# Yo-Yo Despin

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#### jderimig

Hey all you high fliers and knowledge holders on this topic.

I am toying with the design of a very high flight and want to use spin stabilization. Of course there is no point in a high flight without video.

Can someone point me to references on how to roughly size a yo-yo despin system given a starting spin rate and rocket inertia? For example assume I have a 3" 15# sustainer spining at 15 rev/s..... How heavy an long should my yo-yo(s) be?

#### Wingarcher

##### Well-Known Member
TRF Supporter
Why not increase the revs to 30/sec and shoot 30FPS video? :wink:

N

#### jderimig

Why not increase the revs to 30/sec and shoot 30FPS video? :wink:

N
This is why you have been called a genius.

#### Reinhard

##### Well-Known Member
TRF Supporter
The equation can be found here:
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19620006811.pdf (Equation 1, simpler than I expected)

The moment of inertia of the rocket can be determined in different ways. As a first order approximation, I'd model it as a thick walled cylindrical tube:
https://en.wikipedia.org/wiki/List_of_moments_of_inertia

For a bit more precision, it can be modeled as the sum of its (simplified) parts. For those parts, that aren't centered on the rockets axis of rotation, the Parallel Axis Theorem can be used:
https://en.wikipedia.org/wiki/Parallel_axis_theorem

If you're using CAD software, like SOLIDWORKS, the moments of inertia can be easily determined, if the model is accurate.

Alternatively, the moment of inertia can be measured too. A simple approach is to wrap a line around the rockets circumference around the center of gravity. If you let it drop, so that it unwraps like a Yo-Yo, its potential energy doesn't only get converted into translational kinetic energy ("falling down") but also into rotational kinetic energy -> m*g*h = m*v^2/2 + I*omega^2/2. The relation between mass and moment of inertia will determine how fast it falls and how fast it rotates. Drag of the fins will influence the result a bit, but it should be close enough to get a reasonable estimate. In a practical test, two lines on both sides of the cg will be much easier than balancing the rocket on a single line.
Another approach would be to suspend the rocket from the nosecone from some kind of torsional bar or string to create a rotational pendulum and measure it's resonant frequency. For calibration, use some piece with a simple enough geometry, that it can be easily measured and calculated (e.g. metal rod or sphere).

Reinhard

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#### jderimig

Reinhard,

Thank you. I am designing the rocket in Inventor so yes it can calculate the I for me, assuming by material properties are correct.

For measurement I remember a lab back in college physics where I think another method is to hang the rocket with a small cord and getting it moving like a pendulum. (hanging with the airframe parallel to the floor). Then use the frequency and the parallel axis theorem to calculate I about its own axis.

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#### watheyak

##### Barnstormer
TRF Supporter
I was present for a discussion about yo-yo de-spin at Balls. Kent Newman and Chuck Rogers were mentioned as resources.

#### Binder Design

##### Well-Known Member
Why not increase the revs to 30/sec and shoot 30FPS video? :wink:

N
What would be even cooler is if you could sync the video to the rate of spin via feedback loop so the video would be stable even if the spin rate was variable.

#### Ravenex

Our get high spread video and sync it into stable 360 degree panoramic video after the fact.

#### Binder Design

##### Well-Known Member
I see that you are not Occam's Razor kind of guys.....

https://i.imgur.com/olKzaoZ.mp4
Despinning the video via manipulation/editing is a perfect example of Occam's Razor. It adds no extra complexity to the flight vehicle.

#### jderimig

Despinning the video via manipulation/editing is a perfect example of Occam's Razor. It adds no extra complexity to the flight vehicle.
Well since I do not know how to do that nor can I be certain that the single frames will not be blurry that approach seems complex to me.....

#### H_Rocket

##### Death by Powerpoint
So I'll bite - What the heck is a YoYo Despin?

And no, I did not try to look it up.

#### jderimig

So I'll bite - What the heck is a YoYo Despin?

And no, I did not try to look it up.
A method used to stop the spin of rockets and payloads in use since the early sixties. Weights (commonly 2) on a string that is wrapped around the rocket or spinning body you want to stop. When you want to stop the spinning you release the weights and they unwrap the tube applying torque on the tube opposite of the spin. When the weights are fully unwrapped they are released and the body is left de-spun.

#### H_Rocket

##### Death by Powerpoint
No concern about those weights coming in ballistic?

#### Kip_Daugirdas

##### Well-Known Member
I'm looking at this for the bit larger version of Workbench. However, it will be frowned upon if you release the weights and have them intentionally fall and litter the Black Rock playa.

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#### mikec

##### Well-Known Member
Keep in mind that if you spin at a high rate for stability you are probably going to have to do a much better job of spin-balancing the rocket than is typically done by amateurs.

#### ksaves2

I'm looking at this for the bit larger version of Workbench. However, it will be frowned upon if you release the weights and have them intentionally fall and litter the Black Rock playa.
Simple, put "little" parachutes on 'em so they are there when the cutter activates. That way no rocks,lizards or rattlesnakes will be harmed:shock: and any human the comes upon them can pick them up and put 'em in their litter bag! :wink:

oint:Kurt (Go for it John!)

#### jderimig

No concern about those weights coming in ballistic?
More concerned about the rocket coming in ballistic. The weights will be around 9g each. Much lighter than other rocket parts that frequently come in ballistic at BALLS. I can put streamers on them if requested.

I'm looking at this for the bit larger version of Workbench. However, it will be frowned upon if you release the weights and have them intentionally fall and litter the Black Rock playa.
I will make the weights biodegradable if the launch organizers require.

Keep in mind that if you spin at a high rate for stability you are probably going to have to do a much better job of spin-balancing the rocket than is typically done by amateurs.
Looking at 5-6 revs/s but good point.

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#### Kip_Daugirdas

##### Well-Known Member
So this is an old report that Eric Gates' son put together. I remember when this came out I was still in high school lol. Anyways, the testing was really well done. The flights were done using fin tabs instead of canted fins but the info is still applicable.

https://docslide.us/documents/spin-stabilization.html

There are two big takeaways:
1.) Roll rate has a big effect on apogee altitude. Yes you minimize dispersion, but it's not without a price.
2.) De-spin is needed to prevent possible recovery issues especially at higher roll rates. Or some type of swivel.

Another article worth looking into is this Arcas report. Introducing spin can cause other aero problems to surface.

https://www.dtic.mil/dtic/tr/fulltext/u2/437681.pdf

On page 12 it talks briefly about issues the Arcas was having with pitch-roll coupling at spin rates of 5 rps. The spin rate was increased to 15-20 rps in later builds.

There's lots that I have to still research but hopefully one can settle on a fin design/cant angle without needing a bunch of test flights...?

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#### jderimig

On page 12 it talks briefly about issues the Arcas was having with pitch-roll coupling at spin rates of 5 rps. The spin rate was increased to 15-20 rps in later builds.

There's lots that I have to still research but hopefully one can settle on a fin design/cant angle without needing a bunch of test flights...?
Introducing spin with fin tabs is a relatively draggy way to get spin and will consume energy in beating the air with the fin rotation. Fin cant is much more efficient. Some motor energy will go into increasing the angular momentum of the rocket but it will be much less than the drag losses of the former.

I am familiar with the Arcas report and was initially considering 15 rps. However than option eliminates of the option of using tilt sensing to inhibit staging ignition because consumer gyro's popular in this application have limits of 2000 deg/s. Hence the target of 5 rps at least for boost portion of the flight.

#### jderimig

Here is a pic of the yo-yo release mechanism concept I have been toying with. Release arm activated by a servo.

#### jderimig

Here a pic of the 3D printed prototype of the YoYo weight release bulkhead.

#### ksaves2

Nice John, What is the cable/cord configuration out of curiosity? Kurt

#### jderimig

Nice John, What is the cable/cord configuration out of curiosity? Kurt
Its the RC Servo electrical connection.

#### mikec

##### Well-Known Member
He might have been asking about how the cable that the weights attach to that unspools and then gets cut. Yo-yo doesn't work if you don't have a cable.