Reference / Paper · 2017
Peak of Flight #440: Expandable-Foam Core Fins
Apogee Components' Peak of Flight newsletter, issue 440 (2017-04-04).
- peak-of-flight
- apogee-components
- model-rocketry
Reference / Paper · 2017
Apogee Components' Peak of Flight newsletter, issue 440 (2017-04-04).
Apogee Components' Peak of Flight newsletter, issue 440 (2017-04-04).
ISSUE 440 April 4th 2017 IN THIS ISSUE Expandable-Foam Core Fins www.ApogeeRockets.com/Rocket-Kits/Skill-Level-2-Model-Rocket-Kits/Vapor Expandable-Foam Core Fins sanded into them. In most model rockets this isn’t a huge issue, because the rockets only have to My kids are just like me. They dislike sanding go “fairly straight” when launched to be safe and airfoils into fins. We use every short-cut possible successful. to avoid the task. I have even used the laser-enMolded plastic fins are uniform in shape, but graver that we have here at Apogee Components typically they are heavy and therefore limit the to burn away part of the wood so there is less altitude of the rocket. However a fin with a tough sanding to do (https://www.apogeerockets.com/ skin bonded to a lightweight molded foam can be downloads/2015_streamlining_fins_with_a_laser. strong, lightweight, and have a consistent shape pdf). from fin to fin. That was the goal of my experiIn this article, I’ll tell you my process for making mentation last summer when my daughters were fins that need hardly any sanding at all. They are getting ready to attend the World Space Modeling molded with urethane expandable foam inside a Championships in Ukraine. It especially solved the shell made from paper. The result is a lightweight one issue they hated to deal with, which was sandfin that looks great, as seen in Figure 1. ing them to shape. By Tim Van Milligan The Mold The drawback of molded fins is that they require a special mold. Creating the mold is the hardest part of the process. But once you have it made, the individual fins can be manufactured quickly. I started the process of designing the fin in a CAD program on my computer. In Peak-of-Flight Newsletter #408 (https://www.apogeerockets.com/ education/downloads/Newsletter408.pdf) I talk about the software, and how you often have to design in the 3D environment by removing material. That is exactly how the mold was designed. I designed the fin first, and then used that as a pattern to remove material from the parts that make up the walls of the mold. Continued on page 3 Figure 1: Foam fins used on model rockets Balsa wood is one of my favorite building products. It’s strength-to-weight is incredible, and it is easy to work with because it shapes easily and can be glued and painted. However, the one problem with sanding it, besides being time consuming and monotonous, is that you can never be sure that all the fins have the identical airfoil 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 Newsletter Staff Writer: Tim Van Milligan Layout/Cover Artist: Chris Duran Proofreader: Michelle Mason Issue 440 | April 4th, 2017 Expandable-Foam Core Fins Continued from page 2 Figure 2: The fin was designed first so it could be used as a tool. #431, #432, and #434 (https://www.apogeerockets.com/Peak-of-Flight?pof_list=archives&m=education). My backup plan if this didn’t yield anything useful was to just make balsa wood fins. By 3D printing the molds, I was able to test several different ideas in just a few days. As you may have seen with 3D printed parts, the surface isn’t totally smooth. It has layers, like terrace farming. You have to come back and either sand out the little ridges, or try to fill them with some type of filler material. I chose to sand them out. The finished molds are seen in Figure 4. The mold also had location pins added to the side to keep the plates aligned during the molding process. Figure 3: The mold sides were designed using the 3D CAD software. The next step was to 3D print the molds. If it wasn’t for the 3D printing process, I don’t think I would have experimented with making fins this way. When I was doing this process, I was under a severe time crunch because we had to have the models built in time to leave on the trip for the competition. I was more concerned about the other aspects of the contest like making the fiberglass body tubes, which was discussed in Peak-of-Flight Newsletters #430, Page 3 Figure 4: The 3D printed molds Continued on page 4 www.apogeerockets.com/RockSim/RockSim_Information Issue 440 | April 4th, 2017 Expandable-Foam Core Fins Continued from page 3 Pouring the foam The process of molding fins with the grow foam is pretty easy. You simply pour in some mixed foam into the halves and bring them together. I used a clamp to hold them together while the foaming process took place. The foam expands to fill all the nooks and crevices, and the excess comes out of the side of the mold with the vent hole. In my molds, the vent is the root edge of the fin as shown in Figure 7. Figure 7: The excess resin oozes out of the vent hole, which in this mold was the root edge of the fin. Figure 5: Mixing up the foam in a disposable cup. You have to mix thoroughly but quickly. Figure 6: The foam expands 25 times its original volume within 20 seconds. My first experiments dealt with different mold releases, just like I had to do when I was making the fiberglass tubes. The urethane foam is very sticky, and if you don’t have a mold release on the surface of the mold, you won’t get the part out. In the end, the wrong release means you’ll have to spend a lot of time scraping it off. It is easy to damage the 3D printed mold because it is made out of a plastic resin itself. I knew the hazards of not having a good mold release because we make the foam nose cones for TARC competitors (https://www.apogeerockets.com/ Building_Supplies/Nose_Cones/Low_Mid_Power_Nose_Cones/56mm_Foam_Nose_Cone) in much the same manner. The difference is that this is a rigid foam, so it isn’t very forgiving of mistakes. Continued on page 5 www.ApogeeRockets.com/Rocket_Kits/Scale_Rockets Page 4 Issue 440 | April 4th, 2017 Expandable-Foam Core Fins I then tried putting paper skins in the mold. And this worked great. The liquid silicone mold release worked best, I simply cut a paper pattern just slightly bigger and the parts would almost jump out of the mold. than the fin, and laid it into the mold. I buttered But the greasy finish was something I didn’t like. up the paper with foam mix (as shown in Figure I know that silicone is hard to remove, and if you 8), and laid another sheet on top and closed the don’t get it all off, then whatever glue you use mold. to attach the fins to the rocket won’t really stick. After about 20 minutes, the foam is hard and I couldn’t risk the fins falling off in flight with the the part can be removed from the mold. The first competition models we were building. thing I do is to cut off the excess with a razor While playing with mold releases, my first exblade as seen in Figure 9. The foam cuts really periment was just to grow the foam in the molds easily, but it helps to have a very sharp blade in by themselves. But the results weren’t what I order to slice through the paper without tearing it. was hoping for. The biggest problem was that the The edge of the mold is used as a straight fins were very weak. The foam by itself has no edge, so the edge is actually good enough to use bending strength. It would simply crack in half if when attaching it to the rocket. you tried to flex it. This would not do for a rocket, because if they cracked in flight from flutter, then the rocket would go unstable. The other issue that I didn’t like was the surface finish. While it was smooth, you could tell that some of the bubbles in the foam had popped on the surface, so it was slightly irregular. Not bad, but not good enough for what I wanted. My next experiment was to put a small balsa wood core in the mold for the foam to grow around. The hope was that it would stiffen up the foam. The problem was that when the resin Figure 9: Cut off the excess foam with a sharp razor would foam up, it pushed out the wooden core blade. or enough to make the rest of the fin weak. Continued from page 4 Continued on page 6 Minimum Diameter Motor Retainers! Apogee is your one stop shop for your minimum diameter rockets projects! • Fly High • Fly Fast • Impress Your Figure 8: The paper skins laid into the mold before the resin was applied. Page 5 We Have: • Minimum Diameter Retainers • Motor Extenders • Threaded Forward Closures • Adapters for Cesaroni Cases Friends! www.ApogeeRockets.com/Building_Supplies/Motor_Retainers_Hooks Issue 440 | April 4th, 2017 Expandable-Foam Core Fins Continued from page 5 To open the mold, I use the razor blade and wedge it between the two halfs of the mold as shown in Figure 10. You have to be careful here not to cut into the fin itself. I just get it in a little bit, and twist it gently so the mold halves separate. the paper skins come together. Ideally, you want the leading edge of the fin to have a rounded edge, like shown in Figure 2 (Page 3). Figure 11: The finished fin has a pillow like quality, with sharp edges around the perimeter. The edge can be sanded so that it is round, but that is going to expose the foam inside the fin. Since the foam is rough when sanded, you’ll Figure 10: The mold halves are opened by sliding a razor end up with a rough leading edge of the fin. So blade in the gap. on the competition models my daughters’ used in Ukraine, they had sharp edges as shown here. With the paper skins in the mold, the two One thing that I didn’t have a chance to test halves open very easily. But you still have to use was to use a folded piece of paper on the leading mold release in cavity of the mold because some foam does leak out between the skins around the edge to see if that would help round off the edge like I wanted. Someday, I’ll have to play with that. entire perimeter. The other cool thing, as you can see here, is Therefore, the edges of the paper are not that you can pre-print a pattern on the paper prior perfect as they come out of the mold and they to putting them into the mold. That allows you to do have to be trimmed with a hobby knife and a customize the look of your rocket without having straight edge. The paper can shift a little bit too to paint it later. inside of the mold because of the foaming and The outer texture of the paper will be the flowing material. Figure 11 shows the completed fin after it has surface finish of your completed rocket. If you use ordinary bond paper like I did for the ones in been trimmed and attached to the rocket. The leading edge is fairly sharp because that is where Figure 11, it will have a dull appearance. Continued on page 7 www.apogeerockets.com/Rocket-Kits/Skill-Level-2-Model-Rocket-Kits/SkyMetra Page 6 Issue 440 | April 4th, 2017 Expandable-Foam Core Fins Continued from page 6 It is possible to use other papers and different materials. In Figure 12 I test aluminized mylar and it worked well. You do have to start with a piece of mylar that has no crinkles in it. The foam doesn’t push out all the little wrinkles. And the edge is a little harder to work with, because you can’t sand the mylar. So you have to trim it very carefully when it comes out of the mold. Figure 12: Different skin materials can be used to make foam fins. Here are aluminized mylar and tissue paper. Tissue paper also works well for the skins. I tried the material because I wanted to see how light I could make a fin. Tissue paper was the lightest weight, with a fin coming out about 0.3 grams. There are a couple of disadvantages of tissue paper as a material. First, the fin is more flexible and not as strong as one made from bond paper. If the rocket’s speed is fairly slow like the A-engine powered rockets that I was flying, this is fine. My daughters used tissue paper skinned fins in competition, and they worked just fine. The other issue is that the resin will push through the looser weave and get to the surface of the fin. That ruins the look of the fin, and as seen in Figure 12, the smoothness isn’t quite as nice. One discovery I made is that if you use the liquid silicone release with the tissue paper, the silicone soaks into the tissue and prevents the urethane resin from making its way to the surface. You just have to clean them really good after they come out of the mold to make sure the silicone is removed so that they glue will stick them to the tube of the rocket. Conclusion While I was making foam fins for competition style rockets, I’m sure that they would be even better for sport rockets where it doesn’t matter as much what airfoil shape is used. I would say they could also be used to make scale model fins, like the wedged shaped fins on a Nike-Smoke rocket. For the competition rockets like I was making, I’m still a little bit undecided if I’d use them again. They worked, but one thing that I discovered was that the weights were not as uniform as I wished. That means that the density of the foam can vary from one fin to the next. I wish for a bit more consistency, along with a better leading edge. But I did like that I could pre-color the fin with a printed pattern on it. It really helped our rockets to stand out. Continued on page 8 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 Page 7 Issue 440 | April 4th, 2017 Expandable-Foam Core Fins Continued from page 7 Resources: Rigid Foam https://publicmissiles.com/product/adhesives https://bjbenterprises.com/index.php/polyurethanes/foams/rigid/tc-300-a-b/ Mold release: https://bjbenterprises.com/index. php/misc-products/release-agents/partall-hitemp-mold-release-wax-3059/ 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. 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. https://www.apogeerockets.com/Rocket-Kits/Skill-Level-3-Model-Rocket-Kits/Slo-Mo High Power Nose Cones • MONSTER Nose Cones from LOC-Precision • Durable Heavy-Duty Plastic • Fits Standard LOC Tube and Blue Tube • Get That Big Project Off The Ground • Affordable! www.ApogeeRockets.com/Building_Supplies/Nose_Cones/ Page 8 Issue 440 | April 4th, 2017