Control Torquers

We have already briefly mentioned control torquers in our walk around the control loop in Figure 8.1. These are items of ACS hardware that are essentially the muscles of the ACS, converting the virtual commands produced by the on-board computer into physical torques that rotate the spacecraft.

Thrusters

Perhaps the most obvious way of producing a control torque is to fire two thrusters in opposite directions, as illustrated in Figure 8.4a. These small rocket engines are set up in groups, called thruster clusters, which are located at a number of positions around the spacecraft to ensure that the attitude and orbit control functions can be effectively achieved. By firing thrusters in pairs using different clusters, we can rotate the spacecraft in any direction as shown in Figure 8.4b.

Magnetorquers

When I was a child, I made an electromagnet using a large 6-inch nail, some wire, and a battery. I wound the wire many times around the nail, and

Magnetic Torquers
appropriately chosen thruster clusters allows the spacecraft to be rotated in any direction in three dimensions.

attached the ends of the wire to the battery, and magically when the circuit was made the nail became a magnet. I remember being intrigued by this, and enjoyed the childish pleasure of picking up paper clips and toy cars with this magnet, which I could turn on and off simply by making or breaking the contact with the battery. This simple homemade electromagnet is the basis for another form of control torquer, the magnetorquer rod, although the real thing is a little more precisely engineered and a bit bigger! The nail is replaced by a metal rod, usually made from an alloy containing iron, and it can range in length from half a meter to about 2 meters, depending on the size of the spacecraft that needs to be torqued. A considerable length of wire is then wound around this core, giving us an electromagnet that can be activated on command by passing an electrical current through the device.

How is this used to rotate a spacecraft? Consider a compass; the compass needle is simply a magnet mounted on a pivot to allow it freedom to move. It points north because, as a magnet, it tries to align itself with the local magnetic field lines, which at Earth's surface run south to north (see Figure 6.3a in Chapter 6). In the same way, if a current is passed through a magnetorquer, it becomes a magnet, and as a consequence it too will tend to rotate to align itself with the local magnetic field at its orbital position, as shown in Figure 8.5a. Since the magnetorquer rods are firmly attached to the spacecraft (Fig. 8.5b), the vehicle will also share this rotation. So, if we know where we are in orbit, and what the magnetic field is like there, we can produce control torques to rotate the spacecraft on command, by simply passing electrical current through the appropriate magnetorquer. The idea sounds simple enough, but the implementation of this type of control is a fairly complicated business for the ACS engineer. Despite this, however, magnetorquers are commonly used on spacecraft. For example, the Hubble Space Telescope (HST) is equipped with magnetorquer rods to generate torques on the vehicle. An advantage of using magnetorquers on spacecraft like the HST is that they are clean, unlike thrusters that squirt out propellant each time they are used, which could end up contaminating the sensitive telescope optics. Figure 8.5c shows a couple of 1-m magnetorquer rods mounted on a test bed prior to installation in a spacecraft.

Reaction Wheels

Another commonly used type of control torquer is the reaction wheel. These are precisely engineered wheels, usually about 15 to 30 cm (6 to 12 inches) in diameter, with a mass on the order of a few kilograms. Their actual dimensions are governed by the size of the spacecraft in which they are installed, and how fast the spacecraft needs to rotate to achieve its pointing mission. To rotate the spacecraft in any direction in three dimensions, three

Figure 8.5: (a) When turned on, the magnetorquer rods align themselves with the local magnetic field, (b) The magnetorquer rods are firmly attached to the spacecraft, so that their rotation is shared by the vehicle as a whole, (c) Two magnetorquers under test, (Image courtesy of Dutch Space,)

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Figure 8.5: (a) When turned on, the magnetorquer rods align themselves with the local magnetic field, (b) The magnetorquer rods are firmly attached to the spacecraft, so that their rotation is shared by the vehicle as a whole, (c) Two magnetorquers under test, (Image courtesy of Dutch Space,)

Figure 8.6: (a) Three reaction wheels are mounted rigidly in the spacecraft with their axes perpendicular to one another, to allow rotation of the spacecraft in any direction in three dimensions. (b) A typical reaction wheel, The wheel is usually sealed inside a disk-shaped canister, the lid of which has been removed in the picture, The electronics that control the wheel can be seen underneath the wheel in the base of the canister, (Image courtesy of Rockwell Collins Deutschland,)

Figure 8.6: (a) Three reaction wheels are mounted rigidly in the spacecraft with their axes perpendicular to one another, to allow rotation of the spacecraft in any direction in three dimensions. (b) A typical reaction wheel, The wheel is usually sealed inside a disk-shaped canister, the lid of which has been removed in the picture, The electronics that control the wheel can be seen underneath the wheel in the base of the canister, (Image courtesy of Rockwell Collins Deutschland,)

wheels are usually employed with their spin axes mounted at right angles to each other, as shown in Figure 8.6a. However, the ACS engineer will usually mount a fourth wheel for redundancy reasons, with its spin axis canted at an angle to the other three, to allow control of the spacecraft in the event of the failure of one of the three primary wheels. An example of what a reaction wheel looks like is shown in Figure 8.6b.

To see how reaction wheels work to rotate the spacecraft, let's focus on just one of them. The wheel is connected to a torque motor, which itself is rigidly attached to the structure of the spacecraft. A torque motor is simply an electric motor that can be used to spin the wheel, a bit like the motor found in a domestic power drill; when we squeeze the drill's trigger, an electric current passes through the torque motor in the drill to rotate the business end—and so it is with the reaction wheel. To rotate the spacecraft around the axis of the wheel, electric current is passed through the wheel's torque motor, and as a consequence the wheel spins. To understand how this causes the spacecraft to rotate, we return to the power drill. If we "tweak" the drill trigger, the chuck and drill bit will spin in one direction, but the handle of the drill kicks back (in a rotational sense) in the opposite direction. This is why astronauts have trouble using power tools when working during space walks; the reaction causes them to rotate as well as the tool, so they have to be firmly attached to the spacecraft to prevent a rather undignified pirouette! It is this same reaction that kicks the spacecraft into rotational motion about the wheel axis, but in the opposite direction to the wheel's rotation. To summarize: To rotate the spacecraft about the wheel's axis, an electric current is applied to the wheel's torque motor. The wheel spins, and as it does so it produces a rotational kick in the opposite direction on the torque motor. Since the torque motor is attached to the spacecraft's structure, this kick is transferred to the spacecraft, which in turn begins to rotate about the wheel axis. This is an application of Newton's third law of motion, which we talked about in Chapter 1: for every action there is an equal and opposite reaction.

But how can the spacecraft be brought to rest again, because once set rotating, the spacecraft will continue to spin forever, as there are no frictional or other forces in space that will stop it. To stop the rotation of the spacecraft, the wheel is brought to rest. The braking (slowing) of the wheel produces a reaction on the spacecraft in the opposite direction that will slow and stop the rotation.

This method of changing a spacecraft's attitude is clean, efficient—elegant even—and just requires a bit of electrical power, which is (usually) freely available through the conversion of sunlight by solar panels. So it doesn't cost propellant, as is the case for thrusters, and generally it has a larger torque capability than magnetorquers. It also works equally well in finely pointing a spacecraft in a particular direction as it does in rotating the vehicle through large angles. Thus, it is the most commonly used technique by the ACS engineer to control the rotation of the spacecraft.

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