As a L3CC it would be a no.... I would fail it
If you came up to me BEFORE your flight and asked me to 'do your cert' , I would ask the question, how do you know the rocket is strong enough?
If the answer is 'i built a kit and am using a motor the kit recommends' I would ask questions about construction, methods, adhesives (dont get me started about adhesives) for construction.
If the answer is 'I changed the kit using a different way of holding the fins on' There would be many more questions about how do you know it is strong enough. Lots of good ways to answer that question, and a lot of bad ways to answer that too. Dont get me wrong, my level 1 and level 2 rockets were scratch built (I built the tubes from fiber and resin, fins were laid up, I'm talking totally scratch built. I like that people scratch build, but if you do, be ready to 'defend' the materials, methods and design of the rocket. My level 3 was a LARGE rocket with bolted on fins, I'm not against that either. I fly 3D printed fincans too, I'm not saying no for any of those reasons.
Here are the issues I have, and I say these without talking to the flyer;
- Nylon screws is a pretty generic statement, how did you size them, spec then?
- it looks like the edges of the washers are up against the fillet, the engineering term for that is a no-no, causes a bending in the screws, preloading them.
- No washer under the nut, did you 'twist preload' the screws during installation? Really easy to do with nylon screws.
- It is difficult to get the right torque on a nylon screw, have a metal screw, with a significant margin (most do) and this isn't very critical.
- If the fins are pop off, how did the flyer determine what is strong enough to fly but weak enough to pop off, any testing?
- Did the 3d printed flanges have chamfers on the edges (to keep the fin from rotating about the edge), most parts like that will have a lip at the edge (corner is proud not recessed). Were the flanges truly 120 degrees apart or did the parts have snap in. Either of these causes the screws to be loaded in bad ways as the fin 'pivots' on the edges. Like a claw hammer tanking out a nail.
- If the fins were able to slide in the slot, when the fin tip hits the ground the fin acts like a lever shearing the aft fin with little resistance from the other screws Easy to check for this during assembly. Problem is worse due to the shape fin he used. Nothing wrong with the fin but adds load to the screws / joint.
- The fact that it doesn't appear that the flyer knows why the screws failed is a problem to me. Were the screws marginal, installed with preload, and hit by a shock cord on deployment, or nosecone? Or if they did snap off on the ground (mechanical fuse) was the load HIGH enough that with a larger motor or on a different day the fins wouldn't have had any issue during the flight.
- When you pick materials like nylon and 3D printed parts, have you taken into account temperature (BTW hot OR cold). Easy for a rocket (black one) sitting on the pad to get warm enough that - the CTE of 3D printed part causes the, lower strength due to temperature nylon screws, to be REALLY preloaded.
Just a couple of thoughts I had. I really encourage scratch building, new materials, and unique designs, BUT for a level 2 cert that lets you fly some pretty 'fun' motors, it has to be a bit more than a let see what happens build. I'm not saying he needed to do a full material allowable testing program / stress analysis, but, IMHO needs to do some justification for the design. This is where mentors / experienced flyers can help.
Again I'm making LOTS of assumptions here, if the flyer did some testing / analysis ahead of time, my apologies.
Mike K
