Securing the shock cord to the body tube dilema

[dbryantphoto]

I just finished building a beautiful Estes Executioner. After having nearly completed everything I decided that I would like to install a better shock cord.. maybe some sort of nylon cord instead of the rubber band thing.

There's really no way for me to dig out the installed rubber band cord which is secured deep inside the rocket tube.

What is the best way to install a shock cord to the inside of the tube other than folding a piece of paper and gluing it inside? I was thinking of drilling a hole in the tube, tying a new and better piece of shock cord and securing it with a nice knot. The remaining knot that would be protruding outside the body of the tube... i was thinking of designing some sort of smooth cowl that would be placed over the knot... gluing it neatly and then painting to match.

Is there a better way?

[stefanj]

LOC used a simple and effective method:

Tie a loop in the nylon shock cord . . . a circle maybe 2" wide.

Mix up a batch of epoxy.

Lay the body tube flat.

Use a tongue depressor taped to a dowel as a spoon to transfer a nice pool of epoxy down the inside of the body tube a ways. Smear it around a little.

Lay the loop of cord right in the pool of epoxy. Use a dowel with waxed paper wrapped around it to press it firmly against the body tube. Pour on a little more epoxy so the cord is encased.

Let dry, keeping the model horizontal.

[dbryantphoto]

That sounds like a great idea. So basically the cord will be attached to the inside of the body tube encased in a little pool of epoxy. I will try to get the pool of epoxy deep inside. Thanks so much. Are there any pics of this online anywhere just so I can compare my mental visual with pictures?

[dbryantphoto]

Is it necessary to use the rubber band type shock cord or can I just use a nice long piece of nylon cord? Does a portion of the cord need to be eleastic?

[Night Tripper]

I use elastic on all of my low-power builds. You can pick up a small roll in the fabric section of most any Walmart.

And btw, Welcome to YORF, Don!

[mycrofte]

After ordering a shock cord, just to find out they use elastic bands, I went to Wal-Mart too. I started with the same width as the old rubber bands. But they tend to give out faster. Until my supply runs out (18 feet) I started doubling them up...

But I still use the old folded paper to mount them. Some of these guys post about them giving out but mine never have. But, I use so much Elmer's on them they are hard to conform to a round tube...

[GregGleason]

I do a modified pool of epoxy, in that I take fiberglass cloth, about a 2" x 2" section, sometimes bigger or smaller as the airframe dictates. I make a "U" in the cord, then tack it to the cloth with some CA. I then mix epoxy laminate resin and wet it out, then place some wax or parchment paper over the cloth. Lastly, I either lay on the side or inflate a balloon in the airframe press to press the airframe. After the epoxy cures, remove the balloon and/or paper. Using this method I have never had a failure at this area of the rocket.

CAVEAT: watch that the place you attach your cord gives sufficient room for the nose cone shoulder so that there is no interference.

Also, you may want to consider braided Kevlar. It is strong, and heat resistant and will likely be the last part of the recovery train to fail. I know that Gus (a.k.a, Steve) had a post a few months back on where to get it.

Greg

[jeffyjeep]

I nearly always attach a kevlar leader to the engine mount (Quest style) to serve as the shock cord mount. I just tie a loop in the forward end of the kevlar and make sure it's long enough to extend past the forward end of the body tube to facilitate replacement of the elastic shock cord itself. Jeff

[dbryantphoto]

Not sure what you mean.. sort of... you're saying that you attach a length of kevlar shock cord to the engine mount on and then run said cord all the way through the entire length of the body tube? then at the top end connect your elastic cord? easy access to the elastic cord for replacement if need be? i think i got it. however my entire body tube is complete. i'll do that on my next build.

[stefanj]

Use a barrel swivel to attach the kevlar to the shock cord.

The kevlar should NOT extend past the front end of the body tube.

Feed it out the rear of the model when the time comes to attached the barrel swivel and shock cord.

[tfischer]

I don't think I've ever had an "estes-style" triflold mount fail. I've had the shock cord break off (due to age and heat) at the mount, but never the mount fail. So I'm a little curious about some of these options. I can understand the Kevlar past the body tube (to eliminate the break-off I just described), but the other options just seem to solve a problem that I've never had...

[snaquin]

I don't think I've ever had an "estes-style" triflold mount fail. I've had the shock cord break off (due to age and heat) at the mount, but never the mount fail. So I'm a little curious about some of these options. I can understand the Kevlar past the body tube (to eliminate the break-off I just described), but the other options just seem to solve a problem that I've never had...

Here's the instructions to the LOC shock cord mount "SCM" that Stefan referenced in his post to better visualize what he described. It's a tried and true method.

Not putting the Kevlar past the edge of the body tube will prevent the Kevlar from slicing through the tube and causing a nasty zipper.

.

[garmtn]

All these ideas r good I'm sure but, extremely messy. I stumbled upon the article in "Sport Rocketry," the "Anti-Zipper " shock cord attachment. My first attempt was in a dual egglofter, using 1500 lb "tubular nylon." Never came close to breaking or pulling out. Today I took my "tail/fin" section, and rebuilt the body. Found another terrific idea, using a "jig" to smooth out center Plate. I'll take some photos and drawings and post here tomorrow. Basically, just like last guy said, Fins separate from body "rearward".
The build: Made a center plate using the jig to fit a BT-80. Once I had the fit, ran a piece of 200lb kevlar thru hole in middle about 3 ft long. Then plugged hole with "wall anchor" and glued with CA and wood glue. Tied rear end to fin assembly. There's a "baffle system" in fin assembly, with an "eye bolt" sticking out to attach parachute and kevlar. Plate was made from 1/8" fiber board. Other end of kevlar gets tied to nose cone. In my case, this "payload" section going to carry lights and battery, so it will b seen @ night @ Naram 51. There u have it! I swear by this system. Baffle works great and pulls parachute rearward, without wadding. I did use a small piece of elastic from kevlar to tail/fin assembly but, can always be replaced, unlike other shock cord mounts. :)

[jeffyjeep]

Yes, that's it exactly. And you're right, you have to do it diring assembly of the engine mount. Jeff

[garmtn]

Heres a description of attached Photos. 1) regular "cloned BB 0." 3) "Goony" Bluebird. 4) "Baffle system," w/eye bolt attachment; 5) Baffle, main body, & blast plate. Note, the kevlar passes right on thru it, but plugged and glued; 6) whole rocket in separate pieces attached w/kevlar shock cord; 7/8) side by side BB and BB0.
Forgot to take pictures of "center plate" jig. Will do that next. Glenn NAR 89197 ;)

[foose4string]

Choosing the wrong delay will cause a nasty zipper. That's almost always the cuplrit...not the kevlar hanging out the end. The kevlar may help facilitate the zippering phenomenon quicker than say, elastic alone, but it's not the main cause. I allow the kevlar anchor on all my rockets to hang past the end of the tube using the method Jeffyjeep described and have never had a zipper (I have many and fly often :) ). Am I lucky? I'd like to think it's because I've made smart motor choices.

I know some people use a piece of tape(or something along those lines) stuck to the kevlar line where it intersects the end of the tube. This is used as a means to buffer or cushion the line against tube and acts as an "anti zipper device", but I don't do that either. ;)

None of what I'm saying really pertains to your original question, and to that I'd say try the LOC method.

I'm not implying that any of the previous suggestions about anti-zippering aren't good advice...they absolutely are!
But just be careful with your motor and delay choices and you'll be golden.

[barone]

Referring back to your original post.......

If you don't want to use the tri-fold paper mounting system (which, by the way, has never failed for me either), you can use the mounting system used by FSI.

Basically, stiff paper or card stock, about 3/4 to an inch wide and about an inch and half long, maybe two inches. Three holes equally spaced centered on the long axis. Check this link for a description of how to mount it.

http://plans.rocketshoppe.com/fsi/fsi1004/FSI_Voyager.pdf

[foose4string]

Referring back to your original post.......

If you don't want to use the tri-fold paper mounting system (which, by the way, has never failed for me either), you can use the mounting system used by FSI.

Basically, stiff paper or card stock, about 3/4 to an inch wide and about an inch and half long, maybe two inches. Three holes equally spaced centered on the long axis. Check this link for a description of how to mount it.

http://plans.rocketshoppe.com/fsi/fsi1004/FSI_Voyager.pdf

I used this method in my Orbital Transport clone after forgetting to attach the kevlar before the motor mount was installed, and it worked very well. I sort of combined it with the LOC method, still using kevlar(knotted at the anchor end) as the anchor line and using epoxy as the adhesive. The tri-fold works just fine, I just prefer the other methods, mainly because think they are a little less obtrusive.

[Jeff Walther]

I don't think I've ever had an "estes-style" triflold mount fail. I've had the shock cord break off (due to age and heat) at the mount, but never the mount fail. So I'm a little curious about some of these options. I can understand the Kevlar past the body tube (to eliminate the break-off I just described), but the other options just seem to solve a problem that I've never had...

I think the idea is that when the shock cord breaks it is much easier to replace with these other methods. The reason being that the shock cord is not integrated into the mount; it's tied to a more durable cord which is integrated into the mount.

Now, all that said, you could just put some kevlar in a trifold mount and tie the shock cord to the kevlar and get much the same benefit. When the shock cord breaks, tie a new one to the durable kevlar.

However, to some extent it is a solution for a non-problem. As I recall from way back when, I could usually dig an old trifold mount out with a #11 Xacto blade and a lot of patience. And even if you can't dig the old mount out, you can always just glue in a new one part way around the tube from the old one.

I think it's kind of fun to use these other mounting methods and I get to play with kevlar and wire fishing leaders.

Still, about half the time, I have the engine mount assembled before I remember than I meant to integrate a cord mount into it...even when the last thing I think before starting assembly is, "Don't forget the cord mount, this time."

[MKP]

My problem with the tri-fold isn't that it fails, its that it creates a obstruction in the path of the parachute. There's enough problems with a getting a parachute to deploy properly, I don't need to add another.

Like Micromeister I us stainless steel fishing leads and beading wire mounted to the top centering ring.

[dbryantphoto]

Hey Glenn, I love and appreciate the pics but is there any way you can make them larger? the full size pics aren't much bigger than the thumbnails. This is exactly what i'd like to do with my next build.



Heres a description of attached Photos. 1) regular "cloned BB 0." 3) "Goony" Bluebird. 4) "Baffle system," w/eye bolt attachment; 5) Baffle, main body, & blast plate. Note, the kevlar passes right on thru it, but plugged and glued; 6) whole rocket in separate pieces attached w/kevlar shock cord; 7/8) side by side BB and BB0.
Forgot to take pictures of "center plate" jig. Will do that next. Glenn NAR 89197 ;)