Reference / Paper · 2016
Peak of Flight #420: Designing and Constructing Fins for Low-Powered Rockets
Apogee Components' Peak of Flight newsletter, issue 420 (2016-06-28).
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Reference / Paper · 2016
Apogee Components' Peak of Flight newsletter, issue 420 (2016-06-28).
Apogee Components' Peak of Flight newsletter, issue 420 (2016-06-28).
ISSUE 420 JUNE 28th 2016 IN THIS ISSUE Designing and Constructing Fins for Low-Power Rockets www.ApogeeRockets.com/Rocket_Kits/Skill_Level_2_Kits/Grappler_Rocket_Kit Designing and Constructing Fins for Low-Power Rockets By Tim Van Milligan {Editorial note: This article was taken from the describes the uses for the various thicknesses of book: “Model Rocket Design and Construcbalsa available. tion.” This book is will give you all sorts of ideas and techniques for designing and build- Chart: Uses for Different Thicknesses of Balsa ing your own rocketry creations. You’ll find 0.79 mm (1/32”) Hollow built-up fins it at: https://www.apogeerockets.com/Rock1.59 mm (1/16”) Typical small rockets et_Books_Videos/Books/Model_Rocket_De2.38 mm (3/32”) Medium-size rockets sign_And_Construction} 3.18 mm (1/8”) Larger low-power rockets or clusters; “C” or Fins should be constructed so that they can larger engines withstand the forces expected during flight. The 4.76 mm (3/16”) Large cluster engine or materials and methods of construction determine heavy models how much force they can withstand. Small models do not need very strong fins, and simple card6.35 mm (1/4”) Generally too thick; board or balsa fins will suffice. Large models and high drag those intended to fly at extremely high velocities will need more strength. The following list gives If you plan to conseveral materials and construction methods, in struct your fins from order of increasing strength: a solid-sheet material such as cardboard, • Thick cardboard (not corrugated) plastic sheet, or balsa • Balsa wood fins wood, first make a tem• Plastic fins plate of the fin shape • Balsa wood with paper reinforcing so it can be transferred YES Snap fins • Balsa with spruce wood reinforcing to the material. Before NO • Built-up fins marking around the • Foam core fins outside of the template • Fiberglass reinforced fins with a pencil, orient the • Plywood balsa sheet so the Figure 1: Orient the grain on grain direction in the the balsa wood with the leadCardboard is suitable for small rockets using wood is parallel to the ing edge of the fin. up to a full A rocket motor. Straight balsa wood leading edge of the can be used for rockets of 0.91 m (3 feet) or template. This will ensure maximum strength of less in height, using a maximum of a D engine. the fin. If the fin grain is oriented parallel to the Larger rockets or higher-power models should body tube, they will be very weak and will snap use one of the other techniques or materials. easily (Figure 1). These fins can be fixed by lamMost of these methods will be described below. inating both sides with bond paper as shown in Since balsa is a predominant material, the chart Figure 2 (Page 3). About this Newsletter You can subscribe to receive this e-zine FREE at the Apogee Components website www.ApogeeComponents.com, or by clicking the link here Newsletter Sign-Up Page 2 Continued on page 3 Newsletter Staff Writer: Tim Van Milligan Layout/Cover Artist: Chris Duran Proofreader: Michelle Mason Issue 420 | June 28th, 2016 Designing and Constructing Fins Continued from page 2 Paper Skin Figure 2: Adding paper skins to fins is easy and adds a lot of strength. Sometimes it is necessary to make fins out of multiple pieces of wood. Such might be the case if the fin would be wider than a sheet of balsa wood (Figure 3), or if there is an undercut in the fin that would result in a fin that is structurally weak because of the wood grain direction (Figure 4). In either case, cut the wood so that the grain direction is oriented to create a fin with maximum strength. Fins with straight edges are the easiest to cut because you can use a ruler to help guide the knife. Lay the ruler on top of the fin to protect it if the knife blade should stray away from the ruler when you are cutting. Keep the blade at a 90-degree angle to the wood to avoid beveling Figure 4: Multiple pieces of the edges of the fin wood glued together (called (Figure 5). When makcompound fins) should be made ing cuts, make several for fins that have an undercut light passes instead of to them. Note the wood grain one heavy cut. direction. Curved fin shapes are only slightly more difficult to cut. To ensure maximum uniformity between fins, make a template out of thick styrene sheet and use it to guide your knife blade. 4 Figure 3: If the fin is wider than the width of the balsa wood sheet, you’ll have to build it from several pieces. 5 3 2 1 Thicker or harder materials such as plywood are more easily cut with a razor saw than with a knife (FigFigure 5: Place ruler on top of ure 6 Page 4). Use care with a razor saw, fin to prevent it from being since it is much harder sliced if the blade should stray to guide the blade and from the ruler’s edge keep the edges straight. Continued on page 4 https://www.apogeerockets.com/TARC_Supplies Page 3 Issue 420 | June 28th, 2016 Designing and Constructing Fins Continued from page 3 After you’ve cut all the fins, stack them and sand them all together to ensure uniformity of size and shape. Pay particular attention to the edge that will be glued against the Figure 6: Use a razor saw to cut body tube (the root thick balsa and plywood. edge). It should be perfectly straight to maximize the bond strength of the fin. Also at this time, sand the airfoil into the fin. You might just round the edges, but for minimum drag the fins should be streamlined. Using a small wood planer will drastically cut your sanding time on thick fins. If you want to have a definite high point on the fin, as would be needed on a scale model, lay a piece of masking tape along the dividing point (Figure 7). If you sand into the tape, remove it and lay a new piece down. This will give a sharp line on the fin. Masking tape on edge Finally, give the of high point fin an all around surface sanding. Remove any lines you drew with ink pens, as they sometimes bleed through the paint Figure 7: To make a sharp edge on the rocket and on a fin, use a strip of masking ruin its appearance. tape as a guide. You may apply sealer to the fins at this time, too. You’ll find it easier to sand the fins before they are attached to the body tube. Reinforced Balsa Fins: Balsa fins can be strengthened by providing reinforcement. The additional strength will increase their stiffness and make them more resilient to stress incurred during hard landings. To do this you may either apply skins or add stiffer structural members to the fins. Skins are the easiest to apply; you usually bond them directly to the fins with wood glue, instant glue, or epoxy. You can make them from a variety of materials, including tissue paper, ordinary notebook paper, cardboard, plastic sheet, very thin plywood, and fiberglass. Stiff skins are usually applied to flat surfaces, as they are difficult to apply to fins that have airfoils sanded into them. To prevent a balsa wood fin from warping while the glue is drying, place it between two sheets of wax paper and lay a book or other weight on top of it. The waxed paper will protect the table and the book from glue that might ooze out. Flexible skins that drape, such as tissue paper or fiberglass cloth, are better for curved airfoil sections. Apply tissue paper to fins or wings that need only a little reinforcing or to cover the grain of the wood, and use fiberglass cloth for applying maximum strength to any fin or Continued on page 5 Quick-Change Motor Adapters • Allows you to use smaller diameter motors in your rocket kits (adds versatility) • Change out motors in seconds • Works with all single-use and re-loadable motors • Four sizes available www.ApogeeRockets.com/Building_Supplies/Motor_Mount_Kits_Adapters/Ready-to-use_Motor_Adapters Page 4 Issue 420 | June 28th, 2016 Designing and Constructing Fins Continued from page 4 wing. The methods of application are similar, but the adhesives differ. Tissue paper is commonly applied with model-aircraft dope or wood glue Figure 8: Hardwood cap strips thinned with water. add strength and keep fin edges Fiberglass is almost from being dinged. exclusively applied with epoxy, polyester resin, or instant glues. Hardwood spar You can also reinforce balsa fins by strategically adding stronger members to stiffen them. Members placed around Figure 9: A simple hardwood the perimeter of the fin are called spar helps prevent fins from caps, while mem- snapping in two. bers placed inside the perimeter are called spars or ribs, depending on their orientation. Ribs lie parallel to the centerline of the rocket, while spars extend nearly perpendicular from the centerline (Figures 8,9,10). Built-Up Fins: A strong and lightweight fin, generally made hollow, with judicious placement of high-strength structural members inside an outer skin, is called a built-up fin. The advantages (if it is built properly) are low weight and high strength. Built-up structures are mainly used on large wings or Hardwood rib fins, where weight reduction can be Figure 10: Hardwood ribs presubstantial. vents fins from splitting along the grain of the balsa wood. Page 5 Figure 11 shows the key parts of a built-up structure. The spars carry bending loads, the ribs give support and contour shape to the skins, and the skin gives the wing an outer cover. When the skin is used to give strength to the structure, the wing is said to have a semi-monocoque construction. Skins that can be used effectively this way include balsa sheet, thin plywood sheet, and fiberglass sheet. This conSpar Rib struction is also suitSkin able for fins with sharp leading Built-up fin or trailing framework edges. The skins on these have Figure 11: A simple built-up fin using hardto be very wood strips and a skin of light cardboard or thin and balsa. stiff; light cardboard, thin fiberglass sheet, or thin plywood - 0.39 mm (1/64”) or 0.79 mm (1/32”) work well. Figure 12 and Figure 13 (Page 6) illustrate how they could be constructed. Foam Core Fins: To increase the strength of a built-up wing, without significantly increasing its mass, you can insert expanded polystyrene into the hollow areas. This will add strength and stiffContinued on page 6 www.apogeerockets.com/RockSim/RockSim_Information Issue 420 | June 28th, 2016 Designing and Constructing Fins Continued from page 5 en the skins. The Thin cardboard skin added strength is usually remarkable, so that many structural members inside the skin can be Cardboard Balsa spar eliminated, and end-caps this decreases the mass of the Figure 12: Wedge-shaped fins are easily made using a built-up method. wing. Constructing a foam core structure is similar to building a built-up structure. Create a skeleton framework around the fin and cut a piece of foam to fit any hollow area. Shape the foam using a sanding block, so it exactly matches the perimeter defined by the skeletal framework. Bond the skin directly to the foam and all structural members. Epoxy is usually a safe adhesive to use on foam, but polyester resins and instant glues may melt it. Again, the type of skin should be appropriate for the expected loads and the shape of the fin. Flat and stiff skins such as light cardboard, thin balsa, thin plastic, or thin plywood are suitable only to fins with flat surfaces. For curved surfaces, fiberglass cloth soaked in epoxy is generally better. Several layers of fiberglass can be added to give extra strength to the fin. This technique is especially applicable to larger and higher-powered rockets. Figure 13: Folded cardboard fins without spars. Plywood Fins: Plywood is not usually used as a fin material on small rockets, because it is difficult to cut and shape, and its density is too high for the strength it gives. However, it is used extensively in high-powered rockets. For small fins that need accurately shaped airfoils, however, plywood does have some nice characteristics. Use only 0.39 mm (1/64”) or 0.79 mm (1/32”) thick wood. For fins thicker Continued on page 7 Guillotine Fin Alignment Jig The Most Versatile Alignment Jig Ever Manufactured • Get Perfectly Aligned Fins Every Time • Holds the Tube In a Horizontal Orientation to Prevent Glue Drips • Self Adjusts to ANY Size Tube From 13mm (BT-5) to 66mm (BT-80) • Securely Holds The Fin While The Glue Dries • Kid-Friendly! Helps Them Make Stronger Fins, Resulting in Straighter Flights • Can Accomodate Fins Up To 1/2” Thick • Allows Any Number of Fins on the Tube www.ApogeeRockets.com/Building_Supplies/Tools/Guillotine_Fin_Jig Page 6 Issue 420 | June 28th, 2016 Designing and Constructing Fins Continued from page 6 than this on small rockets, balsa is highly superior because the weight is significantly reduced compared to plywood. Plywood is also acceptable for use as a skin on a built-up or foam core fin. When used this way, only thicknesses of 0.39 mm (1/64”) or 0.79 mm (1/32”) are acceptable. Thicker than this, and it adds too much weight without any significant increase in strength. in Space Technology from the Florida Institute of Technology in Melbourne, Florida. Currently, he is the owner of Apogee Components (http://www. apogeerockets.com) and also the author of the books: “Model Rocket Design and Construction,” “69 Simple Science Fair Projects with Model Rockets: Aeronautics” and publisher of the “Peakof-Flight” newsletter, a FREE e-zine newsletter about model rockets. You can email him by using the contact form at: https://www.apogeerockets. com/Contact. Many rocketeers think they need thick, solid plywood fins on larger model rockets and on higher-powered rockets. This is not always true. You can make strong and stiff fins with many of the methods described above. Just take your time making the fins and do the job right. Many modelers can attest that they have built high power rockets using balsa fins that have been reinforced with spars and/or fiberglass skins. About The Author: Tim Van Milligan (a.k.a. “Mr. Rocket”) is a real rocket scientist who likes helping out other rocketeers. He is an avid rocketry competitor, and is Level 3 high power certified. He is often asked what is the biggest rocket he’s ever launched. His answer is that before he started writing articles and books about rocketry, he worked on the Delta II rocket that launched satellites into orbit. He has a B.S. in Aeronautical Engineering from Embry-Riddle Aeronautical University in Daytona Beach, Florida, and has worked toward a M.S. You get: (4) AT 29/13 (4) AT 41/18 (2) AT 56/18 (2) AT 66/18 (1) AC-56 (1) AC-66 You get: (6) AT 13/18 (6) AT 18/18 (6) AT 24/18 (6) AT 33/18 www.ApogeeRockets.com/Building_Supplies/Body_Tubes Quick-Change Motor Adapters • Allows you to use smaller diameter motors in your rocket kits (adds versatility) • Change out motors in seconds • Works with all single-use and re-loadable motors • Four sizes available www.ApogeeRockets.com/Building_Supplies/Motor_Mount_Kits_Adapters/Ready-to-use_Motor_Adapters Page 7 Issue 420 | June 28th, 2016