Estes three fold mount and Kevlar shock cord

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

DankMemes

Well-Known Member
TRF Supporter
Joined
May 14, 2018
Messages
361
Reaction score
77
So I have this red flare e2x kit from Estes, decent looking kit for E size motors, planning on this being a beater rocket for the park launches with the kids. However I can’t believe the shock cord they put on the bag. It’s one of these Estes rubber band cords, I can’t tell you how many I’ve had of those get brittle and snap, after digging through my supply box I found 10 foot of Kevlar cord I had slater for another build that’s toward the back of my build pile. So I’m opting to use this one instead. Not having planned on this originally, like I should have...

im looking at an assembled rocket and wondering if the standard three fold mount will work with the Kevlar cord, my concern is due to its smaller diameter and relatively smooth surface will the Kevlar cord simply pull out of the mount under tension... usually I tie to a screw eye but I’m not planning on adding a baffle to this build

anyone tried this before?
 
Tie a knot in the end that you put inside the 3fold mount. You may want to consider making the mount out of heavier material (card stock, file folder, or cereal box) to keep it from just ripping through.
I have one that I made with card stock in my BT70 Goblin , but it has not flown yet (due to being misplaced).
 
How about using the shock cord and a kevlar cord? Make the kevlar cord longer... that way if the shock cord does fail, the kevlar is there as a back up.

I've been contemplating this for my scratch built X-Wing Alpha.

Kind of the belt AND suspenders approach.

Thoughts? :handshake:
 
I use the tri-fold shock cord mount with Kevlar in pretty much all my LPR builds anymore. It gets the end of the Kevlar off the motor mount (where it typically burns thorugh after 25 flights or so) and seems to hold just fine. I've not had a failure.

I most commonly attach a foot or foot and a half of Kevlar via a tri-fold mount and then tie the supplied rubber shock cord between it and the nose cone. It has been working quite well for me. In a small-diameter model (I did a Viking, which is BT-20-based, the other day) this also works better for getting the recovery system out as the tri-fold is much less of a bump inside the body.
 
I always replace Estes tri fold rubber cords with Kevlar attached to the engine mount for small kit builds.

I’ve retrofitted using thin Kevlar and a paper glued mount when rebuilding or repairing. Never had a failure. PVA combined with paper and cardboard can be remarkably strong. If you’re really concerned you can always run the cord through a slit in the body, or add a length of elastic at the nose to absorb any heavy shock for a little extra insurance.
 
I presume you all are wrapping the Kevlar with something at the point where it exits the body tube, to avoid zippering?

In the future, I think I am going to move to something where the Kevlar is not attached to the motor mount, for the same reasons as described above. Although I haven't had one burn through yet (I don't fly that much), I'd rather not worry about it. Replaceable Kevlar mount strategy is often not practical in my designs. I admit I hadn't though of just putting the Kevlar into the tri-fold mount. That's a simple solution, although I'd prefer not to have the Kevlar exit the body tube. A "perfect" solution has eluded me so far.
 
Zippering is avoidable with correct delay time selection (minimizing the velocity at ejection), long shock cords and stiffening of the tube edge with a little CA glue. 3x the length of the rocket is a good rule of thumb. This is all I do and I’ve had only one zipper due to an early ejection.

If you manage to fly a low or mid power rocket enough to burn through the kevlar without some other tragedy befalling it first, it deserves to be retired IMO but I have a few models where I’ve designed the mount to be serviceable without cutting the fin can up (an eye hook on the engine mount for example). I’ve also braided/twinned multiple smaller strands so that one giving way won’t result in a loss of the rocket.
 
I presume you all are wrapping the Kevlar with something at the point where it exits the body tube, to avoid zippering?

In the future, I think I am going to move to something where the Kevlar is not attached to the motor mount, for the same reasons as described above. Although I haven't had one burn through yet (I don't fly that much), I'd rather not worry about it. Replaceable Kevlar mount strategy is often not practical in my designs. I admit I hadn't though of just putting the Kevlar into the tri-fold mount. That's a simple solution, although I'd prefer not to have the Kevlar exit the body tube. A "perfect" solution has eluded me so far.

Chris Michielssen has written about replaceable kevlar shock cords here

https://www.apogeerockets.com/education/downloads/Newsletter338.pdf

He has also written about this technique several times on his blog

I've done this as a retrofit once -- on a BT55 rocket with a 24mm motor mount, plywood centering rings and through-wall fins. I had a long-enough drill-bit to make more-or-less aligned holes in both centering rings. I didn't try to insert the plastic tube, I just fished a piece of aluminum wire up thought the holes and out the top of the body tube, then taped the kevlar thread to the end of the wire to pull it though.

As to the OPs question, I have also replaced a failed elastic shock cord on a BT60 model by tying a loop in a piece of kevlar kite string and using epoxy to adhere the looped end directly to the inside of the body tube. To install, I laid the rocket on its side with the fins hanging over the end of my bench. I put a piece of paper (not folded over, just a strip of printer paper) over the epoxy and kevlar, then put a cold chisel in the body tube, on top of the paper, to hold the kevlar down while the epoxy hardened. It has last four launches on Estes C and D motors since I made the repair.
 
I have used Kevlar and a tri-fold, but as T-Rex suggested I also use a knot in the Kevlar.

For even more strength you can take a piece of cardstock about 3 or 4 inches long and an inch or two wide. I try to not go more than a third of the way around the inside of the tube as it gets tough to get it to lay flat against the tube if it is too wide. Cut a slit every half or 3/4 inch inch the long way and thread the Kevlar through the slits. An even number of slits works best as you can have both the start and end of the Kevlar on the outside. Again, use a knot at the bottom. Use glue or epoxy and glue this into the rocket like you would a tri-fold. It will be stronger than the tri-fold and have less of a bump. This method works best on larger rockets as you need to get adhesive all along the paper. Use a dowel to get the cardstock to lay flat.
 
neil_q - I haven't had any zippering issues generally. As jnobels noted, good delay choices help. But even when the selection is off I've not had a problem with LPR size/weight models.

jnobels - It was a real surprise to me the first time I had a Kevlar shock cord that had been tied to the motor mount burn through (I was new to the whole idea then). It was a BMS School Rocket somewhere around flight 25. I have a Nova Payloader that was retired after flight 75 because the body above the motor mount was getting soft from ejection charges. The few flights it got on a Aerotech D10 didn't help that. I have an Alpha that's up to 63 now. It got the kevlar-in-tri-fold treatment when the original rubber failed many flights ago. The hardest part of that was cleaning the ejection charge residue off the inside of the tube well enough to glue the new mount in and have it stay in. I've yet to have a failure in a model that had the Kevlar-in-trifold at the outset, but I don't think I have a such model that's passed two dozen flights yet. I have an Alpha III that's close and was bummed that I couldn't get a Q-Jet into it (that's for a different thread)....

Zeus-cat - interesting variation on the shock cord mount. I'll have to try that.
 
What a coincidence, this exact question was addressed on Chris' blog a few days ago:

https://modelrocketbuilding.blogspot.com/2018/05/apogee-payload-altitude-rocket-build_71.html

A Kevlar shock cord was included in a kit by Apogee for payload altitude competition. For lightweight rockets, using Kevlar instead of rubber or elastic should be fine. For example, you may have noticed that Micro Maxx rockets all use a Kevlar thread for the shock cord.For bigger rockets, the shock absorbency ( or lack thereof) of Kevlar comes into play. Some good hints on the blog: Glue on a diagonal to keep the trifold mount flatter, curve it around something round while the glue sets up, etc. I would add swiping a thin film of white or yellow glue over the tri-fold mount after installing to protect it from ejection gases.
As mentioned, there are many strategies to prevent zippering when using Kevlar with a tri-fold mount: masking tape around the cord where it exits the body tube, a swipe of CA inside the end of the body tube, small foam or rubber ball to keep it away from contact with the tube, etc.
As far as Chris' replaceable Kevlar method, you would have needed to incorporate this during your build. Since your Red Flare is already assembled, you can keep this in mind for future builds.
BTW the Red Flare is a great rocket: easy to assemble, snazzy looking, good size but economical, and a great flyer on 24mm engines. Have fun!
 
I conçur with all of Chris' suggestions in that post.

In the few cases where I've gotten the very start of a zipper, I put a bit of masking tape on the Kevlar to help spread the load.

I've routinely hardened the wear ends of tubes with thin CA pretty much since I got back into building rockets 9 years ago. I also do the bottom of motor tubes and, when there is a payload section, where the nose cone goes in and out of the payload section.
 
So what type of paper should be used to make the mount, anything special for such a "critical failure path" component?
 
I just use regular printer paper...I've even been known to use the pattern in the kit's instructions. Now that Estes kits come with instructions in six languages, with three languages per set of instructions, there's a spare right there in the bag. :) Just put a coat of white/yellow glue over the installed mount after it's dried to both encourage the corners to stay stuck down and give it a little protection

I actually think the idea of using cardstock to make a tri-fold suggested above is not such a good idea...it will be harder to get it to conform tightly to both the Kevlar and the inside of the body tube if it's made of cardstock. But I've not done any head-to-head tests to validate this thought....
 
Thanks - Great to know some are having good results with methods presented in the blog.

Laying the shock cord on the mount at an angle was actually the way Estes presented it in their kits in the late 1960s.

In my experience I've had Kevlar burn through near the engine mount at about 15 flights.
That's why I came up with the replaceable Kevlar engine mount.
I do pull out and check the Kevlar after about 10 launches. I keep track of all launches in the "My Rockets" page at rocketreviews.com

I don't know what to expect with Kevlar in a tri-fold paper mount. I'm sure it'll be fine in the Apogee Payload model.
Usually I go with 65 lb. card stock when making a shock cord mount. In the 1970s Estes included a tri-fold mount printed on card stock in their kits.
As I remember the kit tri-fold mount was thinner than 110 pound card stock.
I'm sure the copy paper mount is fine, I feel a little better with it slightly heavier.

What gets me is how many BT-5 based models had bulky tri-fold mounts!
 
So what type of paper should be used to make the mount, anything special for such a "critical failure path" component?

If the diameter of the tube allows for it, I like to use cardstock, only because I've had these mounts start to shred on me when I use regular typing paper and hold down the mount to the tube for a couple of minutes. The glue on my hands lifts the paper being compressed. And I go Grrrrr. For smaller rockets, say BT-50 size and below, I'll do regular paper but go sparingly on the glue, since I'll do a top coat of glue after initial installation to lock everything down. Of course, size the mount for the tube. If you've seen the shock cord mounts included with the PS II Estes kits, you'll know what I mean.
 
Thanks - Great to know some are having good results with methods presented in the blog.

Laying the shock cord on the mount at an angle was actually the way Estes presented it in their kits in the late 1960s.

In my experience I've had Kevlar burn through near the engine mount at about 15 flights.
That's why I came up with the replaceable Kevlar engine mount.
I do pull out and check the Kevlar after about 10 launches. I keep track of all launches in the "My Rockets" page at rocketreviews.com

I don't know what to expect with Kevlar in a tri-fold paper mount. I'm sure it'll be fine in the Apogee Payload model.
Usually I go with 65 lb. card stock when making a shock cord mount. In the 1970s Estes included a tri-fold mount printed on card stock in their kits.
As I remember the kit tri-fold mount was thinner than 110 pound card stock.
I'm sure the copy paper mount is fine, I feel a little better with it slightly heavier.

What gets me is how many BT-5 based models had bulky tri-fold mounts!
That's true, those preprinted cardstock mounts weren't 110 pound stuff for sure.

A tri-fold with 1/8 inch flat rubber in a BT-5 model is - well - certainly not the best way to do it....
 
That's true, those preprinted cardstock mounts weren't 110 pound stuff for sure.

A tri-fold with 1/8 inch flat rubber in a BT-5 model is - well - certainly not the best way to do it....

For the mini sized rockets I think the Stine shock cord mount and round tubular elastic (or Kevlar) is the best solution. If so, tying a knot at the end of the cord would be prudent.
 
Well, if the master says it doesn't work, then I would follow his advice. Chris probably has more rocket building knowledge in his pinkie finger than all of us.
 
Thanks for the tips everyone, swiped a file folder from the office and cut it up for the shock cord mount, tied a loop into the ended of the Kevlar and glued it into the card mount and smashed it as best I could to get it to lay down, after the glue set I was actually surprised how well it layed down flat, the mounted it into the body tube. Dried the glue and layed down a top coat of Pva.

i think it’ll be a strong mount.
 
I have not switched any small models to kevlar, yet, but I intend to do it. (I have had suggestions to use thin elastic and from experience I've found that it works well for slightly larger rockets and seems to last longer than the straight rubber.
It seems that kevlar needs to be anchored better than rubber because it can have a harder pull than rubber as the cord becomes tight. I used to tie a knot in my rubber cords and I think that is a good idea. I think the problems with the tri-fold is that it can be pretty thick, if you make it out of thin paper it could tear easily. Maybe the trick is to use just one or two layers of the card stock but tie a big knot in the kevlar and wrap it around the card stock a few times so it is well anchored.
(In the old days I never liked the tri-fold because of how thick it was. I would tie a knot in the rubber cord and either glue on a single piece of card stock over the cord or take a square piece of card stock, punch a hole in it to feed the rubber through, then fold the card stock in half and glue it in.)
 
Back
Top