Table 7.1 in Chapter 7 stated the purpose of the ACS: "to achieve the spacecraft's pointing mission.'' What does this mean? Most operational spacecraft in orbit have payloads that require pointing. For example,
• a communication satellite (comsat) needs to point its communications dish(es) at a ground station to receive and transmit the stream of telephone conversations that are using the system,
• an Earth observation spacecraft needs to point its imaging cameras at the targets of interest on the ground, and
• a space observatory needs to respond to ground commands to point a telescope at particular objects (planets or galaxies) in the sky.
G. Swinerd, How Spacecraft Fly: Spaceflight Without Formulae, DOI: 10.1007/978-0-387-76572-3_8, © Praxis Publishing, Ltd. 2008
So, the ACS addresses the pointing, or rotation, of the spacecraft. In earlier chapters we talked a lot about orbits, focusing on how the spacecraft's center moved along its orbit. By contrast, in this section about the ACS, we are not concerned with the motion of the spacecraft's center along its orbit, but rather with the way the spacecraft rotates about its center. We can imagine ourselves being in the same orbit as the spacecraft, just a few meters away, and watching it rotate in response to commands to point the payload instruments. The word attitude describes the position of the spacecraft in a rotational sense, and a change in attitude therefore implies a rotation. What causes the spacecraft to rotate (or to stop rotating)? In the earlier chapters on orbits, we saw that it was force that changed the orbit along which the spacecraft moves. In contrast, it is torque that causes the spacecraft to rotate about its center.
We discussed torques in Chapter 3, when we looked at gravity anomaly orbit perturbations. A torque is a rotational force such as the one we apply to remove a bolt from a wheel when we change a flat tire. We apply a force of a number of Newtons in a rotational sense by pushing down on the end of the handle of the wrench. The size of the applied torque is based not only on the amount of force exerted but also on the length of the handle of the wrench. The longer the handle, the greater the moment arm and the more torque there is. It is easier to loosen the wheel bolts if the handle of the wrench is lengthened. Effectively, this increases the amount of torque, without requiring us to apply more force on the handle. The magnitude of the torque is given by the force times the moment arm, and has units of Nm (Newton meters).
The main job of the ACS is to control the rotational state of the spacecraft by using on-board devices called control torquers that produce torques on command to rotate the spacecraft. The most obvious kind of control torquer is a pair of thrusters fired in such a way as to produce a rotation. We will discuss thrusters later, when we talk about the propulsion subsystem, but essentially they are small rocket engines (small enough to be held in the palm of your hand), each producing a thrust force on the order of a few Newtons (a force of 1 Newton is about the weight of a small apple). They are located in clusters around the exterior of the spacecraft, and can be fired in opposed pairs (see Figure 8.4) to produce a torque, and therefore a rotation of the spacecraft. Other kinds of control torquers are discussed later. However, in the interim, it is helpful to briefly describe the main functions of the ACS, as these are the aspects that lead to the ACS design:
• To achieve the pointing mission of the payload, in terms of directions and accuracy. A comsat, for example, may need to point its antenna at a ground station with an accuracy of 0.1 of a degree, whereas a space observatory such as the Hubble Space Telescope may need to point the telescope at a galaxy with an accuracy of less than an arc second. If we divide a degree by 60, we get an arc minute, and if we divide an arc minute by 60 we get an arc second. So an arc second is a tiny angle, being l/3600th of a degree!
• To achieve the pointing requirements of other subsystems, a process sometimes called housekeeping. For example:
- Pointing a solar panel at the Sun to generate electrical power
- Pointing an antenna at a ground station on Earth's surface to downlink payload data
- Pointing thermal radiators to the cold of space to allow heat to escape (this will make more sense after we look at the thermal control subsystem)
- Pointing a rocket engine in the right direction before it is fired, so that the correct change in the orbit is achieved
• Overall, to manage the rotational state of the spacecraft, meaning motion and torques about the spacecraft's center.
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