Not sure what caused this as I built the kit per the instructions and a previous flight on a B6-0/A10-3T combo was picture perfect.
Well, a C6-0 is heavier than a B6-0, so the CG at liftoff was a bit farther back than when it flew stably on a B6-0.
Still, I would expect a kit to have a pretty decent stability margin, and for sure that kit should be stable on a C6-0, I am just pointing out that the B6-0 flight did not prove it would be stable on a C6-0.
Now, the finish of the model looks like it is really nice. Possibly.... too nice? I am wondering if possibly a lot of weight was added to the aft portions of the model, to a greater percentage than to the rest of it, making it more tail-heavy than normal.
Thinking on it a bit more, the design is nice as a "hidden 2-stage" type of thing, but that is not good for stability. For better stability, the ring(s) ought to be farther back. Also, by making the balsa fins so stubby, they are almost more like strakes than fins, reducing their effectiveness for the given area. Then speaking of strakes, those small fins higher up on the body, totally BAD idea by whoever designed this. As much as this model is playing around with stability by the neat back to back "hidden stage" design, the last thing this needed was some extra fins ahead of the CG to reduce the stability margin.
Was it windy or sort of calm when you flew it? Sometimes marginally stable models will fly OK in calmer winds. But in higher winds, when they leave the rod, they are at several degrees angle of attack to the wind and that can be enough to push them unstable. But I still suspect it was the mass difference from the B6-0 to C6-0 that made it fly stably one time and unstable the next.
Oh, let me put on my Swami hat....... the rocket went unstable to head towards the East.... if the wind was from the West. OK, old trick, unstable rockets usually go unstable towards downwind.
One last possibility. The rocket was stable enough, but the nozzle had an issue that caused the thrust to not be perfectly axial. This happened at our BRB December Launch. A Mercury Redstone went unstable, but had flown stably before. It was a mystery. Then the owner saw a liftoff photo I shot, and noticed the exhaust plume was "vectored" a degree or two, aligned to cause it to pitch in the very direction it did go when it went unstable (pitched to the left as seen in the photo). Unfortunately, with the crud that a fired engine has in the nozzle, it might not be possible to see if that was the case here or not. And if the engine had been rotated 90 degrees, then the exhaust would have looked normal in that photo.
- George Gassaway
