# Could Magnetorquers be used on the ISS?

The way that the ISS manages it's attitude now is to use a set of reaction wheels for primary control, and occasionally firing small thrusters to allow the wheels to despin themselves. This isn't always a practical response, as it uses fuel continually.

The way that many satellites manage to do this is with Magnetorquers, which align themselves with the Earths Magnetic Field, albeit in an active way.

What I want to know is,

• Under what conditions would the management of the Space Station be too difficult using only Magnetorquers?
• Could only Magnetorquers be used to control it, and if only some of the time, when would the most difficult times to use them arise?
• Do you have any numbers on how much torque they can output? – Undo Sep 19 '13 at 22:06
• ISS uses CMGs, not RWs. The Zero-Propellant Maneuver (ZPM) can be used to save thrusters propellant. I believe thrusters are still needed to absorb vehicle docking momentum, for debris avoidance which may be too quick events for magnetorquers, and for CMGs desaturation. – mins Jan 12 '15 at 7:01
• A study performed for the use of magnetorquers on CFFL (small space station model), showed that they could be useful to reset CMG momentum. – mins Jan 12 '15 at 7:35
• Could be an interesting answer... – PearsonArtPhoto Jan 12 '15 at 12:47

In addition to Undo's answer, I'll try to give a "zeroth order", back of the envelope analysis for this:

First, from my answer here, we have $$T=DB=2DM/R^3$$ where $T$ is the torque applied by the torquer, $M$ is the magnetic moment of the Earth (about $7.96\times 10^{15}~\text{tesla}\cdot \text{m}^3$), $D$ is the dipole induced by your torquer, and $R$ is the distance from the center of Earth's dipole.

Now, mechanics tells us that $$T=I\alpha$$ where $I$ is the moment of inertia and $\alpha$ is angular acceleration. We'll take $I_{xx}=127908568~\text{kg}\cdot\text{m}^2$ from here for our purposes, and let's say you want to impart a modest angular rate of $0.05~^{\circ}/\text{sec}^2\approx 0.0009~\text{radians}/\text{sec}^2$.

So, the magnetic dipole required in this case would be $$D=\frac{I\alpha R^3}{2M}=\frac{127908568\cdot 0.0009\cdot (6478*1000)^3}{2\cdot 7.96\times 10^{15}}\approx 1.966\times 10^9~\text{A}\cdot \text{m}^2$$

This is a massive dipole. If we calculate the dipole moment of a wrapped coil with $N$ turns as $A\times I\times N$, and even assume a fairly massive 1 meter cross sectional area, and a generous 10 Amps of current, a torquer using 1 mil copper wire would be 1 meter long and have a layer of copper 13 cm thick. This would weigh about 1200 kg, which is pretty damn heavy. Consider this with Undo's more qualitative points, and it's starting to look like a bad idea, even if it's not physically impossible.

I can't give a definitive answer until I have numbers on how much torque that the devices can output, which I wasn't able to find on the Interwebs.

However, Wikipedia paints a grim picture for the possiblity of their use on larger sattelites: