I’m more intrigued by the printed fin can - printed with slots ready for balsa fins? I really like that idea! Joe, can you talk about the concept and design?
That's actually how I got started thinking about producing kits. I wanted rockets to give my daughter and our friends (adults and kids) that would be easy to assemble---potentially even on-the-spot at a launch. My daughter's very good with crafts and is picking up rocketry real quick, but ultimately she's not quite 6 yet. Aligning & setting typical surface-glued fins and similar tasks are challenging for her. So, having done a lot of 3D printing for wargaming, I made printable fin canisters w/ slots to make it easy to align & glue them.
That blue rocket above is a design using one of them that she made playing with parameters in OpenRocket and then built with some help.
Of course there are tradeoffs for that convenience. A plastic fin canister is fairly heavy for an MMX rocket. Not enough to be infeasible by any means, but it does cut altitude and limits the space of stable designs. E.g., the motor mount assembly in the
R3 and
R4 kits, which themselves have 3D printed rings, weigh just under 0.6g. My fin canister designs so far have been more like 1--1.6g. The
original R4 and R6 designs were forced to be a good bit longer than
R3 in order to counteract that weight in the tail.
The natural, easy way to do a printed fin canister also has it just plug into the back of the body tube, as those originals do. But then, if you care, you have to deal with the layers. Sanding and/or coating the layers away is against the goal of being simple to assemble. Printing at higher resolution/smaller layers would improve the look, but would take a lot longer to print and make the design weaker---FDM prints are sensitive to shearing forces in particular, because of the comparatively weak inter-layer bonding, so the more layers you have the more opportunity for the part to just snap. Resin prints are even heavier so I'm hesitant for stability reasons to put "large" resin pieces in the tail. Tight dimensional accuracy with elements of varying thickness as well as small holes in the canister would also be somewhat challenging in resin.
So the production version of
R4 lost the fin canister it originally had and adopted the same motor mount as
R3. That in turn is a modest refinement of the mount in my
initial scratchbuild before there was any thought of making kits.
Fast forward some months though, and when I asked my daughter what she'd want if we made a kit of her design she still said the same thing that got all this started: "A rocket I can make all by myself." But meanwhile my workshop's been heavily re-focused toward (tiny) rockets, I have more experience, and the designs have been refined. So, for her birthday on Wednesday she's going to get the very first production
R6 kit, a rocket she designed & can pretty much build all by herself.
As a conscious design choice it is still very much a kit, not a ready-to-fly model. There's some very minor cutting, gluing, etc. required. But the fin canister eliminates a lot of fiddly steps while still presenting a traditional smooth paper outer surface. You put some glue on it & slide it right into the body tube, no messing about with trying to spread glue inside for centering rings. It integrates the motor tube & block + friction retention, fin slots, shock cord anchor, and launch lug. The launch lug and a matching body tube slot align the canister rotationally and vertically, so the fin slots on body tube and canister all line up and you can't push it in too far. The canister and body tube slots together hold the fins well while drying and make it easy to put together even this 6-finned design. The anchor lets you tie on the shock cord with a simple shoelace knot & drop of glue, or even just a loop & sticker with no knot needed at all. The launch lug of course is already set, so no putting that on askew or gluing your fingers together. All in all I think it reduces the fiddliness just enough to make the kit more approachable for fairly young kids while still looking & feeling just like the "adult" kits and giving them a very similar experience.
That's a lot of words about a tiny little rocket. But that's the story on the fin canisters, and it's a whole big thing for my daughter & I. Thanks for asking
@Scott_650.
View attachment prototype-cut.mp4
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