The above image is the drawing that accompanied editorial bits in the old Model Rocketeer magazine (a 60's/70's precursor to Sport Rocketry). It is very appropriate for this post, as I am about to do a bit of loud editorializing, words which will undoubtedly be lost in the ignorasphere surrounding the TARC universe. However, I feel they need to be said, and now is the time to do it, before TARC season begins in earnest.
Duane, my colleague and the current reigning TARC Geezer, and I are preparing an outline for a TARC workshop we are planning to host at the Space and Rocket Center in Huntsville. His part will consist of the proven techniques he and his teams have employed in building their TARC rockets - through-the-wall plywood fins, thick plywood centering rings, heavy body tubes, and generous applications of epoxy to hold these pieces together. Duane is not alone here, as many, many TARC teams have birds built in a similar fashion, which is a direct application of high power rocket construction techniques to the mid power models used in TARC. Strong and heavy equals survivable in this scheme; if you want your rocket to last, there is no other way.
I have one word for this...
(without the r, of course).
The techniques that Duane and many of my other friends use are overkill, tantamount to using a gun to kill an ant. Yeah, it works, but so does just stepping on the little pest. Let me explain my gripe with these techniques:
- Through the wall fins are unnecessary in low and mid power - Gluing the fins on the body tube is plenty strong for the E and F motors commonly used in TARC. As a matter of fact, surface mounting the fins has been shown to work well all the way up to H impulse (just don't use a Warp 9 motor). The nice thing about surface mount is that cutting out the fins is easier because you don't have to worry about the fin tabs, and there is no need to sacrifice fingers cutting slots in the body tube. This translates to quicker build times.
- White/wood glues are more than strong enough - If you follow the Handbook of Model Rocketry and use Aileen's Tacky Glue in a double glue joint, the fin ain't coming off. The body tube may fail, or the fin may break, but it won't come loose. Some rocketeers will tell you that the heat from the motors will soften white glue and cause failures, but I have NEVER, in all my decades of rocketry, witnessed this. I have seen fins on models left out in the summer sun all day get a little wriggly at the end, but this is easily solved by placing them in shade of your tent or canopy. You are never going to launch a rocket so fast that the heat from repeated firings will loosen the fins, even on minimum diameter models. The same applies to motor mounts and centering rings - white glue is simple to use and works very well in the mid power arena. It saves weight because it is less heavy than epoxy, and in rocketry, weight is everything.
- Balsa/basswood are perfectly good materials for fins - they are lighter than plywood, and far easier to shape/airfoil. Balsa is not only the lightest, but it also can be made very strong with little weight increase by laminating the fin with paper or adhesive labels. This speeds construction because you do not need to apply a sealer to the fins - they will be ready for primer and paint right after fillets are applied.
I know you are wondering why so many TARC teams use HPR construction techniques if the above points are true. I believe I have a answer (not THE answer, but a answer). Many current mentors started out in rocketry with mid power and high power builds; they have little to no low power experience, so they teach what they know. I live in the world of low power rocketry, so I want to build light, efficient, and pretty. Therefore my approach is going to be different from that of my colleagues. And I think they will admit, despite the trash talk, that my birds have survived their flights just as well as their models have. And so I am going to preach a bit at the workshop about the low power approach in building TARC rockets, because I think it will save the teams time, work, and a little bit of money. Duane will have his say, of course, but so will I.
This year, I think I can accomplish the TARC goals on a cluster of 3 C motors, much less impulse than the other guys are using. If I do, they are gonna hear "Build light!" lots over the next several months.