As I was pondering what to do with the SA-666's successor, I went back to read some of Dean Black's write-ups. I don't remember seeing this in his Apogee Newsletter article but I find that, while an air gap behind the CG results in induction stabilization, an air gap ahead of the CG retards it. D'OH, thats what I randomly decided to add to the SA-666. When it comes to rocket science, you just can't make stuff up. But hey, I never claimed to be a rocket scientist.
Soooo, I decided to resurrect this rocket and try again. The first thing I tried to do is find the transition wrap that I threw away when I invented the air gap transition. I didn't find it so a roughed out a new one (which lacks the nice detail...oh well).
I also wrapped the damaged nozzles with some metal tape. In my experience, this stuff will protect the substrate, although the heat melts the glue so it tends to fall off. Oh well again...I only need it to stay on for one flight.
Dean's latest reference design (updated from what's presented in the Apogee article) includes some added holes for long induction tubes. When I sized my air gap, I included the area of those holes in the main gap. There I went, making stuff up again. Anyway, I decided to cut some holes below the air gap. I don't know how this all plays out with the (hopeful) induction stabilization but it might help with any Krushnik problems. On the first flight, the rocket was stable (had a lot of coning, but generally went in an upward direction) but the altitude was a lot lower than expected. That could have been Krushnik or it could have been instant instability during coast. I wish it had been video taped, but I don't think it was.
Random thoughts on the SA-667. I still can't decide on the geometry of, or the material for, the internal, smaller induction tube. I am also thinking about making it a ring tail kind of like those used on water rockets. I just don't know. This build may not be finalized until after the MDRA August launch when the SA-666 will fly again.