# Why does a spacecraft's attitude become stable when it rotates?

What is the logic behind spacecraft spinning to increase stability? How does angular momentum play a role in this?

• Your question conflates two things, stability and trajectory. It shouldn't. You can have a perfect trajectory with a wildly spinning object (although burn timing could be fun...) Spinning is for the object's stability - so you always know where you are pointing. Jul 19, 2017 at 11:19
• That's what I want to ask.. Why spacecraft's pointing become stable on rotating the spacecraft? Jul 19, 2017 at 11:47
• The answers below cover that off Shweta. It's just the question title that didn't fit - I have edited it. Jul 19, 2017 at 11:48
• Possible duplicate of How does spinning helps stabilisation? Jul 19, 2017 at 13:01
• @Nat Thanks for your proposed edit. It takes some getting used to, but spacecraft attitude is a real term. It refers to the spacecraft's orientation with respect to some reference frame (the direction the spacecraft is pointing). There is even an answer here that addresses the multiple meanings of the word "attitude" as it applies to spacecraft; NASA's baseball bat of all things.
– uhoh
Jul 20, 2017 at 3:06

Rotations about a principal axis for an object with three distinct principal moments of inertia are stable if the rotation is about the axis with the least or greatest moment of inertia but unstable if the rotation is about the axis with the intermediate moment of inertia.

Showing that this is the case is one of the torture tests for physics majors. One name for this phenomenon is the tennis racket theorem. You can easily see this by wrapping a rubber band about a book and tossing it with a bit of a spin. Give the book a flip about either the smallest and largest principal axes and you'll see a nice and stable rotation. Give it a flip about the intermediate axis and you'll see something a bit chaotic.

Some satellites take advantage of this phenomenon and establish a rotation that is more or less about either the smallest or largest principal axis. The satellite's control system can detect and correct deviations precisely because these rotations are stable.

• That book trick blew my mind when I was taught it. Jul 19, 2017 at 15:02

The process is called spin stabilization and is not used on every spacecraft, but some. Most notably, it is not used on any manned craft since it would be detrimental to the health of the passengers.

Conservation of angular momentum applies. A body always spins about its principal axis. If the rocket already spins at a high RPM, it is much more diffcult to alter the axis of rotation - the rocket will be much more stable. See it that way: if you add just a little bit of rotation to a body at rest, it will slowly rotate. If you apply the same tiny bit of rotation to a fast-spinning object, its axis of rotation will barely even change.

Furthermore, a rotating rocket smoothes out any individual disturbance.

Its pretty much the same effect effect as a gyroscope or momentum wheel (which "absorb" angular momentum on demand), just with the whole rocket body and only on one axis.

Not that the rocket has to be de-spun from its usually high RPM (50 - 600) once it reaches its target orbit in order to release its payload (typical satellites can handle at most 2-5 RPM with their own attitude control). Various techniques are available, e.g. yoyo-despin, but this technique is not always seen as desirable because of the debris it generates.

• A body most definitely does not always spin about a principal axis. The Earth, for example, does not spin about one of its principal axes, resulting in the Chandler wobble. Spinning exactly about a principal axis is a highly improbably event (probability = 0). A small deviation from a principal axis is less improbable. Spin stabilization takes advantage of the fact that sometimes these small deviations are stable. (Other times they're not, resulting in the polhode rolling without slipping on the herpolhode lying in the invariable plane.) Jul 19, 2017 at 11:37

A symmetric body with no torques applied with even a slight bit of internal damping (as all real object have) will eventually rotate about its principal axis with the lowest moment of inertia. The faster the spin (= higher angular momentum), the more effort it takes to alter the axis of the spin (= greater stability).

• This is not true. A rigid body will spin about whatever axis whatsoever. The body will precess if that happens not to be a principal axis. Jul 19, 2017 at 3:10
• A symmetric body with no torques applied with even a slight bit of internal damping will eventually rotate about its principal axis with the lowest moment of inertia. Edit applied.
– Erik
Jul 19, 2017 at 4:32
• Would the slight damping need to couple the axes in some non-zero way? This is interesting!
– uhoh
Jul 19, 2017 at 9:42
• Here I want to know specifically about Juno spacecraft.. Why does its 'Pointing' become stable on rotating? Jul 19, 2017 at 10:20
• Pointing is stable since you must overcome angular momentum to change the axis of rotation. You have to rotate the angular momentum vector. If it wasn't rotating, this would be much easier.
– Erik
Jul 19, 2017 at 12:43