Hello all:
I'm currently working on the Estes Vegabond with a working tube length of about 26"
I'm planning to replace the stock shockcord mount with the a Kevlar cord\elastic combo attached at the engine mount-what is the general rule of thumb for length of Kevlar cord? Half the length of the body tube or make the Kevlar cord the only material and skip the elastic cord? Thanks guys! And happy Wednesday! Take care, Jeff

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"That's considerably more than minor, David"

Well, with kevlar there is the chance that the cord will slice the body tube on ejection / recovery. It is tough stuff.

So, I always make the kevlar anchor long enough to just reach the end of the tube. I attach it to the elastic with a barrel swivel.

(I do the tying with the kevlar cord fed out the rear of the rocket, through the motor mount tube.)

Really large models sometimes do away with the elastic and just use a really long piece of tubular nylon cord.

I usually make my kevlar leader about 1" short of the forward tube end, and tie the elastic on by threading it out the motor mount (aft) end as well then sliding it back into the tube.

As long as the kevlar ends short of the forward tube end, you cannot get a "kevlar zipper" upon ejection no matter how high the DV is.

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Thanks for your responses, guys!
That's exactly what I needed the hear. I just can't imagine the stock Estes shock system working well with this size kit so I'm glad I saved the extra Kevlar! Take care, Jeff.

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"That's considerably more than minor, David"

The total length of the kevlar and rubber shoick cord should be around 39 inches (1 1/2 times the bt length)

You'd be surprised at how well the tri-fold works for models that size and larger. I normally do replace the elastic with something a bit longer though to avoid the Estes dent.

kj

The tri folds work very well on practically all size models in the Estes range.

You could make your own for the correct rocket size, use your "enlarge" feature on your copier. You could also "clone" a sheet of shock cord mounts as well.

Thanks guys:
I did already setup the Kevlar cord but was thinking about using thicker material like a index card for a trifold in the future. I like how Quest set up their kits with the tied cord. Hopefully this rocket will be a success and won't find out the hard way! Take care, Jeff

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"That's considerably more than minor, David"

A narrow rectangular strip of copier paper or computer printer paper folded over just once and laminated with white glue makes a very strong and tenacious shock cord anchor for Kevlar, elastic, Kevlar/elastic, or "old school" rubber band shock cords, and it can be scaled up or down to fit anything from BT-5 tubing on up. Here's how:

Cut a strip of the paper about five times as long as it is wide (just wide enough to *almost* span the inside diameter of the body tube), then fold it in the middle of its longer dimension. Using a 1/16" or 1/8" office/craft hole puncher (the tip of a knife can be used instead), punch three holes down the middle of one half of the paper strip so that it looks just like a card stock "Stine Shock Lock." Thread the shock cord of your choice through the holes, apply white glue to one side of that half of the paper strip, fold the other side of the strip down onto the glue, press it together firmly to squeeze out excess glue (this can be done by placing the anchor between two sheets of wax paper if desired), and let it dry. Then spread a thin film of white glue on *both* sides of the anchor and let them dry. Finally:

Spread a generous film of white glue on the chosen bonding side of the anchor and carefully lower it into the front end of the rocket's body tube (slightly curling it lengthwise before applying the glue will provide more "wriggle room" inside the tube, especially in tiny BT-5 body tubes). When it is far enough down inside the tube as you want it to be, press the anchor against the inside wall of the body tube with the side of a round-barreled (not faceted) plastic pen. Press the anchor firmly to squeeze out any excess glue and to make the anchor conform to the curvature of the inside wall of the body tube, then let it dry thoroughly before flying the model. Also:

(NOTES: Even in BT-5 size models, this thin shock cord anchor leaves almost all of the volume inside the tube open for stowage of the shock cord and the recovery system. In larger rockets [bigger than BT-50/ST-10 size], the shock cord anchor need not be wide enough to nearly span the inside diameter of the body tube--a width of ~0.8" [~20 mm] is sufficient. Since the shock cord anchor's glue joint inside the body tube is a variation of Stine's "double-glue joint," it is *very* strong. The dried white glue film on the exposed upper surface of the shock cord anchor will protect it from ejection charges.)

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In light of the discussion about shock cords and their lengths, I have gotten together some of the new Estes kits from the 2011 catalog.
I will list the part number, name of the kit, the published length and the calculated length of the shock cord using the 1 1/2 times method.
Part Number Kit Name Length calculated shock cord length

1446 Mini honest John 11.75 17.62
1903 Maxi Alpha 3 33.25 49.88
2447 Astron Elliptic II 23.3 34.95
3217 Vagabond 35.25 52.87
3218 Laser Lance 20.25 30.37
3219 Air Commander 32.5 48.75
7216 EPM - 010 35.25 52.87
7217 Hyper Bat 21.9 32.85
1335 Mega Mosquito 18.6 27.9
2134 MIRV 24.5 36.75
3221 QCC Explorer 35 52.5
3223 Xarconian cruiser 22.7 34.05

As you can see from this small sampling of kits, a company could not possible cut to length shock cords for each kit without creating a inventory nightmare.

You could possibly create a standarized length that is longer than the norm, but, the selection would have to be made to a minimum.

Going way back, we didn't have the trifold method of shock cord mounts, you cut a slit into the top of the body tube, fished the shock cord through and glued the slit closed.

We must also realize that over time the rubber shock cords are going to dry out and become brittle just for the fact of useage and age. Easily replaced.