Shock Cord Breakage?

Let me lay it on the line for ya -

Estes' OEM shock cords are junk. Trash. Offal. Garbage. Kapeesh?

The only thing you're doing wrong is not replacing them with some Kevlar cord and a length of sewing elastic. The Kevlar is heat resistant, and the elastic will absorb the shock. Make the Kevlar:elastic ratio about 3:1 and the total length should be about 5 times the length of the rocket (on average; for Estes-size rockets).

Estes should really study how Quest or Pratt builds their rockets, in terms of shock cords (today's Estes ain't your father's Estes....) BTW, Kevlar cord is available from Pratt hobbies:

Estes shock cords are notoriously shorter than they ought to be. The general rule of thumb is to have your shock cord 2 to 3 times as long as your rocket. And you can pick up more elastic at a fabric store or in the fabric section at a WalMart, etc. Fabric stores have a better selection & you can buy it by the yard off a big spool. Also, if you see any that's not as bright white, looking a little dingy, that's the cotton swimsuit elastic that is better for our purposes than the polyester stuff.....stands up to the heat better. You could do as BB suggested & use the Kevlar/ elstic combination OR go with just elastic. Before any flight with an elastic shock cord, we should check the condition of the elastic, as they becaome weak & charred after a number of flights & need to be replaced periodically. -- Richard "model rocketry is not a crime & I am not a criminal" Hickok

the other posts about using kevlar and longer elastic are spot on. (although I am not really sure Estes shock cord is actually worse than the elastic we get from the sewing store.)

another thing, be certain you pack the wadding lower down the tube than the shock cord & mount. then pack the chute in, and the cord on top of that.

even my kids do it this way (fumblefingers) and we've gone from losing shock cords every flight to not losing a single one in the 40 flights we've done this week.

Jacob Thurman wrote:

I have been very successful with using braided nylon elastic from the fabric stores. I use the same size as Estes unless it is the very smallest, then I use the next larger size. I use at least 2 times the total length of the rocket (including the NC and fins). I rarely use Kevlar. I have had only one shock cord break in over 10 years and that was when I put an AT D21 into a Estes SR-71. It really flew great but it blew the NC completely off!

Karl Perry QUARK, Cincinnati, OH

Are you filleting the fin/tube interface? Patrick

"BB" wrote in news:bhYWa.5355$ snipped-for-privacy@nwrddc01.gnilink.net:

Once more with feeling: whatever shock the elastic "absorbs" it gives back when it contracts, and likely in the wrong direction (i.e., propelling components toward one another).

Think about it. Why is there no elastic in a real parachute's rigging?

len.

Is the jumper's leg attached to the airplane by a piece of rope? ;O)

You talking about bungee jumping? No way. I won't use that stuff in a rocket, much less tie some around my ankle and go jumping off something.

Others have suggested using a piece of Kevlar string tied to a piece of elastic, like Quest kits use. I have built all my Estes kits this way, with great success. However, I'm still trying to perfect my wadding packing technique and I have charred som elastic. No seperations yet, but I figured it's just a matter of time.

Another minor downside to using Kevlar string is that it is so strong and thin that it can easily cut a zipper into the side of a body tube during an energetic deployment.

Then last month I came across a new product sold by Mile High Rockets

called Kevlar-wrapped Elastic Shock Cord. It has the stretchiness of elastic but the fire-resistance and strength of Kevlar. It can also be attached as one continuous piece, eliminating the need for a knot in the middle which can get hung up on the parachute lines. The diameter is larger and softer than a Kevlar string of the same strength, so there is less chance of zippering.

-- Bob Cox

Usual disclaimers: I have no interest in Mile High except as a very satisfied customer.

"Jacob Thurman" wrote:

I'm not exactly sure what you mean by "filleting," but I believe the answer is no. I'm not making any intentional structural changes, and there is no damage to the body tube during separation (just during impact when it comes down streamlined... nothing like watching a 30" rocket shrink to 10" in a split second).

--Jacob

Ah... yes, I do fillet the body tube. I didn't know that's what it's called, but from a structural standpoint, it just makes sense. Thanks for your suggestions!

-- Jacob Thurman

Ah... yes, I do fillet the body tube. I didn't know that's what it's called, but from a structural standpoint, it just makes sense. Thanks for your suggestions!

-- Jacob Thurman

Jacob has made it abundantly clear that he fillets the tube. -- Richard "Arnold will likely be the next governor" Hickok

Max Headroom imitation?

Randy