Attitude Stabilization

One reason why the ACS is considered to be such an important element of the spacecraft is that the type of attitude stabilization used on a particular spacecraft is very influential in determining what it looks like. There are four general types of stabilization, which are illustrated in Figure 8.3. Types 1, 2, and 3 involve spinning all, or a part, of the spacecraft. This spin feature makes the spacecraft's attitude inherently stable; if the spacecraft is affected by a disturbance torque, the change in attitude that results is small. This is a useful feature, as it means the ACS does not have to work so hard to control the spacecraft's attitude.

We can get a good idea of how this inherent stability due to rotation works by looking at a bicycle. The bicycle's tires provide two small points of contact with the ground, each perhaps a couple of centimeters across. The bicycle rider represents a large mass on the top. Other objects with these two characteristics tend to fall over; for example, no matter how hard we try, we cannot balance a nail on its point. But strangely the bike rider is quite happy whizzing along the road without any thought of the bike toppling over. Why? It's basically because the rotation of the wheels give the bike stability; the axes about which the wheels rotate become stiff, in the sense that they want to remain pointing in the same direction. The wheel axles remain horizontal, ensuring that the bike stays upright. As the rider slows down, the wheels' spin rate correspondingly decreases. Eventually the wheels stop rotating, and then the bicycle's stability is lost, and the rider has to put a foot down on the ground to prevent the bike from toppling.

And so it is with a spacecraft. When it spins, the spin axis becomes stiff, tending to make the spacecraft point in a fixed direction. The spin axis becomes less sensitive to disturbances, giving the spacecraft as a whole this characteristic of inherent stability. A spacecraft with this type of spin stability is said to have momentum bias. In Figure 8.3, a spacecraft with type 1 stabilization is called a pure spinner. These are usually cylindrical in shape, and the whole spacecraft rotates at a rate of a few tens of revolutions per minute (rpm), providing spin stability. The example shown is the Meteosat SG (second generation) satellite, which is a spacecraft that provides satellite pictures for weather forecasts. With this type of stabilization, there is no part of the vehicle or its contents that is not rotating; thus, it is difficult to accommodate payloads that need to point in a fixed direction.

Figure 8.3: The four general types of spacecraft attitude stabilization. All spacecraft fall into one of these categories, some examples of which are illustrated. (Image credits: Meteosat SG image, copyright © ESA; Intelsat 6 image, copyright © Boeing; GPS Navstar 2R image, copyright © Lockheed Martin; Hubble Space Telescope image, copyright © NASA.)

Figure 8.3: The four general types of spacecraft attitude stabilization. All spacecraft fall into one of these categories, some examples of which are illustrated. (Image credits: Meteosat SG image, copyright © ESA; Intelsat 6 image, copyright © Boeing; GPS Navstar 2R image, copyright © Lockheed Martin; Hubble Space Telescope image, copyright © NASA.)

This problem is alleviated by the use of type 2 stabilization. Spacecraft with this stabilization type are referred to as dual spinners, and have a cylindrical section spinning at a few tens of rpm (like type 1), giving it spin stability. However, there is a platform mounted on top of the vehicle that is mechanically de-spun, where payloads that need to be pointed in a fixed direction, such as antennas or imaging cameras, can be mounted. The example shown in the figure is the Intelsat 6 communication satellite, which remains (at the time of this writing) the largest example of this type of attitude stabilization.

The third type, which I have called hybrid stabilization for want of a suitable label (although this terminology is not used universally by ACS engineers), is quite an interesting arrangement. Here the spacecraft acquires spin stability by mounting a momentum wheel inside the vehicle (see Figure 8.6). In this case, the spin stability is achieved by spinning a small object very rapidly (the wheel rotates at a typical rate of a few thousand rpm), rather than spinning a big object more slowly (the whole or part of the spacecraft rotating at a few tens of rpm, as is the case for the pure and dual-spinners). The mass of the wheel is typically a few kilograms, and its spin rate is maintained at around 6000 rpm. Given that the wheel is rigidly mounted in the spacecraft, its spin stability is transferred to the spacecraft as a whole. Thus the vehicle has the benefit of inherent stability, while at the same time allowing lots of space on the exterior surfaces of the spacecraft to mount payloads and deploy solar panels. The example shown in Figure 8.3 is that of a Navstar GPS satellite, which is part of the U.S. Department of Defense's constellation of satellites used for navigation. Like all such spacecraft using this type of stabilization, the spin stability is not at all obvious to the casual observer, as the mechanism for achieving this (the momentum wheel) is concealed inside the vehicle.

Type 4 is referred to as three-axis stabilization. In this case the spacecraft has no significant rotating parts, and thus does not have the inherent stability associated with the other types. As a consequence, the ACS has to work a bit harder to achieve the pointing mission. This lack of stability seems on the face of it to be a disadvantage, but often it is the only suitable option. A good example of this is a space observatory, such as the Hubble Space Telescope shown in Figure 8.3. To achieve its pointing mission, it has to rotate freely to point in various directions, and as a consequence there is no axis in the spacecraft about which it is sensible to employ spin stability. This would make the spin axis stiff, which would make no sense at all if it has to be moved around a lot in the process of pointing the telescope.

The different types of stabilization affect the overall shape or configuration of the spacecraft, which is why the control engineer would say that the ACS is the heart of the spacecraft.

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