Nutation Control Game Tips and Tricks

What you see in your "Heads-Up-Display" is the path followed by the spacecraft pointing beam (the spin axis) in the Earth nearby-space. Instead of the "red beam," you see its projection on a giant imaginary screen which is traveling with the Earth. Superimposed (in blue) you see the maximum offpointing circle. Since your view is travelling with the Earth, stationary objects like the stars, or the center of the rosette, appear to drift away in the opposite direction.

The center of the rosette (marked as a small red circle) is what you actually move when you fire the thruster. This centre happens to be the tip of the "Angular Momentum Vector", and remains constant until you fire the thruster. When you fire, it moves in the direction of the torque applied (the orange arrow). In Ulysses case, both the thruster and its corresponding torque, do continuously rotate with the rest of the spacecraft.

The trick is to move the "Angular Momentum Vector" (rosette centre) on top of the elusive spin axis. If you do that, you automatically set the spacecraft to "pure spin", and reduce the nutation amplitude to zero!

Once you have almost no nutation, the next time you fire you will induce some. This happens because you push the "Angular Momentum Vector" away from the spin axis, thereby iniciating the oscillation. Think of it in this way: If you push a swing at the right time you may be able to stop it, but if you push it when at rest, you will set it to oscillate. It is Ok to alternate between the following two actions:

• Fire to move towards the Earth (but inducing some nutation), and then...
• Fire to control nutation (but attempting to move towards the Earth as well).

If left alone, the nutation (in this game) grows "exponentially". This is just another way to say that the bigger the amplitude, the faster it grows. Therefore you are better off by keeping it rather small.

NOTE: Understand that in real life, and under normal circumstances, the "passive nutation dampers" (some clever friction devices) will take care of the nutation, that will go down to zero without "mission control" intervention. The "Nutation Anomaly" only occurs in certain phases of the Ulysses Mission, when the Sun's thermal energy is strong enough to make the flexible booms oscillate (phenomenon discovered in-flight). The nutation feeds on this oscillation, and the Sun "pumps" into the system more energy than the nutation dampers can handle. It is then, when we need to execute the real Nutation Control Manoeuvres, which are based on the same principle than this game (although they are slightly more complex).

Nutation Game copyright European Space Agency 1996
(Developed by Raul Garcia-Perez of the European Space Agency office at JPL).
Send Comments to Bruce E. Goldstein.

This page was last updated August 19, 2009
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