Shock cord material

[Lugnut]

I'm restoring some old rockets that have original shock cords. What do you guys recommend for shock cord replacement material for small stuff (C size and less)? Craft store eleastic, or some other cheapo replacement material?

Mark

 

[Stymye]

one option,

call estes, they are sending out the new rubber cord to anyone who wants to replace their elastic,, the are even offering quantitys to clubs and groups for evaluation... they are better and much more heat resistant than the elastic

 

[powderburner]

Elastic may be more vulnerable to heat than the rubber, but I will still go with elastic.
Rubber has its own problems. When it gets a little older it begins to add the risk of snapping completely through when stretched. At least elastic offers some chance of detecting damage when it is over-worked, before your BT heads for China and the 'chute and NC drift off to neverland.
You shouldn't be roasting your shock cord anyway. It is easy enough to avoid by using a tether for the first part of the anchor line. Some of these TRF guys swear by kevlar; I have used it and it is certainly OK. I have also used woven nylon cord with very good results. Anyway, run this first part of the tether out the front of the rocket and tie your shock absorber on the far end. It can ride out the front of the rocket with the parachute or streamer, separated from hot ejection gas by the wadding.

 

[JStarStar]

I make sure I stuff the wadding ball down deeper into the tube than the SC mount, so the cord doesn't take a straight blast of hot ejection gas. I also dust the interior of the tube, the cord mount, etc., pretty liberally with baby powder before launch.

I don't know if either of these precautions is the magic formula, but I don't recall having an elastic cord burn or snap lately. Anyway, seems to work for me.

 

[rstaff3]

For modrocs I like that method. A leader of Kevlar twine and the stock elastic at the NC end. For retrofits I often just use a long length of the Kevlar, 3 or more x the length of the rocket. On a new rocket I attach the twine through the motor mount. A method I like on retrofits is to punch 2 small holes in the airframe next to the launch lug. The thread is fed through these and is held on with 5-min epoxy.

 

[sandman]

As of this post I have had more Kevlar lines burn, break, fail than I have had elastic fail.

Actually...in 40 years of flying model rockets I have NEVER had and elastic shock cord break or burn through. Had a few wear out! but I catch that in the "pre-flight". With a tri-fold mount they are easy to replace...even in the field.

I have had a few "Estes Dents" even with a 3 foot shock cord!

Sorry all you high tech purists...I am dropping the Kevlar. Not what it's cracked up to be.

Me and Jim Flis! Tri-fold paper mounts from now on!

My $0.02

 

[rstaff3]

I have had an elastic cord or two fail on me. I have never had a Kevlar leader burn through, but have had them come loose from the attachment point.

I have had many more dents on short cords than long ones, but rebounds are not the only cause. If the rocket is moving too fast at ejection the body can still fly into the cone.

On the other hand I have 3 mid power and one high-power rockets that use elastic along with a leader and these show no wear or burning.

Still, I am not giving up on the Kevlar leader. Blame it on the user if you will, but I have less problems with it.

If its in good shape, I still fly old estes rubber too

 

[bobkrech]

Lugnut

I replace the short stock shock cords with polyester elastic from Jo Ann's fabrics. Comes in a variety of sizes starting at 1/8" and going all the way to 1" width.

It cheap, and readily available. It's about 70% fabric and 30% elastic so if the elastic breaks, you still should't separate.

Just make the shock cord longer than stock, 3 to 4 body lengths will eliminate the "ESTES" dent.

Bob Krech

 

[JStarStar]

I've come up with a new method (well, not really new, I'm sure) of shock cord anchoring.

Like, I am sure, most everybody else, I have a number of scrap ends of body tubes sitting around the workshop. I was actually cleaning the place up and throwing some of these scraps out when it occurred to me, "Hey, I could make some use out of these."

So, to make a shock cord anchor, I cut about a 1" long segment of scrap body tube. Looks like a small stage coupler.

Use a smaller tube size than the tube you're installing the anchor into - use a BT-20 tube scrap if you're installing the mount into a BT-50 rocket, use a BT-50 or -55 scrap for a BT-60 bird, etc etc.

Cut the tube scrap in half vertically. This will leave you with two C-shaped pieces.

Take your long elastic shock cord and tie a flat loop around one of the pieces. Apply a healthy dose of glue to the inside of the other piece, then use plastic mini-clamps to clamp the two together. Let dry 15-20 minutes until tacky.

Hand-bend the anchor so it matches the interior tube curvature of the rocket. Apply a good dose of glue to the backside and glue an inch or so down inside the tube. When it's somewhat dry, smear a glue layer over the edges of the mount and you're set.

This anchor method gives you a flatter, thinner, neater anchor on the inside of your body tube than the trifold method, which helps allow your laundry to slide out easily come ejection time.

The body tube scraps are also stronger than most cardstock or paper used for the trifolds.

Not exactly rocket science (hehe)... but it works for me. On rockets with stuffer tubes and large centering-ring motor mounts, I'll cut a slot through the top ring and epoxy the cord onto the other side, that seems to nail it down pretty well. But on most small-caliber birds, the tube-scrap anchors work OK for me.



I'm not sold on the kevlar anchors - first of all, kevlar itself is kinda expensive - and I've seen tube zippers using thread/string - I'm more confident with a 1/4" or so wide shock cord.

I agree with bobkretch, and I am sure multitudes of others - I just replace stock shock cords in kits with elastic from JoAnn's fabrics. I usually use 1/4" elastic, about 3 ' or more. The heck with these 6" cords.

 

[Kitch]

Although I now have some Kevlar string, I've been considering a compromise method on my next build after seeing other threads on this issue.

Since I've always been ok with the classic tri-fold method, I figure I could use a length of kevlar loop in a trifold mount so it extends to near the end of the bt then I'll tie the elastic at that point. That way I can easily replace the elastic as needed while the mount with the kevlar should last the life of the rocket.

I also like JStarStar's suggestion and may give it a try.

I've able to find the elastic (in the common sizes) in Wal-Mart's craft section.


-Kitch

"One, Two, Five!!"

 

[RevKeith]

My method for BT55 and larger tubes is to take Fix-It epoxy clay sold through Apogee Components and use it like a paper mount and embed the kevlar string in the Fix-It. It will smooth out real nice so no hang ups with chutes or shock cords. Also adds a little weight on the nose cone end of your rocket.

Apogee also sells 300 and 500 lbs kevlar cord. Very heavy duty and works well with larger modrocks.

 

[wwattles]

I know this kind of goes against the grain of this particular thread, but I LIKE kevlar, and have never had one fail. I also have discovered that Mile High Rockets has Kevlar Elastic which is absolutely wonderful for times when you need the strength/heat resistance of kevlar, but some elasticity as well. I used it in combo with regular kevlar for my 4xE9 FrankenPhoenix, and it did quite nicely with the 1.4 pound nose cone.

Wonderful stuff!

WW

 

[William]

I,M glad this subjecked was brought up, I know i am going with a longer shock cored, but i to am not shure of the type. i am going to check michelles, franks, for elastic, and also going to check wal-mart, for kevlar fishing line. has any one used fishing line??

 

[GL-P]

I've used steel fishing line for pull pins on screamers.

 

[North Star]

I once made the mistake of using thin Kevlar line as an anchor and nylon as the shock cord. The Kevlar cut the nylon like a knife

I still like Kevlar but I use the braided sort now (mostly Pratt's) and I attach it by the LOC 'epoxy on the knot' system in the same place as the tri fold would be. Not had a problem since.

 

[Kitch]

Thanks North Star! I hadn't fully considered that possibilty. I'll try another attachment method.

-Kitch

"One, Two, Five!!"

 

[Stones]

I've had great success with elastic epoxied directly to the BT, just below where the NC shoulder rests in the rocket.
Pic here.

In fact, this rocket, which weighs about 1 pound loaded, is using this method with 1/4" elastic.

 

[teflonrocketry1]

I have been thinking about using fiberglass rope for a shock cord like the stuff McMaster-Carr (https://www.mcmaster.com) sells:
Catalog number 8818K42 Braided Fiberglass Rope Seal 1/8" Diameter. Looks like it might work and its considerably cheaper than Kevlar. If the fliber glass rope is too friable I could always ewncase it in PTFE sleeving. McMaster also sells a fiberglass sleeving that could be used to protect Nylon or Kevlar shock cords.

Bruce S. Levison, NAR #69055

 

[Lugnut]

For shock cord mounts, I'm going to make reproductions of the old Centuri SC mounts, which can be seen here:

https://www.dars.org/jimz/centuri/kf-4c.tif

I'm taking file folders from Office Max (which have a thin paper-stock thickness), gluing aluminum foil as a backing (facing the inside of the tube) and cutting them the same shape as the original, and drilling holes into them, using an original as a template. I'll make a batch of them as replacements for the originals, which have dried up over the years. Ought to kill an evening to make a dozen or so.

 

[Elapid]

Originally posted by teflonrocketry1
I have been thinking about using fiberglass rope for a shock cord like the stuff McMaster-Carr (https://www.mcmaster.com) sells:
Catalog number 8818K42 Braided Fiberglass Rope Seal 1/8" Diameter. Looks like it might work and its considerably cheaper than Kevlar. If the fliber glass rope is too friable I could always ewncase it in PTFE sleeving. McMaster also sells a fiberglass sleeving that could be used to protect Nylon or Kevlar shock cords.

Bruce S. Levison, NAR #69055

just thinking about it.

glass?
c'mon... there are better possibilities than using woodstove gaskets.