Why did my rocket go unstable?

[diyaerospace]

Hi,
I recently flew a spin-stabilized model rocket on a C6-7 motor. The rocket is not statically stable but the fins are canted which spins the rocket and spin stabilizes it.
This HAS worked before. I flew this rocket on B4-4 and A8-3 motors multiple times and it worked just fine. After this I gave the rocket a paint job and flew it on a C6-7 motor, it went unstable. Openrocket simulations said it would fly straight on a C but that clearly wasn't the case. I tried an A motor and the flew straight during the burn but as the rocket slowed down it spun out. This same design worked fine on an A motor before.
I then added some weight to the nose and flew the rocket on a C motor again. The rocket was stable during the burn but after burnout, the rocket spun out. I was very confused and ran lots of simulations to try and figure out what is causing the problem. I was unable to find fault in the simulations or the rocket.
Does anyone have any suggestions?
Thanks.

The videos below show the flights described above

the first video is of the 2nd flight on the C motor
the second video is of the flight on the A motor
the third video is of the unstable flight on the C motor

 

[Highpowerrocketyahoo]

Upper wind shear that was counteracted under thrust perhaps?

An expended A weighs less than an expended C so perhaps that slight weight difference created an offset COG?

 

[UhClem]

Spin stabilization in rockets is mostly a myth. It works in things with low L/D like artillery but not in rockets. Spin, at the correct rate, can reduce dispersion.

I remember that Topics In Advanced Model Rocketry had something to say about it.

 

[ZoomieG]

Hi,
I recently flew a spin-stabilized model rocket on a C6-7 motor. The rocket is not statically stable but the fins are canted which spins the rocket and spin stabilizes it.
This HAS worked before.
I then added some weight to the nose and flew the rocket on a C motor again. The rocket was stable during the burn but after burnout, the rocket spun out. I was very confused and ran lots of simulations to try and figure out what is causing the problem. I was unable to find fault in the simulations or the rocket.
Does anyone have any suggestions?
Thanks.

I suggest you read "The Handbook of Model Rocketry", by G. Harry Stine, in particular chapter 9.

A rocket that is statically unstable cannot realistically be made so by fin induced spin. The fins cannot spin the rocket fast enough to reliably overcome the instability. Additionally, the rate of spin changes based on velocity, which is likely why it tumbles after burnout.

Attempting to stabilize gyroscopically can also lead to a phenomenon known as "pitch-roll coupling", which is a form of dynamic instability in which the roll actually magnifies the pitch oscillations.

Which might be why you can't find an error in the sims. Most sims are pretty good at calculating and displaying static stability, but to account for dynamic stability you have to dig into the simulation parameters quite a bit, or they just aren't factored for. The calculations for dynamics are much more involved, and much less predictable due to the number of variables.

Hope this helps!

 

[diyaerospace]

Thanks for everyone's help and time!

 

[rocket_troy]

Spin stabilization in rockets is mostly a myth. It works in things with low L/D like artillery but not in rockets. Spin, at the correct rate, can reduce dispersion.

That's not really a useful comment. There have been countless sounding and a few small orbital launch vehicles utilising spin for stability for at least one of their stages. If we added a "for model rocketry and general HPR" qualifier, then perhaps it might be correct - I'm not sure.
You do need *mass* and *balance* as minimum prerequisites for the gyroscopic forces to overcome other (upsetting) aerodynamic and misc. inertial forces ie. to become effective at stabilising the rocket - things seldom available in model rocketry.
Saying that, we had one guy come to 1 or 2 of our club launches many years ago who only flew 2 fin spin stabilized rockets in the LPR -MPR power range. Every single one of his flights were perfectly stable.

TP

 

[Grog6]

I would bet it was unbalanced in rotation; it eventually sky's off in some random direction. It was suggested to me as a way to alleviate weathercocking. I broke off a fin, unbalancing it, and had similar results.

 

[icyclops]

Hi,
I recently flew a spin-stabilized model rocket on a C6-7 motor. The rocket is not statically stable but the fins are canted which spins the rocket and spin stabilizes it.
This HAS worked before. I flew this rocket on B4-4 and A8-3 motors multiple times and it worked just fine. After this I gave the rocket a paint job and flew it on a C6-7 motor, it went unstable. Openrocket simulations said it would fly straight on a C but that clearly wasn't the case. I tried an A motor and the flew straight during the burn but as the rocket slowed down it spun out. This same design worked fine on an A motor before.
I then added some weight to the nose and flew the rocket on a C motor again. The rocket was stable during the burn but after burnout, the rocket spun out. I was very confused and ran lots of simulations to try and figure out what is causing the problem. I was unable to find fault in the simulations or the rocket.
Does anyone have any suggestions?
Thanks.

The videos below show the flights described above

the first video is of the 2nd flight on the C motor
the second video is of the flight on the A motor
the third video is of the unstable flight on the C motor

Show us the fins….please

 

[gldknght]

You said it yourself. Your rocket is not statically stable. So as the motor thrust decreases toward burnout and the rocket slows down, the (questionable) stability induced by the spin loses effectiveness. This will be even more apparent at altitude where the wind can be much stronger than what you feel on the ground.

As others have pointed out, a C motor is heavier than an A motor, making your rocket even more tail-heavy. This should NEVER be considered a safe condition for launching your rocket.