3 ms·

From the article, this is the second reaction wheel to fail in 8 months. They had redundancy, but I wonder what the cost/benefit would have been to have more ba

From the article, this is the second reaction wheel to fail in 8 months. They had redundancy, but I wonder what the cost/benefit would have been to have more backup wheels. Obviously, I'm an armchair/hindsight 20/20 observer here. I have no clue whether it would have been an been an infeasible engineering feat to have more than 4 wheels. Also, I don't really have a clue what a reaction wheel is. EDIT: Wikipedia to the rescue http://en.wikipedia.org/wiki/Reaction_wheel http://en.wikipedia.org/wiki/Reaction_wheel

mikeash 13y ago
In your hindsight bit, don't forget to look at not just wheels but every other piece of equipment on the spacecraft. Otherwise, you're just playing a game of whack-a-mole. Add more reaction wheels, then the coolant system fails. Add redundancies there, then the computer fails. Sure, you can include twice as much of everything, but then you end up with something that's vastly more expensive.
- Daniel_Newby 13y ago
  Reaction wheel bearings are notorious for failing, having killed numerous science spacecraft. I remain amazed that they have not standardized on magnetic bearings.
  - sbierwagen 13y ago
    Power draw?
  - BlackAura 13y ago
    It's possible that there's some engineering reason why they haven't. From a quick search and skimming a couple of research papers, it looks like magnetic bearings are used on smaller craft, but it sounds like they're still being actively developed for larger craft. I guess the tech might not have been available for use on Kepler when they were designing it (they seem to have started design work in the late '90s sometime), or it had some kind of limitation or drawback that they couldn't accept.
speeder 13y ago
Reaction.Wheels.are vert heavy, adding too much redundancy of then make they all more prone to failure, add to rocket fuel costs, and also increase the needed fuel on the ship thrusters and battery size, thus making everything even more heavy. In short, add more reaction wheels and end with a much more heavy machine, more complex, expensive, and more expensive to launch.
- EvanKelly 13y ago
  I had a hunch that was the case, but this quote from Wikipedia suggests otherwise > Since the reaction wheel is a small fraction of the spacecraft's total mass... The gist from the Wikipedia article suggests momentum wheels (used in Hubble) are very heavy, but reaction wheels, not so much, due to their use for precision movement. Obviously I defer to someone that has more than a cursory Wikipedia articles breadth of knowledge on the subject. Am I misinterpreting Wikipedia, or are the reaction wheels on Keppler more than a "small fraction of the mass"?
  - sbierwagen 13y ago
    Momentum wheels are reaction wheels. I'm not sure why the wikipedia article on them thinks differently-- Hubble wasn't gyrostablized at all, it's a telescope, it can't be allowed to spin. Hubble also masses eleven times that of Kepler. (11,000kg vs 1,000kg) I don't know the actual numbers, but attitude control systems possibly take up a larger percentage of spacecraft mass on Kepler than it does on Hubble.

mikeash 13y ago

In your hindsight bit, don't forget to look at not just wheels but every other piece of equipment on the spacecraft. Otherwise, you're just playing a game of whack-a-mole. Add more reaction wheels, then the coolant system fails. Add redundancies there, then the computer fails. Sure, you can include twice as much of everything, but then you end up with something that's vastly more expensive.

Daniel_Newby 13y ago
Reaction wheel bearings are notorious for failing, having killed numerous science spacecraft. I remain amazed that they have not standardized on magnetic bearings.
- sbierwagen 13y ago
  Power draw?
- BlackAura 13y ago
  It's possible that there's some engineering reason why they haven't. From a quick search and skimming a couple of research papers, it looks like magnetic bearings are used on smaller craft, but it sounds like they're still being actively developed for larger craft. I guess the tech might not have been available for use on Kepler when they were designing it (they seem to have started design work in the late '90s sometime), or it had some kind of limitation or drawback that they couldn't accept.

speeder 13y ago

Reaction.Wheels.are vert heavy, adding too much redundancy of then make they all more prone to failure, add to rocket fuel costs, and also increase the needed fuel on the ship thrusters and battery size, thus making everything even more heavy. In short, add more reaction wheels and end with a much more heavy machine, more complex, expensive, and more expensive to launch.

EvanKelly 13y ago
I had a hunch that was the case, but this quote from Wikipedia suggests otherwise > Since the reaction wheel is a small fraction of the spacecraft's total mass... The gist from the Wikipedia article suggests momentum wheels (used in Hubble) are very heavy, but reaction wheels, not so much, due to their use for precision movement. Obviously I defer to someone that has more than a cursory Wikipedia articles breadth of knowledge on the subject. Am I misinterpreting Wikipedia, or are the reaction wheels on Keppler more than a "small fraction of the mass"?
- sbierwagen 13y ago
  Momentum wheels are reaction wheels. I'm not sure why the wikipedia article on them thinks differently-- Hubble wasn't gyrostablized at all, it's a telescope, it can't be allowed to spin. Hubble also masses eleven times that of Kepler. (11,000kg vs 1,000kg) I don't know the actual numbers, but attitude control systems possibly take up a larger percentage of spacecraft mass on Kepler than it does on Hubble.