1. The ship that points by spinning wheels

James S. A. Corey’s The Expanse returns as the recalled work because the Rocinante changes orientation by firing thrusters and by managing its own angular momentum. A spacecraft in orbit does the same, except it prefers not to spend propellant on every pointing adjustment. Instead it stores and releases angular momentum in spinning wheels.

This entry reads about GNC actuators and attitude control.

2. Reaction wheels

A reaction wheel overview describes the basic principle: a motor-driven flywheel exchanges angular momentum with the spacecraft. Spin the wheel one way and the spacecraft rotates the other way. Three wheels in a nominally orthogonal arrangement enable three-axis control; a fourth wheel in a pyramid or tetrahedral arrangement provides redundancy.

Reaction wheels are precise, electrically powered, and propellant-free. Their limitations are:

  • Momentum saturation: environmental torques slowly accelerate the wheels until they reach maximum speed.
  • Coulomb and viscous friction: cause small disturbances at low speeds.
  • Bearing wear: limits life and can cause failure or vibration.
  • Tumble at launch: wheels cannot detumble a rapidly spinning spacecraft from scratch.

3. Control moment gyroscopes

A TensorTech comparison of reaction wheels and control moment gyroscopes notes that CMGs produce torque by gimbaling a high-speed rotor rather than changing its speed. They can deliver much higher torque for fast slews but are mechanically more complex and heavier. CMGs are common on large agile spacecraft; reaction wheels dominate small satellites.

4. Magnetorquers

Magnetorquers create a magnetic dipole that interacts with Earth’s magnetic field to produce torque. They are simple, robust, and require no moving parts, but their torque is small and only available where the field is strong. Their main uses are:

  • Detumbling: removing initial spin after deployment.
  • Momentum dumping: unloading accumulated angular momentum from reaction wheels.
  • Safe-mode control: providing coarse attitude authority when wheels are off or saturated.

A ScienceDirect paper on reaction wheels plus asymmetric magnetorquers demonstrates how magnetorquers can provide backup control if reaction wheels fail.

5. Thrusters

Thrusters produce torque by firing off-center from the center of mass. They are used for:

  • Large slews or orbit corrections that wheels cannot provide.
  • Desaturation when wheels and magnetorquers are insufficient.
  • Emergency attitude control.

Their cost is propellant. For a long-life platform, thrusters are a scarce resource and are used sparingly.

6. Control laws

The software that commands actuators is the control law. Common approaches include:

  • Proportional-derivative (PD) control: simple and widely used for three-axis stabilization.
  • Quaternion feedback control: avoids singularities associated with Euler angles.
  • Momentum management: schedules wheel unloading and keeps the system away from saturation.
  • Slew planning: computes trajectories that respect rate and torque limits.

A NASA S3VI document on space vehicle control systems surveys these actuator and control strategies.

7. What this changes

  • The desktop will almost certainly use reaction wheels as its primary actuators, with magnetorquers for detumbling and momentum dumping.
  • Thrusters may be needed for large slews or desaturation, depending on the platform’s inertia and disturbance environment.
  • Redundant wheel arrangements and magnetorquer backup improve reliability.
  • The next entry will read about GNC modes and fault protection.