# Get this straight (Part 1)

Putting fins on a rocket tends to be the hard part.

There are three angles you can get right… or wrong. There’s the angle between the position on the circumference of the airframe where the fin should be and where it is (A. in the diagram) — the position discrepancy. There’s the angle between the perpendicular to the airframe and the fin mid-chord (B.) — the tilt angle. Finally there’s the angle between the airframe’s longitudinal axis and the fin root (C.) — the cant angle. (Of these three terms only one, cant angle, seems to be standard. The other two are what I’ve chosen to call them; someone else might have different names.) Sometimes a rocket is designed with fins that are not equally spaced around the airframe, or with nonzero tilt or cant angles. Maybe even all three. Usually, though, the positions are multiples of 120° (for 3 fins) or 90° (for 4 fins) or 360°/n generally (for n fins), and the tilt and cant are zero.

(There’s one more way to misposition a fin: wrong location forward/backward. But that’s an issue for another time.)

Getting the right position, tilt, and cant is tricky enough that people have invented a number of different tools and techniques to help out. Techniques include through the wall fins, motivated primarily by strength considerations, but a side benefit is that if the slots are cut carefully, the position and cant are well controlled. As for tools, the commercially available ones I know of are:

Qualman Rocketry’s Fin Guides. There also are similar “free” (modulo the cost of cardboard) tools available. They’re just cardstock pieces with holes cut a little larger than the airframe and slots matching the desired fin positions. You slide one onto the rocket, over the fins after they’re glued but before the glue’s dried, and that’s that. Cheap, and some people swear by them. You do need different ones for each combination of body tube size and fin thickness, though, and they don’t address zeroing of the cant angle.

Estes Fin Alignment Guide. You set up three or four vertical panels, in one orientation for 3/32″ fins and another for 1/8″; put one or more nested motor casings in the center hole; put the rocket in the middle, using the motor casing in the motor mount; then use clips to hold the fins against the panels. This tool addresses position, tilt, and cant. Inexpensive at around \$22 list. I have one of these and find it useful. But it has its limitations — maximum body tube size of course (true really of any fin alignment tool, but some are more limited than others), but it also doesn’t work well with very swept back fins (e.g. K-6 Ranger) or with cluster motor mounts without a tube in the center (e.g. K-6 Ranger), or with other fin thicknesses. Set up only for 3- and 4- fin rockets with fins uniformly spaced, though you can always just glue one fin at a time in other cases. Also, it’s made of plastic, not particularly durable, nor very precise — there’s a bit of “play” in both the panels and the rocket position which can lead to cant errors if you’re not careful.

ibuildstuff4u’s Ultimate Fin Guide and  bmi CAD’s Model Rocket Fin Alignment Jigs. These are pretty similar but as far as I can tell are made by unrelated businesses. They’re essentially the same kind of thing as the Estes Fin Alignment Guide, but machined out of aluminum. I’ve never used either but presumably both are a lot more durable than the Estes, and more precise with less play in the parts. Limitations are similar to the Estes Guide, though as far as maximum rocket diameter goes the bmi CAD one comes in sizes up to 18″ square (that one also does 5-fin rockets) and ibuildstuff4u’s can handle 1/16″ as well as 3/32″ and 1/8″ fins, using shims; so can bmi CAD’s but it’s not clear shims are provided. Prices range from \$40 to \$250.

Macklin Guillotine Fin Jig. A different approach: essentially a plywood box with v-notches in both ends, and sliding panels with diamond cutout holes. The rocket rests horizontally in the notches, with the panels securing it from above. Then a couple of aluminum angle rails attached on top hold one fin in position. The spacing between the rails can be adjusted to accommodate any fin thickness, and there’s no dependence on the motor mount. There’s a large version for fatter rockets. This one handles tilt and cant, but for position you’re on your own. Swept back fins aren’t a problem. Cost is \$90 for the standard size, \$195 for the large. You used to be able to (maybe still can) buy plans to build your own, and for that matter the pictures online make its design pretty clear so it wouldn’t be too hard to build one without the plans — if you have the tools and skills. (Same is true of the machined aluminum ones above, too, I suppose.) Don’t try to sell your own version, though; Mr. Macklin defends his intellectual property with vigor.

High Tech Rocketry Rose Fin Jig. Design by Art Rose. This looks to be a highly evolved device, with a dial and gears and lots of machined metal. I’ve never used one, and with a selling price of \$440 I doubt I ever will. It’s probably worth that, though, to a certain segment of the market.

I’ve heard good things about most of these devices, and have considered buying one or two besides the Estes. Mainly it’s cost that’s held me back — psychologically I just have a problem talking myself into a \$100-ish single purpose tool when that same money could go to buy rockets! Or, you know, food and shelter. And I can’t machine aluminum, and the Guillotine looks just tricky enough to build from scratch that I’d rather not try.

But there’s another design kicking around. This one I haven’t seen done commercially, but there are a number of people who’ve built their own versions. There’s a thread about it here. It’s similar to the Guillotine in that it handles tilt and cant (but not position), does any fin thickness (using shims, which can just be leftover pieces of the same wood used for the fins) with any sweep, and doesn’t depend on the motor mount (so can be used on clusters). It’s a simpler design, though. In its most basic form it’s just two pieces of wood, one longer than the other, with beveled edges forming a V-shaped groove when put together. Put shims between the wood pieces, secure the airframe in the groove with string or bungee cords, and the surface of the longer board then provides a place to put the fin at zero cant and tilt. Add legs, some bolts or all-thread to hold the pieces together, maybe cup hooks or something to tie onto, and there you have it.

The ones on that page generally seem to use 45° bevels, but another version I’ve seen elsewhere (can’t find it now) made them 30°, easier I think to cut (at least with a circular saw), resulting in a shallower but still serviceable groove. Scroll down in that thread and you find one made out of scrap Melamine, where instead of cutting a bevel off a thick board, two thinner pieces are glued together, beveled edge to beveled edge. Melamine or MDF should be nice and flat and warp-free, good things in a tool like this. I put together the ideas I liked best plus one or two of my own, and then started building…

But that’s Part 2.