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I have never heard about this distinction between "Momentum wheel" and "Reaction wheel" I think these are just two names for the same thing. The only reason to avoid zero (or low) rotation rate is possible lubrication problems. Do you really mean that "Reaction wheel" are those having no lubrication problem at low revs?
- The desaturation of the wheels is a major operational constraint and should be explained somewhere. — Preceding unsigned comment added by 22.214.171.124 (talk) 08:38, 8 September 2011 (UTC)
Why is Momentum Wheel and Reaction Wheel lumped together in the same article?
I used to design both Reaction Wheels and Momentum Wheels for spacecraft. Although they each have similarities between them, they are different animals.
A reaction wheel imparts a instantaneous reaction torque into the spacecraft structure when accelerated / decelerated from 'coasting' RPM. The maximum available torque is typically in the order of 0.1 to 0.5 N/m. The reaction torque is proportional to the rate of change of angular motion in conjunction with the mass of the flywheel. The wheel 'integrates' the torque together with time and this manifests itself as a increasing/decreasing RPM. The RPM speed is monitored on board, and reported as telemetry to the Ground. The maximum RPM is limited for safety locally by the wheel's own hardware. To prevent 'running out of RPM', solar sails would be trimmed, or torque poles or attitude correction thrusters deployed. A wheel will 'spin down' and eventually stop if deactivated, but this may take more than an hour.
A Momentum Wheel spins at a constant high RPM, typically 6000 rpm. The speed is sometimes stabilised to prevent unwanted torque reaction. The internal friction losses are minimised by design. The momentum wheel(s) on a spacecraft is used in conjunction with reaction wheels. A set of momentum wheels 'translates' applied torque into a programmed direction. A momentum wheel can be configured as a CW or CCW unit. Reconfiguration between these modes requires a braking period to reduce the RPM below a safe level before operating in the new direction.
We hear about how these things fail on a regular basis and how they impact the success of the overall missions. Could someone fill in how/why these things fail? Loss of lubricant? Drive electronics? --Hooperbloob (talk) 14:37, 24 July 2012 (UTC)
Cool explanation of Reaction Wheels, BUT WHY DO THEY NEED TO SPdamn capslock! to spin at all? Why not push against the inertial mass of a motionless wheel when you want to rotate the telescope? When you stop pushing, the rotation stops. This eliminates high-speed bearings, and the maintenance of a "base" rpm. This would give you exceedingly fine control of attitude, because a low mass, small wheel (like a penny) can be pushed just 1º. Also, it doesn't have to "store" momentum which then needs to be dumped with a rube-goldberg magnetic brake.
- That would "work" (in principle), but it would only have an infinitesimal range of adjustment. If the controlling mass is going to be smaller that the vehicle, then it has to spin faster. Andy Dingley (talk) 16:04, 16 October 2012 (UTC)
Size of a reaction wheel
To understand this better, I'd like the article to say how big a reaction wheel is. The size of a thimble or the size of a washing machine? If it varies, what is the range and what are the reasons for variation. I'd also like it to be more clear how a reaction wheel differs from a gyroscope in controlling spacecraft attitude. Thank you. SchreiberBike (talk) 20:38, 2 February 2013 (UTC)