NASA - Reaction wheel blues

[Ming]

Damn, they must be version .21! Houston we need a hotfix!

Kepler Mission Manager Update: Recovery Begins ... http://www.nasa.gov/content/kepler-mission-manager-update-recovery-begins/#.Ughm0pzpxhN

And ... http://www.scientificamerican.com/article.cfm?id=the-wheels-come-off-kepler-planet-finding-mission

Edited August 12, 2013 by Ming

[King Arthur]

Given the slew of problems with reaction wheels from that same manufacturer that NASA has experienced or was witness to, as well as having problems with control authority systems failing in general (read: Hubble Space Telescope gyroscopes), one would think NASA could do something right for a change.

Alas, I guess the one good thing to come out of this is that remaining funding meant for Kepler can be released and rerouted to other projects once Kepler is officially killed off. Sad to see it go, but with no way to get astronauts there to service Kepler (and I'm sure Kepler wasn't designed with in-flight servicing in mind anyway), unlike the cases with Hubble, the only remaining choices are to either scrap the mission or try and drive a car that have two broken wheel assemblies.

[lazarus1024]

Kepler is usable, but not for planet detection. It can still operate with the single remaining reaction control wheel, but it isn't stable enough to stare at the same spot in the sky for hours/days at a time. So it is likely Kepler will kept being used, but not for planetary detection.

Also AFAIK, the issue with that manufacturer's control wheels didn't pop up until after Kepler was launched, it appeared in missions that had already gone up with the control wheels from that manufacturer. I think.

[cypher_00]

In a more recent article they say the two bad wheels still are able to spin. They are just worried about the fricton level. Not all is lost if the friction level stays low enough for the craft not to enter safemode.

[Kerbart]

Well in all fairness

Borucki notes that, at the time, potential problems with the reaction wheels seemed less of an issue because the mission was supposed to last for only 3.5 years. That was how long it was expected to take for Kepler to achieve its main goal: surveying a group of about 150,000 Sun-like stars up to 920 parsecs (3,000 light years) away to determine how common Earth-sized planets in Earth-like orbits are in the Galaxy.

[...]

After launch, however, scientists discovered that most stars are more variable than the Sun (see Nature 477, 142–143; 2011). The mission would need more time to distinguish a random fluctuation in a star’s brightness from a dip in starlight due to a passing planet, so last year, NASA extended the mission until 2016. But around the time of that decision, the first reaction wheel went bad.

So as far as I can see, NASA went with a SAS from Jebediah Kerman's Junkyard and Spaceship Parts Co where they should have gotten a more expensive Rockomax model. When looking at launch dates, etc, the failures are 4 years into a mission that was supposed to last only 3½  so they lasted long enough for the initial mission.

Then again, the design specs might have listed "10 years" in a better-safe-than-sorry attitude in the first place. NASA missions tend to run a lot longer than initially planned and I wouldn't be shocked if budgetary constraints have to do with that (a 10 yr mission means budgetting a 10 yr headcount for support & research. A 5 yr mission counts for only 5. "Well it's still working it would be a waste to ditch it now" and then another 5-10 year. I can totally see that happen).

Space is an environment that is incredibly harsh and you can't go out to squirt some oil in a bearing or replace worn parts. I'm always amazed how they manage to keep things working for a so long, in an environment with huge temperature gradients, severe radiation, etc. Maybe NASA should take a more low-budget KSP style approach and accept the occasional failure? I for one would love to see wobbly contraptions with boosters jerry-rigged onto the main rocket with duct tape and tie-wraps. "We don't know if she's gonna make it into orbits but we've learned from our last failure and just doubled on the solid fuel boosters this time"

[Mefi282]

Radial attached Reaction wheels:blush:

[The Jedi Master]

NASA disapproves of the new SAS.

Seriously, if NASA had actual funding, this sort of thing would happen a lot less often. Usually, I'm against spending billions of bucks on things like probes and flag-and-footprints missions (much to the disdain of my fellow forum members ), but come on, give them enough money to MAINTAIN THE REACTION WHEELS!

[cypher_00]

Stupid thing is that they actually knew about the problem before the launch. They did send the 4 units back to the manufacturer Ithaco Space Systems and they did some "repairs" and then they still failed...

Better not to sub contract stuff to that company any longer I think..

[3_bit]

Those long-term mission spacecraft really need some ion thrusters. They'd be perfect for the task of being a temporary attitude control system.

[softweir]

Real-world space/science/technology topics such as this really belong in The Science Labs - so over it goes!

[Nuke]

Those long-term mission spacecraft really need some ion thrusters. They'd be perfect for the task of being a temporary attitude control system.

i think the main reason for using gyros in these cases is for precision tracking of celestial objects. things like hydrazine thrusters are a bit too clunky for that kind of precision. perhaps the possibility of using feep drives, which are very good at precision throttling (throttling probibly isn't the correct term, i think these kinda thrusters use pwm switching to fire off small poofs of thrust really fast).

Edited August 14, 2013 by Nuke

[Fuzzy Dunlop]

i think the main reason for using gyros in these cases is for precision tracking of celestial objects. things like hydrazine thrusters are a bit too clunky for that kind of precision. perhaps the possibility of using feep drives, which are very good at precision throttling (throttling probibly isn't the correct term, i think these kinda thrusters use pwm switching to fire off small poofs of thrust really fast).

I belive the LISA pathfinder spacecraft is planned to use FEEP based attitude control

[Bunsen]

The trouble with using thrusters for fine attitude control is that every little tiny twitch expends propellant. When you're keeping something pointed with a precision that's right down near the attitude sensor noise level, you inevitably do a lot of back-and-forth and no-wait-back-the-other-way actuation, and that bleeds off propellant rather quickly. Reaction wheels and CMGs only burn electricity to do that.

But you always need thrusters (unless you can get away with magnetorquers, but we're talking about beyond-LEO missions here) to handle net torques, and there's a definite advantage to using finely throttleable thrusters so that you can burn propellant as a backup in case the wheels fail. That backup plan won't buy a ton of time, though, because those thrusters or their propellant supply always have a limited lifespan. I'm not sure how much benefit you actually get from having that contingency option, nor how the cost balance plays out between hydrazine and electric thrusters.

As for LISA Pathfinder, that requires fine thruster control because it's compensating for disturbance forces, not just torques. I'm guessing that attitude control imposes a minor increase in the delta-v requirement compared to canceling out radiation pressure, so it was simpler to do attitude control with the thrusters. It's also possible that a wheel-based system would have created too much vibration for that mission.