Elastic shock cord?

[tfischer]

Hi all, potentially dumb question alert:

I have a number of old (circa 20-year-old) rockets from my youth that are in great flyable condition except for the britle old shock cord.

I was with my wife at JoAnn Etc. the other night (craft/fabric store) and they had bulk elastic in 1/8" and 1/4" (and greater) widths, for very cheap (something like 28 cents a yard). A few yards could do quite a few rockets for next to no money.

The only difference is that instead of just being the "elastic rubber band" type stuff that Estes has always shipped, this stuff has some fabric/thread component to it. It seems like this should still work very well as a shock cord, but I thought I'd ask the experts before trying it myself and perhaps ruining an old classic rocket.

Another question: I know most of you here seem to swear by using a kevlar thread attached to the engine thrust ring or some such, then hooked to the shock cord. I also know a lot of people talk down about the standard Estes Tri-fold-paper cord mount. What exactly is the reason for this? In all my launches, I think I've had exactly one of these mounts fail, and that wasn't during a flight, but when it was sitting in the box for a number of years (and was probably due to impropper installation. Am I lucky, or what exactly is the problem with this setup?

Thanks bunches in advance for the answer to these basic questions.

-Tim

[CPMcGraw]

I buy a similar product from Wally World, and probably have about 30+ packs of 1/16" and 1/8" round stock. Maybe 6-8 packs of the 1/4" wide stock.

To get the most from these products, you still need a means of attaching it to the rocket in a way that is repairable. It will still go bad, through hot particles hitting it, and through other rough handling. You have to be able to change it out. Using a Kevlar thread is one way, as this usually allows you to tie the elastic to the thread outside of the body. Now, you can cut it away and replace it as needed without having to repair the attachment point inside the tube.

It's hard to replace rubber strips with the tri-fold method, as you have to remove the tri-fold from the tube without delaminating the tube. Granted, it worked and was simple to fabricate, but the Kevlar thread & elastic method is much easier to repair when the time comes.

[CraigF]

One other method is to use stainless steel braided wire (i.e. fishing leaders, for smaller rockets) around the engine tube instead of Kevlar thread. It's a *tiny* bit heavier, but I like that it's a bit stiff because of my next "tip". Which is to leave the loop in the thread/wire, where you attach the elastic, a little bit below the lip of the BT. I have had BTs ripped by the Kevlar cord rubbing against the BT edge...not a huge deal, but nice to avoid by having the elastic rub against the BT edge instead. The wire, instead of Kevlar, is stiffer and thus easier to grab the loop than the floppy Kevlar is when not sticking out the end of the BT. The SS leader will never wear out either, Kevlar can from the heat, if you're lucky enough to have the rocket last that long. Also, you can get 24/36" SS leaders for cheap at Wal-Mart, your local rocket recovery stuff store...

[barone]

As Craig said, that stuff will work fine and it sounds like a great price. I use that material exclusively in my rockets...throw the rubber band away.

Using the Kevlar mounted to the motor mount/centering ring/thrust block also provides a "clean" tube for the recovery system to deploy through. Nothing for it to possibly hang up on. The Kevlar is burn resistant and can be mounted close to the ejection charge, something you can't do with elastic. If you don't want to use Kevlar, you can still use the tri-fold method of attaching the shock cord using the elastic braid material (I've done that also).

One thing nice about the Kevlar, as Craig said, is the ease of replacement of your shock cord system. The Kevlar should outlast your elastic braid. It should also be of a length that doesn't protrude from the body tube. As mentioned by CraigF, it more easily "zippers" body tubes. I actually thread mine out the aft end of the body tube to make my repairs (when building, I tie them together before mounting the motor mount, the Quest way). When it's put back into the body tube, the knot (Kevlar to elastic) is inside the body tube, not protruding out the forward end of the body tube, allowing only the elastic braid material to encounter the body tube's forward end.

Now, if the kevlar is damaged, the repair is a little more difficult. I haven't even attempted a repair for this. I just take a length of Kevlar/elastic braid and loop the Kevlar around the motor, taping it in place (normally near the nozzle), thread it into the rocket as I install the motor and attach it to the recovery system. :D

[tfischer]

Appreciate the replys -- thanks!

-Tim

[kurtschachner]

Only one other thing, if you can find the cotton-faced elastic (sometimes called swim suit elastic) itwill hold up better than the polyester stuff. The cotton type is often brownish or tan in color whereas the polyester is white.

I'll tell you though, I have given up on elastic and use Kevlar almost all the time now. But I do still use the tri-fold mounts, I just like them better.

[Royatl]

Only one other thing, if you can find the cotton-faced elastic (sometimes called swim suit elastic) itwill hold up better than the polyester stuff. The cotton type is often brownish or tan in color whereas the polyester is white.

I'll tell you though, I have given up on elastic and use Kevlar almost all the time now. But I do still use the tri-fold mounts, I just like them better.


Thanks for the tip about "swim suit elastic". Almost all the elastic I got in the 70's was cotton-faced. but in the 90's couldn't find a bit of it. When Estes started using elastic it was the polyester stuff, with expected results (i.e. it was worse than the old and new-again rubber).

[CPMcGraw]

...As mentioned by CraigF, it (long Kevlar threads reaching outside the body tube) more easily "zippers"...

This is why, when we have our discussions about new designs, I always talk about the Dv that RockSim gives on different motors. If you reduce the Dv, or "Deployment Velocity", to something under 20 FPS, your zippers will be greatly reduced. Zippers are the result of "explosive" recovery system action, when the opening of the parachute 'yanks back' so hard that the Kevlar is dragged through the tube. If you can hear the parachute opening with a loud gunshot-like report, then your Dv is higher than it should be. Below 20 FPS, the parachutes don't react as violently. Below 10 FPS, the opening is vigorous but not damaging. Below 5 FPS and it's gentle and quiet, just barely 'ticking over' at the top.

Recovery failures can be minimized with the proper selection of the motor and delay. It's not always the design of the harness (Kevlar, steel leader line, or rubber strip) that makes the difference; it's how the system is used (or abused).

[CraigF]

I try to use RMS when possible and it makes sense...if you know about the regularity of their delays, you'll know why I went to SS shock cord anchor, and kept it below the BT lip. :) They can also be a bit explosive until you learn to judge the amount of powder. I also use a Kevlar safety line on my shock cords JIC. You get into these (over-)building habits typically by bad experiences...sometimes you're just happy with ANY deployment.

[PaulK]

...The only difference is that instead of just being the "elastic rubber band" type stuff that Estes has always shipped, this stuff has some fabric/thread component to it. It seems like this should still work very well as a shock cord, but I thought I'd ask the experts before trying it myself and perhaps ruining an old classic rocket....I regularly use the fabric elastic stuff from craft stores on low power birds. The upside is it has a bit of redundancy in the multiple rubber bands wrapped with thread. The downside is the fabric burns easier than pure elastic. Not sure which really has a longer life. Kevlar is nice, but it has failure modes as well. The mounting point needs to be well out of the direct path of the ejection charge, or it will burn through in a few flights. (not the forward motor block for example). This makes it tough to use in minimum diameter models, unless there is a coupler upstream. The tri-fold is less than ideal, but I've never had one fail in-flight. On older models, the glue is crusty enough to easily remove & replace with a new tri-fold.