Aerodynamics curiosity - Rocketarium Terraformer - Why does it spin?

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Guess you'll just have to buy a kit and launch it down under.
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One wouldn't be enough. To be statistically significant I usually use 30 as a starting N. You can build and fly them there, then ship them down to me :) .

Actually Hawaii might be too close to the equator to be useful for testing this. A higher latitude might give more consistent results.
 
Any pitch or yaw on a 3-finned rocket will induce roll. Easy to visualize if you point a 3-fin rocket at one eye and then move it off-axis, you can see the asymmetric fin areas exposed to the airflow, which will induce a roll moment. On a 4-finned rocket, the exposed areas are generally symmetric. There are other effects, but this is basic.

One of the most stable flight videos I've ever seen was with a model using a printed fin can. Perfect fin alignment, perfect fillets.
 
Any pitch or yaw on a 3-finned rocket will induce roll. Easy to visualize if you point a 3-fin rocket at one eye and then move it off-axis, you can see the asymmetric fin areas exposed to the airflow, which will induce a roll moment. On a 4-finned rocket, the exposed areas are generally symmetric. There are other effects, but this is basic.

One of the most stable flight videos I've ever seen was with a model using a printed fin can. Perfect fin alignment, perfect fillets.
Looking at the maths of this (in a missile aerodynamics book), roll-pitch coupling is a VERY small effect. For our rockets it is almost exclusively fin alignment and other asymmetries that cause roll. I am pretty careful with my builds, milling slots accurately on a milling machine and using other techniques. I still have not got zero roll rate. Here is my 1/2-scale Nike Apache flight. It is curious to note that the roll rate was 265deg/s with the motor on, and only 47deg/s after burnout, for the sustainer. Not a bad roll rate considering it was a t M2.14.
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Could it be a combination of imperfect building AND design?

very few low power rockets are built with near perfect fin alignment, although @neil_w and @Ronz Rocketz do pretty well. But the Terraformer design doesnā€™t have much and ā€œflat lateralā€ surface to resist spinning once it starts.

although interestingly, some sounding rockets used a twisted launch rail to provide spin stabilization from the get go.

https://afspacemuseum.org/artifacts/loki-and-launcher/
 
One of the most stable flight videos I've ever seen was with a model using a printed fin can. Perfect fin alignment, perfect fillets.
maybe my Estes Astrocam printed fin can got bent in shipping, it has a mild spin, although I have seen some other videos of that rocket with little or no spin.

definitely agree, however, that the prime culprit for most spin is fin asymmetry. Printed fins should make near perfect alignment a heck of a lot easier.
 
Looking at the maths of this (in a missile aerodynamics book), roll-pitch coupling is a VERY small effect. For our rockets it is almost exclusively fin alignment and other asymmetries that cause roll. I am pretty careful with my builds, milling slots accurately on a milling machine and using other techniques. I still have not got zero roll rate. Here is my 1/2-scale Nike Apache flight. It is curious to note that the roll rate was 265deg/s with the motor on, and only 47deg/s after burnout, for the sustainer. Not a bad roll rate considering it was a t M2.14.
View attachment 554997
Interesting indeed. For sustainer roll rates, are those AVERAGES (ā€motor onā€ refers to time from ignition to burnout, and ā€œmotor offā€ refers to burn out to deployment?)

I would assume Maxximum booster velocity would be at staging/booster burn out, while I would also assume that Max sustainer velocity would be at sustainer motor burned out. Therefore the sustainer for the ā€œmotor onā€œ time span begins at Mach 1.144 and finishes at Mach 2.14, while motor off starts at Mach 2.14 ands at near 0. So velocity averages would be significantly different.

so roll rate would likely be close to directly If not directly proportional to velocity, which would make sense for any rocket Asymmetry, fin or otherwise.
 
Interesting indeed. For sustainer roll rates, are those AVERAGES (ā€motor onā€ refers to time from ignition to burnout, and ā€œmotor offā€ refers to burn out to deployment?)

I would assume Maxximum booster velocity would be at staging/booster burn out, while I would also assume that Max sustainer velocity would be at sustainer motor burned out. Therefore the sustainer for the ā€œmotor onā€œ time span begins at Mach 1.144 and finishes at Mach 2.14, while motor off starts at Mach 2.14 ands at near 0. So velocity averages would be significantly different.
I think it will be peak roll rate when under boost, and for the sustainer I remember it dropped almost immediately (not tailing off proportionally to velocity) to the low value of 47deg/s, at burnout. It was peculiar and hence why I reported it in the figures.
 
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