# Why does spinning help stabilisation?

I don't really understand why do we use spin stabilisation for rocket launches?

If I understand correctly, the idea is to make the rocket spin really fast so that any asymmetry in the thrust/aerodynamics will average out and keep the rocket going straight?

Bonus question: How can one provide guidance for such launches? Especially how can they achieve a gravity turn...

• Have you take a look at bullet spin stabilization? Bullet going out of a gun spins very fast thanks to stripes inside the gun. I thinks same principle applies to rockets. +1 for asking how to make a gravity turn while spinning Mar 21, 2017 at 6:59
• Some rocket have Rollerons they spin really fast to pre vent the rocket from spinning. Rollerons were first used in the AIM-9 Sidewinder missile. en.wikipedia.org/wiki/Rolleron I am guessing you don't want the rocket to spin because it would mess with the guidance.
– Muze
Dec 18, 2017 at 3:17

## 4 Answers

I made some researchs since my comment, and especially on rifles:

In firearms, rifling consists of helical grooves in the internal (bore) surface of a gun's barrel, which impart a spin to a projectile around its long axis. This spin serves to gyroscopically stabilize the projectile, improving its aerodynamic stability and accuracy.

On the rifling depends the bullet rotational speed, and figures can go up to 270,000RPM, a rotation speed a rocket will hopefully never achieve.

Talking about rockket, spin stabilization is not communly used, especially during ascent and it has been superseded by three-axis stabilisation.

In addition, it seems that this method is more often used while in space, and I found a very interesting video here (but I did not succeed to put it here embedded).

As you said in OP, the spacecraft is successively spun, fired, and de-spun (a solution to de-spun for free can be a yo-yo de-spin).

Bonus answer to your bonus question: if spin stabilization is not use during ascent but once the payload reaches space, then there is actually no question about how to manage the gravity turn while spining.

• I noticed that that animation seems to show the craft's rotation to continue to slow down after yo-yo release - is that supposed to happen somehow? Also - a cool video of another way to de-spin can be found here..
– uhoh
Mar 21, 2017 at 9:09
• @uhoh I noticed that too, and it seems to me that this is really unrealistic. Mar 21, 2017 at 9:12
• I understand that it is used for stabilisation, but my question is more about how does spinning helps ? Mar 21, 2017 at 9:14
• @Antzi maybe then maybe change the title to "How (or Why) does spinning provide or hep stabilization" where everyone can see it, not down here in a comment.
– uhoh
Mar 21, 2017 at 9:17

This answer answers the title "Why does spinning help stabilisation" rather than specifically for launches.

Context

Before jumping into the answer it is worth looking at the background. With rigid bodies then adding a spin will help average out asymmetries. However neither rockets or satellites are rigid, they contain liquids and flexible appendages and these are both means of energy damping, and means by which energy can transfer from one axis of rotation to another.

The basic ideas of stability in this context (stability is a big subject, even in maths before one tries to apply it to any real object) are that objects that spin can be stable if they spin around their axes of maximum or minimum inertia. Good examples would be the central axis of a coin (like a windmill) and pencil (like a barbeque) respectively.

This doesn't hold for bodies with energy transfer modes from propellant sloshing and appendages. A pencil like rocket spinning longitudinally can, through flexible modes, transfer all of its rotational energy into a rotation about its axis of maximum inertia. This means it will adopt an "end over end" motion. Please note I'm not claiming that its angular momentum has changed here. There is a case of this really happening with an early small launch vehicle (late 50's I think).

The answer

If there are flexible modes then the only rotation that is stable will be about the axis of maximum inertial, i.e. a windmill rotation rather than barbeque mode.

Spin stabilisation is used on many satellite designs though in every case the designers have to pay close attention to the shape of the satellite so that it is behaving firmly in windmill mode, i.e. spinning around its axis of maximum inertia.

Extra detail (EDIT)

The early small launch vehicle I mentioned was the Explorer 1 / Juno 1 mission the first US satellite. It was intended to spin at 750 rpm along the long axis but was found to be spinning at 7.5 - 8 rpm which corresponded to the energy having transferred into one of the transverse axes. Its well known enough to be a class room dynamics example. This image below shows the whip antennas which are, I believe, the physical mechanism for the energy transfer between axes.

• This should be the accepted answer. Dec 21, 2017 at 5:45
• clearly I should have put some pictures in! Dec 24, 2017 at 0:18

It's counter-intuitive, but true as per the laws of physics, that rotating objects will be harder to change direction in the axis perpendicular to their rotation.

It's the same principle of the bicycle. Almost no one can balance on a stationary bicycle, but, when the wheels are spinning, it tumbles slowly enough to allow a trained human to sense it and shift their weight in time.

A coin will roll on a smooth surface, but will not stay upright on its side if not rolling.

The easiest way to experience it directly is with one of those fidget spinner toys that are in fashion. It takes more muscular effort to roll one around while it's spinning.

The same principle is used in rockets. A given force asymmetry will change the direction of the rocket much more slowly if the rocket is spinning. This gives the control system more time to react.

(the fact that the force came from either thrust or aerodynamics is not relevant)

• Nitpick: the angular momentum of a bicycle's wheels is not the reason for its stability (see e.g. en.m.wikipedia.org/wiki/…)
– matz
Jan 27, 2019 at 21:03
• Spin does not make it difficult to change an object's axis of rotation in a direction perpendicular to spin, but instead make an object respond to applied torque that's perpendicular to spin as though the torque were applied in a different direction. For something like a spinning top, this will mean that torque which would cause the top to fall over will instead cause it to precess. If a bullet were pointed at a direction 10 degrees below its direction of travel, torque which would try to change the direction to be further from the direction of travel will instead change it to be e.g... Jul 18, 2020 at 16:48
• ...10 degrees to the left of travel, and then 10 degrees above the direction of travel, then 10 right, then 10 down, etc. so that any sideways force produced by the bullet's heading would tend to propel the bullet in a spiral. Jul 18, 2020 at 16:49

It is all about angular momentum. A spinning object has angular momentum -- which can be visualized as a vector around which the object spins. It takes a force to change that angular momentum vector and therefore the axis around which the object spins.