I noticed that my 24mm Deuce's Goblin spun a but (but not bad) on the way up at NSL. I double checked the alignment of everything with a laser sight I have for work, and everthing seems to be in order. So I began hypothesizing, and the only thing I came up with may be what causes that spinning on the Tres.
BTW, Jim, I thought I noticed a little spin/waggle on yours when you flew it Sunday at NSL. Once again, not bad, and nothing to make you take pause and worry or anything - but perhaps I'm not remembering correctly.
Anyway - my only theory on my Goblin that makes any sense to me is attributed to slight inconsistencies between the two E9-8 motors used. Although they are directly opposing each other in the horizontal direction, a slightly inconsistent burn/thrust-curve between the two motors might cause "one side" to thrust a tad higher than the other at finite moments during the motor burn. This is probably amplified a bit with the higher thrust of the 24mm engine and the longer burn of the E's. Perhaps a phenomenon like this caused just enough non-vertical force to be applied by the engines to cause a slight spinning effect (with the aerodymic stability of the rocket itself in flight keeping her going straight up overall)
If this motor inconsistency theory can possibly be applied to the Tres: Sure, you have three canted motors whose horizontal thrust vectors cancel each other out when the thrust is equal. However, if there are finite inconsistencies in the individual thrust curves of the three motors in question (more possible with three than two as well) then you could have a somewhat oscilating net horizontal thrust vector during the motor burn, possibly in three different directions (rather than just two in the Deuce case.) This could be a cause.
I've not modelled the Tres on RockSim to find the static margin of the rocket - increasing the static margin might fix the waggle if it's an oscillating horizontal thrust vector (enough overstability in the aerodynamic stability to overcome it.)
This seems to be the case in the Deuce design - as I've never seen one waggle or spin (unless the spin was imparted on purpose.) The other helping hand on the stock Deuce design is that it only uses 2 motors, 18mm in size (a manageable amount of thrust and duration in this case) and the possibilities of horizontal thrust vectors due to inconsistent or non-exact engines is not as great with only two motors directly opposing each other.
Actually, after thinking a bit more about the Tres case, this oscillating thrust vector would not necessarily be limited to three directions, it could oscillate a full 360 degrees to the opposing angles and the possible differences in the thrust curves. And also, this dynamic horizontal vector has the greatest possibility to be largest at MaxQ.
**Disclaimer - I'm no rocket engineer, but this hypothesis makes sense to me. Any of you "real" rocket scientists want to chime in?
