When playing KSP if you click a Key (A,S,D,W) then the rocket turns without using fuel. I thought that a rocket needed RCS to turn. Is it even possible to turn a rocket in space without using any fuel?

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    $\begingroup$ Perhaps worth noting that while the principle is real, reaction wheels in KSP are unrealistically powerful by a factor of about 100 - this is done for the sake of the game so you don't have to wait several minutes/hours to reorient your craft. $\endgroup$ Jul 28, 2022 at 20:51
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    $\begingroup$ Have a read through Intuition as to why the orientation (of a 3D object) is not a conserved quantity? $\endgroup$ Jul 29, 2022 at 5:33
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    $\begingroup$ It's worth pointing out KSP parts really do include (unrealistically powerful) reaction wheels - it's not just out-of-universe magic plot torque. I think all the manned capsules include reaction wheels, so you'd be forgiven for thinking all spacecraft have them magically. $\endgroup$
    – user253751
    Jul 29, 2022 at 13:27
  • $\begingroup$ Can you define "without fuel", please? Why is it not obvious that although gizmoes such as gyroscopes need no fuel to wind down, winding them up did consume fuel? $\endgroup$ Jul 30, 2022 at 21:21

1 Answer 1


Is it even possible to turn a rocket in space without using any fuel?

That's the primary reason reaction wheels, control moment gyros, magnetic torquers, etc. (I'm sure I'm missing some) were invented. They don't need propellant. The ones I cited do need electrical power, but not propellant.

Eventually, reaction wheels and control moment gyros do become saturated, in which case propellant is needed to desaturate them.

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    $\begingroup$ @TheRocketfan That's how reaction wheels work. Control moment gyros are a different beast. They spin rapidly. You push on them in a direction orthogonal to the spin axis, and the result is a torque normal to both the spin axis and the push axis. Magnetic torquers (aka magnetorquers) have no moving parts. They use the Earth's magnetic field to help them turn. $\endgroup$ Jul 28, 2022 at 12:58
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    $\begingroup$ @TheRocketfan Pretty much, primarily because of the $1/R^3$ drop off in the strength in the Earth's magnetic field. $\endgroup$ Jul 28, 2022 at 13:06
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    $\begingroup$ @TheRocketfan, note that KSP reaction wheels are massively overpowered compared to real-world reaction wheels. Reaction wheels in the real world tend to produce speeds on the order of rotations per hour, not rotations per second. $\endgroup$
    – Mark
    Jul 29, 2022 at 3:11
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    $\begingroup$ Magnetic torques also work in some Jupiter orbits $\endgroup$ Jul 29, 2022 at 5:09
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    $\begingroup$ @OscarSmith I'd assume they would also work around other gas giants, which also have significant magnetic fields? $\endgroup$ Jul 29, 2022 at 8:30

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