Keeping a satellite or spacecraft correctly aligned, and on course, with efficient attitude actuation is vital for a successful mission.
Ensuring that a space system maintains a reliable orientation, with precise attitude actuation, in orbit is crucial for the following reasons:
- Navigation – to provide accurate orbital maneuvers and adjustments to remain on course
- Communication – to orient the system towards the relevant ground assets
- Data collection – to point onboard cameras or other instruments to relevant points of interest
- Power generation – to maneuver so that solar panels are facing the Sun
- Re-orientation – before making larger maneuvers, for example; lining up thrusters ready for collision avoidance
Attitude control systems are made up of attitude sensors that collect information on the system’s orientation relative to a reference frame, along with attitude actuators, that change the body frame’s orientation by applying torque where required.
On this page you can see information on the 4 main types of attitude actuator, some of the key performance criteria for each of them, and links to up-to-date information, directly from suppliers, on attitude actuator solutions available on the global supply chain.
Magnetorquers
Magnetorquers (also known as magnetic torquers or torque rods) produce a magnetic field which interacts with the Earth’s own magnetic field to produce a torque on the satellite and control its attitude.
Magnetorquer key performance criteria include:
- Coil material – usually copper or aluminium; aluminium is lighter but copper has lower resistance
- Core material – typically air or ferromagnetic material
- Permeability
- Linear region
- Length & cross-sectional area of coil and rod
- Time constant – should be a reasonable value in order for steady state to be achieved quickly
Reaction wheels
Reaction wheels (also known as momentum wheels) store rotational energy, providing spacecraft with 3-axis attitude control without requiring external sources of torque.
Reaction wheel key performance criteria include:
- Attitude control sensitivity and precision
- Maximum momentum storage, torque, and angular speed
- Angular speed control accuracy
- Static and dynamic imbalance
- Control equipment, mechanisms and procedures
- Installation procedures and equipment
- Redundancy system (e.g. a 4th wheel)
Control Moment Gyroscopes (CMGs)
Control Moment Gyroscopes produce gyroscopic torque that rotates a spacecraft by changing the direction of a spinning rotor’s angular momentum.
CMG key performance criteria include:
- Type of CMG: single-gimbal, dual-gimbal or variable speed control moment gyroscopes (VSCMG)
- Angular momentum storage
- Output torque (maximum and nominal)
- Control sensitivity
Thrusters
Thrusters are in-space propulsion units that utilize fuel to create thrust and control attitude. Thrusters have a variety of other applications in space missions alongside attitude control.
Thruster key performance criteria include:
- Specific and total impulse
- Minimum impulse bit
- Thrust range
- Format – electric or chemical
- Propellant type
- Operating power
- Delta-V capability
- Operational firing time and sequences/pulses
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General procurement advice for attitude actuators
Here are some additional criteria to assess when weighing up any attitude actuation or control solution for your next mission:
- SWAP-C (size, weight, power and cost)
- Interoperability options
- Redundancy
- Customization options
- Operating (and storage) temperature ranges
- Radiation and vibration tolerance limits
- Operating lifetime
- Flight heritage and reliability
- Assembly and integration requirements
- After-sales support
- Supplier reputation
- Availability and lead time
- Export controls
