Reaction wheels are electric flywheels inside a spacecraft that help point cameras, antennas, solar panels, and scientific instruments. They let a spacecraft rotate without firing thrusters, so they save fuel and allow very precise aiming. This matters because many missions need steady, accurate pointing for imaging Earth, studying stars, or communicating with ground stations.
In a typical spacecraft bus, three wheels are mounted along different axes so the craft can control roll, pitch, and yaw.
Key Facts
- Angular momentum is conserved when no external torque acts: Ltotal = Lwheel + Lspacecraft.
- Wheel angular momentum is L = Iω, where I is moment of inertia and ω is angular velocity.
- Changing wheel speed creates a torque on the spacecraft: τ = dL/dt.
- If a wheel speeds up one way, the spacecraft rotates the opposite way.
- Three reaction wheels on different axes can control rotation in 3D: roll, pitch, and yaw.
- Desaturation removes built-up wheel momentum using external torques from thrusters, magnetorquers, or environmental forces.
Vocabulary
- Reaction wheel
- A motor-driven spinning wheel inside a spacecraft used to rotate the spacecraft by changing the wheel's speed.
- Angular momentum
- A measure of rotational motion that depends on moment of inertia and angular velocity.
- Torque
- A twisting effect that changes an object's rotational motion.
- Attitude control
- The process of controlling which direction a spacecraft is pointing.
- Desaturation
- The process of reducing stored angular momentum in reaction wheels so they do not reach their speed limits.
Common Mistakes to Avoid
- Thinking reaction wheels push against space, which is wrong because they rotate the spacecraft by exchanging angular momentum internally.
- Assuming reaction wheels move a spacecraft from place to place, which is wrong because they mainly change orientation, not position.
- Forgetting the spacecraft turns opposite the wheel's change in spin, which is wrong because total angular momentum must be conserved.
- Ignoring desaturation, which is wrong because wheels can reach maximum speed after repeated pointing changes or external disturbances.
Practice Questions
- 1 A reaction wheel has moment of inertia 0.08 kg m^2 and speeds up from 0 rad/s to 500 rad/s. What angular momentum does the wheel gain?
- 2 A spacecraft body has moment of inertia 120 kg m^2 about one axis. A reaction wheel on that axis changes its angular momentum by +6 kg m^2/s. If the spacecraft starts at rest and no external torque acts, what angular velocity does the spacecraft gain?
- 3 A satellite in low Earth orbit keeps speeding up one reaction wheel in the same direction over many orbits. Explain why desaturation may become necessary and name one device that could help.