Rocket started OK but then started making strange turns, why?

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billdz

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This rocket flew beautifully a couple of times with two D12-5 motors. Last week we tried it with two E9-6s. The first second or two was fine, then the rocket started making strange turns. Initial speculation was that one of the motors had failed to ignite, but the photos show that both did properly ignite. Perhaps one motor cut out prematurely? See the below photos, any thoughts on what might have happened?

Double O composite.jpg

Double O composite 2.jpg

Double O composite 3.jpg

Double O composite 4.jpg
 
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Did you check for stability with two E9 motors? It looks long enough that stability should not have been a problem, but E9s are heavier and less powerful than D12s. The slower speed may have made you more prone to weathercocking. Remember, that winds aloft may be stronger and in a different direction than on the ground.
 
If this was a weathercock, it was different from any I have seen before. The rocket just kept turning in different directions. And the trajectory was good at liftoff, so it should have been even more stable when the turning started. Maybe it was some kind of different wind aloft.
 
Stability with the E9's should be double checked, in terms of static stability and speed off the rod. It could also be that one motor started a tad earlier than the other.
 
Check nozzles after flight and compare to new unfired motor nozzles. There could be uneven erosion or a chunk missing from the nozzle leading to vectored thrust.

Otherwise, the rocket has fins in the middle which are BAD for stability. That, combined with the very heavy E9 motors shifting the center of gravity aft could result in neutral stability. If the rocket is neutrally stable, any gust of wind or slight bending of the airframe will result in unstable flight since it cannot correct itself.

Neutrally Stable = meandering wherever it feels like going.

Unstable = doing loops - usually tight loops - end over end.
 
Pulled the spent motors out of the garbage, here's a pic. They look pretty normal to me. Fred's idea of "neutral stability" is the best explanation I've heard so far. I've seen several unstable launches where the rocket goes crazy right on liftoff, often straightening after a couple of seconds once the propellant burns off and the CG shifts more towards the top. This is the first time I've seen the opposite -- a smooth takeoff but then turns after a couple of seconds.

E9 motors.jpg
 
Put the spent motors next to a new unfired motor to compare the nozzles.

Zoom in as close as possible.

Take photos from several angles as well as looking directly into the aft end. This will help compare the diameter pre and post firing and if there was any asymmetrical erosion.


Pulled the spent motors out of the garbage, here's a pic. They look pretty normal to me. Fred's idea of "neutral stability" is the best explanation I've heard so far. I've seen several unstable launches where the rocket goes crazy right on liftoff, often straightening after a couple of seconds once the propellant burns off and the CG shifts more towards the top. This is the first time I've seen the opposite -- a smooth takeoff but then turns after a couple of seconds.
 
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