5 ms·

How can they constantly reorient the spacecraft for each shot without quickly running out of propellant?

• Evidlo 4y ago
  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]
• hughes 4y ago
  It likely has reaction wheels and magnetorquers on board for attitude control. The propellant is only used for orbit raising.
  • dylan604 4y ago
    raising and lowering. sometimes, you have to move your bird out of the way of something else. with enough notice, you speed up or slow down to change altitude cost effectively. direct lifting in altitude is expensive in use of fuel
  • 7952 4y ago
    I think you also use propellent to reset the reaction control wheels when they run too quickly.
    • sgtnoodle 4y ago
      I believe that's the reason for the magnetorquer? There should be very little reason for a satellite to build up angular velocity beyond very small imbalances like from atmospheric drag. The needle of a high quality compass seems to have plenty of torque. It probably used propellant initially, though. Using the propulsion system frivolously could even be detrimental for various reasons. Depending on what it is, it could cause degradation to parts of the satellite like its optics or solar panels. During scheduled thrusts, the satellite could have to orient mechanisms a certain way or close an aperture.