Strange Oscillation In Flight

[Rocketholic]

(I tried to find references to this phenomenon using the search function but didn't really see anything.)

At the club launch this past Saturday we witnessed an MPR that, shortly after takeoff, started to oscillate. To be honest, it may be classified as an HPR but that is beside the point. Anyway, at somewhere around 500', with the nose still pointed in the direction of travel trajectory, the aft end started to make a circular motion about the long axis. The diameter of the circle was somewhere around 1/4 to 1/3 the length of the rocket. In this case it was around 1 to 1-1/2'. I don't think my friends had ever seen something like that and started to debate the cause.
Illustration:
Take a writing utensil and stand it perpendicular to a surface, pointy-end up, and hold the tip motionless. Now with the other hand gently trace a small circle on the surface about an inch in diameter. The period for each oscillation circle, while slightly varying, was around 0.5 to 0.75 seconds.
Once the rocket started slowing down after motor burnout the oscillations stopped.

Initially we thought the fins were simply out of alignment. Later, this same thing happened to my MPR!

My rocket, which happens to be in my avatar photograph, is around 46" long, just less than 19 oz. w/o motor, and has a three-fin setup. Stability was around 2 x diameter (CG forward of CP). I have extensively simulated many motors in a very detailed Open Rocket design file and powered it accordingly. Due to only having 3 instead of 4 fins, I made them bigger (more surface area) to help with stability.
This hasn't happed with E26, F30, and F32 Aerotech motors. It DID happen with an F52 RMS. (My rocket didn't oscillate nearly as much or as long as the first one we saw.)

Notes:
- First rocket had smallish fins (I thought) and may have only had a 3 fin set up like mine.
- The fins on my rocket are very straight and perpendicular to the tube (as best as I could get them).
- Flights with less acceleration and lower max velocity were clean, stable, flights - no oscillation.

So, is a lack of sufficient fin area to keep it stable at higher velocities the cause?
Would winglets on the aft main fins (not the pseudo-booster fins) help with this?
Am I missing something completely?

 

[James Duffy]

That motion is called "coning," and takes place when there is adequate nose weight to maintain stability, yet fin area is insufficient to prevent the motion. The mid-body fins on your rocket are moving the center of aerodynamic pressure forward, which allows the phenomenon to occur.

Solve it by increasing fin area as far aft on the rocket as possible. That can be tough to do on a nice scale-ish model such as yours, however.

 

[Rocketholic]

That motion is called "coning," and takes place when there is adequate nose weight to maintain stability, yet fin area is insufficient to prevent the motion. The mid-body fins on your rocket are moving the center of aerodynamic pressure forward, which allows the phenomenon to occur.

Solve it by increasing fin area as far aft on the rocket as possible. That can be tough to do on a nice scale-ish model such as yours, however.

Thank you for your explanation and the compliment! It wasn't supposed to turn out missile-like but the more I worked on it it seemed the way to go! LOL (It was supposed to be a quick, down n dirty, test bed for sugar motors.)

Then it is as I suspected. I'll have to keep the power to an F40 or lower unless I'm feeling the need to trust a fart.

The F32 flew it very nicely and reached 826'.

 

[rocketguy101]

Just out of curiosity, have you plotted the "Angle of attack and orientation vs. time" using wind conditions similar to the actual flight for the different motors you listed? Combining the vertical and lateral orientations might show the coning??

 

[Rocketholic]

Just out of curiosity, have you plotted the "Angle of attack and orientation vs. time" using wind conditions similar to the actual flight for the different motors you listed? Combining the vertical and lateral orientations might show the coning??

I tried this but the graph looks pretty much the same for all of the other rockets I've modeled in OR.
Honestly, I don't know what I'm looking at/for.

The small amount of research I've done indicates that an over stable rocket with the masses concentrated at the fore and aft ends plus not having sufficient aft fin surface area to combat the pitching and yawing will be most likely to exhibit the coning phenomena. This describes my rocket to a T.

I have a way over-stable rocket with BT-55 tubes but with 4 fins and it flies fantastic on 18mm A-D motors. Can't wait to try it on a 24mm D class motor.

 

[Rocketholic]

Just out of curiosity, have you plotted the "Angle of attack and orientation vs. time" using wind conditions similar to the actual flight for the different motors you listed? Combining the vertical and lateral orientations might show the coning??

I just did some mass wrangling in the sim that I can complete in the rocket and by reducing the stability caliber closer to and below 2.0 the graph smooths out considerably. It is interesting (and makes logical sense) that right when the rocket is around max Q, the motor is around 1/2 or more burned away. It just so happens that the burning away of the propellant (reducing aft weight) shortens the CG (moves it forward) making the stability caliber greater than at takeoff.
Granted, the stability is a dynamic thing based on many factors. It would appear keeping mine between 1.5 and 2 is paramount to a clean flight. Huh, so the books were right... Whooda guessed THAT!?

Thanks for the tip!!