estes shock cord replacement

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watermelonman

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Is there anything to keep in mind or watch out for when replacing Estes shock cords and mounts? I am inclined to do the paper three fold with some 300lb kevlar line, or some rubber bands, but wanted to check if there was anything special about the paper or cord material they supply.
 
The following answers are based on the assumption that you are working on typical lightweight, low-power Estes kits.

1. The rubber shock cords they supply are too short. Whether you replace the cord with elastic, or kevlar, or a combination of both, make the replacement 3 times longer than the original one (that is, as long as you have enough space in the airframe)

2. A lot of people will automatically tell you to ditch the three-fold mount. Many old-timers will attest to this: A properly installed three-fold mount will outlast the cord itself - by DECADES. I have models I built in the 70s with the mounts still securely in place. The rubber shock cord dry-rotted away years ago, but the mount is still as secure as the day it was installed.

The mistake that too many people do with these "tea bag" mounts is that they do not properly "mold' them into the shape of the tube - leaving edges that are not glued down, causing obstructions. Now, the card stock Estes supplies makes it more difficult for the anchor to be properly shaped so that this snuggles down into the tube contour. My material of choice has become plain old brown paper bags. Once the three folds have been glued together, and the glue has set up some (I use Titebond II), I take the still pliable mount and press it into the tube, moving it side-to-side and in and out until it matches the shape of the tube interior. Then while it is still pliable, I glue with in place, making sure all edges are burnished down against the tube interior.
 
Yes, definitely low power lightweight rockets here.

I will use brown bag paper for these replacements. Thanks! Is there anything special about the elastic they supply, or would you expect kevlar or rubber band to work as well as the commercial part?
 
What *I* would use in this situation would be elastic. Go somewhere that sells cloth and other supplies for sewing. Get some 1/8" wide elastic band. It looks like this:

3294824.jpg
 
I personally much prefer sewing elastic to Kevlar thread with the tri-fold mounts. Thin Kevlar has a tendency to zipper, and I suspect would be more likely to pull out of the mount if not glued in perfectly.

I do like thin Kevlar when using the "shock cord attached to motor mount" approach for its better durability in heat, but in those cases I'll tie a loop in the Kevlar and add sewing elastic for the part extending past the body tube. I do like this setup more than the "teabag" for narrow body tubes (BT-50 or less) where chute space is at a premium.

That said I've never had a tri-fold mount fail at the mount, even under stress. The shock cord (rubber band or elastic) has always failed first.
 
I do like thin Kevlar when using the "shock cord attached to motor mount" approach for its better durability in heat, but in those cases I'll tie a loop in the Kevlar and add sewing elastic for the part extending past the body tube. I do like this setup more than the "teabag" for narrow body tubes (BT-50 or less) where chute space is at a premium.

Excellent point Oberon! Kevlar is actually a better choice for smaller tubes. I still do elastic in BT-50s, but kevlar in anything with a smaller diameter. When I use kevlar, I attach to the motor mount or thrust ring.
 
Is there anything to keep in mind or watch out for when replacing Estes shock cords and mounts? I am inclined to do the paper three fold with some 300lb kevlar line, or some rubber bands, but wanted to check if there was anything special about the paper or cord material they supply.

I personally can't stand the Estes Teabag shock-cord mount as it can cause recovery system deployment problems particularly on smaller body models.
I perfer motor mount wrapped Kevlar/Elastic or Stainless Leader/Kevlar/Elastic permanent and replacement Shock cord mounts. Retro-fitting This type mount only requires the addition of an airframe size centering ring and a little epoxy.

All that said: If you feel you must use the Estes Teabag shock-cord mount there are a couple really important things to consider:
* Make sure the widest part of the teabag mouint is facing the inside of the body tube placing all the folds and snag corners Under this solid wide paper tab.
* Use maximum of 100lb Kevlar for shock-cords. With the TeaBag the thinner the better!, I've found 70-100lb kevlar is plenty for LPR models up to 2.5lbs. I nearly always use 70lb in BT-5, 20 & 50 models.
* When smoothing out the Teabag bump use white glue or carpenters glue to smooth and ease any paper corners that protude toward the aft end of the model.
* If using only Kevlar be sure the shock-line is long enough for the model to slow down before the nosecone hits the end of the shock-line bouncing back creating the dreaded Estes Dent. For most LPR's 36 -48" will work just fine. Some heavier, and longer models might need 48" to 60" Kevlar only lines.
* If using a combination Kevlar/Elastic shock line I've found 30" of 70-100lb Kevlar with 30" of 1/8" Oval elastic makes a great shock-cord that eliminates the dreaded "Estes Dent" Where the NC recoils back into the bodytube at ejection. Usually caused by Far To Short a shock-cord.
Hope these suggestions and comments help.
 
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I use elastic shock cord for nearly everything.

However, I like to use either wire cable or Kevlar to attach deep inside the body tube, come out to the front end, and THEN attach the elastic shock cord. Here's the wire cable shock cord mount for my recent 1/45 Little Joe-II build:

5v6xVXn.jpg


I'll be attaching 1/4" elastic to it, many feet of it.

One problem I have with the old-style methods of gluing a mount to the inside of the tube, is that it is a jamming restriction. Chutes sometimes get stuck if they are not packed tight enough to slide freely enough. I'd rather keep the tube smooth inside.

The other problem is when the old cords get worn our, scorched, brittle, etc. It's a huge hassle to try to remove and replace a mount glued to the side of the tube. I have not even tried to replace one. I gave up on those methods before my 10th rocket, after my most successful early reliable rocket (MPC Viper), broke its shock cord on its 10th flight. Just before it flew, I saw how badly worn the cord was, but being glued 2" down inside of the tube, there was no way for me to fix or replace it. So I flew and hoped it would hold.... it didn't. I hated those mounts ever since and never used them again.

That is the benefit of the wire cable, or Kevlar, being mounted in deep then attaching Elastic to it, it is very easy to replace the elastic. The wire cable never wears out. Kevlar.... could, it depends on the size, hot it is attached, and how exposed it it to ejection heat. For example D12 ejection charges will fry 100 pound Kevlar, but I've not had an 18mm motor burn up 100 pound Kevlar (yet). Still I try to minimize heat damage to Kevlar, if I attach it to a front centering rig, I will add 2" or more of heat shrink tubing over it where it is most exposed to the ejection charge as heat protection.
 
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I have mostly replaced the Estes rubbers with good braided elastic, but one time I ran out of elastic so I went back to my big bag of let over rubbers and carefully tied two of the thicker ones together. When the Pad Fuhrer saw the deployment of the tied shock cords he just shook his head. So I kept flying it, again and again. Eventually he commented that I sure know how to tie crappy Estes rubber shock cords together. I was so happy, he actually said something almost nice about my rockets and or techniques! In most cases using a thin paper trifold with a good soaking of tacky glue can give a very sturdy, thin and durable mount for your tied and extended stock rubber shock cords. I now use some of the rubbers and I find it is not as sinful as I was taught. You must check the quality of the rubbers, I have found some that had thin/weak spots and they go into the trash. Don't use them in designs where they can get burned, like a stubby rocket. Sometimes the rubbers even wear better than the elastic, a bit more heat resistant. You must keep them maintained. You must try and fly your rockets in a decent manner for good deployment.

Enough nice talk about rubbers. Motor mounted steel cords and kevlar rule. They are cool, trifold rubbers are not. Super long, non elastic shock cords are cool, makes you look like you are flying high power style. Steel - Kevlar show you have some class and are not a cheapskate. Bulletproofing and anti-zippering techniques rule.

Can a well installed trifold mount fail - yes, I finally did it, the outer layer of paper on the inside of the tube failed before the elastic cord broke. Is kevlar tied to the motor mount the godsend many think it is? No. heat will degrade it and it will break, much to the horror of the kevlar faithful. But I still love those mounts. A few wraps of electric tape can insulate the kevlar or elastic if you can get by with the reduction of flexibility at the mount. Good if you have limited recovery room and are desperate, a condition I have often found myself in when dealing with questionable designs.
 
Lots of great advice here.
One thing Estes doesn't bring up anymore is how to set the elastic onto a tri-fold mount at an angle.
This keeps the finished mount flatter and less likely to block the ejection, especially in a smaller diameter tube.

Shock Mount 1_WEB.jpg

When folded and glued it ends up like this:

Shock Mount 3_WEB.jpg

I rarely use a tri-fold mount. When I do I use this "flatter" glue method:
https://modelrocketbuilding.blogspot.com/2010/10/estes-shock-cord-mounts-tip.html

I find the mounts printed on instruction sheets to be too thin.
I use 110 lb. card stock for larger models, 65 lb. for smaller rockets.
 
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