Sport Rocketry

By Mark Newton Yes, it is the always the first rule in construc- tion: read and follow the directions. But now you want to build a rocket using your own parts, or you want to modify a manufacturer's kit for more radical flights. What can you do to keep it together with that F motor, when the kit was really designed for a D motor? Simple: use your bag of building tricks to rein- force key parts of the rocket so it can "return alive" from that flight you've planned. The Fundamental Rule of Building All right, maybe there are more rules than one to remember. But over time this rule keeps moving to the top of the list: MORE ATTACHMENT SURFACE AREA IS BET- TER. You will notice how much the surface area impacts the various components when the rocket is assembled, for example, sand- ing a surface before gluing cuts micro-ridges in the surface. These ridges provide more sur- face area for the glue to bond to, increasing the strength of the bond. You will build stron- ger rockets if you do things to increase surface area as you build. Choose Your Adhesive Wisely Rocket parts come in a variety of materi- als: molded plastic, paper, cardboard, ply- wood, balsa wood/basswood, phenolic tubing, Quantum tubing, fiberglass, and aluminum are some of the materials in your rocket. No single glue has the ideal properties to bond all these materials together. To achieve the greatest strength with the least weight, you must choose adhesives with the best proper- ties for the materials you want to bond. For example, two-part epoxy gives a strong bond to hold wood fins on a paper body tube, but it adds more weight to the rocket than yellow (aliphatic) glue does. For bonding paper and wood products together, yellow glue is your best choice. It is stronger than the materials you will bond. Once glued, the parts will strip away from the glue before the glue itself fails, and yellow glue is much easier to handle than epoxy: no mixing, no gloves, less expense, and water clean-up. Outside of wood and paper, most other materials adhere best with epoxy, including phenolic tubes, G-10 fins, Quantum tubing, plastic nose cones, and aluminum. Epoxy is the rocketeer's friend when bonding two items made from different materials. NOTE: You will become hypersensitive (al- lergic) to epoxy over time if you work with it without wearing protective equipment such as nitrile gloves. Latex gloves do not protect you from epoxy. Also, use epoxy cement only in areas with good ventilation. Now, having talked up epoxy for most mate- rials, let's look at the exceptions. When build- ing many of the new ready-to-fly kits, you can use modeling cement to bond plastic to plas- tic. Also, most epoxies do not hold up under high temperatures. For bonding metal motor mounts or retainers to motor tubes, special high-temperature epoxies are recommended. For field repairs, nothing beats cyanoacry- late (CA), also known as "super glue." It will repair broken fins and launch lugs. It is won- derful stuff to have in your field box. CA also has another feature: when soaked into paper or wood, it acts like an instant sealer, making the fibers stiff. CA-soaked items sand easily, without the "fuzz" that normally accompanies sanded wood or paper. You have to pick the right CA for the job, as it comes in thin, me- dium, and thick viscosities. Thin CA is best for soaking into wood/paper, while medium or thick CA is best for bonding materials in construction or field repair. Some rocket manufacturers recommend CA for construct- ing their kits. Fins For the typical rocket, fins and motor mounts are the parts that receive the most stress in flight. Motor thrust and wind shear both at- tempt to shred fins from the airframe. The fins usually also receive a beating when the rocket hits the ground. Reinforcement and building techniques can increase the strength of fins. Let's discuss some techniques for wringing more performance from your fins: Reinforce your fin stock. Balsa or plywood fins can be reinforced with a layer of typing paper. Coat the fins on both sides with a thin coat of yellow glue, cover each side with typ- ing paper, then put a layer of wax paper over the typing paper. Press the whole setup un- der books, bricks, or something else flat and heavy for several hours while the glue dries. After the glue has dried, remove the fins, peel off the wax paper, and carefully coat the typ- ing paper with CA. The CA will stiffen the pa- per and wood so that it sands easily. From this point, the fin can be sanded, cut, or treated like any other balsa fin. This technique also eliminates the need for sealing balsa fins prior to painting. Reinforce your fin joints. Increasing the surface area of your fin joint will keep fins attached through those high-performance flights, especially if your fins are attached di- rectly to the airframe (surface mounted). The simplest reinforcement is a glue fillet, which forms a smooth joint between airframe and fin surfaces. Proper fillet technique is discussed in most basic rocket kits. For more strength, use reinforcement materials. Tissue paper and yellow glue work great for model rockets. Cut a rectangle of tissue paper as long as the fin joint to be reinforced, and about two inches in width. Cover the tissue with a thin coat of yellow glue and gently push it into the joint— a Popsicle stick helps. You can increase the strength further with a second layer of tissue. After you've done several, you will be able to create a smooth reinforcement that keeps 3 NAR MEMBER GUIDEBOOK, January 2020 Edition Rocketry Basics Rocketry Basics Building a Rocket

• Cover