Reaction wheels are crucial components for satellites of all shapes and sizes. They are becoming particularly important in the modern space industry, due to increasing demands for better for maneuverability, agility and precision control of small satellites.
This demand for improved performance and control follows the trend we’re seeing in the in-space propulsion sector, which you can read more about on our recent post discussing CubeSat thrusters.
In this article we take a high-level look at reaction wheel design, how reaction wheels work, and what to take into consideration when searching for a reaction wheel for your satellite. We also give an overview of some of the products currently available on the global marketplace for space.
If you would like to skip the introductory material and go straight to view the available products, please click here, otherwise please read on.
What are reaction wheels?
Reaction wheels are internal mechanical components of controllable satellites and spacecraft that enable them to reposition while in orbit. They are sometimes referred to as momentum wheels.
Reaction wheels store rotational energy, providing satellites or spacecraft with three-axis attitude control without requiring external sources of torque (such as rockets or propellants), saving on weight and the available space.
Note that attitude in this context refers to the orientation of the satellite with respect to another object or frame of reference (such as a celestial sphere centred on Earth) and three-axis control refers to the typical x-y-z cartesian system used to specify an object’s location in three dimensions.
Reaction wheels control a satellite’s attitude with very high precision, which is critical for applications that require excellent pointing accuracy, such as for Earth Observation purposes, or to keep a telescope fixed towards a particular region of interest.
Please note that this is just a brief introduction to reaction wheels of course; in a future article as part of our Let’s talk about series on the blog we’ll go deeper into the topic of attitude control – if you’d like to be informed when this is published, please sign up to our mailing list at the link below.
How do reaction wheels work?
Reaction wheel dynamics are relatively simple to understand. The satellite undergoes external torque from various sources (e.g. solar radiation pressure, Low Earth Orbit (LEO) aerodynamic forces and due to gravity gradient torque) that can disturb its position and path.
Reaction wheels are flywheels – devices that store rotational energy by conserving angular momentum and that enable the exchange of momentum within the satellite body in order to provide stability and counteract such disturbances.
They also provide a high pointing accuracy and so can precisely reorient a satellite to align an Earth Observation (EO) payload for example.
The benefit of using reaction wheels, rather than thrusters, to perform momentum exchange in these cases is that there is no need to use any fuel, saving costs and mass budget, and avoiding propellant safety issues.
Instead reaction wheels use a brushless DC electric motor connected to a spinning wheel, with the rotational speed controlled by the satellite’s on-board computer (OBC).
When operated, the reaction wheel causes the satellite body to counter-rotate – i.e. to rotate in the opposite direction to the wheel’s direction of rotation.
This rotation is carried out around the satellite’s center of mass. As the satellite is a closed system the total angular momentum must be conserved, therefore a reaction wheel cannot change a satellite’s location.
The direction of the satellite’s counter-rotation in response to a wheel’s rotation will only take place along one axis, depending on the axis of the wheel. Therefore for complete control of the entire system three reaction wheels must be used, in orthogonal orientation to each other.
How to select the best reaction wheel for your needs
In our recent product overview article on CubeSat thrusters we suggested a basic four-step approach to selecting the right in-space propulsion system for your needs.
This framework can also be used to select the optimum reaction wheel, and an overview is given below:
- Specify your exact mission parameters – ensure you are clear on the full range of activities your spacecraft needs to perform.
- Record all known design specifications of the satellite – keep to hand the current specifications of the craft (understanding these may change as the design evolves) – for example, it is vital that reaction wheel sizing is correct due to the impact on the system’s available volume.
- Consider the range of technology that will be used in the system – take into account the results of all decisions on what other components and sub-systems have been made. It is important that the reaction wheel you choose will work effectively with the other parts of the satellite.
- Take into account the key performance criteria – understand how to evaluate available reaction wheel products according to the criteria most relevant for your applications. More on this below.
For a reaction wheel the key performance criteria will include characteristics such as:
- Size/volume and weight
- Attitude control sensitivity and precision
- Control equipment, mechanisms and procedures
- Installation procedures and equipment
- Redundancy system (e.g. 4th wheel)
- Flight heritage, testing results and/or technology readiness level (TRL)
Reaction wheels available on the global market
In the article section below we have included a selection of the reaction wheel products currently available on the market.
Please note that this list will be updated when new products are added to the global marketplace for space – so please check back for more.
The RW210 and RW400 series reaction wheels are aimed at applications in 1-3 U CubeSats and 6-12 U CubeSats (or similar platforms) respectively. Both systems feature a small envelope (of 25 x 25 x 15 mm for the RW210 and 50 x 50 x 27 mm for the RW400) that includes a dedicated controller, offering simple fire-and-forget control to the user.
The RW210 comes in three models that offer 1.5, 3.0 or 6.0 mN.m.s of angular momentum storage respectively in both directions and 0.1 mN.m of torque that allows for rapid slewing operations with small satellites.
The RW400 comes in three models that offer 15, 30 or 50 mN.m.s of angular momentum storage respectively in both directions and Up to 12 mN.m of torque that also allows for rapid slewing operations with small satellites.
The CubeWheel Small, Medium and Large by CubeSpace
CubeSpace offers the CubeWheel range of reaction wheels in three distinct sizes to suit different satellite requirements. CubeWheels include a highly balanced flywheel disc, a brushless electrical motor, an electronic driver and a microcontroller. The wheels can be given a speed reference command through I2C, UART or CAN, and they integrate easily with any ADCS.
The modular wheels are provided with all driving and interfacing electronics integrated into a complete package. They are also individually precision-balanced to reduce vibration and are able to operate at a wide range of voltages, making them compatible with satellites running on battery voltages between 6.5V and 18V.
NanoAvionics provides reaction/momentum wheels as a separate component (RW0) or an integral four- reaction wheels redundant 3-axis control system (4RW0) to enable precision pointing of the small satellite.
The NRWA-T2 and NRWA-T065 by NewSpace Systems (Pty) Ltd.
NewSpace Systems offers reaction wheels in a range of sizes. Typically, three to four reaction wheels are needed to provide full three-axis control of a spacecraft and the additional integration of these wheels with external gyroscopes for full three-axis control using inertial rates. With high torque stability, wheel momentum capacity and accurate speed control; the NewSpace reaction wheels are ideal for agile small satellite missions requiring highly precise pointing.
Reaction Wheel Units by Bradford Space
Bradford Space manufactures a range of Reaction Wheel Units (RWUs) consisting of a Wheel Drive Electronics Box (WDE) and a Reaction Wheel Assembly (RWA); a rotating inertial mass, driven by a brushless DC electric motor. The wheel accelerates when power is applied to the motor causing the satellite body to rotate in the opposite direction, due to the induced counter-torque. A minimum of three sets of RWAs are needed per satellite to allow rotational control around the three axes. An extra RWA is generally used for redundancy purposes. The portfolio consists of:
- Reaction Wheel Unit W18 – a 5.20 kg mass system with a maximum torque of 0.265 Nm.
- Reaction Wheel Unit W18ES – a 6.02 kg mass system with a maximum torque of 0.265 Nm.
- Reaction Wheel Unit W45 – a 6.70 kg mass system with a maximum torque of 0.248 Nm.
Traditional satellite attitude control systems require 3 single-axis reaction wheels and 3 single-axis magnetorquers in hardware. The reaction wheels are equipped to provide precise internal torque and the magnetorquers are equipped to provide rough external torque. The reaction sphere is based on a unique magnetic field design and control methodology and is designed to replace all these components.
Rotating in 3 degrees of freedom, the multi-axis rotor can replace 3 traditional reaction wheels in X, Y, and Z axes. In terms of rotational dynamics, the sphere exerts gyroscopic torque as a control moment gyro does. Moreover, bias-currents are applied to the motor coils to enable its magnetorquer functionalities. However, the torque ripple of the reaction sphere will be higher when bias currents are turned on.
The RWA-05 by Berlin Space Technologies is a reaction wheel assembly with an integrated fibre optical Gyro (RWA) for small satellites. The integrated gyro enables highly accurate control loops such as precise speed, angular rate and angle control loops. These features have been pioneered on Berlin satellites since 1994.
The reaction wheels module – an autonomous actuator module of 4 flywheels, with drivers and control electronics, which is designed to be integrated into 3U or larger nanosatellites. The flywheels and electronics are balanced and designed to provide enough momentum to rotate the nanosatellites. The system uses a single PC/104 connection with power and CAN buses and each flywheel driver can be driven separately with its own CAN ID.
The SX-RW-50-2.0 reaction wheel – a brushless dc-motor system that can be used in up to 200 kg microsatellites. The reaction wheel can be utilised in a microsatellite ADCS for three-axis orientation building in-orbit, which may be used in remote sensing, communication, celestial bodies observation, and other applications.
The RW-0.01 by Sinclair Interplanetary
Sinclair Interplanetary has pioneered high-reliability, vacuum-lubricated reaction wheels for picosatellites, nanosatellites and microsats. There are currently more than 75 wheels in-orbit, all working well. All reaction wheels in the range incorporate a built-in digital processor. They can be controlled over a serial bus to produce a desired speed, momentum or torque, and report digital telemetry concerning speed, temperature, voltage, current, and other health parameters.
The full portfolio of reaction wheel products listed on satsearch consists of:
Reaction Wheel 1 and other products by Astro- und Feinwerktechnik Adlershof GmbH (ASTROFEIN)
Astrofein manufactures a range of reaction wheel products for different size satellites. The suite of products ranges from one of the world’s smallest commercial reaction wheels, the RW 1 (10-4 Nms), to the RW 250 (4 Nms) system. Note that the RW 90 and RW 1 are flight proven and four RW 90 reaction wheels are used in the small satellite TET-1 (launched 2012).
The full portfolio of reaction wheel products listed on satsearch consists of:
RW1 and other products by Blue Canyon Technologies Inc.
Blue Canyon Technologies reaction wheels provide an efficient, high-performance solution for spacecraft attitude control. The wheels are available in a range of sizes, providing the combination of torque and momentum storage to suit different spacecraft and mission requirements. Control electronics are available in each reaction wheel, simplifying system integration and operations.
The full range of reaction wheel products manufactured by Blue Canyon Technologies available on satsearch are included below:
CubeSat Pointing reaction wheels enable accurate control of the attitude of CubeSats. The wheels have been tested for a 3-year lifetime and have the required torque and momentum capacity for typical 3U CubeSat missions. Their compact design and easy interface facilitate integration in any satellite.
GSW-600 by GomSpace
The GomSpace NanoTorque GSW-600 is a compact and high performance reaction wheel designed and qualified for an equivalent of 3 years in-orbit operations. The envelope of the reaction wheel is designed to allow 4 wheels to be placed in a redundant setup within the envelope of a standard CubeSat. Available versions are 4-wheel 30° pyramid and cold-spare setup. The GSW-600 wheel is available in a version with SPI and I2C interface and one with CAN and I2C interface.
The HR 0610 Reaction Wheel provides a high-speed, low-weight solution for small spacecraft. Standard microelectronics provide flexibility for wheel performance over a wide range of spacecraft interfaces.
The Flywheel by Chang Guang Satellite
The flywheel can play an essential role in controlling the satellite’s flying attitude in a steady state. Its control system is designed to ensure the reliability and lifecycle of satellites in orbit. The system uses an integrated design of flywheel body and control circuits has been applied to many of the in-orbit satellites of Jilin-1 series. The product features a rated output torque of 6,000 rpm and rated angular momentum of 2 Nm.s at this level. The system weighs approximately 3kg and has a typical lifecycle of 5 years. It also supports both speed and torque controlling modes.
The MicroWheel is a “smart” sensor/actuator pair. It includes both a reaction wheel, and an optional solid-state angular rate sensor, both of which are operated locally by an embedded microcontroller. The microcontroller implements local speed and torque control loops for the reaction wheel, allowing the reaction wheel to accurately track speed or torque commands from a satellite’s attitude control computer. Commands as well as telemetry (e.g., measured wheel speed, momentum, acceleration, torque, rate sensor measurements, temperatures, voltages, internal pressure, etc.) are communicated via a serial interface.
Tamagawa seiki’s reaction wheel for microsatellites weighs less than 1.1 kg and features angular momentum storage of 0.3 Nms. The unit is just 65 mm long, 98 mm in diameter, and is designed for use with a range of microsatellites for various applications and orbits. It can deliver a torque of 0.02 Nm on a power input of less than 10 W.
The RW1000 by O.C.E. Technology
The RW1000 is an integrated digital reaction flywheel consisting of a brushless motor, photoelectric encoding sensor, rotating mass, sealed housing, ball bearing unit, and digital electronic control. It can provide a torque of 1 Nm and an angular momentum of 15 Nms simultaneously and is primarily used in zero momentum satellites and light, fast reaction satellites which require rapid attitude manoeuvre in middle or high orbits.
The RW-10NMS reaction wheel is designed for small to medium size LEO satellites which require highly stabilised, 3-axis controlled, agile platforms. It can be used as both a torque actuator and momentum storage device and can be controlled with speed and torque commands.
The compact cyber® reaction wheel 2 is specifically designed for attitude control in 1U – 8U satellites. It has a 20 x 20 x 20 mm^3 cubic shaped with integrated electronic controls, enabling use in several configurations. The reaction wheel is also provided with vacuum-rated bearings and components resisting radiation of up to 200 Gy.
The cyber® reaction wheel 2 provides 2.0 mNms of angular momentum storage, with a maximum rotation rate of 20,000 rpm, and a nominal power consumption rating of 200 mW at 8,000 rpm. It features a high dynamic acceleration and precise sensorless control designed to provide high robustness, low vibration, and fast manoeuvring capability. The cyber® reaction wheel 2 can also be provided with a reaction wheel starter kit, enabling the system to be put into operation rapidly using a PC and the MotionGUI 2 graphical user interface. This also allows diagnostics, optimisation and parameterisation to be carried out.
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