Magnetorquers

[Enthalpy]

Hello you all!

Magnetorquers serve to control the orientation of smaller satellites in low-Earth orbit. A set of big air coils, or small coils on soft magnetic rods, create a torque in the geomagnetic field.

I propose instead to use an aggregate of magnetic materials and tilt it versus the geomagnetic field when wanting a torque. One aggegate serves for all directions, gaining mass over fixed rods with varied orientations. Creating a field but stronger than Earth's one suffices to orient the permanent magnet, and this takes far less power than usual magnetorquers.

A big Leo satellite can be 6m*3m*3m and 15t with 56*10³kg*m² inertia. If willing to suppress 1turn/mn=0.1rad/s rotation in one day, it needs mean 65mN*m, I take peak 0.13N*m. 4.5µPa radiation pressure on a single deployed solar array, 3m*15m at 10m, makes 2mN*m only. This torque resembles more the drag on very low orbits, or radiation pressure on a small space station.

The aggregate can comprise a rod of FeCo: Vacoflux 48 offers 2.35T for 8120kg/m3, 6% better than pure Fe Vacofer S1, far better than permanent magnets. 1.7dm³ create the 0.13N*m in 40µT geomagnetic field, the L=0.5m R=0.033m rod weighs 14kg and consumes 0.5kA. The ends spread to R=0.08m for 0.40T, adding some 2.2kg per end. Modelled as hemispherical, the emission of 8.0mWb from R=0.08m takes 12.7kA per end. At 0.4T and 0.88MA/m, 15mm thick Neorec53b NdFeB magnets provide the 13kA and weigh 2.3kg per end. The aggregate weighs 23kg.

The huge magnets are badly dangerous. Spreading the flux more, possibly beyond the magnets, would improve. Many thin parallel FeCo rods would also reduce the leaks.

A three-stage Cardan lets the aggregate rotate freely. Balls in a cage between two spherical shells can be more robust. Or levitate magnetically, using the field spread at the ends.

Air coils that create 0.3mT or 5* the geomagnetic field, orient the aggregate. The depicted spherical coils are more efficient, but Helmholtz coils ease the assembly and with R=0.35m they consume 0.12kA*turn. 2*2kg aluminum windings have 0.18mohm/turn² each and they consume 5W.

Four coil planes would give redundancy, their orientation can be perpendicular to C-H in methane: (+1 +1 +1), (+1 -1 -1), (-1 +1 -1) and (-1 -1 +1). Six planes can be regularly spaced too as (1 1 0) etc.

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Could bare air coils compete? 0.13N*m and 40µT need 3.2kA*m². Coils 3m*3m big take 0.36kA*turn. With 3*17kg aluminum, the resistance is 0.6mohm/turn² and they consume 81W. The advantage of a rotated aggregate improves for stronger torques.

Static rods need more rods hence are heavier.

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A static magnet, without ferromagnetic rod hence heavier than needed, oriented the nacelle of my stratospheric balloon, so more uses exist.

Marc Schaefer, aka Enthalpy