(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?
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?