North American Aviation workers fit fin to Little Joe booster
Fin Construction
The "Little Joe" fins are ideally suited to a spiffy scale model, as they are large, full of detail, and very distinctive. The draggy cross-section of the fins will help the model fly a slow, scale-like flight profile.
1/17.44 scale Little Joe fin surface detail (click for PDF version)
Fin construction begins by whipping up a drawing in Vectorworks. In addition to the basic outline of the component, this drawing also shows the general arrangement of rivet and panel lines on the surface of the fin. This drawing also serves as the template for the fins that we'll use on the boilerplate version of the model.
I was all set to forge ahead with what I now call the "heavy panel line" fins, but Kevin Johnson, a fellow member of the US Team headed to Baikonur, threw the bullshit flag on my plans. Kevin sagely pointed out what I already knew: the fins just aren't right. I decided to absolve myself of this modeling sin and prepare fin masters that would accurately replicate the riveted appearance of the prototype, also abandoning the vacuum-bagged fiberglass fascia approach. Frankly, I had avoided producing the riveted fin masters in the first place because I dreaded the thought of drilling hundreds of tiny little holes with a pin vise, a task that would require far too much beer to get through with sanity intact. The obvious solution was to go out and buy a Dremel, something I had formerly considered a barbarian's tool. Even with the Dremel at hand, it took roughly six hours to drill the rivet holes in the fin masters.
With that finished it was a simple matter to throw the masters on the vacuform table, heat up the oven, and pop the parts out of .015" styrene sheet. The results are wonderful, with subtle rivet detail that realistically mimics the appearance of the prototype. After cutting the parts from the backing sheets, the styrene fascias were attached to 3/32" balsa with 3M permanent double-sided tape. Note that the rivet detail does not show up in these photos, as my old digital camera outlived it's usefulness long ago.
A bevel is cut and sanded into the balsa on the leading edge of each fin surface, allowing the two plastic fascia pieces to meet at a sharp point. After the fit is right the fin halves are joined with Titebond, and two spacer pieces are inserted inside the fin to hold the halves at the correct fore and aft spacing. Next, Tenax 7R is wicked into the plastic joint at the leading edge of the fin, joining the plastic fascia pieces. (Tenax is a less-aggressive plastic adhesive than the ProWeld used on the escape tower lattice, and is better suited to the thin styrene used on the fins.) When dry, the leading edge is finished with a series of sanding sticks, from coarse grit to fine. Cutting the bevel into the fin halves has been one of the most time-consuming steps in the construction process, with each piece taking about 30 minutes to prepare. That's eight hours I'll want back someday...
A long pie-shaped piece of 3/32" balsa is cut and sanded to fit into the gap on the bottom of the fin, and is then covered with a piece of .015" styrene sheet. (If you're interested in seeing some of the rivet detail be sure to click on the left photo, as some of the detail is visible in the large photo linked there.) Tenax is applied to the joint of the bottom and side styrene fascia pieces, then trimmed to size with a #11 blade when dry.
Next, a drawing of the fin root plate is created in Vectorworks to assist in cutting these critical parts. The fins are designed to be removable for easy packing and shipment, a critical quality for any model that needs to survive a trip to the far side of the globe to Kazakhstan. The small hash marks on the centerline indicate the locations where blind nuts will be inset into the fin root.
The fin root plates have been cut from 1/16" plywood using the template shown above as a guide, then backed with a scrap sheet of 3/32" balsa. The balsa backing will provide a bit of extra thickness for the teeth of the blind nuts to bite into, as well as some additional glue surface once the plates are fitted into the fins. (The right photo is horribly composed, further evidence of: A) my incompetence as a photographer, B) the inadequacy of my camera, or C) both.)
Once the fin root plates are in place, the locator holes for each fin are carefully marked on the body tube, then drilled out to accept the 2-56 allen head bolts. This is by far the most critical step in the project so far, and screwing up now will set the project back more than a week. I took this as an opportunity to invoke The Astronaut's Prayer, an entirely appropriate choice given the subject. The photo on the right shows each fin in place, as well as the bare balsa that still needs to be covered with styrene. The tip of each fin will receive a cover with vacuformed detail, and a simple piece of styrene sheet will cover the underside of each root.
I managed to mark and drill all of the locator holes successfully, so I'm very close to completion of the fins. Ideally they will be finished within a week, complete with paint!
The tip plates of the fin have four distinctive depressions that we'll need to model, as shown in this photo. To duplicate this, a drawing of the details is whipped together in Vectorworks, then that drawing is used as a template to cut vacuform masters out of .020" styrene sheet. We'll do four masters so that all four parts can be produced with a single sheet on the vacuformer, as it takes less time to make three extra masters than to reload the vacuform frame three extra times.
The small opening at the base of each fin root is addressed next by gluing a piece of .015" styrene sheet over the gap. We'll trim and sand the piece after the Tenax dries...
The fin tip plate masters are placed on the vacuformer, then molded out of .015" styrene sheet. We don't need to be especially precise when cutting the parts from the backing sheet, as the final trimming and shaping will occur after the oversize tip plate is glued into place on the fin.
The Dremel tool makes an encore appearance, as we need to gently grind away a bit of balsa from the interior of each fin to provide clearance for the concave details on the tip plates. Once the fit is right, we can glue the oversized tip plate to the end of the fin with Tenax.
The final two photos show three trimmed fins, as well as a fin with an untrimmed tip plate. With that little chore behind us, we can start in on final prep of the completed fins, which consists of finish sanding and some Squadron white filler in a few spots. The fins are now ready for paint!
Back to part 1: Capsule assembly...
Back to part 2: Escape Tower assembly...
Back to part 3: Transition assembly...
Back to part 4: Boilerplate assembly...
On to part 6: Airframe assembly...
On to part 7: Paint and Markings...
On to part 8: Motor Mount assembly...
