Reaction wheels are mechanical devices used for attitude control on spacecraft and satellites. They provide torque to rotate the spacecraft by exchanging angular momentum with a spinning mass inside the wheel. Reaction wheels offer precise 3-axis control without requiring propellant or external torques, making them a popular choice for modern spacecraft.
How Reaction Wheels Work
A reaction wheel contains a flywheel attached to an electric motor. By accelerating or decelerating the flywheel, the motor can apply torque to the wheel’s spin axis. Due to Newton’s third law, this torque is transferred to the spacecraft, causing it to rotate in the opposite direction to conserve total angular momentum.
Most spacecraft use three reaction wheels mounted orthogonally to provide full 3-axis control. The wheels are spun at a constant speed, storing angular momentum. When the spacecraft needs to rotate, the wheel speeds are adjusted to transfer momentum to the spacecraft body. For example, speeding up Wheel 1 will induce a torque on the spacecraft, causing it to pitch down. Slowing down Wheel 2 causes a yaw torque, and Wheel 3 controls roll.
Advantages of Reaction Wheels
Reaction wheels offer several key benefits compared to other attitude control systems:
- High precision pointing – Reaction wheels can provide extremely fine attitude control on the order of millidegrees or microradians. This makes them well-suited for missions requiring accurate pointing like Earth observation and astronomy.
- No propellant required – Unlike thrusters, reaction wheels function without expendable propellant. This reduces overall spacecraft mass and eliminates concerns over propellant depletion.
- Internal system – Being fully contained within the spacecraft, reaction wheels do not interact with the space environment. This prevents contamination from thruster plumes interfering with sensitive instrumentation.
- Power efficient – Reaction wheels only draw tens of watts during operation. They can be powered by the spacecraft’s solar arrays without issue.
- Long lifetime – With quality bearings and motors, reaction wheels can operate reliably for 10-15 years on orbit.
Reaction Wheel Design
The core components of a reaction wheel are the flywheel, motor, bearings, spin axis, and housing.
- Flywheel – The flywheel provides the angular momentum storage. High density materials like tungsten or titanium are often used to maximize momentum for a given size. The flywheel is balanced precisely to avoid inducing vibrations.
- Motor – Brushless DC motors spin the flywheel. They offer high torque, efficiency, and controllability. Redundant motor windings improve reliability.
- Bearings – Smooth ball bearings allow near-frictionless rotation of the flywheel. Special lubricants withstand the space environment. High quality bearings are critical for long life.
- Housing – The wheel must be contained in a rigid housing that interfaces with the spacecraft. Aluminum housings are common. Vibration damping prevents structural coupling.
- Electronics – Control electronics adjust the motor speed on command and provide telemetry on wheel speed, temperature, torque, etc.
Reaction Wheel Specifications
Key performance parameters of reaction wheels include:
- Momentum storage – Amount of angular momentum available, in Nms. More storage allows faster slewing.
- Torque – Rotational force produced by the motor, in Nm. Higher torque enables rapid response.
- Top speed – Maximum flywheel rotation rate, often 4000-6000 rpm. Limits momentum storage.
- Jitter – Small torque fluctuations introduce pointing errors or “jitter”. Quality bearings minimize jitter.
- Mass – Total reaction wheel mass, typically 1-5 kg. Impacts spacecraft mass budget.
- Power – Electrical power drawn during operation, ~10-100 W. Affects solar array sizing.
- Lifetime – Bearing lifetime until failure, ideally 10-15 years. Determines number of required spares.
Reaction Wheel Operations
Reaction wheels are operated by the spacecraft’s attitude control subsystem. This involves:
- Sensing – Star trackers, sun sensors, gyroscopes and magnetometers provide attitude and rate telemetry.
- Planning – Using current vs desired attitude, torque commands are calculated.
- Actuation – Motor commands spin wheels up/down to induce desired torques.
- Monitoring – Housekeeping data monitors wheel health and detects impending failures.
- Momentum Management – Buildup of stored momentum is dumped using magnetorquers or thrusters.
The attitude control system runs control loops to translate pointing requirements into appropriate reaction wheel commands many times per second.
Reaction Wheel Failure Modes
Despite their reliability, reaction wheels are still prone to certain failure modes, including:
- Bearing wear – Mechanical wear degrades bearing smoothness, increasing friction and jitter.
- Motor failure – Motor windings short out or break, preventing acceleration.
- Electronics failure – Issues with control boards or power systems disable the wheel.
- Mechanical jam – Debris or bearing defects cause the flywheel to seize.
- Lubrication loss – Lubricant escapes in vacuum, leading to increased wear.
- Speed limits – Damage occurs if operational speed limits are exceeded.
To mitigate risks, most spacecraft carry redundant reaction wheels. If one fails, the others can take over attitude control until the failed unit is replaced.
Reaction Wheel Applications
Reaction wheels are widely used on many types of spacecraft:
- Satellites – Communications, Earth observation, technology demonstration satellites commonly use reaction wheels for 3-axis control.
- Space telescopes – Precision pointing requirements make reaction wheels ideal for space telescopes like Hubble and James Webb.
- Human spacecraft – Vehicles like the Space Shuttle and ISS utilize reaction wheels for attitude control and jitter reduction.
- Interplanetary probes – Reaction wheels help orient planetary probes for trajectory maneuvers and science observations.
Even small satellites and CubeSats can benefit from miniaturized reaction wheels. The precise control enables advanced performance from even tiny platforms.
Future Reaction Wheel Developments
Upcoming improvements in reaction wheel technology include:
- Miniaturization – Smaller satellites demand smaller, lower mass wheels with high performance.
- Improved lifetime – Longer design lifetimes up to 20 years between failures.
- Lower power – Reduced power consumption allows use on small satellites.
- Lower jitter – Bearing and balance improvements to minimize torque noise.
- Better electronics – More reliable control boards with radiation hardening.
- Active vibration isolation – Actively canceling bearing noise to further reduce jitter.
- Alternate torque generation – Exploring non-mechanical torque generation mechanisms.
Summary
Reaction wheels have become an essential technology for modern spacecraft attitude control. Their ability to deliver precise pointing without expendable propellant has made them a key component of many space missions. With continual improvements in performance, reliability, and miniaturization, reaction wheels will remain critical for future space applications ranging from CubeSats to deep space science probes.