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Basics of Vacuum Bagging Fins Continued from page 4 Continued on page 6 www.ApogeeRockets.com/Building_Supplies/Parachutes_Recovery_Equipment/Parachutes Why Vacuum Bag a fin? What the vacuum bagging process does is to smooth the epoxy out so that it has a more uniform thickness and a better surface finish. This is particularly useful when applying a fiberglass or carbon fiber skin to the fin, because it can be difficult to avoid air bubbles trapped under the cloth. If you have a gap of air under the skin, then you lose all the advantages of putting on a skin to increase fin strength. Vacuum bagging is a way to suck those air bubbles out while the epoxy is still wet, so the skin is really laminated to the surface, and strength is maximized. I repeated the goal of vacuum bagging, so you don’t assume it can do something that is not possible. Most people think that the vacuum bagging process will squeeze out as much epoxy out of the cloth as possible. In my experience, that is not what it does. If you put too much epoxy into the cloth or onto the surface of the fin, it doesn’t squeeze it all out. I read somewhere and unfortunately I can’t find the source that vacuum bagging is equivalent to putting a max of 8 psi of pressure on the surface. So think of it like putting some heavy weights on the fin to smooth out the epoxy on the surface of the fin. Unfortunately, 8 psi isn’t all that much for squeezing out excess epoxy. If your goal is to squeeze out all the excess epoxy to make it as light as possible, what you really want to do is put your fin in an autoclave (a pressure chamber that also adds heat to speed up the cure time). That’s what they do in the aerospace industry. I’ve also done it for fiberglass tubes (https://www.apogeerockets.com/education/ downloads/Newsletter434.pdf), but it was a hassle. My pressure chamber was only capable of about 20 psi of pressure. But it is certainly better at squeezing out epoxy than the 8 psi you’d get from vacuum bagging. Other than the situation where you’re attaching fiberglass or carbon fiber cloth skins for extreme strength, the technique isn’t used a lot in rocketry because it is rare that you couldn’t achieve the same results with other finishing methods; particularly if you don’t need to optimize three variables (weight, surface finish, and strength) all at the same time. In my situation, where I found vacuum bagging useful is fins with airfoils (compound curves). I was looking for a glass-smooth surface finish without having to paint it. This is possible because the plastic of the bag can stretch a little bit to follow the curvature of your airfoil, smoothing out the epoxy nicely. The Process of Making Competition Fins with a Smooth Surface You want to start out with fins that are pre-airfoiled and as smooth as possible. In the fins that I was making for competition rocketry, I was using low-density 1/32-inch thick balsa wood. Even 1/32” thick wood was too thick, and I had sanded it down a lot thinner than that to lose even more mass. Then I sealed the surface with wood filler. All the little grooves and valleys in the surface of the wood have to be filled, or the epoxy would eventually fill them. Epoxy is heavy, and if weight is critical, then they should be filled with something that has a lower density. That is why I chose to use wood filler initially to seal the fins, because it has lower density. Page 5 Issue 569 / Mar. 15th, 2022PDF Image | Basics of Vacuum Bagging Fins
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