Reaction wheels adjust spacecraft orientation via conservation of angular momentum. Impulse wheels stabilize moving spaceships. Drive wheels maintain orientation against disturbances. Rocket boosters can counter reaction wheel unloading. Reaction wheel technology is well-developed, while rocket boosters have limited history.
A reaction wheel is a device used to make fine adjustments to the orientation of a spacecraft. It works via the principle of conservation of angular momentum: accelerating a reaction wheel produces a proportional response from the rest of a spacecraft. In this way, the orientation of a spacecraft can be controlled with the same high level of precision as reaction wheels. Impulse wheels, on the other hand, are used to stabilize a moving spaceship.
In an isolated system, the total amount of angular momentum must remain the same. Angular momentum is analogous to linear momentum, except that it refers to rotary motion rather than linear motion. If one ice skater pushes another stationary ice skater, they both move apart to conserve net linear momentum. Similarly, if part of a system starts to rotate in one direction, the rest of the system must rotate in the opposite direction. Otherwise, the total angular momentum would change.
In the case of a spacecraft, a reaction wheel can be turned to induce a response from the rest of the spacecraft. Without this induced response, the total angular momentum would change. However, the final rotation speed of the spacecraft will normally not be the same as the speed of the reaction wheel. This is because angular momentum must be conserved, not angular velocity. The spacecraft will move more slowly if it is much more massive than the reaction wheel.
Reaction wheel technology is generally well developed. Metal wheels have been used for over a century on trains. Electronics and equipment to make fine adjustments to reaction wheels, likewise, have many terrestrial applications. Small rocket boosters, on the other hand, have a limited history of development and refinement.
Occasionally the reaction wheels need to “unload” if they are spinning too fast. On an unpredictable mission, engineers may not know which way a spacecraft should point; therefore, a method is needed to slow down the reaction wheels without inducing a response. Rocket boosters can be fired to counter the response of slowing down a reaction wheel.
A drive wheel also uses the principle of conservation of angular momentum. However, its purpose in a spaceship is to maintain a certain orientation rather than change it. Various forces on a spacecraft, including air friction and varying gravitational and magnetic fields, can produce torques that perturb a spacecraft’s orientation. This is experienced in the Hubble Space Telescope, which relies on a stable orientation to focus on distant objects. When spinning rapidly, drive wheels can withstand such disturbances and help maintain a stable orientation.
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