The reaction wheel (RW) is a type of flywheel used primarily by spaceships for attitude control without the use of fuel for rockets or other reaction devices. They are very useful when the spacecraft must be rotated with a very small amount, like keeping the telescope pointing at the star. They can also reduce the mass fraction required for fuel. This is accomplished by equipping the spacecraft with an electric motor attached to the flywheel which, when its rotational speed changes, causes the spacecraft to start spinning proportionally through the conservation of angular momentum. The reaction wheel can rotate the spacecraft just around its mass center (see torque); they are unable to move the spacecraft from one place to another (see translational power). The reaction wheel works around the nominal zero rotation speed. However, the external torque on the spacecraft may require a gradual buildup of the speed of the reaction wheel spin to keep the spacecraft in a fixed orientation.
A reaction wheel is sometimes operated as (and is called) the momentum wheel , by operating it at constant rotation speed (or near constant), to inspire satellites with a large amount of stored angular momentum. Doing so changes the dynamics of spacecraft rotation so that torque disturbances perpendicular to one satellite axis (the axis parallel to the wheel axis) does not produce directly in the space motion of the spacecraft about the same axis as the distortion torque; instead, they produce a (generally smaller) angular movement (precession) of the spacecraft's axis about the perpendicular axis. This has the effect of tending to stabilize the axis of the spacecraft to point in an almost certain direction, allowing for a less complicated attitude control system. The satellites use this "momentum-bias" stabilization approach including SCISAT-1; by orienting the momentum wheel shaft to parallel the orbital-vector vector, this satellite is in a "momentum pitch bias" configuration.
A current gyroscope of control (CMG) is a type of actuator of related but different attitudes, generally consisting of a momentum wheel mounted in one-axis or two axes. When mounted onto a rigid spacecraft, applying a constant torque to the wheels using one of the dreadlock motors causes the spacecraft to develop a constant angular velocity about the perpendicular axis, thus allowing control of the direction of the spacecraft. CMG is generally capable of producing greater continuous torque than RW with less motor heating, and is preferably used in larger and/or more agile spacecraft, including Skylab, Mir, and the International Space Station.
Video Reaction wheel
Implementation
The reaction wheel is usually implemented as a special electric motor, installed along at least three directions: for example, as long as the x, y and z axes do not provide redundancy; while mounting four along the tetrahedral axis provides redundancy. The change of velocity (in any direction) is electronically controlled by the computer. The strength of the material used in the reaction wheel determines the speed at which the wheels will be released, and therefore how much angular momentum can be stored.
Since the reaction wheel is a fraction of the total mass of the spacecraft, easily controlled, temporary changes in speed result in small angular changes. Therefore the wheel allows a very precise change in the attitude of the spacecraft. For this reason, the reaction wheel is often used to direct spacecraft carrying cameras or telescopes.
Over time, the reaction wheel can build up stored momentum that needs to be canceled. Therefore, the designer equips the reaction wheel system with other attitude control mechanisms. In the presence of a magnetic field (such as in low Earth orbit), the spacecraft can use magnetorquers (more commonly known as torque rods) to transfer angular momentum to Earth through its planet's magnetic field. In the absence of a magnetic field, the most efficient practice is to use high-performance jets such as ion boosters, or small lightweight solar screens placed in locations far from the center of the spacecraft, such as in the solar cell or projection cell. Most spacecraft, however, also need to be quick to point, and can not afford the extra mass of three attitude control systems. Therefore designers typically use conventional monopropellant vernier machines for all of these purposes.
Maps Reaction wheel
Failure and replacement
Failure of one or more reaction wheels may cause the spacecraft to lose its ability to maintain position and thus potentially lead to mission failure.
Two service missions to the Hubble Space Telescope have replaced the reaction wheel. In February 1997, the Second Service Mission (STS-82) replaced one after the 'electrical anomaly', not a mechanical problem. Studies on the mechanisms returned provide a rare opportunity to study equipment that has undergone long-term service (7 years) in space, especially for the vacuum effect on lubricants. It's found in 'excellent condition'. In 2002, the 3B Mission Service (STS-109), astronauts from the Columbia shuttle replaced the other reaction wheels. None of these wheels have failed and Hubble is designed with four redundant wheels, and retains three functional pointing abilities.
In 2004, during the spacecraft mission Hayabusa , the X-axis reaction wheel failed. The Y-axis wheels failed in 2005, causing the aircraft to rely on chemical boosters to maintain control of attitudes.
From July 2012 to May 11, 2013, two of the four reaction wheels in the Kepler Telescope failed. This disadvantage greatly hampers Kepler's ability to maintain a sufficiently precise orientation to continue its original mission. On August 15, 2013, the engineers concluded that the Kepler's reaction wheel was irreversible and planetary search using the transit method (measuring changes in star brightness caused by an orbiting planet) could not be continued. Although the failed reaction wheel is still functioning, they experience friction over an acceptable level, and consequently impedes the ability of the telescope to adjust itself properly. The Kepler telescope was restored to a "resting point", a stable configuration that uses a small amount of fuel to compensate for a failed reaction wheel, while Kepler's team considers the alternative use of Kepler that does not require extreme accuracy in the orientation required by the initial mission. On May 16, 2014, NASA extended Kepler's mission to a new mission named K2 , which uses Kepler differently, but allowed it to continue searching for exoplanets.
See also
- The reaction control system
- Space Propulsion
- ROSAT, satel- lite is lost when the limit in the control envelope is exceeded UTC Aerospace Systems, the owner of Ithaco Space Systems, Inc., who built the fateful reaction wheel for (among others) the Kepler spacecraft, Dawn, and Hayabusa
References
External links
- Sinclair, Doug; Grant, C. Cordell; Zee, Robert E. (2007). "Enabling Reaction Wheel Technology for High Performance Nanosatellite Performance Control" (PDF) .
- "Reaction Wheel at Wolfram Research". June 2008.
- Markley, F. Landis; Reid G. Reynolds; Frank X. Liu; Kenneth L. Lebsock (2009). "Maximum Torque and Momentum Envelope for Array Wheel Reactions" (PDF) .
Source of the article : Wikipedia
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