Building Stiff Lightweight Fins for an Estes Aries SST upscale?

I am planning to build a upscale of the 1980's classic Aries SST, the reason I am posting this in the MPR section is that I hope to be able to construct in lightly enough to fly on G motors mainly and Hs too.

The biggest issue on the build is that I will be the main fins/wings will be about 8" from root chord to tip chord (semi-span), root chord will be approximately 10" and the tip chord will be about 4" long. My concern is how to build the fins light enough, yet stiff enough that they will survive moderate thrusting H motors ( most likely I will fly it with H90s and the other lower thrusting motors).

I am currently considering using 1/64" ply skins over 3/32" balsa (or basswood) with either two carbon or spruce spars running from TTW root edge to tip edge, the spars would also be part of attaching the winglets more securely, winglets would be ply skinned basswood or balsa as well. Obviously I am trying to avoid using solid plywood fins or much in the way of epoxy.

The rest of the rocket is to be built from a BMS PNC-300K modded to look like the PNC-50S, and BMS T300 airframe (same as Estes 3"). I could easily build this from LOC tube and plywood but then it would only fly on HPR motors most likely.

I have also considered depron cored wings which I have never worked with.

I am looking for other suggestions or comments on my current construction ideas or alternatives and how to apply them.

Rich,

Perhaps this can give you some insight. I did an upscale of the Estes Orbital Interceptor, which was a BT-50 based kit very similar in size to the Aries SST. Using standard low-power parts, I upscaled it to BT-80 and used just plain old 1/8" balsa with no skinning or any other reinforcement. It flies beautifully on composite Fs and Gs.

OTOH, I also used 1/8" balsa (papered) on my BT-80 upscale of a Big Bertha. Yesterday, I had the lower half of one of the fins snap perpendicular across the grain from a hard landing on a soft field. The only thing that held it together was the paint & paper. My personal feeling is that plain 1/8" balsa is insufficient for rockets of this size/mass, unless the landing can be made very soft. I would go with basswood or 1/8" ply at the least. I was looking at doing a BT80 Orbital Interceptor next, once I can get my hands on a suitable cone. I'd designed it using 1/8" balsa, but am now reconsidering. I would be especially leery with the tip fins on the SST, since I've also had repeat issues with that type of feature on my Pro-Series Jayhawk (having just repaired it again over the weekend).

The tip fins are a concern and most likely will have solid spruce/hardwood around the edges. Good to know that balsa worked on the BT 80 upscale. I may even work a very small amount of .75 oz FG into strategic locations ( I will try and avoid FG as much as possible).

I think one of your issues will be the winglet joints. The winglets will very likely be the point which hits the ground first. The heavier the rocket, the more stress those joints will have to endure. I have an SR-99 Hyperswift which popped a fin tip just about every other landing. Eventually, I drove steel pins through the wing and into the root of the fin tips. Were I to build this kit again, I would definitely beef up that joint.

The winglet joints are a special area of concern. Definite reinforcement there.

Keep in mind, the joint itself isn't the only area of concern. This ties back into the original question - if the stress of landing is high, then the whole wing system has to be strengthened. Like mentioned above, on my Jayhawk, I popped a tip fin every (or every other) flight. When I glued it up stronger, the wing itself cracked along the grain inboard aways. Eventually, I built up fairly large (and non-scale) epoxy fillets, which seemed to help for a while. On the last flight, one of the lower tip fins popped free of the attachment and pivoted around / above the epoxy fillet. So, unless the strength of the wings and winglets is matched to the joint strength - and all is up to the task - simply strengthening the joint just pushes the stress further down the line to the next weakest area. You will break wings instead of joints, if the landing stress hits them instead and they can't handle the load.

I would think that the laminated plywood, with the winglets somehow tabbed on, epoxied together and then maybe lightly glassed would be the way to approach that. Or - alternately - construct a recovery harness that drops the model horizontally so the tip fins aren't the first point of contact. But that's not a guarantee either.

I addition to construction the parachute will be over sized to a descent rate near 10 fps.

That would certainly help, assuming launching on a calm day and a soft surface.

Eastern Washington desert sand ( not playa) and sod will be the landing area.