Attitude control is the part of astronautics that keeps a spacecraft pointed in the right direction. A telescope must aim steadily at stars, an antenna must point toward Earth, and solar panels must face the Sun to make power. Even small unwanted rotations can ruin images, weaken communication, or reduce mission performance.
Engineers use sensors and actuators together to measure orientation and correct it in space.
Reaction wheels rotate inside the spacecraft and change the spacecraft attitude by conservation of angular momentum. Thrusters can also rotate the spacecraft by firing in pairs to create a torque, and they are often used when reaction wheels become saturated with too much stored momentum. Sensors such as star trackers, Sun sensors, gyroscopes, and inertial measurement units estimate where the spacecraft is pointing.
A control computer compares the desired attitude with the measured attitude, then commands wheels or thrusters to reduce the error.
Key Facts
- Torque changes angular momentum: τ = dL/dt
- For a rigid spacecraft about one principal axis: τ = Iα
- Angular momentum of a spinning wheel is approximately L = Iω
- Reaction wheels rotate the spacecraft by changing wheel speed in the opposite angular direction.
- Thruster torque depends on force and lever arm: τ = rF sinθ
- Momentum dumping uses thrusters or magnetic torquers to remove stored reaction wheel angular momentum.
Vocabulary
- Attitude
- Attitude is the orientation of a spacecraft in space, usually described by its roll, pitch, and yaw angles or by a quaternion.
- Reaction wheel
- A reaction wheel is a motorized spinning wheel inside a spacecraft that changes spacecraft orientation by speeding up or slowing down.
- Thruster
- A thruster is a small rocket engine that produces force and can create torque when its force acts away from the spacecraft center of mass.
- Star tracker
- A star tracker is an optical sensor that identifies star patterns to determine a spacecraft's orientation very accurately.
- Momentum saturation
- Momentum saturation occurs when a reaction wheel reaches its speed limit and can no longer provide the commanded torque effectively.
Common Mistakes to Avoid
- Treating attitude as position is wrong because attitude describes where the spacecraft points, not where it is located in orbit.
- Assuming reaction wheels push on space is wrong because they work internally by conservation of angular momentum between the wheel and the spacecraft body.
- Ignoring the center of mass is wrong because a thruster only creates rotational torque if its line of action does not pass through the center of mass.
- Forgetting momentum dumping is wrong because small environmental torques can build up wheel momentum over time until the wheels saturate.
Practice Questions
- 1 A spacecraft has a moment of inertia of 240 kg m^2 about its yaw axis. What torque is needed to produce an angular acceleration of 0.015 rad/s^2?
- 2 A thruster produces 0.40 N of force at a perpendicular distance of 1.5 m from the spacecraft center of mass. What torque does it create?
- 3 A spacecraft telescope must keep a star centered for a long exposure. Explain why reaction wheels are useful for fine pointing, and why thrusters may still be needed later.