shock cord mount studies

[bjphoenix]

I've been retrofitting some old rockets so I end up studying or reinventing old concepts, trying to do them easier and better. My old rockets have deteriorated rubber shock cords plus I've lost nose cones, I've had nose cones with circular cuts and damaged body tubes. So I'm switching to kevlar. I wanted to try it with tri-fold mounts and found that there wasn't much strength so I did other experiments. All of these tests used Titebond II yellow wood glue.
First the basic Estes paper mount only provides one layer of paper to tear through as shown in the first picture. There is some strength there but not a lot.

I pondered ways to make this stronger and did some tests. I concluded that it needed more layers of material to tear through. To get something done quickly I doubled up the paper and figured out a different way of orienting the folds so that the cord would have to tear through 2 layers, which means 4 layers since I doubled up on the paper.

Later when I had more time I tested more ideas. I thought maybe cloth or fiberglass would be stronger so I tied a loop in the cord and ran the material through the loop per the photo below. I found that the kevlar cord would just zipper through the material. In this situation the cloth and the fiberglass would each zipper with surprisingly little strength.


The folded cloth has more strength because it can bunch up a little bit and the cord has to tear more fibers at one time. I don't know anything about materials that cloth is made out of, there are surely some types of cloth that are stronger than others. This red cloth was kind of a thin flannel material. From this experiment I decided that I needed some way to bunch up the fabric so it would all have to tear at one time. I think this could be done but it really increases the thickness of he mount. However the v-shaped piece of cloth has more strength than the 4-layer tri-fold mount and is easy to make. This all lead me to the last experiment which was to take a piece of heavier kevlar, fan out the threads and glue it on in a U-shaped configuration. This had the most strength, limited by how much contact area I could get between the fibers and glue and the cardboard. But not everybody has pieces of heavy kevlar laying around that they could use.

 

[Lee]

I found fiber washers work

 

[Lee]

Tie the kevlar to the washer then embed in the trifold mount

 

[GlenP]

If you don't mind adding weight, a baffle, or partial baffle, is a good place to attach a Kevlar line. If the tea-bag has come out, then you can install a baffle through the top, just mount it deep enough so you have room for the chute, maybe just a bit of wadding, and the nose cone shoulder.

 

[bjphoenix]

If you don't mind adding weight, a baffle, or partial baffle, is a good place to attach a Kevlar line. If the tea-bag has come out, then you can install a baffle through the top, just mount it deep enough so you have room for the chute, maybe just a bit of wadding, and the nose cone shoulder.

I was retrofitting old rockets, some of which didn't have much room in them and all of them had a tri-fold mount down in there from the rubber days. It is possible to remove the trifold mounts if you are careful. I didn't want to go for the trouble of the baffle even though I like them in my MPR, I was looking for the simplest easiest retrofit that would give good strength.

I had a further idea for the folded red cloth- I thought if I embedded a very small straw under the red cloth then it would be possible to replace the kevlar by threading through a new piece.

 

[waltr]

I have used a short coupler and Kevlar to replace broken cords.
Cut a coupler to about 1 BT diameter, knot the end on the kevlar.
Mix some 30 minute epoxy and sear it way down near the MMT. Lay the knoted end of kevlar just inside the BT then force the coupler in with the kevlar between the BT wal the the coupler. Push all the way in to the epoxy.

 

[lakeroadster]

Replaceable shock chord options. Both are retrofittable and neither restrict the inside of the body tube like tri-fold mounts do.

 

[dhbarr]

I like to punch three holes in a piece of cereal box. I don't fold it, just weave the cord in-and-out.

 

[hermanjc]

I've been retrofitting some old rockets so I end up studying or reinventing old concepts, trying to do them easier and better. My old rockets have deteriorated rubber shock cords plus I've lost nose cones, I've had nose cones with circular cuts and damaged body tubes. So I'm switching to kevlar. I wanted to try it with tri-fold mounts and found that there wasn't much strength so I did other experiments. All of these tests used Titebond II yellow wood glue.
First the basic Estes paper mount only provides one layer of paper to tear through as shown in the first picture. There is some strength there but not a lot.
View attachment 476417
I pondered ways to make this stronger and did some tests. I concluded that it needed more layers of material to tear through. To get something done quickly I doubled up the paper and figured out a different way of orienting the folds so that the cord would have to tear through 2 layers, which means 4 layers since I doubled up on the paper.
View attachment 476418
Later when I had more time I tested more ideas. I thought maybe cloth or fiberglass would be stronger so I tied a loop in the cord and ran the material through the loop per the photo below. I found that the kevlar cord would just zipper through the material. In this situation the cloth and the fiberglass would each zipper with surprisingly little strength.
View attachment 476419
View attachment 476420
The folded cloth has more strength because it can bunch up a little bit and the cord has to tear more fibers at one time. I don't know anything about materials that cloth is made out of, there are surely some types of cloth that are stronger than others. This red cloth was kind of a thin flannel material. From this experiment I decided that I needed some way to bunch up the fabric so it would all have to tear at one time. I think this could be done but it really increases the thickness of he mount. However the v-shaped piece of cloth has more strength than the 4-layer tri-fold mount and is easy to make. This all lead me to the last experiment which was to take a piece of heavier kevlar, fan out the threads and glue it on in a U-shaped configuration. This had the most strength, limited by how much contact area I could get between the fibers and glue and the cardboard. But not everybody has pieces of heavy kevlar laying around that they could use.
View attachment 476421

As an engineer I love a good design of experiments! Good work, and hopefully between this and the suggestions of the community you can get some long lasting repairs in your rockets.

 

[Bravo52]

Great thread. Speaking of thread, in your example of using "heavy kevlar", I think any bulky string would work as long as you could fan the fibers out and attach with glue. The strength will come in the contact area between the glue, fibers and body tube. Using a multi-strand string would provide superior strength in the loop. In fact, based on the forces applied at ejection, a mcuh smaller diameter of kevlar would likely work if you could fan and attach as you show. Nice job!

 

[Bill S]

I like to punch three holes in a piece of cereal box. I don't fold it, just weave the cord in-and-out.

That's what I've been doing on rockets that aren't going to be easy to convert to a removeable kevlar system.

 

[brockrwood]

I like to punch three holes in a piece of cereal box. I don't fold it, just weave the cord in-and-out.

The G. Harry Stine type shock cord mount.

 

[Bill S]

The G. Harry Stine type shock cord mount.

That's what I've been using as of late. I don't care for the trifold mounts. I even use the Stine mount for kevlar (up to #200).

 

[prfesser]

The folded cloth has more strength because it can bunch up a little bit and the cord has to tear more fibers at one time. I don't know anything about materials that cloth is made out of, there are surely some types of cloth that are stronger than others. This red cloth was kind of a thin flannel material. From this experiment I decided that I needed some way to bunch up the fabric so it would all have to tear at one time. I think this could be done but it really increases the thickness of he mount. However the v-shaped piece of cloth has more strength than the 4-layer tri-fold mount and is easy to make. This all lead me to the last experiment which was to take a piece of heavier kevlar, fan out the threads and glue it on in a U-shaped configuration. This had the most strength, limited by how much contact area I could get between the fibers and glue and the cardboard. But not everybody has pieces of heavy kevlar laying around that they could use.
View attachment 476421

Great post, excellent experimentation! The arrangement on the right was the way my L1 from LOC described, about 25 years ago. Short nylon cord with ends epoxied to the inside of the airframe. The cord was attached to that loop. Worked well.

 

[Back_at_it]

Great post. I'll add my experience with retro fitting.

How I go about it all depends on the rockets diameter and length. If the rocket is BT55 or larger and I have enough length in the body tube I'm more than likely going to build a baffle for it. I can then attach the Kevlar to the baffle using a screw eye and that becomes my permeant attachment point for an Elastic shock cord.

If I'm retro fitting shorter rockets I will first try to get the motor mount out of the rocket. A lot of my restorations are rockets that I picked up from old forgotten collections and you would be SHOCKED how poorly some of them are assembled. If you look back and some of my resto posts you can see a few where the motor mount literally pulled out. The reason for removing the motor mount is that it allows me to add the Kevlar around the motor mount or replace the upper centering ring with plywood and attach there.

Other rockets like an Alpha or similar small diameter, short rockets I will remove the Estes tri-fold mount. I'll then build a new tri-fold mount exactly as pictured in the second illustration of the original post. Basically I place a loop in the Kevlar before gluing. Excessive glue is squeezed out and the mount is placed in the body tube as low as I can get it as to not interfere with the parachute. One additional thing I add is a layer of heat shrink tubing over the first couple of inches of the Kevlar. Since these Kevlar needs to be relatively thin in small rockets, there is the possibility that it could burn through. A little shrink tubing gives it extra life as I really don't want to go back in there to replace it again.

Good news is that I have never had to replace a Kevlar leader on any of my builds and I have not had one fail..... Yet..



One more comment. The Kevlar loop glued directly to the inside of the tube also works extremely well. I have done this on a couple of larger rockets. When I do this I make it a complete loop with the knot at the bottom and I use epoxy to attach it to the body tube.

 

[mh9162013]

Here's something that might be of help:

https://www.apogeerockets.com/education/downloads/Newsletter573.pdf

 

[bjphoenix]

Replaceable shock chord options. Both are retrofittable and neither restrict the inside of the body tube like tri-fold mounts do.

View attachment 476713

I actually did this on one of my old rockets with BT55 and cardboard centering rings. I had a small diameter drill bit long enough to go through both rings so I just drilled a hole from the back, taped the cord to a piece of wire and fed it through there. I pulled the loop around the end of the motor tube and added a couple of dabs of Titebond II to hold it in place. I don't know how long before the cord burns through right at the end of the motor tube. I suppose a better solution would be to feed some stainless steel fishing leader through there but I didn't have any of that.

 

[BigMacDaddy]

Cool thread

When I saw your first picture I thought you had tied the Kevlar shock chord to a paper clip and after I realized that was not the case I wondered how that would work. Tie line around paperclip and glue / tape the paperclip to side of body tube.

I have also taken to putting 3D printed parachute support rings in a bunch of LPR to hold the parachute forward in the rocket. Those also makes a good object to tie a shock chord to (just glue it to body tube) .I can stuff wadding or dog barf past these supports into the tube.

 

[BEC]

Note that the original tri-fold (which first appeared in the first K-25 Alpha, though later versions of earlier kits also show it) had a different arrangement, which had all three layers of paper over the shock cord. Of course that would also be thicker in the tube…. Here’s what those instructions showed:



I have no idea when the tri-fold was “simplified” to the arrangement we see now — the first diagram in the OP’s first post.

 

[bjphoenix]

Cool thread

When I saw your first picture I thought you had tied the Kevlar shock chord to a paper clip and after I realized that was not the case I wondered how that would work. Tie line around paperclip and glue / tape the paperclip to side of body tube.

I have also taken to putting 3D printed parachute support rings in a bunch of LPR to hold the parachute forward in the rocket. Those also makes a good object to tie a shock chord to (just glue it to body tube) .I can stuff wadding or dog barf past these supports into the tube.

View attachment 522390

What I learned from the experiments is that the shock cord needs to connect to something that will distribute it evenly to a large area of cardboard body tube. That something needs to be relatively solid. When I tried using strips of tri-fold paper, strips of cloth and even strips of fiberglass, the thin strong kevlar could just cut through it. Anything that can transfer the load to the entire area at the same time could work well. Anything that yields a little bit can lead to the tearing.
I tried some later examples that had a small piece of wire in the front of the folded cloth. This worked well because the kevlar couldn't pull through the wire. I have no doubt that some form of paper clip could be made to work but might be harder to get in place. When I used cloth somewhat soaked in Titebond glue it was fairly easy to put in and not overly messy.

 

[brockrwood]

What I learned from the experiments is that the shock cord needs to connect to something that will distribute it evenly to a large area of cardboard body tube. That something needs to be relatively solid. When I tried using strips of tri-fold paper, strips of cloth and even strips of fiberglass, the thin strong kevlar could just cut through it. Anything that can transfer the load to the entire area at the same time could work well. Anything that yields a little bit can lead to the tearing.
I tried some later examples that had a small piece of wire in the front of the folded cloth. This worked well because the kevlar couldn't pull through the wire. I have no doubt that some form of paper clip could be made to work but might be harder to get in place. When I used cloth somewhat soaked in Titebond glue it was fairly easy to put in and not overly messy.

What about a G. Harry Stine type cardstock mount but the holes in the cardstock are across from each other in two rows so that the kevlar laces into the piece of cardstock like shoe laces?

 

[mh9162013]

What about a G. Harry Stine type cardstock mount but the holes in the cardstock are across from each other in two rows so that the kevlar laces into the piece of cardstock like shoe laces?

I just read the part of Stine's book that discusses that. While it doesn't explictly say the Stine "shock lock" can handle Kevlar, it's implied that it can. I conclude this b/c the book implies that the shock lock can work with cotton twine, which has almost no give...at least compared to an elastic shock cord.

The force placed on the Kevlar will be in a largely parallel direction as the shock lock, so cutting through the paper is less likely. Also, it's my understanding that the shock lock should be literally coated/encased in epoxy, wood glue or white glue.

 

[mh9162013]

If you're worried about the Stine shock lock, try the LOC/Precision SCM (it's in Figure 3)

 

[Bluegrass Rocket]

How about using Tyvek for the paper in a tri-fold mount?

 

[mh9162013]

How about using Tyvek for the paper in a tri-fold mount?

The problem with Tyvek is finding a glue that will effectively adhere to it.

 

[brockrwood]

How about using Tyvek for the paper in a tri-fold mount?

Will wood glue stick to Tyvek?

 

[brockrwood]

The problem with Tyvek is finding a glue that will effectively adhere to it.

Too fast for me!

 

[bjphoenix]

Will wood glue stick to Tyvek?

It might stick a little bit because there seem to be paper fibers mixed into some of it, but mostly Tyvek is some form of plastic and wood glue doesn't stick to it very well. I tried some of it with a Tyvek envelope I dug out of the trash. Then I studied what Tyvek was and found out that it is sensitive to heat so it wouldn't be good for inside the airframe.

 

[BigMacDaddy]

What I learned from the experiments is that the shock cord needs to connect to something that will distribute it evenly to a large area of cardboard body tube. That something needs to be relatively solid. When I tried using strips of tri-fold paper, strips of cloth and even strips of fiberglass, the thin strong kevlar could just cut through it. Anything that can transfer the load to the entire area at the same time could work well. Anything that yields a little bit can lead to the tearing.
I tried some later examples that had a small piece of wire in the front of the folded cloth. This worked well because the kevlar couldn't pull through the wire. I have no doubt that some form of paper clip could be made to work but might be harder to get in place. When I used cloth somewhat soaked in Titebond glue it was fairly easy to put in and not overly messy.

I like the idea of a paper clip inside of folded fabric soaked in glue but maybe I just need to 3D print something - small, low profile, good adhesion, etc...

 

[prfesser]

Tyvek is polypropylene (same as large nose cones ) fibers "spunbonded" into a thin sheet. It might be okay with gorilla glue but I wouldn't trust it.

A small piece of kevlar cloth? Ought to resist being cut quite so easily by kevlar shock cord.