Magnetic Sail - Principles of Operation and Design

Principles of Operation and Design

The solar wind is a tenuous stream of plasma that flows outwards from the Sun: near the Earth's orbit, it contains several million protons and electrons per cubic meter and flows at 400 to 600 km/s (250 to 370 mi/s). The magnetic sail introduces a magnetic field into this plasma flow, perpendicular to the motion of the charged particles, which can deflect the particles from their original trajectory: the momentum of the particles is then transferred to the sail, leading to a thrust on the sail. One advantage of magnetic or solar sails over (chemical or ion) reaction thrusters is that no reaction mass is depleted or carried in the craft.

In typical magnetic sail designs, the magnetic field is generated by a loop of superconducting wire. Because loops of current-carrying conductors tend to be forced outwards towards a circular shape by their own magnetic field, the sail could be deployed simply by unspooling the conductor and applying a current through it.

For a sail in the solar wind one AU away from the Sun, the field strength required to resist the dynamic pressure of the solar wind is 50 nT. Zubrin's proposed magnetic sail design would create a bubble of space of 100 km in diameter (62 mi) where solar-wind ions are substantially deflected using a hoop 50 km (31 mi) in radius. The minimum mass of such a coil is constrained by material strength limitations at roughly 40 tonnes (44 tons) and it would generate 70 N (16 lb_f) of thrust, giving a mass/thrust ratio of 600 kg/N. It is not clear how such a coil would be cooled.

The operation of magnetic sails using plasma wind is analogous to the operation of solar sails using the radiation pressure of photons emitted by the Sun. Although solar wind particles have rest mass and photons do not, sunlight has thousands of times more momentum than the solar wind. Therefore, a magnetic sail must deflect a proportionally larger area of the solar wind than a comparable solar sail to generate the same amount of thrust. However, it need not be as massive as a solar sail because the solar wind is deflected by a magnetic field instead of a large physical sail. Conventional materials for solar sails weigh around 7 g/m² (0.0014 lb/sq ft), giving a thrust of 0.01 mPa (1.5×10−9 psi) at 1 AU (150,000,000 km; 93,000,000 mi). This gives a mass/thrust ratio of at least 700 kg/N, similar to a magnetic sail, neglecting other structural components.

The solar and magnetic sails have a thrust that falls off as the square of the distance from the Sun.

When close to a planet with a strong magnetosphere such as Earth or a gas giant, the magnetic sail could generate more thrust by interacting with the magnetosphere instead of the solar wind, and may therefore be more efficient.