Two of four reaction wheels seized up, critical for precise photometry
Excerpt from the Kepler Mission Manager Update. (h/t to Dr. Leif Svalgaard)
At our semi-weekly contact on Tuesday, May 14, 2013, we found the Kepler spacecraft once again in safe mode. As was the case earlier this month, this was a Thruster-Controlled Safe Mode. The root cause is not yet known, however the proximate cause appears to be an attitude error….
…We attempted to return to reaction wheel control as the spacecraft rotated into communication…but reaction wheel 4 remained at full torque while the spin rate dropped to zero. This is a clear indication that there has been an internal failure within the reaction wheel, likely a structural failure of the wheel bearing. The spacecraft was then transitioned back to Thruster-Controlled Safe Mode.
An Anomaly Review Board concurred that the data appear to unambiguously indicate a wheel 4 failure, and that the team’s priority is to complete preparations to enter Point Rest State. Point Rest State is a loosely-pointed, thruster-controlled state that minimizes fuels usage while providing a continuous X-band communication downlink. The software to execute that state was loaded to the spacecraft last week, and last night the team completed the upload of the parameters the software will use.
The spacecraft is stable and safe, if still burning fuel…In its current mode, our fuel will last for several months. Point Rest State would extend that period to years.
…We will take the next several days and weeks to assess our options and develop new command products. These options are likely to include steps to attempt to recover wheel functionality and to investigate the utility of a hybrid mode, using both wheels and thrusters.
With the failure of a second reaction wheel, it’s unlikely that the spacecraft will be able to return to the high pointing accuracy that enables its high-precision photometry. However, no decision has been made to end data collection.
Kepler had successfully completed its primary three-and-a-half year mission and entered an extended mission phase in November 2012.
Even if data collection were to end, the mission has substantial quantities of data on the ground yet to be fully analyzed, and the string of scientific discoveries is expected to continue for years to come….
Read the full Mission Manager Update.
In a New York Times article, astronaut John Grunsfeld, now in charge of NASA Science Mission Directorate under which Kepler operates, is quoted: “For Mr. Grunsfeld, who played mechanic to the Hubble telescope during several lengthy spacewalks, the Kepler malfunction looked particularly frustrating. ‘Unfortunately, it’s not in a place where I can go and fix it,’ he said.”
What does reaction wheel 4 look like?
(see photo at right)
Where are the reaction wheels on the spacecraft?
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Great article. And that answers a previous question in the thread. This gyro is lubricated, not magnetically suspended.
TomB says:
Magnetic suspension on reaction wheels? Sounds sketchy. The point of reaction wheels is attitude control. If you magnetically suspend them you’re degrading the performance of your control because the physical coupling is now imprecise. This sounds like something I would avoid like the plague on a telescope satellite, but RCS’ are something I’ve never looked at for a failure analysis.
> “This is a clear indication that there has been an internal failure within the reaction wheel, likely a structural failure of the wheel bearing.”
These reactions-wheels are clearly not using magnetic-hubs … likely due to the fact that their purpose is not *just* to turn something but to impart steering-forces upon the entire space-craft
IMHO, these things are THE single-point-of-failure most likely to render a spacecraft useless and I wish they were more redundant than the have-4-need-3 paradigm … they are not a vast expense nor a large weight, and their function is mission-critical … it would be far better for cost-effective-long-term-survival to go with have 9-need-3 etc
It is really disappointing how often reaction wheels fail – in this case two out of four within 4 years. After 40-50 years in development you would think we could do better. Hubble had trouble with them too, as did Hayabusa and mars global surveyor amongst others. Given ongoing failures like this, there will probably be a bit more effort put into their development – they should be a simple, cheap, off the shelf, and multiply redundant item for space astronomical applications.
Magnetic bearings are not a particularly great solution – they require active electromagnetic control systems to maintain stable operation if operating through zero speed (checkout Earnshaw’s theorum for explanation), or superconducting elements that are just not feasible on a spacecraft. They also need mechanical bearing backups for when/if power or control fails.
I’m shocked that this telescope has only one spare reaction wheel. A mission critical moving part.
Not quite fair to bitch about it failing after it had already exceeded its design life. That’s like moping about a car going “just” 300k miles instead of 200k. The unexpected durability of the first Mars rovers has probably spoiled us.
Would it be possible to repair it with a remote controlled robot, a set of versatile arms and hands some maneuvering boosters and a tool belt. It may be a good Idea to design future satellites with repair droids in mind, HA! r2d2.
I speak heretically, I’m ready for a reaction, but what I ask is “who gives a shit?”
Uh pat, that’s not really his account even * … you’re good at research, I need not add more …
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(OFA property)
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Kaboom says:
May 17, 2013 at 9:33 am
Not quite fair to bitch about it failing after it had already exceeded its design life. That’s like moping about a car going “just” 300k miles instead of 200k. The unexpected durability of the first Mars rovers has probably spoiled us.
Kaboom,
Actually the most common reason why the primary service expectation low in satellites is because of the attitude control… gyroscope and such. So, the general lament, ala
Rob L says:
May 17, 2013 at 8:44 am
is quite valid.
Still wondering why we cannot have lot more redundancy in the pieces that are most prone to break… eg. 9 or 12 reaction wheels, where just 3 will do???
@Mark Wood says:
Control wheels are indeed very heavy. The point of an RCS is rotational inertia that is capable of being pushed off against to change the attitude of the craft. This eliminates the need for traditional reaction control thrusters which use up fuel. In theory, a reaction control wheel if it lasts, can change the attitude of the craft so long as there is electrical power. Unfortunately, since the craft we put into space need to be small and light, those wheels need to spin at fairly high RPMs, this imparts significant wear on the bearings, even in the weightlessness of space. It virtually guarantees they won’t last forever. If you’ve seen the reaction control wheels on the ISS, you’ve seen that they’re HUGE, 2 of them are the size of one medium-sized satellite, and I think the ISS has 4 or more of them. They’ve also had to replace some of them.
There is a solution… Ion propulsion reaction control thrusters. They still use fuel, but at a VERY low rate. The problem with ion propulsion is it’s expensive, and the RF power you generate will generally cause significant communication problems with your satellite.
Hi, Wamron (“I’m ready for a reaction”), here’s one.
On the one hand, I see your point. A wonderful machine that has accomplished much of great value, long past the end of its projected useful life, is dying.
On the other hand, there is something in a lot of us humans (not all, not all, I read you, W.), that deeply mourns the loss of our faithful, hardworking, creations (even if we didn’t build them, as the Kepler engineers did). I remember the day we drove “Brownie,” an old, ugly, Chevy Apache (1961, I think), to the junk yard a few years ago and left him there. We kept his turn indicator light, “Brownie’s soul,” we called it. We still have it. I wept as we drove away. Why was that? Because we loved that old truck. Irrational, yes, but, true.
Thus, I care about the Kepler. I care because there are some fine engineers (and many other professionals) who will mourn the loss of their amazing machine. I care about them.
David L. masterfully handled Dudley Horscroft’s attempted affront. Even the heights of scientific snobbery are no match for true humility. Well done David.
Mmmmm, I’m not so sure. It’s been 25 years or so since I was in the chemical process industry, but even then we were using magnetically coupled process pumps to move fluids you just didn’t want leaking out. There was a limit to the amount of torque you could transfer, but that was before the widespread availability of rare earth magnets. I might be more concerned about the magnetic fields you would be creating, but I’m sure there are engineering fixes or SOP’s to deal with that.
@Janice Moore….well that was a vastly more thoughtful and engaging reaction than I expected. Totally blind-sided me. I was expecting things like “oh you are an ignorant pillock Wamron” like the amateur astronomer who slated me as some kind of wife-beating savage. But no, you caught me out with the love of machines. Touche.
You are welcome to view my home: you would find I have a much greater variety of, mostly old, some ancient, photographic appliances on display in glass cases than you’ll see in most camera shops. I have cameras well over a century old and also some of the most recent. The entire range of formats and sizes up to half-plate (bigger than that are too large for me). Whats more Ive used both pre-WW2 and post-war era cameras and have superb images out of them.
So, yes, I do love machines. But I prefer ones I can handle, or even viewi n a museum (which my home has been compared to) not hundreds of thousands of miles away. I never considered it that way.
OK so if you love Kepler you must utterly worship the Voyager spacecraft!
@Feller with a big name:
“Still wondering why we cannot have lot more redundancy in the pieces that are most prone to break… eg. 9 or 12 reaction wheels, where just 3 will do???”
PAYLOAD MASS!
Dear Wamron,
What a pleasure to make a connection with you across cyber space. I’m so glad that you understood what I was trying to say. I would love to see your home. Maybe someday I will (I and someone else, if you are a man! — to avoid even the appearance of impropriety). It sounds like a warmly peaceful place of fascinating “decor” and happy nostalgia. Your photographic devices collection sounds impressive to say the least. Do you have one where you light off that powder (I can’t remember what it was – phosphorous?) to make a flash? Can you make a Daguerretype (sp)? I, too, am an amateur photographer (very basic). I still haven’t found a digital camera (at least one that I could afford!) that can perform as well as my old 35mm SLR. Except for trivial use, I can barely stand to use a digital camera — the lighting and, even more, the action-stopping (aaaarrrrrgh!) are, for me, very frustrating. I don’t WANT the camera to do it for me!!
On the other hand, it sure is wonderful to play around with all the digital processing on the computer… . Would be nice to have both worlds in one camera. I need to start reading up (I’m about 10 years out of date on camera technology!).
Well, must cut this short, since it is totally off topic. I, too, much prefer purely mechanical machines (esp. motor vehicles) with only basic electrical support systems. I don’t know diddly about how to repair them, but my brothers do. Yeah, I know quite a few “gear heads” and “motor heads.” That is why I have such deep sympathy for those Kepler engineers. Engineers are born fixers. Sitting here on earth watching a problem they know how to fix but can’t must be excruciating.
Take care,
Janice
Janice, sadly I have long been forced into relying upon digital. The cost of using 120 roll film on the Mamiya 645 and Press 23 that I used to love is too vast at the rate I took to shooting (several hundred exposures per session of a couple of hours). Plus Ive never had adequate scans from transparencies. But if you look at newer cameras, particularly SLRs, you’ll find that annoying shutter lag of ten years ago has effectively much vanished.You can use allyour oldest lenses on most systems too! Now Im being pushed increasingly from still to video anyway. Thats how they make us buy gear!
I guess you could shoot Dageurrotypes on one or two of mine with plates (its the process I believe, not the camera) but I dont have a tray for flash powder.I do however have some very kranky early flash bulb gear.
When I was young we would snigger at old electronic gear that took time to “warm up” whereas the new kit started immediately. I find it hilarious and also significant that todays digital gear takes ages to “boot up”. Is it progress? The ergonomics of modern cameras is superb, but I find most other digital appliances are appalling in practice with an interface dependent upon having dainty fingers and perfect, youthful dexterity.
Thanks for all that great info., Wamron. Re: digital appliance interfaces generally — they STINK. Obviously there were no first-class engineers allowed in on the camera, phone, and other products’ designs!
Two steps forward, 1 back…
Galane, that would work except it’s a hardware error… maybe they could pull the RAM, blow on the contacts, and reseat it 😉
Janice, if you’re looking for a decent digital camera, try a Sony NEX5 (or 6 or 7). I bought one second hand for $250 and had no idea what it was, the original owner had his camera bag stolen and got the whole kit replaced on insurance so he sold me the camera and lens. I ordered new batteries and charger from China on ebay and had to wait weeks to see how good it was.
I was blown away. It’s a pro quality camera but it’s SMALL – since it doesn’t have a flip up mirror and prism like a traditional SLR (totally not needed for digital work, but essential for film) it uses a screen on the back for framing, like a cheapo.
At 14 megapixels the images are large, the quality of the images is amazing, with a perfect balance of color and contrast right off the card. I had been lamenting the loss of my beloved 35mm rig and didn’t want to spend thousands on something new, this was a perfect solution. When it breaks or is lost or whatever I’ll buy another like it. You can throw it into manual mode and adjust everything, or you can slap it into full auto and get good shots.
No learning curve, and I was able to use my Nikon 35mm lenses and accessories with a simple $12 adapter ring (also China, ebay). It’s ready to shoot immediately, saves images as quick as you can take them, and has none of the other issues I had with digital cameras. It even shoots HD video with better than “acceptable” quality.
(Re: the vehicle… I kept my last car for 23 years… it was my daily driver for 22. That last year I just left it in the driveway because I couldn’t bear to part with it… so many memories wrapped up in one piece of hardware)
Code Tech…..everyone swears by the camera they like.BUT..the Sony NEX series HAS NO FECKING VIEWFINDER!!!!!!!!!!!!!!
No, a viewfinder is essential for good camera stance. Sorry, you need an SLR or if you want no mirror, an SLT.
Code Tech…re your car, Im the same with an old pair of shios. Where theyve been on my feet.heck I cannot just bin them! Maybe I will need to bury them with a ceremony.
Hi, Codetech — thanks for the tip on the Sony digital camera. That’s good to know.
Yeah, Wamron, I really do like a viewfinder. But, I’ll have to check out the latest in that back window display screen (I always feel like I’m looking in a mirror, it feels like left and right are reversed as I move it or something, the “something” is me, no doubt, heh, heh) feature.
It sure is fun to frame things on the computer — so many artistically pleasing possibilities.
Thanks, Codetech, for the empathy about loving a motor vehicle. Some of them sure are extra special. It’s nice to know others understand the phenomenon. I even weep whenever I read (AndyG, it has been years! #[:)]) about Mike Mulligan and his steam shovel, Maryanne, who tries SO HARD to keep up with that big new diesel excavator. Sure, the artist put a face on Maryanne, but, I can see those faces on machines like her without any aid.
Wamron, just find a quiet corner and KEEP THE OLD SHOES. I doubt, at your useage rate, you will end up with an unmanageably large hoard of them. Now, if you start fondly treasuring all your milk containers …. and empty paint cans…. and… . Might want to ask a friend for help. [;)]
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BACK ON TOPIC!
@Mr.. Horsecroft, what a fine response by you. I hope that those other fellows will acknowledge your thoughtful post just above. “IF THEY CAN WADE THROUGH ALL YOUR OFF TOPIC STUFF,” you probably would like to yell at me. {:[)
Janice, I recommend this site:
http://www.imaging-resource.com/
Under “cameras” at top left you will find everything digital made now or in this century described in comprehensive detail.