A reaction wheel is a type of flywheel used primarily by spacecraft for
attitude control without using fuel for rockets or other reaction devices. They
are particularly useful when the spacecraft must be rotated by very small
amounts, such as keeping a telescope pointed at a star. They may also reduce
the mass fraction needed for fuel. This is accomplished by equipping the
spacecraft with an electric motor attached to a flywheel which, when its
rotation speed is changed, causes the spacecraft to begin to counter-rotate
proportionately through conservation of angular momentum. Reaction wheels can
only rotate a spacecraft around its center of mass; they are not capable of
moving the spacecraft from one place to another. Reaction wheels work around a
nominal zero rotation speed. However, external torques on the spacecraft may
require a gradual buildup of reaction wheel rotation speed to maintain the
spacecraft in a fixed orientation. A reaction wheel is sometimes operated
as a momentum wheel, by operating it at a constant rotation speed, in order to
imbue a satellite with a large amount of stored angular momentum. Doing so alters
the spacecraft's rotational dynamics so that disturbance torques perpendicular
to one axis of the satellite do not result directly in spacecraft angular
motion about the same axis as the disturbance torque; instead, they result
in angular motion of that spacecraft axis about a perpendicular axis. This
has the effect of tending to stabilize that spacecraft axis to point in a
nearly-fixed direction, allowing for a less-complicated attitude control
system. Satellites using this "momentum-bias" stabilization approach
include SCISAT-1; by orienting the momentum wheel's axis to be parallel to
the orbit-normal vector, this satellite is in a "pitch momentum bias"
configuration. A control moment gyroscope is a related
but different type of attitude actuator, generally consisting of a momentum wheel
mounted in a one-axis or two-axis gimbal. When mounted to a rigid
spacecraft, applying a constant torque to the wheel using one of the gimbal
motors causes the spacecraft to develop a constant angular velocity about a
perpendicular axis, thus allowing control of the spacecraft's pointing
direction. CMGs are generally able to produce larger sustained torques than
RWs with less motor heating, and are preferentially used in larger and/or
more-agile spacecraft, including Skylab and the International Space Station.
Implementation Reaction wheels are usually implemented
as special electric motors, mounted along at least three directions: for
example along the x, y and z axes provides no redundancy; while mounting
four along tetrahedral axes provides redundancy. Changes in speed are
controlled electronically by computer. The strength of the materials of a
reaction wheel determine the speed at which the wheel would come apart, and
therefore how much angular momentum it can store.
Since the reaction wheel is a small fraction of the spacecraft's total mass,
easily controlled temporary changes in its speed result in small changes in
angle. The wheels therefore permit very precise changes in a spacecraft's
attitude. For this reason, reaction wheels are often used to aim spacecraft
carrying cameras or telescopes. Over time, reaction wheels may build up
stored momentum that needs to be cancelled. Designers therefore
supplement reaction wheel systems with other attitude control mechanisms. In
the presence of a magnetic field, a spacecraft can employ magnetorquers to
transfer angular momentum to the Earth through its planetary magnetic field. In
the absence of a magnetic field, the most efficient practice is to use either
high-efficiency attitude jets such as ion thrusters, or small, lightweight
solar sails placed in locations away from the spacecraft's center of mass,
such as on solar cell arrays or projecting masts. Most spacecraft,
however, also need fast pointing, and cannot afford the extra mass of three
attitude control systems. Designers therefore usually use conventional
monopropellant vernier engines for all these purposes.
Failures and replacement The failure of one or more reaction
wheels can cause a spacecraft to lose its ability to maintain position and
thus potentially cause a mission failure.
Two servicing missions to the Hubble Space Telescope have replaced a reaction
wheel. In February 1997, the Second Servicing Mission replaced one after
'electrical anomalies', rather than any mechanical problem. Study of the
returned mechanism provided a rare opportunity to study equipment that had
undergone long-term service in space, particularly for the effects of vacuum
on lubricants. This was found to be in 'excellent condition'. In 2002,
Servicing Mission 3B, astronauts from the shuttle Columbia replaced another
reaction wheel. Neither of these wheels had failed and Hubble was designed with
four redundant wheels, and maintained pointing ability so long as three were
functional. In 2004, during the mission of the
Hayabusa spacecraft, an X-axis reaction wheel failed. The Y-axis wheel failed in
2005 causing the craft to rely on chemical thrusters to maintain attitude
control. From July 2012 to May 11, 2013, two out
of four reaction wheels in the Kepler telescope failed. This loss severely
hampered Kepler '​s ability to maintain a precise enough orientation to continue
its original mission. On August 15, 2013, engineers concluded that Kepler's
reaction wheels cannot be recovered and that planet searching using the transit
method could not continue. Although the failed reaction wheels still function,
they are experiencing friction exceeding acceptable levels, and consequently
hindering the ability of the telescope to properly orient itself. The Kepler
telescope was returned to its "point rest state", a stable configuration that
uses small amounts of thruster fuel to compensate for the failed reaction
wheels, while the Kepler team considered alternative uses for Kepler that do not
require the extreme accuracy in its orientation as needed by the original
mission. On May 16, 2014, NASA extended the Kepler mission to a new mission
named K2, which uses Kepler differently, but allows it to continue searching for
exoplanets. See also
Reaction control system Spacecraft propulsion
ROSAT, a satellite lost when limitations in its control envelope were exceeded
UTC Aerospace Systems, the owner of Ithaco Space Systems, Inc., which built
the ill-fated reaction wheels for the Kepler, Dawn and Hayabusa spacecraft
References External links
Sinclair, Doug; Grant, C. Cordell; Zee, Robert E.. "Enabling Reaction Wheel
Technology for High Performance Nanosatellite Attitude Control".
"Reaction Wheel at Wolfram Research". June 2008.
Markley, F. Landis; Reid G. Reynolds; Frank X. Liu; Kenneth L. Lebsock.
"Maximum Torque and Momentum Envelopes for Reaction Wheel Arrays".