While I realise that "frequently" is relative, why exactly do reaction wheels fail so frequently? Is it simply wear and tear? What happens to their mechanisms?
In the case of the recent Kepler failure, how were NASA going about trying to fix the wheel remotely?
If it's possible to include redundancy by adding a fourth wheel in tetrahedral formation, can't more points be added for additional redundancy?
[Background: I'm writing this as a developer whose firmware's in flight on several substantial satellite missions. I've developed attitude control systems, working directly with RW hardware engineers.]
As Hash says, there is a lubrication distribution issue with conventional mechanical-bearing reaction wheels. The result is increased wear, leading to wear particulates that become embedded into the lubricant, further accelerating the wear process.
There is a solution to this: magnetic-bearing RWs. These have no physical contact between the rotor and the static assembly, so have a mechanically indefinite lifetime. They also have the benefit of greater torque, and less micro-vibration. Unfortunately, these are both much more expensive, and can be rather heavier, than mechanical designs.
If I may expand the scope of the question slightly (but remain on topic): the situation is compounded by the finance model for space missions. In order to be able to declare a mission success (and hence funding for future projects), mission designers set a very low bar for success. Many, perhaps most, missions are designed with an unspoken expectation that they will exceed the success threshold by a very substantial margin. The problem is that the mission manifest will be based upon the bill of materials to achieve that low success threshold, something that will be under heavy scrutiny from budget-conscious managers and/or customers.
Thus, reaction wheels will be chosen that satisfy the base mission requirements, but are not necessarily fit for purpose for a substantially extended mission. Unsurprisingly, we then see early failures during the extended missions.
Reaction wheels consist of an electric motor attached to a flywheel. There are two causes of failure:
mechanical
electrical
MECHANICAL FAILURES
Both the flywheels and the motor can be damaged by the G-forces and the vibration caused during launch. Once in space, lubricating the bearing is almost impossible which leads to increase in friction and eventually to failure.
ELECTRONIC FAILURE:
Once in orbit or in space radiation possesses a greater threat to the electronic circuits.
Example:
In the case of Kepler, one of the four reaction wheels was lost because of erratic friction over several months.
In an exercise of caution, mission managers switched off Kepler's reaction wheels for 10 days in January, hoping the break would redistribute lubricant inside the wheel assemblies, reducing friction and allowing the units to cool down.
That did not happen: it actually showed increased friction after the shutdown.
But hopefully the two remaining reaction wheels are working properly.
A bit more detail on the causes of mechanical failures: Bearing cage instability (scroll to the sections on Momentum and reaction wheels) is a common phenomenon. It states:
Bearing lubricant depletion between the ball race retainer causes cage instability and subsequent pointing errors, increased bearing torque, and wheel vibration.
If this isn't immediately obvious,try this as a primer. The "ball race" is the circular track of a ball bearing that contains the balls. The retainer or cage is a non-metallic, usually phenolic, that sits in the ball race and keeps the balls spaced apart. The balls therefore bear against both the race, which takes the main load of the mechanism, and the cage, which takes no load. Bearing cage instability refers to unintended vibrational modes of the cage caused by a lack of lubricant.
This has affected at least the Cassini, also here, and XMM-Newton missions.
Typical solutions mentioned in the latter papers, learned on a trial and error basis, include stopping the wheel for a while to allow the lubricant to redistribute, and also avoiding certain operational speeds, either very high or near zero.
This short answer is meant to highlight the information in its more thorough form within this answer.
If you find this interesting, read the full answer there, and if you are so inclined to up vote, consider doing it there rather than here.
Since this and the other question are somewhat different, and each has good and non-identical answers, I don't see a way to cast either question as a duplicate of the other.
