Paperclip "Shock Lock" design
 

Hello All,

Today I was thinking about the small metal loop shock cord anchors that were used in the Cox Ready-To-Fly model rockets that Estes later re-released. A cross between this anchor and the card stock "Shock Lock" that G. Harry Stine developed in 1969 (it is illustrated on pages 5 and 55 of the Seventh Edition of his "Handbook of Model Rocketry") would be a durable shock cord anchor that would also permit worn shock cords to be replaced *without* the need to install a new anchor! It is a very simple and easy-to-build design that can be built using household materials. Below is a description of how it is constructed.

For those who may not have seen a "stock" Stine "Shock Lock" (they were used in MPC's model rocket kits of the early 1970s, for which he was a design consultant), it was a card stock rectangle about 5/8" X 1" in size (if memory serves) that had a row of three small punched holes running down the middle of its longer dimension. They are very easy to make from scratch. The shock cord is "snaked" through the holes in the Shock Lock, first coming up through the first hole from below (the side that is glued to the inside wall of the body tube), then down through the second hole and below the Shock Lock, and then finally back up through the third hole.

Using one's fingers, the Shock Lock is then given a slight curl along its longer dimension which closely approximates the curvature of the body tube. White or yellow glue is applied to the underside of the Shock Lock, and then it is pressed against the inside wall of the body tube at a distance of a minimum of an inch or so back from the front edge of the body tube. Before being glued inside the body tube, the Shock Lock can (optionally) be coated with a film of glue on its upper side (which is allowed to dry before installation) to protect it from ejection charges. (Coating the underside of the Shock Lock with a film of glue which is allowed to dry before installation creates a tremendously strong double-glue joint when the Shock Lock is glued inside the body tube.) Shock Locks work well with shock cords made of round elastic, cotton kite twine, and Kevlar. The metal paperclip variation of the "stock" Shock Lock is built as follows:

After the card stock rectangle is cut out (its size can be varied depending on the size of the rocket and the diameter of the shock cord that is used), two parallel rows of small holes (three holes per row) are punched through the Shock Lock using an ice pick or a small "finishing" carpentry nail. The distance between the holes is the same as the width of a paperclip. (For BT-55 and larger models, "jumbo" paperclips can be used, with the smaller paperclips being used for narrower rockets.) Only one of the elongated "U-shaped" sections of a paperclip is used to fabricate the Shock Lock, so one paperclip can be used to make two Shock Locks.

Once one of the paperclip's elongated "U-shaped" sections is chosen, it is cut free from the rest of the paperclip using a pair of wire cutters (some pairs of pliers have wire-cutter blades incorporated into their jaws). Using two pairs of pliers (preferably with both having broad, flat jaw surfaces for grasping), about 1/4" - 1/2" (depending on the size of the paperclip) of the closed end of the "U" is carefully bent at an angle of about 15°. This can also be done by using a small vise to hold the "U-shaped" section of paperclip while bending its closed end with a pair of pliers.

After the section of paperclip is bent in this way, its two cut ends are "snaked" through the two rows of holes in the Shock Lock by gently flexing the card stock rectangle to line up the holes with the wire ends of the paperclip section. The paperclip section is oriented so that its bent closed end is angled (upward) away from the upper side of the card stock rectangle. This makes it easier to "feed" one end of the shock cord through the closed end of the paperclip section when the completed Shock Lock is installed inside the rocket's body tube.

When the section of paperclip is in place in the Shock Lock, about 1/8" - 1/4" of each of its two cut wire ends is bent 90° (with these bent ends being parallel to the card stock--that is, not sticking up) using a pair of "needle nose" pliers so that the paperclip section can't be pulled out of the holes in the Shock Lock. After this is done, both sides of the Shock Lock are coated with white or yellow glue, then allowed to dry. Then the Shock Lock is given a slight curl and is glued inside the rocket's body tube.

The eraser end of a pencil can be used to press the card stock rectangle into the glue to ensure that (1) the glue joint is strong, and (2) no edges of the card stock stick up and remain in the way of the shock cord and the parachute or streamer. Once the glue joint has dried thoroughly, one end of the shock cord can be "fed" through the closed end of the paperclip section. If desired, a "U-shaped" piece of stainless steel wire (bent to the same shape as the section of paperclip) can be used instead of a paperclip.

I hope this information will be helpful.

Black Shire- I love it! I've been thinking of using the wire "fishing lead" method discussed on the boards, but this appeals to my desire to keep weight down yet still have a serviceable recovery system. Thanks for the clear description, I at first thought this would be too subtle to describe clearly, but your effort painted such a precise picture in my mind!

My standard method is to attach a length of either Kevlar or stainless steel fishing leader to the motor mount, and make it just long enough to reach the top of the tube. I finish off the end with a small split ring. This is my shock cord anchor. I tie the actual cord or elastic to the split ring. It is therefore easy to replace if I need to do so; I can just push the end of the anchor down through the motor mount so that it extends out the aft end, which gives me access to the ring. Or else I attach the anchor to the baffle, if I install one. With this arrangement, all of the anchor and shock cord is below the recovery device when it is packed, and none of it is in a position to interfere with the ejection of the device out of the tube. I haven't built any long, Mean Machine-style rockets yet, but when I do, I will attach the anchor to a baffle if I use one, or else consider attaching it to the inner wall of the tube as you describe, but deep inside of it, below the area where the recovery device will reside.

In nine total years as a rocketeer, including five years as a BAR, I have only had to replace shock cords twice. In both cases, the cords were made of thin, flat elastic (1/8") that melted due to poorly packing the recovery systems. In my experience, 1/8" flat elastic is much more prone to damage from ejection charges than anything else I use, including other widths of elastic. I try to use wider material when I can (in rockets that are large enough to allow it to deploy without jamming). But when I do use the 1/8" material, this anchoring method allows me to easily remove and replace the cord.

MarkII

Chris Michielssen, who created the "How to Build Model Rockets" web site (see: http://www.howtobuildmodelrockets.2...Part2Page9.html ), commented to me that the paperclip Shock Lock might not fit well in small diameter (ST-7 or BT-20 and smaller) rockets, so I thought of a simplified variation of the design that would take up less space.

A modified paperclip Shock Lock should work well in narrower rockets if, instead of using a "U-shaped" section of a paperclip, a *single* length of wire with a small closed "eye" loop on its top end was passed through the three holes in a standard Shock Lock. The bottom end of the wire would be bent at 90° (the bent end being flat against the Shock Lock) to prevent it from pulling out. The wire could come from a paperclip, or it could be stainless steel wire. An enlarged version could be used in bigger rockets.

Steel fishing leader doesn't add very much weight; if it does, then you are using something that is too thick. But there are limits, of course. Heavier rockets would require steel cable that is much too heavy, if the builder wanted to install one in the manner that I described. (Kevlar would be used instead.) Black Shire's method is variation of a classic anchoring method that has a proven history. It can usually be used with no problem and it provides all of the advantages that he mentioned. I just wanted to point out that the more contemporary method that is frequently used today also addresses these same issues.

One advantage of the paper clip anchor design is that the materials can be obtained locally, from almost anywhere. To obtain Kevlar, on the other hand, you usually have to order it online.

MarkII

If memory serves, the Centuri MINI-MAX large-scale (using "D," "E," and "F" motors) model rockets such as the Lil' Hustler, Hustler, Scorpion, Jaguar, Explorer, and Aero-Dart used steel cable (it may have been fishing leader) for the lower section of the shock cord and fabric/rubber elastic for the upper portion. I would use this for large, heavy rockets using "E" and higher impulse motors. If stainless steel cable is available in these sizes, it should be even better.Not anymore. I found that I could buy "Stren"-brand Kevlar by the spool locally (in yellow and "sea green" colors) from a bait & tackle shop. It was at a tiny (unfortunately now gone) shack-size establishment in my neighborhood called "House of Rods." The hunting & fishing supplies sections of the hardware stores also carry it, and bigger outlets such as Wal-Mart probably have it in their sporting goods sections.

I have looked for that locally and online and have never found it. What is the test on it? Here is a link to the fishing leader that I use. It is 7x7, or 49-strand nylon-coated leader. With 49 strands, it is quite limp and easy to work with. I have the 90 lb., and it is so flexible that I can tie fishing knots in it.

http://www.cabelas.com/cabelas/en/t...equestid=198811

MarkII

That is similar to the Stren Powerbraid that I bought from "House of Rods"--I think it was either 90 pound or 100 pound (I've long since used it all). I just looked it up online and found that Stren dropped it from production in 1999 because it quickly wore away eyelets in fishing tackle, but existing stocks of it are still available from vendors. (I'm not surprised, given how effectively it "zippers" body tubes if not sheathed with masking tape where it exits the end of the tube.) Luckily, braided Kevlar line of this size is made by other companies and (presumably) has uses other than fishing and model rocketry, which keep it in production.