Author Topic: Reaction wheel failures  (Read 62967 times)

Offline FOXP2

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Reaction wheel failures
« on: 04/12/2013 04:16 pm »
Dawn, Kepler, Mars Odyssey, etc, they even have backup wheels that fail. So why do they fail so often? This company here claims 20+ year lifespan for their wheels so are they just BSing or are most of the reaction wheels warranted for much shorter lifespans?

Offline JazzFan

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Re: Reaction wheel failures
« Reply #1 on: 04/12/2013 04:25 pm »
Any mechanical device under constant motion is doomed to fail at one point of time.  The manufacture's timelines just seem to be in conflict with what the device is actually designed or capable of doing.

Offline kevin-rf

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Re: Reaction wheel failures
« Reply #2 on: 04/12/2013 04:27 pm »
There have been some threads on it, part of the HST problem that led to a change of manufacturing procedure was free O2 was ending up in the working fluid (oil) resulting in corrosion of wires (or was so believed). They have since switched to an N2 purge for this step.
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Offline LegendCJS

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Re: Reaction wheel failures
« Reply #3 on: 04/12/2013 04:28 pm »
I'm aware of only one long term dependable space moving part technology, and that is flexures, and devices built with them, such as cryo pumps.  Properly constructed, flexures seem to be the only mechanical with the life span of solid state parts.  Unfortunately, flexures seem to be incompatible with continuous rotational motion applications, like reaction wheels.
« Last Edit: 04/12/2013 06:18 pm by LegendCJS »
Remember: if we want this whole space thing to work out we have to optimize for cost!

Offline Robotbeat

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Re: Reaction wheel failures
« Reply #4 on: 04/12/2013 04:31 pm »
Those missions have, I think, already surpassed their primary mission (in some cases, by a very large margin). And better reaction wheels are expensive.

But I agree, it does seem to be a pretty common failure point that's worth addressing more. NASA has sponsored research into longer lifetime reaction wheels. Here's one:
http://sbir.gov/sbirsearch/detail/279945
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Offline muomega0

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Re: Reaction wheel failures
« Reply #5 on: 04/12/2013 04:36 pm »
Dawn, Kepler, Mars Odyssey, etc, they even have backup wheels that fail. So why do they fail so often? This company here claims 20+ year lifespan for their wheels so are they just BSing or are most of the reaction wheels warranted for much shorter lifespans?

The causes are well known.   the solutions are not funded.  Recall that NASA does not like to develop technology to solve problems. 

Simply stated, lubricants do not want to stay on rotating surfaces in a vacuum, but there are other issues too.

Offline Robotbeat

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Re: Reaction wheel failures
« Reply #6 on: 04/12/2013 04:41 pm »
Dawn, Kepler, Mars Odyssey, etc, they even have backup wheels that fail. So why do they fail so often? This company here claims 20+ year lifespan for their wheels so are they just BSing or are most of the reaction wheels warranted for much shorter lifespans?

The causes are well known.   the solutions are not funded.  Recall that NASA does not like to develop technology to solve problems. 
...
NASA does lots of SBIR grants, but the technology often gets stuck in the TRL "valley of death" that's below what a space mission planner is comfortable with putting on his spacecraft (unless no other option) and above what SBIR grants can develop.

NASA used to have more tech dev programs, but much of the funding for that was canceled during run-up to Constellation. Obama's FY11 budget had a lot more resources available to do tech dev to get things out of the TRL valley of death, but Congress didn't like it.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline FOXP2

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Re: Reaction wheel failures
« Reply #7 on: 04/12/2013 06:58 pm »
Questions
1: I'm assuming mechanical bearing need lubricants, why would a magnetic bearing need lubricant?
2: Has a magnetic bearing reaction wheel ever been flow in space yet?

I'm assuming that the bearing is the problem, and that a magnetic bearing would be the solution. Magnetic bearings bring in the problem of control circuits needing to operate for many years without fault or wheel collides with containment vessel, but this problem is replaced with the advantage of no friction, no rubbing parts, and thus hypothetically no wear.

Offline Zed_Noir

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Re: Reaction wheel failures
« Reply #8 on: 04/12/2013 07:19 pm »
I recall someone (maybe Blackstar) mention on another thread that the reaction wheel lifespan difference between a $60k unit and a $600k unit is about a magnitude. :o Guess which unit usually get chosen.  ::)

Offline LegendCJS

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Re: Reaction wheel failures
« Reply #9 on: 04/12/2013 08:48 pm »
I recall someone (maybe Blackstar) mention on another thread that the reaction wheel lifespan difference between a $60k unit and a $600k unit is about a magnitude. :o Guess which unit usually get chosen.  ::)

Especially because budget can only be justified for parts that satisfy the primary mission life span requirement.  Extra years of mission life is a bonus when it happens, but to design for extra years of mission life probably raises cost more than linearly (despite the cited example) and primary mission life span is carefully optimized to have the highest science ROI taking into account opportunity cost for other missions entirely. 

Conclusion: it is really not a problem for the space science community. Nor would (in my opinion) longer lasting reaction wheels open up new science opportunities, which explains the lack of TRL development priority.
Remember: if we want this whole space thing to work out we have to optimize for cost!

Offline FOXP2

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Re: Reaction wheel failures
« Reply #10 on: 04/12/2013 10:06 pm »
So because Keplar had to operate longer then its original primary mission schedule, we should blame the astronomers that made the wrong estimate on star output consistency and not the Kepler spacecraft designers that put in crappy reaction wheels? 

I don't think there needs to be much investment in development magnetic bearing reaction wheels already exist, is it just that they are too expensive? Or untested in space or what?
« Last Edit: 04/12/2013 10:08 pm by FOXP2 »

Offline kevin-rf

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Re: Reaction wheel failures
« Reply #11 on: 04/13/2013 01:42 am »
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Offline Jim

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Re: Reaction wheel failures
« Reply #12 on: 04/13/2013 03:17 am »
So because Keplar had to operate longer then its original primary mission schedule, we should blame the astronomers that made the wrong estimate on star output consistency and not the Kepler spacecraft designers that put in crappy reaction wheels? 

Huh?  It doesn't "have" to operate longer than its primary mission.  It only does it because it can.  Its reaction wheels met the primary mission design life, so how can they be "defined" as crappy?

This is just a nonsensical post and thread.  I don't see where there is an problem.  Reaction wheels are meeting their design lives.


Offline kevin-rf

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Re: Reaction wheel failures
« Reply #13 on: 04/13/2013 04:21 am »
I think the problem is so many people feel that the primary mission is a low bar and exceeding the design life for an extended mission should be designed for.

Just look at the Mars Rovers, Voyager, DSP, GPS, and so many other systems that regularly exceed the design life. There is a benefit, not just in data return, but also in replacement cost with active constellations.

That said, may all your missions exceed the primary mission and may the funding rise up to meet the extended missions!

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Offline JohnFornaro

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Re: Reaction wheel failures
« Reply #14 on: 04/13/2013 01:22 pm »
Well, you all are right about bearings and lubrication being the problem in this area.  Seems like for the longest bearing life, keep the spacecraft on Earth, at least for the duration of the mission.

Excellent paper.  Plus, from the "Learn Something New Every Day" department, we learn about Tribology:

Quote
The word "tribology" was first introduced in the publication named "Department of Education and Science Report" England in 1966, and is defined as the science and technology of interacting surfaces in relative motion and of the practices related thereto (Hamrock, et.al., 1994). In otherwords, it is the study of friction, wear and tear, and lubrication of interacting surfaces.

The article doesn't discuss magnetic bearings though.  It would be nice to read up on them too.
Sometimes I just flat out don't get it.

Offline FOXP2

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Re: Reaction wheel failures
« Reply #15 on: 04/16/2013 07:47 pm »
Again magnetic bearings should get over this problem, they already been developed and are in operations in space:

Rockwell Collins link

Perhaps its a problem of price as Zed was saying.
« Last Edit: 12/14/2016 10:52 pm by Lar »

Offline manboy

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Re: Reaction wheel failures
« Reply #16 on: 04/16/2013 11:27 pm »
Again magnetic bearings should get over this problem, they already been developed and are in operations in space:

Rockwell Collins link

Perhaps its a problem of price as Zed was saying.

"An expected lifetime of more than 20 years"

That's impressive.
« Last Edit: 12/14/2016 10:52 pm by Lar »
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Offline iamlucky13

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Re: Reaction wheel failures
« Reply #17 on: 04/17/2013 12:17 am »
Again magnetic bearings should get over this problem, they already been developed and are in operations in space:

Rockwell Collins link

Perhaps its a problem of price as Zed was saying.


The site says "low vibration" but while that may be fine for a comsat, it doesn't necessarily mean good enough for a telescope.

I don't know much about magnetic bearings, but if the idea behind reducing friction is to magnetically maintain a gap, then that is a potential situation to allow vibration, either due to the inevitable imperfections in balance, or due to perturbations when rotating about a different axis.

I do know that with conventional bearings, if vibration or play is a critical concern, you don't just eliminate the gaps; you actually design with a small negative tolerance (preloaded bearings) so that a very small amount of compression takes place. This does add friction, but it's the only way high end machine tools can achieve the tolerances they do.

Preload is also actually necessary with ball bearings if the load is extremely light to ensure they roll, rather than skid, but that's not relevant to the question.
« Last Edit: 12/14/2016 10:53 pm by Lar »

Offline tnphysics

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Re: Reaction wheel failures
« Reply #18 on: 04/17/2013 06:25 am »
Remember that satellites cannot be serviced over their lifetime. In most machines back on Earth, the bearings are often a wear item that are not expected to last the life of the machine. In space, however, they cannot be replaced.

Magnetic bearings have the advantage of absolute non-contact operation. Therefore, their lifetime should be limited only by mechanical fatigue of the shaft and flywheel - which is avoidable - and by electronics.

Another design that works especially well for control moment gyroscopes (which never stop spinning) is hydrodynamic bearings. Since shaft seals for vacuum applications have high friction and short lifetime, this would require that the entire rotating assembly be enclosed in a sealed, pressurized container filled with lubricant. The lubricant may be a gas in certain cases, such as foil bearings. These bearings have zero wear and very low friction. The trade-off, again, is the need for pressurization.

Offline FOXP2

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Re: Reaction wheel failures
« Reply #19 on: 04/17/2013 10:13 pm »
Again magnetic bearings should get over this problem, they already been developed and are in operations in space:

Rockwell Collins link

Perhaps its a problem of price as Zed was saying.


The site says "low vibration" but while that may be fine for a comsat, it doesn't necessarily mean good enough for a telescope.

I don't know much about magnetic bearings, but if the idea behind reducing friction is to magnetically maintain a gap, then that is a potential situation to allow vibration, either due to the inevitable imperfections in balance, or due to perturbations when rotating about a different axis.

I do know that with conventional bearings, if vibration or play is a critical concern, you don't just eliminate the gaps; you actually design with a small negative tolerance (preloaded bearings) so that a very small amount of compression takes place. This does add friction, but it's the only way high end machine tools can achieve the tolerances they do.

Preload is also actually necessary with ball bearings if the load is extremely light to ensure they roll, rather than skid, but that's not relevant to the question.

Logical, but counter to their claims: google search

"Magnetic bearing reaction wheels provide a number of interesting advantages over ball bearing wheels. They allow high-speed operation with minimum loss torque and generate substantially less micro-vibrations."

"MBRW's show a low mean loss torque and do not exhibit loss
torque fluctuations due to oil distribution changes over time or temperature."

"Even in this non-optimal test environment, the MBRW demonstrated a better micro-vibration performance compared to conventional ball bearing
wheels with similar rotating mass. Much better results are to be expected without the adverse effects of the levitation coil, vacuum pump and vacuum chamber."
« Last Edit: 12/14/2016 10:53 pm by Lar »

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