# How to Prevent a Rocket from Spinning

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#### Sooner Boomer

##### Well-Known Member
I concur tube fins spin very little.
Except for the Corkscrew - a rocket with a single, off-center tube fin. It doesn't just spin, it spirals.

#### Sooner Boomer

##### Well-Known Member
It does if let's say the rocket was built unevenly or has an imbalance in weight or drag and would otherwise veer to one side...then sufficient spin would prevent that imo.
Something I've noticed, esp. with rockets that have heavy electronics packages (batteries) - the rocket will rotate on its longitudinal axis (direction of flight) around the "vertical" centar of gravity. The further the mass is from the center, the worse the rotation (spin).

#### BABAR

##### Builds Rockets for NASA
TRF Supporter
I've read that 4FNC rockets have less tendency to spin, all else being equal, than 3FNC rockets. I believe I understand why, and it's pretty simple. A cross wind perpendicular to one fin "sees" more area on that side than on the other, so that side is pushed downwind wind more, and, voila, spin. So for the minimum spin, in addition to symmetry and all the other good stuff, if you're using "regular" fins, use four rather than thee.
Maybe. I'd buy that for weathercocking. Assuming same net surface area spread among three fins vs 4*, at any given time the average cross sectional area of the fin can (in 2 dimensions perpendicular to net wind vector) will be greater for 3 find than 4, as the three fins each have a longer hemi-span. But even if on fin is slightly larger, as the rocket rotates it goes from one side to the other, so rotational forces should balance out.

@neil_w point on ring fins is intriguing, and matches my experience with box fins. Any asymmetry on these fins would cause pitch or yaw force, which should be at least partially self correcting. But no roll force, as these fins are always perpendicular to the roll axis..

For standard planar fins (which are PARALLEL TO the roll axis), any asymmetry (canting or non-neutral airfoil) will cause potential pitch or yaw force, which again will be at least partially self correcting. But they will also cause a NON-CORRECTING roll force, i.e., spin.

#### jqavins

TRF Supporter
That reminds me of a point I forgot to make yesterday. Since asymmetric airfoils will cause spin, one method to prevent spin would be don't airfoil the fins at all. Maybe round the corners, but that's it. If you don't try then you can't mess it up. And that is an OK strategy if you're not too concerned with altitude.

Also, you do have a point, using my 3 fin vs. 4 fin reasoning, that the rotational torque would switch sides as the rocket rotates. That means I may have the explanation wrong, but doesn't change what I've read, that most sounding rockets have 4, and that's for the purpose of reducing spin. (Note the word most, so please don't bother raising the counterexamples that are unquestionably out there.)

#### Alan15578

##### Well-Known Member
Something I've noticed, esp. with rockets that have heavy electronics packages (batteries) - the rocket will rotate on its longitudinal axis (direction of flight) around the "vertical" center of gravity. The further the mass is from the center, the worse the rotation (spin).
Spin balancing, to align the principle inertial axis to the airframe, is a very good thing.

#### Alan15578

##### Well-Known Member
It does if let's say the rocket was built unevenly or has an imbalance in weight or drag and would otherwise veer to one side...then sufficient spin would prevent that imo.
Not quite. Intentional roll or spin is used to get more predictable trajectories, not to improve stability. The spin tends to average out the effects of asymmetries. For most of us, the factors you cited are small potatoes, compared to thrust misalignment, which can be alarmingly large and uncharacterized for sport rocket motors.

#### KenECoyote

##### Well-Known Member
TRF Supporter
Not quite. Intentional roll or spin is used to get more predictable trajectories, not to improve stability. The spin tends to average out the effects of asymmetries. For most of us, the factors you cited are small potatoes, compared to thrust misalignment, which can be alarmingly large and uncharacterized for sport rocket motors.
Actually my response was to a question about stabilizing "the flight" of a rocket vs. making the rocket itself more stable. I still believe that is the case that a roll or spin can prevent an unevenly built or imbalanced rocket have a straighter flight (what I interpreted to be "stable flight". This may be a matter of semantics.

Regarding thrust misalignment (with the axis of the rocket), that can be used in a positive way as well if used to cancel out the imbalance...I believe the fliskits Duece can still fly on one canted motor. There are other examples of imbalanced rockets that fly fine due to spin or angled thrust, etc.

#### KenECoyote

##### Well-Known Member
TRF Supporter
Here's a wild thought...regarding the OP's initial inquiry on how to reduce spin for a camera, how about having it mounted on a roller/thrust bearing sleeve over the bt or nose with two fins added to it? I believe that should help keep it more stable and less rotation even if the rest of the rocket rotates.

I actually investigated using such bearings for a spin-can rocket I was building years ago, but that's a separate story.

#### jqavins

TRF Supporter
Regarding thrust misalignment (with the axis of the rocket), that can be used in a positive way as well if used to cancel out the imbalance...I believe the fliskits Duece can still fly on one canted motor. There are other examples of imbalanced rockets that fly fine due to spin or angled thrust, etc.
Yes, regarding canted motor mounts. By I think that what Alan15578 was talking about is defects in the motors' nozzles, which occur randomly. Sometimes radically, which can cause skywriting in an aerodynamically stable rocket, but sometimes only slightly.
For most of us, the factors you cited are small potatoes, compared to thrust misalignment, which can be alarmingly large and uncharacterized for sport rocket motors.

Here's a wild thought...regarding the OP's initial inquiry on how to reduce spin for a camera, how about having it mounted on a roller/thrust bearing sleeve over the bt or nose with two fins added to it? I believe that should help keep it more stable and less rotation even if the rest of the rocket rotates.
It seems like decoupling the camera's (potential) spin from the rocket body's (potential) spin would mostly only help if all the same things are done to keep the camera from spinning that should be done for the whole rocket if they are not decoupled. It would help regarding nozzle defects, but otherwise I don't see the point.
I actually investigated using such bearings for a spin-can rocket I was building years ago, but that's a separate story.
I remember that one. That was a great project.

#### KenECoyote

##### Well-Known Member
TRF Supporter
It seems like decoupling the camera's (potential) spin from the rocket body's (potential) spin would mostly only help if all the same things are done to keep the camera from spinning that should be done for the whole rocket if they are not decoupled. It would help regarding nozzle defects, but otherwise I don't see the point.
Actually I believe it would make a significant difference. The reasoning is that creating two small fins on a collar should be much more precise and able to be built straight vs the rest of the rocket since there are significantly less build variables involved. Basically mostly the two fins in the airstream vs. everything on the rocket and the nozzle (good point about the nozzle...never really thought of that).
Assume the rocket had a moderate spin...not too crazy. The two fins on a collar freely rotating should mostly stay straighter/rotate less than the rest of the rocket. Of course the hard part is making it freely rotate with as little binding as possible, which is very tricky since there are a few forces involved on the bearing(s). I'd LOVE to test this, but alas, my honeydew list it too long as it is lol. I do plan some counter-rotating bladed rocket one day using bearings, but that's still a while off.
*Edit: adding a camera's mass to the collar would also help in this case.

#### BABAR

##### Builds Rockets for NASA
TRF Supporter
That reminds me of a point I forgot to make yesterday. Since asymmetric airfoils will cause spin, one method to prevent spin would be don't airfoil the fins at all. Maybe round the corners, but that's it. If you don't try then you can't mess it up. And that is an OK strategy if you're not too concerned with altitude.

Also, you do have a point, using my 3 fin vs. 4 fin reasoning, that the rotational torque would switch sides as the rocket rotates. That means I may have the explanation wrong, but doesn't change what I've read, that most sounding rockets have 4, and that's for the purpose of reducing spin. (Note the word most, so please don't bother raising the counterexamples that are unquestionably out there.)
As usual, you are probably right, I just don’t conceptually understand why, that’s on me!

#### BABAR

##### Builds Rockets for NASA
TRF Supporter
Yes, regarding canted motor mounts. By I think that what Alan15578 was talking about is defects in the motors' nozzles, which occur randomly. Sometimes radically, which can cause skywriting in an aerodynamically stable rocket, but sometimes only slightly.

It seems like decoupling the camera's (potential) spin from the rocket body's (potential) spin would mostly only help if all the same things are done to keep the camera from spinning that should be done for the whole rocket if they are not decoupled. It would help regarding nozzle defects, but otherwise I don't see the point.
I remember that one. That was a great project.
I think a free spinning can would likely benefit from a gyroscope. May be difficult to accomplish on a LPR rocket.

@JimJarvis50 does some cool stuff, not sure if he is using gyros but definitely not something that’s gonna fit in a BT-50.

#### KenECoyote

##### Well-Known Member
TRF Supporter
Another idea for the OP...tape the nose cone to the bt, then attach/epoxy fishing line to the very tip of the nose cone and suspended the line on a metal rod supported high enough over the ground, then use a ladder and a leaf blower pointing straight down on the rocket and observe for any spin and make corrections. That should get you pretty close without having a lot of launching and trial and error. Well, this is what crazy me would do sans a big enough wind tunnel.

#### JoePfeiffer

##### Well-Known Member
It does if let's say the rocket was built unevenly or has an imbalance in weight or drag and would otherwise veer to one side...then sufficient spin would prevent that imo.
Yes, and it takes very little spin to accomplish that.

#### JoePfeiffer

##### Well-Known Member
I've read that 4FNC rockets have less tendency to spin, all else being equal, than 3FNC rockets. I believe I understand why, and it's pretty simple. A cross wind perpendicular to one fin "sees" more area on that side than on the other, so that side is pushed downwind wind more, and, voila, spin. So for the minimum spin, in addition to symmetry and all the other good stuff, if you're using "regular" fins, use four rather than thee.
I'd have to devote some thought to it, but that doesn't match my intuition. My expectation would be that the fin area on each side of the rocket, taking angle into account, will be the same. It doesn't seem like you have to worry about the upwind fin shading the downwind, since the crosswind will never be great enough in comparison to the speed of the rocket for that to happen.

#### BABAR

##### Builds Rockets for NASA
TRF Supporter
Except for the Corkscrew - a rocket with a single, off-center tube fin. It doesn't just spin, it spirals.
Not sure that is correct.

I think it ”Precesses”

from wikipedia

Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called nutation. In physics, there are two types of precession: torque-free and torque-induced.

#### KenECoyote

##### Well-Known Member
TRF Supporter
Not sure that is correct.

I think it ”Precesses”

from wikipedia

Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called nutation. In physics, there are two types of precession: torque-free and torque-induced.
View attachment 459209
Ah, very interesting! I never questioned the usage nor definition of "spiral" since it's so commonly used. Based on the main definition I got off Google, it seems incorrect, however then that also means what we call spirals in the tubes to be incorrect and "helix" is more appropriate.
"
1. winding in a continuous and gradually widening (or tightening) curve, either around a central point on a flat plane or about an axis so as to form a cone.
"a spiral pattern"
noun
1. 1.
a spiral curve, shape, or pattern.
"he spotted a spiral of smoke"
"
However, I think our common usage is similar to "spiral staircase". In this case, we commonly refer to a rocket Precessing as spiralling due to the spiral smoke trail left.

#### BABAR

##### Builds Rockets for NASA
TRF Supporter
Ah, very interesting! I never questioned the usage nor definition of "spiral" since it's so commonly used. Based on the main definition I got off Google, it seems incorrect, however then that also means what we call spirals in the tubes to be incorrect and "helix" is more appropriate.
"
1. winding in a continuous and gradually widening (or tightening) curve, either around a central point on a flat plane or about an axis so as to form a cone.
"a spiral pattern"
noun
1. 1.
a spiral curve, shape, or pattern.
"he spotted a spiral of smoke"
"
However, I think our common usage is similar to "spiral staircase". In this case, we commonly refer to a rocket Precessing as spiralling due to the spiral smoke trail left.
“A rose by any other name is still a rose.”

the smoke trail, since it has a third dimension, is best described as a helix. I was originally going to say it had a stable radius of curvature, but according to the definition below, it can track along a cylinder OR a curve. My experience (and since I am at best mediocre at finishing skills, balancing gliders, and building rockets that are competitive is any of the traditional categories) is in that realm of expertise somewhere between @kuririn ‘s a “avant garde” and Gary Larson’s “off-the-wall”. I think I have built as many or more “spiraling” rockets than anyone. Kudos to the High Power Guys, but by day three of NSL in 2019 the FLIGHTS all starting looking the same, except the one that did a 320 degree vertical turn (think vertical incomplete Hot Wheels track loop ). THAT one got my attention! Final trajectory was mostly vertical and fortunately AWAY from the flight line.

from Britannica.com

Spiral, plane curve that, in general, winds around a point while moving ever farther from the point.

From the free dictionary
he·lix
(hē′lĭks)
n. pl. he·lix·es or hel·i·ces(hĕl′ĭ-sēz′, hē′lĭ-)
1. Mathematics A three-dimensional curve that lies on acylinder or cone, so that its angle to a plane perpendicular tothe axis is constant.
2. A spiral form or structure.

from a rocket or airplane perspective, I think helix is more appropriate. But “spiral” is used in one of the definitions of helix.

you read about an airplane or rocket spiraling out of control. You never hear of one helixing (helicking?) out of control

all in fun, amigo, thinking about my videos when I am narrating them I call it spiraling too!

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#### Ez2cDave

##### Well-Known Member
TRF Supporter
I'm wondering if anyone knows anything about preventing a rocket from spinning, and if so, what are the tricks. It probably helps to have fins with perfectly symmetrical airfoils, but I'm wondering if a square or triangular body shape would help (instead of a standard cylindrical body tube). Maybe there are other ways but nothing comes to mind yet (excluding gyros and electronics for now).
( 1 ) Perfect longitudinal alignment of fins.

( 2 ) Perfect axial alignment of fins.

( 3 ) Perfect symmetry of fin airfoils.

( 4 ) Fins with zero flex / flutter.

Dave F.

#### KenECoyote

##### Well-Known Member
TRF Supporter
This is spiralling out if control!
LOL!