paracute, separate payload recovery system. I'm either going to attach the upgrade flat elastic shockcord directly to the mid body tube attach point, or setup a two part kevlar cord/elastic shockcord system similar to the one pictured at the top of page 113 of Tim Van Milligan's book "Model Rocket Design and Construction". Since the motor mount of the Oracle is already factory assembled, it can't be used as an attachment point. Fortunatly, there is a shockcord attachment point built into the tube coupler.

 

[powderburner]

Stitching is fine. It is used in lots of load-critical places to secure and join flat straps of material (nylon, dyneema, spectra, lots of other exotic fibers/fabrics) for rock climbing, industrial straps and load suspension gear, sports equipment, and many others. The advantage of stitching is that it flexes with the strap and is basically self-load-leveling (high-stress portions stretch until the load is evenly distributed).

Yes, glue in a knot can weaken things, but that is only a serious consideration if you are working right up to the limits of the material strength. I would not recommend glued knot for climbing gear, especially if it was MY tail hanging on the end of the rope or strap, or if a crane/strap combo was hanging a 10,000 lb load over my head. However, for rocketry purposes, we are seldom ever working so close to strength limits that you couldn't go ahead and use glue (but I mean glue as in white glue, not epoxy or CA). IMHO, we make a mountain out of a molehill on this issue when nine out of ten instances work just fine. We are already causing a strength "knock-down" in the rope/cord/strap simply by having any knot at all; it's not usually the end of the world if you go ahead and use some glue.

If you actually think your rocketry item is seriously compromised by using glue on a knot, then A) don't use glue, or B) use stitches, or C) use a bigger size of rope/cord/strap so you will still have the necessary strength after you use your glue. If you are designing that close to the strength limits of a material, you should probably be using the next bigger size anyway, or a different (stronger) material in the first place. Use your head. Be practical.

If you are going to use glue in a knot, one way to do it "safely" is to soak a short length of tail, and let it dry/cure/whatever in a straight line. Then, if you are worried about a nearby knot slipping, you can rest assured that the straight piece of "tail" will not pull through the knot. The un-glued "head" of the knot will not be affected.

The other way to use glue is to limit your glue to stuff like elmers, which "dries" to the naked eye but actually never quite gets rock solid (like epoxies and CA can do). Elmers glue in a fabric will stiffen when it dries but it also definitely remains flexible to a small degree. A few dots of thinned white glue can help lock up a knot solidly, but it only takes a little bit or you will run into the hard-knot problems that these other guys have described.

There. That ought to spark some discussion.

 

[Peartree]

With flat elastic I have also taken to using a little glue to reassure myself. Again, I don't use the glue in the knot but tie a good knot and with a decent amount of free end in the tail. I take the tail, lay it flat against the original length of elastic and glue them together. I realize that the glue may eventually cause issues, but it hasn't happened yet (and I haven't been around as long as some of the folk here) and I'm too lazy to sew it and too insecure in my own abilities to trust the knot alone.

 

[Bravo52]

Not sure why people (above) recommend cutting slits in the body tube. Granted, that method is as old as dirt (like many rocketeers) but I suspect that technique was a result of inadequate glue. With new gluing ability, seems to me you can get just about anything to bond to paper given the right glue. I bond kevlar to the paper with epoxy (like LOC mounts) and have never had one come out. I use paper tri-folds with kevlar and epoxy with great success. The only thing I might do is tie a knot in the end of the kevlar before I fold it up in the paper. Then again, I also think it is a little over kill to use kevlar on a BT mount shock cord. If I need kevlar, I'll mount it to the motor mount tube. Obviously, your mileage may vary.....

 

[Micromeister]

Thanks for the replies asm109 and Micromeister.

You are absolutely right about the extra strain that occurs where glue terminates on an elastic cord if glue were to be used. I'll pass on any glue at that location. But I'm also looking to make the attachment more bulletproof than an unsecured two halfhitches. If it can, it will. And it will when you're not looking. So I want to make it so it absolutely cannot. It's the mountain climber in me that's looking for the extra security of some kind of retainer knot or backup knot or something. I don't want to rely on only the friction.

I'm also considering a few stitches or wraps of kevlar thread through and/or around the tail end of the elastic to secure the tail end back to the load end (didn't they do something similar to the standing rigging on old square rigger sailing ships?).

I want to be sure my camera payload will be returning safely. And I'll be doing this twice, once on the rocket for its deployment system and again on the nose section for its deployment system.

So what do you think of a few stitches of kevlar thread through the flat tail end and the knots, with a firm wrapping of the kevlar cord around the knots turning the flat elastic cord into a short tube between the attachment point on the rocket body and the end of the knot section? I have seen metal Snap Rivits put through the loop end of a flat shock cord on a factory rocket kit for security purposes I suppose, but there's not enough room on the 1/4" elastic shock cord which I'm discussing here for a rivit. I should add that the current project is to retrofit an Oracle for a two paracute, separate payload recovery system. I'm either going to attach the upgrade flat elastic shockcord directly to the mid body tube attach point, or setup a two part kevlar cord/elastic shockcord system similar to the one pictured at the top of page 113 of Tim Van Milligan's book "Model Rocket Design and Construction". Since the motor mount of the Oracle is already factory assembled, it can't be used as an attachment point. Fortunatly, there is a shockcord attachment point built into the tube coupler.

After reading your reply a couple times I have to say: Sounds like you shouldn't use knots at all. If your opinion is that knots are unsecure then don't use them. However, just so ya know; they are indeed secure and Friction is part of what holds ALL knots together. Further; particularly in use on LPR/MPR models there is absoluetely NO reason for making anything "bulletproof". Oracle, Priceless electronic payload or what have you aside, the Use of Properly tied knots are as secure as anything else suggested.
When it comes right down to it we're relying on a single loop or bend of Kevlar or Elastic to "Hold" whatever is dangling below our chutes. This bend of elastic or line has MORE chance of failing then the weakest knot.
Why do we use knots in the first place? So they can be untied for repairs, changes and replacement as needed. If there is no chance of ever needing to replace your line or elastic then why not splice in the eye and sew on the elastic....No knot needed It's all about convenence and time.

While I'm not a mountain climber, as a rescue worker and lifeguard I have hung my butt on single knot lines enough to know I didn't have to worry about them any more then I worry about the line itself snapping. Back-up or retainer knots are unnecessary in the lines and materials we talking about here.

Choice of knots for a specific job can be a personal thing; But That Choice has to take into account all the factors involved in the application. In our particular scheme here (LOW and MID POWER) Model Rockets and their Payloads regardless of expense, Overhand Loops, Butterflys, and 2-half hitches are and have proven to be as dependable as any other attachment method, while combining speed, flexability, and Changeability over the more labor intensive permanent attachment methods.
You can choose any method that suits your application; Rather then stitching I'd be more inclined to whip the free end of an elastic or line tail to the standing line if I didn't want to take the time to splice in an eye. Either way your totally removing the reason for use of a knot in the first place....ease of removal for change.

 

[traveler]

[snip]

You can choose any method that suits your application; Rather then stitching I'd be more inclined to whip the free end of an elastic or line tail to the standing line if I didn't want to take the time to splice in an eye. Either way your totally removing the reason for use of a knot in the first place....ease of removal for change.

Thanks Micromeister. Whipping. That's the knot term I couldn't remember. Tie off the end with the knots and whip the tail. Then, at the end of the main shockcord's useful life (if it happens at all if kevlar is used), a knife will be the tool for ease of removal of the deteriorated material.

 

[Peartree]

You can choose any method that suits your application; Rather then stitching I'd be more inclined to whip the free end of an elastic or line tail to the standing line if I didn't want to take the time to splice in an eye. Either way your totally removing the reason for use of a knot in the first place....ease of removal for change.

I see what you're saying Micro, but FWIW, if I'm tying a knot in a recovery harness that is attached to a motor mount that is going to be glued inside of a long body tube, then the chances of me ever needing to remove it or change it pretty much go straight to zero.

I tie knots because a) they are easy, b) secure, c) fast and d) cheap. I appreciate that knots can be untied but many of the places they end up in a model rocket are, in the end, inaccessible. For those that are accessible, it is definitely a good thing to be able to untie them.

 

[Micromeister]

I see what you're saying Micro, but FWIW, if I'm tying a knot in a recovery harness that is attached to a motor mount that is going to be glued inside of a long body tube, then the chances of me ever needing to remove it or change it pretty much go straight to zero.

I tie knots because a) they are easy, b) secure, c) fast and d) cheap. I appreciate that knots can be untied but many of the places they end up in a model rocket are, in the end, inaccessible. For those that are accessible, it is definitely a good thing to be able to untie them.

I understand what your saying John:
I can identify with much of your assessments when it come to practicing new or renewed skills.
What I'm getting at is we; as a whole hobby, seem to put ourselves in situations that don't lend themselves to Mod-Roc maintenance.
Sure I understand when we build a kit it's usually only given a life span of a year more or less... However is that really a well founded case????

Generally those Motor mount anchored shock-lines are for the most part inaccessible. But that doesn't mean we can't construct them for alteration and replacement. True, the MMT Anchor point itself will to be permanent; but the Stretchy portions can to be placed in such a way that if you fly the model often it can be accessed for changing. That's were unglued Knots are useful.

Starting with either a very heavy Kevlar, Stainless or Titanium Leader anchor that will outlast the model, just a tiny bit of pre-planning can place the elastic or rubber portion(s) where they may be changed several times over the models flyable life. At least that's what i've leaned over the years My aging fleet has a good number of model built in the 80-90's that I did not make these arrangements for and I'm now having to retrofit New Shocklines & anchor points to remedy the problem. Some of these models have over 50 flights others suffer from old age atomsphereic elastic (rubber) deteroration, either way they have to be changed Frankly it's a PAIN installing recessed Shockline anchors in existing models but they are so much better/cleaner then the Tea Bag bulges that it's worth the aggravation working them in. Really the only inaccessable termination point on any Shockcord should be the MMT anchor point. I'm hoping with these suggestions, I can help some of you Not make the same mistake I made back then

A couple more points about Spliced in or Crimped eyes on Shockline Anchors.
Long splices in braided/tubular kevlar while possible are truely a test of your patients and dexterity. Short eye splices are fairly easy in 3 strand twisted lines and steel cables sometimes requiring use of a marlin spike or nail to help hold the strands apart.
If you must use compression crimped sleeves for you cable anchors or leaders be sure to use a dremel to grind as many edges as possible round to prevent snagging and/or ripping of your laundry.

 

[Peartree]

Frankly it's a PAIN installing recessed Shockline anchors in existing models but they are so much better/cleaner then the Tea Bag bulges that it's worth the aggravation working them in. Really the only inaccessable termination point on any Shockcord should be the MMT anchor point.

Micro,

I'm not sure what you're saying here. If you are not repairing with a teabag style attachment and not gutting the rocket to install a new motor mount attachment, then what sort of "recessed shockline anchor" are you using? I'm just not visualizing what method you're using and it sounds interesting.

 

[Micromeister]

Oh Man; I thought I'd made the process pretty clear with the retrofit pictorial posted earlier. In the pictorial the AR-2050 Centering ring is installed atop the existing forward motor mount ring....or as close as possible.

I absolutely detest the bulge and snagable corners created with the old Tea bag mounts. Back in the Day I used to spend more time trying to smooth out, coverover those things then it took to build the model. When I first started using a motor mount or Thurst ring anchored shockcord method in the mid 70's..I've never gone back.

Thats what I ment by "it's a Pain" retrofitting Shockcord mounts 12 to 30inched deep in a model from the front end. It takes a little planning
This method envolves applying epoxy to the forward ring area DEEP inside the model without taking it apart or spreading(slopping) the glue all over the inside of the tube while installing the Ring/anchor/shockcord replacement into this glue or epoxy from the forward end of the model, making sure it's set tight and flush with the existing MMT and inside body tube without clogging the ejection opening.
To accomplish these tasks long dowels are tipped with cotton balls to act as swabbs to carefully apply epoxy to the area after cleaning out the soot and grime with others first. Setting the new ring is the easier part with the use of larger dowel rods, tubes or what have you that makes a fairly decent fit with the ID of the model body to slide the ring/shockcord into place in the epoxy or glue and retracting the "pusher" without disturbing the newly seated ring. Actually the setup is the longest part of the process LOL. I generally follow up with a fillet of glue or epoxy applied with the same long swabs after the first application has set.
Humm! guess I need to take some progress photos of one of these operations in the near future...LOL! Sorry...we think were being clear when often our understanding of what others precieve is I guess clouded

 

[Peartree]

Thanks Micro, that's how I've been retrofitting some of mine where the teabag attachment has already failed. The word "recessed" threw me off. Thanks for clearing that up.