Removable Fins
The pages of this web site are often devoted to the technical challenges associated with extreme rocketry. Construction, materials, electronics, telemetry, propulsion, and others have all been addressed in various ways; but I can think of three significant issues that plague the large project builder. These are, in order:
- Storage;
- Transportation;
- Storage
Make that five issues. Did I mention transportation and storage?
The fact is, some rockets are too darned BIG to fit just anywhere. I've heard stories about the guy who built his Level 3 rocket in his basement only to discover he was unable get it up the stairs. (No, it wasn't me....) But I do have a 14-footer that stands in the portico outside my front door. My wife hated it until she realized that it frightens away solicitors, and now she tolerates it with resigned annoyance. At least it is no longer standing in the living room.
Cutaway drawing of the removable fin design used on the Hindrocket. Note the interlocking parts.
I know of several clever rocketeers who have defeated this problem by making their rockets portable with removable fins. Once the fins come off the rocket fits neatly into the back seat of a sedan, a storage space in a garage, or even an air freight container (the most expensive factor in shipping a rocket is volume, not weight). Another advantage is that damaged fins are easily replaceable. Certification rules allow for frangible parts, so you can still certify if with a broken fin if you can demonstrate that it was designed to be replaced.
It's said that there is more than one way to skin a cat. However, since PETA and the SPCA have clamped a virtual death-grip on the genteel pastime of imaginative feline epidermis removal, innovative cat-skinners have turned their creative energies to the solving of other useful conundrums. That's good news to rocket builders, because at least three of them have devised nifty designs for removable fins.
One I can attribute to my brother, Rick. He was up-scaling a 1950's rocket-shaped coin bank -- The Hindrocket -- into a spiffy J-powered craft, and he doubted the pointy fins would survive the landing. So he designed the rocket with replaceable fins. There are tabs on the forward edges of the fin tenons that slide forward into receiving slots cut into the forward centering ring. The fins fasten into place when an interlock ring is screwed into the aft end of the rocket; the interlock ring intersects with slots cut in the aft edges of the tenons (see illustration). The fins are intentionally breakable, absorbing the shock of landing by crumpling on impact. He conserved a lot of his weight budget since he didn't need to heavily reinforce his fins (made from cheap 1/4-inch Masonite), and the shock absorption preserved the intricate details of his stylistic airframe.
Click [HERE!] to view an exhaustive (and excellent) article on how Rick designed and built this innovative solution.
There are larger projects, like the Gila Monster built by Ed Wilke and Bob Heninger, or Eric Burch's 16-inch diameter Fat Boy, that incorporated heavily reinforced removable fins purely for reasons of portability. Their designs varied slightly, but used common elements. The photos are from Bob's Mini Gila Monster.
The concept is pretty elegant. A plywood 'box' for each fin tenon is installed in the fin can and reinforced with fiberglass. The fin can is mounted into place in the aft of the airframe, bonded into place with epoxy, and fortified with expanding foam. The slots can be opened up using a router and a straight rabbet bit with a roller bearing on the end; the roller follows the inside of the 'box' just like a router guide. Screws extend through the forward and aft centering rings and into the fin edges to anchor the fins in place.
Bob used brass threaded inserts in the fins of his Gila Monster to receive the machine screws: Eric used threaded rod in his Fat Boy, which extended through the entire length of the fin tenon and through both centering rings. Both designs performed as intended, as both rockets have flown successfully multiple times and both have still fins with the capability to slide in and out of their respective slots with majestic ease.
Bob cuts the fin slots
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The perfect fin slot
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Threaded inserts used to anchor the fin
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The fin installed between centering rings
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These designs represent only a couple out of (perhaps) an infinite number of viable solutions for the same problem. But more than that, they also represent three content households without large launch vehicles on permanent display, and, three more domestic mousers are saved the embarrassment of slinking about skinless.
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