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Most players of Kerbal Space Program live and die by their reaction wheels that turn electricity into momentum that allows you to point your spacecraft in any which way a lot like RCS that uses monopropellant. The problem with this is that reaction wheels aren't as easy to use in the real world and that you need multiple to point in all the directions instead of the just one you would need. I wanted to know if there was anything that was similar to the KSP Reaction Wheels that didn't consume propellant to move the spacecraft in a similar matter.

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    $\begingroup$ Momentum wheels don't "turn electricity into momentum", they exchange angular momentum with the spacecraft. You still have to unload the reaction wheels later if they get too "full'. $\endgroup$
    – uhoh
    Apr 13, 2021 at 15:21
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    $\begingroup$ Would CMGs (en.wikipedia.org/wiki/Control_moment_gyroscope) qualify or are you excluding anything based on exchanging angular momentum between the craft and spinning wheels? $\endgroup$ Apr 13, 2021 at 16:21
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    $\begingroup$ All attitude actuators are difficult to use in their own regard. Reaction wheels and alike under high spin rates can produce gyroscopic effects. Magnetorquers have controlability limits. Propulsion systems can have sloshing propellants. These are only few issues of those actuator types. For RCS you also need multiple thrusters, more than three. Multiple mangetorquers as well. Proper selection of the actuators needs careful analysis of the situation and goals of the attitude system. If you have more specific information to share, it would be easier to recommend. $\endgroup$
    – Nemanja
    Apr 13, 2021 at 19:00
  • $\begingroup$ @ChristopherJamesHuff How capable are CMG's at moving a spacecraft around? $\endgroup$ Apr 13, 2021 at 20:22
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    $\begingroup$ @JamesErvin Not capable at all to move around, but capable to change the rotation. In general CMGs are able to produce higher torques and have better energy efficiency, at the cost of higher volume, mass and somewhat lesser momentum storage. And unless you have two CMGs per axis, they are quite tricky to operate. On the other hand, double gimballed CMGs provide control of two axes (or even three if some performance hit is OK) at the same time. CMGs and RWs are not always interchangeable options. $\endgroup$
    – Nemanja
    Apr 15, 2021 at 6:41

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Magnetorquers are the most similar thing. They only work if one is around a strong magnetic field, usually low Earth orbit. They work by orienting the spacecraft to the magnetic field lines of the Earth, which will allow them to slowly change their positions with time, similar to a compass.

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  • $\begingroup$ Did the Transit satellites use this method? They have a fairly long boom arm with some mass at the end. I've read that the gravity gradient between that and the main body of the satellite kept it oriented correctly, but I've always been skeptical. $\endgroup$
    – Brad
    Apr 14, 2021 at 5:05
  • $\begingroup$ Gravity gradient is another way that can work reasonably well, similar to a pendulum. The trick with that method is there are usually more than one stable orientation, so one has to make sure the orientation the spacecraft is in stays there, but they can work. $\endgroup$
    – PearsonArtPhoto
    Apr 14, 2021 at 13:09

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