3 ms·

Nevermind, I guess they only need to use the gyros for changing attitude.

• mlyle 4y ago
  Just a nitpick: they are reaction wheels (which work on the basis of conservation of angular momentum), not gyros (which work on the effect of gyroscopic precession).
  • _a_a_a_ 4y ago
    Could you explain further please.
    • mlyle 4y ago
      Angular momentum is conserved. If you're in space, and subject to no outside forces, and start off stationary... while you spin a wheel one way, you move in the other way (an equal and opposite reaction). When you stop the wheel, you stop. These are called reaction wheels. (There's also reaction spheres, etc.). A control moment gyro works on a different principle. They're big (generally only used on space stations) but more versatile and power efficient. They are a wheel spinning quickly. A motor "tilts" the wheel and the resultant gyroscopic force imparts a torque to the station. https://en.wikipedia.org/wiki/Control_moment_gyroscope https://en.wikipedia.org/wiki/Control_moment_gyroscope https://en.wikipedia.org/wiki/Reaction_wheel https://en.wikipedia.org/wiki/Reaction_wheel
      • detrites 4y ago
        Does this mean you'd always need three reaction wheels (or possibly just two?) to be able to orient in any direction, or is a single wheel itself able to orient to face different directions before activation, to achieve the same thing?
        • mlyle 4y ago
          Two wheels, or a movable wheel, or a "ball" that you can actuate in two directions do it. Of course, you probably also have thrusters (to "spin down the wheels" when they accumulate speed from outside forces), magnetorquers, passive stabilization mechanisms, etc.
          • detrites 4y ago
            Magnetorquers, very interesting - electromagnets that couple with the earth's magnetic field to assist with attitude/stabilisation: https://en.m.wikipedia.org/wiki/Magnetorquer https://en.m.wikipedia.org/wiki/Magnetorquer
            • mlyle 4y ago
              Yup. There have even been satellites that have used only passive stabilization--- a permanent magnet + dark/light surfaces that spin it up from photon pressure.
            • addaon 4y ago
              And even more entertaining, these are off-the-shelf parts at this point. https://www.cubesatshop.com/product/isis-magnetorquer-board/ https://www.cubesatshop.com/product/isis-magnetorquer-board/
      • _a_a_a_ 4y ago
        Very interesting, the reaction wheel is obvious but to the gyro is not. I can't see any efficiency problem though, electrical spin-up and spin-down with regenerative braking should reclaim the energy. I'll read the wiki article. Thanks.
        • mlyle 4y ago
          > I can't see any efficiency problem though, electrical spin-up and spin-down with regenerative braking should reclaim the energy. A lot of uses of reaction wheels requires them to constantly spin because there's been an outside force applied. The wheel mass also tends to be low because it's divided between multiple axes and so speeds of wheels need to be pretty high.
        • outworlder 4y ago
          > with regenerative braking should reclaim the energy. Nah. You start braking, you start spinning again. You have to combine that with thrusters, which means you are probably braking pretty quickly.
          • mlyle 4y ago
            If the wheels have not accumulated steady rates from outside forces, you spin a wheel and move while it's spinning. Then you stop the wheel and stop. That was the (trivial) case I'd described that the person you replied to is responding to.
      • deleted 4y ago
        [deleted]