I fly ARLISS.
Most of our 6" M birds use sheet metal fins, with an
internal can. The can is a pre-fab item. The can goes inside the HM 6" glass a/f tube, with the fins screwed to the outside. (It is not a "fincan" like the Acme cans.) Don't know if there are any left to be purchased. They were originally made by Tom Rouse, of Rouse-Tech. Just a thin-walled aluminum can, pre-drilled and tapped for the screws to hold 4 sheet metal fins. A second "batch" was made thru Chris of Erwin Automation, now in Carson City, that were rolled sheet aluminum, with Pem-nuts installed. (Not 100% on this, but I'm pretty sure Chris contracted those to a friend in sheet metal work.) The cans also have a hole for a rail guide.
These birds are assembled using #8-32 button head socket head cap screws. You will find very little epoxy in an ARLISS-M. This makes for easy fin replacement, and easy rebuild, should that be necessary. Fly a lot of ARLISS, and you will bend fins.
Can't speak to TRA rules, but it's absolutely "legal" for a NAR L3 cert. (I'm a former L3CC. Gave it up due to health.)
I haven't talked with Becky in a while, are there more fins available? There was talk of contracting a big batch of fins @ $20 a copy. (I know some folks have spares, but I don't think they'll want to sell them.)
If you can get the fins, making a booster shouldn't be a problem. I know where I can _borrow_ an internal can. (I'm sure that can will not be for sale.) I position the internal can in the booster tube, and use a 90° off-set pneumatic drill to drill the first hole, from the inside. Remove the can, drill out the hole for #8 clearance. Reinstall the can, install that first screw to hold things tight, and drill the rest of the holes. Remove the metal can, replace it with a fiberglass coupler, position it and drill it for #8 clearance for all the holes. Then install T-nuts inside the coupler with some J-B Weld. (Not-so) Instant internal fin can. Then you can finish building the bird.
The upper section is crucial. You'll need a tether and charge well above the av bay, then in goes the carrier, then a spacer plate as a lid on the carrier, then the nosecone gets shear pinned in place. There must be NO gaps. The n/c bulkhead is recessed, to make room for the n/c chute and the tether that attaches to the carrier lid. Mains are below the av bay.
ARLISS is a great program. But it's not for the inexperienced. You will need to demonstrate a proficiency for getting the laundry out. And you'll need to do a test flight for the ARLISS committee, demonstrating that you and your bird can not only get the laundry out, but deploy the payload, too.
ARLISS started in 1999. Over the years, we have over 1000 flights, and in that time, we have lost only 3 student payloads to rocket failure.
Because some of those student projects represent years of work, a grade, and possibly that student's future, that's 3 TOO MANY.
If you want to get fancy, you can always contract to have aluminum centering rings made. Like I said, these things are assembled using screws.
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