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anbeck

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Hi there,

I'm interested in finding out whether stabilizing a model rocket by making it spin is worth trying out or not, which has lead me to several questions:

1) Can anybody explain to me which forces are at work in this process?

2) How many rotations are necessary to stabilize a given rocket, and how can this be calculated in advance?

3) If the aim were to reduce drag in order to make a rocket fly higher (given the same amout of thrust, gravity and other forces we cannot influence) reducing the fin size should be one of the easiest methods. On the other hand to make a model spin, you will have to give the fins an angle of attack, which I suppose would increase drag.
Is there a point where the pay-off is worth it, that is, where the decrease of drag due to smaller fins is bigger than the increase of drag due to the angle of attack needed to make the model spin?

I hope I was able to put my thoughts into sentences that make sense :rolleyes:

Thanks a lot!
André
 

TheAviator

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While spin stabilization is really cool, there really isn't a point where it becomes more efficient than standard fins. I've seen people that have successfully done rockets based on the technique, and they need to spin at least 50-100ish times per second ala audible buzz. If you really want to get into the physics, this is a matter of making the rocket act like a gyroscope (not the ones in active stabilization systems, those gyroscopes act as a reference.) As the rocket gets torqued on the pitch axis, the gyroscopic effect is a resultant torque on the yaw axis. If the rocket is spinning fast enough, the result is a coning flight. If it is spinning even faster, the coning isn't even noticable.

This is analogous to a top. If you spin it really slowly, only barely enough to keep the top stable, it cones like crazy. However, if you spin it as fast as you can, the top seems to stand straight up without any coning. It is there, but so minute that it isn't noticable.
 

jorpet

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You could try the Estes Spin Control rocket. Not sure if it comes anyway except as a starter pack. It sounds interesting as you can adjust the fin and it has a spin counter in the nose. Would let you experiment with spinning the rocket and correlate spin to height and stabilization.
 
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Micromeister

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Spin Stablilzation has been used in full size rocketry for many years. Most if not all of the unguided fin stablilzed sounding rockets of the 40-80's were in one form or another spin to increase vertical trajectory. I don't recall the spin rates but I'm sure if you visit the Wallops Island facility website there well be some info on it there.
However for our purpose spin stablility ALWAYS decreases attained altitude for the very reason you mentioned in your first post increased drag of the fins as they turn in the air.
There are several ways to induce spin in your models. Canting the fin or fins is a common practice. or airfoiling all fins mounting them to ensure they are all facing the same direction. I made the mistake of doing this on my very first Omage 2-stage to fly my Cineroc back in the stone age. While the flight of the model was arrow straight the spin made the movie rotate so fast that even at slow motion it made everyone in the room dizzy watching LOL!!!! At full size it caused headaches.
Avaitor is right on with regard to the gyroscopic effect on the model and coning if the rate is slow enough. Sometimes you'll see this effect in a model if one fin is a little off line as the model slows down after burnout.
As long as your not using spin to stabilize your performance or competition models it's a fine way to help ensure nice vertical flights with even slightly uneven or misbalanced models.
It's also used often on assymetric model designs to help maintain an upward safe flight path.
 
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anbeck

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Thanks a lot for sharing your insights!

I didn't know there was an Estes Kit with a rotation counter, I might look into that one day.

I've found this document on the internet, which is quite an interesting read:
http://www.rocketryplanet.com/images/pdf/Spin-Stabilization.pdf

It analyses the influence on drag by inducing rotation. Of course, he concludes that fin-induced rotation reduces altitude performance, which he had expected.
Unfortunately he does not continue his research as to whether spin stabilization allows reduction of fin area, which would make possible to regain some of the lost altitude.
For example: His booster with a 0 degree fin angle reached an average of 1139 ft, while the booster with an 7.2 degree angle had a spin rate of 16 to 18 revolutions per second, but reached only 937 ft.

I put together a simple model in the Rocksim trial version, where the fins are responsible of 21% of the Cd value.
Reducing the fin area by a guesstimated 50% got this down to 13%, but unfortunately the model isn't stable anymore (or can I make RockSim simulate spin stabilization?)...

But I'm afraid that you are right and spin stabilization costs more than you can gain by the reduction of fin area it allows. Still, I find it interesting enough to maybe test exactly how much you can reduce fins by this method.

I hadn't even thought of technical problems such as parachute lines getting twisted due to the spinning.
And I hadn't thought of airfoiling the fins instead of attaching them with an angle of attack or installing the engine at an angle. When I have the time and money, I might get the roll rate counter thing and do some tests as to which method is the one that costs the least altitude performance.

a.
 

Handeman

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I build a finless spin stabilized rocket. I used an oil funnel from Walmart. I put an aluminum tube through the center, a motor holder with four 18mm motors canted at 10º and a chute compartment for that chute that is pulled out when the motor holder ejects.

It actually flies quite well on very calm days. On windy days, it gets a little squirrelly.
 

Zeus-cat

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If your looking for a cool spin stabilized rocket, try the TAO. It is 2 stage. The booster spins, and the sustainer is finless. Haven't built it myself, but have printed the plans and looked at them many times.

Plan starts on page 2
http://www.spacemodeling.org/JimZ/mrn/mrn3701b.pdf
That looks very interesting. Does anyone have any idea of how high it goes? A D12-0 and a B6-6 sounds like it would get up there. I'm thinking a streamer is a better choice than a parachute for recovery of the upper stage.


I did a little checking and George Pike built the Tao and posted the results of two flights on EMRR. Not pretty. Apparently, the design is inherently unstable.

http://www.rocketreviews.com/reviews/all/plan_est_doq_tao.shtml
 

JAL3

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That looks very interesting. Does anyone have any idea of how high it goes? A D12-0 and a B6-6 sounds like it would get up there. I'm thinking a streamer is a better choice than a parachute for recovery of the upper stage.


I did a little checking and George Pike built the Tao and posted the results of two flights on EMRR. Not pretty. Apparently, the design is inherently unstable.

http://www.rocketreviews.com/reviews/all/plan_est_doq_tao.shtml
I'm so dissapointed. I've had this plan for a while and it seemed interesting.
 

Micromeister

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Basicly spin stabilazation has one huge drawback. As the rotation slows the body become increasingly unstable. With no other form of directional control Finless or under finned vehicles tend to end their flights badly:(

Spinning a finned vehicle to increase stability and directional control is one thing, spinning a Model for the sole purpose of reducing or eliminating fin area would have very limited benifit with our smaller short burn models.
 
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DaveHein

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The Tao seems to have a couple of problems. A light rocket on a D12 is going to accelerate pretty fast, and the fin attachment doesn't seem to be strong enough to hold them on. The aerodynamic force on the fins is going to be higher than usual because they are angled. The rocket won't be able to spin until it's cleared the launch rod, so the fins will have quite a bit of drag until the rocket is able to spin up to full speed.

Ideally, the entire fin should be positioned at the same angle. This will minimize the drag on the fins once the rocket is spinning. If the entire fin is angled the spin rate is determined by multiplying the rocket speed times the tangent of the angle, and divide by the average circumference of the fins. As an example, a 1" diameter rocket with 1" wide fins will have an average fin-diameter of 2" or a circumference of half a foot. If the fins are angled at 1 degree the tangent is 0.017. At a speed of 200 MPH, which is 293 feet/second the rotational speed will be 293 * 0.017 / 0.5 = 10 revolutions/second, or 600 RPM.

The width of the fins should be minimized so the entire surface area has about the same circumference around the rocket. Otherwise, the rotational speed will be slower than optimal at the fin-root, and faster than optimal at the fin-tip. The Tao has short wide fins, and part of the fin is straight while the bottom edge of the fin is angled. This is going to cause a lot of stress and drag on the fins. The fins should be tall and narrow to minimize the stress and drag.

The other problem with the Tao is that the Cg of the second stage is well below the Cp. This causes it to be so unstable that even the spin can't stabalize it. A football has the Cg and Cp located at the same spot, so a little bit of spin is enough to stabalize it. Some nose weight could be added to the Tao to move the Cg up to where the Cp is. It still will not be statically stable, but a little bit of spin should make it dynamically stable.

Dave
 

GlennW

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At NERRF this past weekend we were treated to a high power spin stabilized rocket launching, one of the last launches of the festival on Sunday. Really cool flight! I had not seen this type of rocket before and to see a High power version was really something. Very neat..

Glenn
 

Micromeister

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At NERRF this past weekend we were treated to a high power spin stabilized rocket launching, one of the last launches of the festival on Sunday. Really cool flight! I had not seen this type of rocket before and to see a High power version was really something. Very neat..

Glenn
Glenn:
Was the rocket fins or finless? mechanically per-spun or fin spun at launch? a Little more discription would be helpful;)
 

jcsalem

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Glenn:
Was the rocket fins or finless? mechanically per-spun or fin spun at launch? a Little more discription would be helpful;)
I enjoyed that rocket at NERRF as well. Assuming we're talking about the same one, it was called the "Two Fin" and flew arrow straight due to the spin stabilization. It had (obviously) two fins which I assume were canted to induce the spin. It flew on a single central motor.

Anyone have more details?

Jim
 

Lentamental

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Due to the issues stated above, with the rocket slowing down, and thus losing stability, as it flies, the real benefits of spin stabilization come when the rocket is pre-spun, or is launched from a helictical tower. The reason being that it provides extra stability as it leaves the pad, when it is most susceptible to weather cocking, and then slows down, decreasing the drag, as it flies up.
I am thinking of doing something along those lines for my TARC rocket, but not really sure if it is worth the added complications.
 

GlennW

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Yes Jim that was the one, the "Two Fin." I had forgotton the name. Only the bottoms of the fins were canted, almost like fixed flaps. Big orange rocket about 7 or 8 ft tall.

Glenn
 

DaveHein

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Due to the issues stated above, with the rocket slowing down, and thus losing stability, as it flies, the real benefits of spin stabilization come when the rocket is pre-spun, or is launched from a helictical tower. The reason being that it provides extra stability as it leaves the pad, when it is most susceptible to weather cocking, and then slows down, decreasing the drag, as it flies up.
I am thinking of doing something along those lines for my TARC rocket, but not really sure if it is worth the added complications.
It seems like spin stabalization would work well for a two-stage rocket like the Tau. It just needs to be modified slightly so the fins are stronger and the second stage is marginally stable without spin. It might be interesting to launch a pre-spun finless rocket.:) Without fins it should be able to maintain its spin during the entire flight, and it would minimize the drag.

Yes Jim that was the one, the "Two Fin." I had forgotton the name. Only the bottoms of the fins were canted, almost like fixed flaps. Big orange rocket about 7 or 8 ft tall.

Glenn
Are there any pictures or video of the Two Fin rocket?

Dave
 

JStarStar

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I am certainly not a stability specialist, but my guess is that with spin stabilization, you may be edging into the territory of problems with inertial coupling instability (aka pitch-roll coupling).

http://www.nakka-rocketry.net/ep_lr7.html

Scroll about 3/4 of the way down where this phenomenon is discussed at much greater length and detail than I could ever do.

I believe G. Harry Stine touched on it briefly in one of the editions of the Handbook. My understanding is the problem manifests when the spin rate coincides with the rotation rate around one of the other two axes (pitch or yaw), essentially causing the vehicle to corkscrew off into a full tumble.

:y::y:
 

MarkII

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Spin stabilization was used in various sounding rockets ever since the end of WWII; some rockets may use it even now. It is often used in the upper stages, possibly because they operate in the upper reaches of the atmosphere where the air density is too low for fins to be of much help. The upper stage of many satellite launch vehicles employ spin stabilization. I also recall seeing videos of satellites being deployed from the payload bay of the Space Shuttle that are spinning as they are released.

The Estes Spin Control is only the latest example of a model rocket that was designed to spin in flight. Estes sold a rocket in the 1980's called the Spin Fin, and Canaroc produced the Tornado Two and the Super Tornado. More recently, Quest also produced a kit called the Spin Fin. I would not be surprised if there were others as well.

ADDENDUM: The Estes Astron Space Plane, Kit #3, was designed with tabs that would cause the rocket to spin slowly as it boosted in order to help it boost straight.

ADDENDUM TWO: In addition to the Tao, there was another Estes Design of the Month plan called The Flyin' Stovepipe. It was a boost glider that featured a cylindrical glider that was spun by canted fins on the booster prior to being released. The spin enabled the cylinder to glide in a manner that was similar to the way a thrown football is spun to stabilize its flight.

The Art Applewhite Rockets Stealth and Scimitar Qubits also spin during boost and recovery. My Priority Stealth could clearly be seen spinning around its vertical axis during its descent when I launched at at NARAM-51.

MarkII
 
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MarkII

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Another well-known historical rocket that was spin-stabilized was the US Army Honest John. I'm surprised that no one has mentioned this example.

MarkII
 

kramer714

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couple of thoughts on this...

When I launched the football, http://www.rocketryforumarchive.com/showthread.php?t=49339&highlight=football I originally was thinking of small canted fins or thrust vectoring to bring it up to speed. Angling the thrust would take away quite a bit of the vertical vector. The other reason I went away from that was to control the spin BEFORE it launches to ensure stability.

The launcher used a variable speed drill connected to a belt and pulley to spin up. I could get any speed I wanted with the setup. 400 rpm seemed to keep the football stable on the pad.

The football has quite a large moment of inertia compared to a rocket, heavy skins, and a much larger diameter aids in stability when it is rotating. I did notice that after launch there was a gyroscopic progression that caused the ball to start to progress away from straight. The rotational speed kept up there but the nose and tail started to follow a circle. The CG of the system did maintain a straight line during the entire flight.

After burn out and during recovery (fumble recovery) the ball transitioned to almost a flat spin. After a few flights the balance of the football got a little off as the foam inside got crushed in a few spots. The last flight (month ago) the ball was a bit wobbly on the launcher.

Looking at the results of the launch I would make 2 changes, 1) rebalance or check balance after each flight. 2) I think a slower rotation would have worked better, i think I would have dropped down to 200 RPM or so.

I am thinking of a spin stabilized pumpkin rocket for an October launch. For this one I am thinking of adding 'belly weights' to the plastic 10" diameter plastic pumpkin to increase the moment of inertia.
 

shockie

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G. Harry Stine first experimented with spin stabilization, as a means of trajectory control (to control weathecocking in winds) for model rockets in February 1957.

He added some thin brass spin tabs to the ends of his MARK II's fins.


Terry Dean
 
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gizza2

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And don't forget that Korey Kline successfully flew a spin-stabilized FINLESS rocket in 1982:

http://www.v-serv.com/crp/CRm/4-82/CRm.4-82.htm (cover and page 8)

I viewed that flight in person and it was truly amazing! Korey remains one of my heroes. I've considered replicating his design on a larger scale.
 

DaveHein

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I am certainly not a stability specialist, but my guess is that with spin stabilization, you may be edging into the territory of problems with inertial coupling instability (aka pitch-roll coupling).

http://www.nakka-rocketry.net/ep_lr7.html

'''
I clicked on the link and read the report on the Frostfire One rocket. It was very interesting, but it seems like there isn't much evidence that that inertial coupling caused the failure of the rocket. The author (which I assume is Richard Nakka) didn't say how large the spin tabs were on the rocket -- only that they were canted at 5 degrees. His data showed that the rocket achieved a velocity of around 375 MPH just before the destruction happened.

Without knowing the size of his spin tabs I would guess that the rocket was spinning somewhere between 1 to 10 times per second. At a speed of once per second it is doubtful that there would be much inertial coupling. At 10 times per second, the rocket may have started flexing due to the material he used or the tightness of the couplers.

I believe the main purpose for spin stabalization in rockets is to average out the thrust and aerodynamic forces. There isn't much gyroscopic stability happening in a long skinny rocket. Just try spinning a pencil on its tip like a gyroscope. The pencil will fall over as fast as it would without spin.

Dave
 

rstaff3

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I enjoyed that rocket at NERRF as well. Assuming we're talking about the same one, it was called the "Two Fin" and flew arrow straight due to the spin stabilization. It had (obviously) two fins which I assume were canted to induce the spin. It flew on a single central motor.

Anyone have more details?

Jim
Could it have been Steve Bayak's 'Slightly Insane Deuce'? I have some photos on page 2 of this album:
http://our.rocketryplanet.com/photo/albums/mdras-red-glare-v-day-1
 

dedleytedley

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I've scratch-built a fin-spun rocket and would like to find an interesting paint scheme. Does anyone know of a pattern that will show some effects during rotation? Ted
 

dedleytedley

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I hadn't thought of that maybe I could use mirror for the white? Ted
 

Lentamental

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Thats the Black Brant, no?

You would probably get a more distinct visual effect if you painted on entire half white and the other half black. I am assuming from other threads that you are using the paint scheme to be able to measure rpm from video footage, but even if you aren't, half and half split down the middle should get you the best effect.

For an even better effect, put a rotational sensor on board, and have it radio down to a strobe light on the ground, and then you can make it look like it is not rotating at all! :p
 
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