A Trivial Shock Cord Mount

by Roger L. Deran, LUNAR #569

Why is recovery separation so common? In smaller rockets it is dangerous and embarrassing, and with high power there is no way it can be tolerated. It seems that usually the problem is the shock cord mount. Let's take a hint from H.P. and apply it to the smaller rockets; glue the shock cord or a nylon loop directly onto the inside of the airframe. This method is recommended by LOC Precision for use in rockets as large as, or larger than the EZI-65 – my own favorite. I have used it in a variety of low power rockets including kits and scratch models. It has worked flawlessly (so far!)

It may seem that gluing directly to the inside of the cardboard tube would create a weak joint, but done correctly it will be far stronger and safer than the folded paper recommended by at least one important manufacturer. The secret is to cover a wide area with epoxy and set the cord into it. The epoxy should be built up to form a solid, smoothed unit at least as wide as the folded paper type, preferably much larger. With a 98mm (4") airframe this is easy, since you can get your entire hand down into the tube to spread the layers of epoxy. But how can you get such a joint inside a 24mm (.976") tube?

The technique I use is even easier than using folded paper. Just squeeze the epoxy into the end of the shock cord and stick it onto the end of a rolled-up 5" square piece of waxed paper. Put plenty of extra epoxy around and on the cord to form a 1" square. Now tighten up the waxed paper to fit inside the body tube and push it down inside far enough to allow the nose cone or payload section bulkhead to fit forwards of the mount. Unroll the waxed paper so that it fits snugly, pressing the cord and epoxy onto the inside of the tube. When it dries, pull the waxed paper out. Done!

The joint formed this way will be almost perfectly flat due to the pressure from the waxed paper. Flatness is very important, since any burr or blob of epoxy will tend to catch the recovery system on the way out. If you just try to use the natural viscosity of the epoxy to hold the cord during setting, the cord tends to rise up in some places and form rough spots, jagged edges, or even a genuine aft-pointing hook! These problems have to be corrected by filing, sanding, and covering with more epoxy. The waxed paper trick avoids these difficulties and requires only one mix of epoxy. I don't think it would work as well in larger tubes, but it is worth a try.

For a really perfect mount, fill the waxed paper tube with cotton balls after it is unrolled inside the body tube. This will put pressure on the waxed paper from inside and insure that there are absolutely no burrs on the edge of the epoxy. Even without the cotton ball filling, the epoxy mount is much flatter and smoother than the folded paper type. You can feel the difference by sliding a 'chute through the tube by hand. One more improvement is building the epoxy layer up even further with additional coats of glue. The additional coats will add strength and increase smoothness.

LOC Precision also recommends gluing in a loop of nylon cord instead of the shock cord. The cord is a single piece, without knots, doubled up to form the loop. The two loose ends are glued into the body tube side by side, providing a two-place connection that is even more reliable. Tie the shock cord to the loop so that if one end of the loop breaks, the cord will remain attached to the other end rather than sliding off. With this nylon loop, the cord can be removed for convenience or replaced if damaged. Use extra thick nylon cord.

One last thing: don't use the shock cord supplied with the kit! The manufacturers seem to want to save money on the cord by giving you thin, short pieces. Such cords are really insufficient, especially when the model is heavy. If there is significant weight on the forward end of the cord – perhaps a payload section – then there is going to be even more tension in the cord. If the model is moving at high speed when the recovery system is deployed, the force of deceleration will be transferred to the model through the shock cord. If the cord stretches to its limit, the tension will suddenly go sky high.

Go to a fabric store and get some of that same elastic for almost nothing. It comes in black or white, in various widths, and in any length. Use a piece as long as you can comfortably work with. Four feet would not be extreme. There is no reason to skimp. Don't worry about weight either; safety is the number one priority.

Have fun with this waxed paper shock cord trick. Please send me any comments about it at rlderan@boilerbay.com.

Information date: November 14, 1998 lk