Magnetic sail
A magnetic sail is a proposed method of spacecraft propulsion where an onboard magnetic field source interacts with a plasma wind to form an [|artificial magnetosphere] that acts as a sail, transferring force from the wind to the spacecraft requiring little to no propellant as detailed for each proposed magnetic sail design in this article.
The animation and the following text summarize the magnetic sail physical principles involved. The spacecraft's magnetic field source, represented by the purple dot, generates a magnetic field, shown as expanding black circles. Under conditions summarized in the [|overview] section, this field creates a magnetosphere whose leading edge is a magnetopause and a bow shock composed of charged particles captured from the wind by the magnetic field, as shown in blue, which deflects subsequent charged particles from the plasma wind coming from the left.
Specific attributes of the artificial magnetosphere around the spacecraft for a specific design significantly affect performance as summarized in the overview section. A magnetohydrodynamic model predicts that the interaction of the artificial magnetosphere with the oncoming plasma wind creates an effective sail blocking area that transfers force as shown by a sequence of labeled arrows from the plasma wind, to the spacecraft's magnetic field, to the spacecraft's field source, which accelerates the spacecraft in the same direction as the plasma wind.
These concepts apply to all [|proposed magnetic sail system designs], with the difference how the design generates the magnetic field and how efficiently the field source creates the artificial magnetosphere described above. The [|History of concept] section summarizes key aspects of the proposed designs and relationships between them as background. The cited references are technical with many equations and in order to make the information more accessible, this article first describes in text beginning in the overview section and prior to each design, section or groups of equations and plots intended for the technically oriented reader. The beginning of each proposed design section also contains a summary of the important aspects so that a reader can skip the equations for that design. The differences in the designs determine [|performance measures], such as the mass of the field source and necessary power, which in turn determine force, mass and hence acceleration and velocity that enable a [|performance comparison] between magnetic sail designs at the end of this article. A comparison with other spacecraft propulsion methods includes some magnetic sail designs where the reader can click on the column headers to compare magnetic sail performance with other propulsion methods. The following observations result from this comparison: magnetic sail designs have insufficient thrust to launch from Earth, thrust for deceleration for the magsail in the interstellar medium is relatively large, and both the magsail and [|magnetoplasma sail] have significant thrust for travel away from Earth using the force from the solar wind.
History of concept
An overview of many of the magnetic sail proposed designs with illustrations from the references was published in 2018 by Djojodihardjo. The earliest method proposed by Andrews and Zubrin in 1988, dubbed the magsail, has the significant advantage of requiring no propellant and is thus a form of field propulsion that can operate indefinitely. A drawback of the magsail design was that it required a large superconducting loop carrying large currents with a mass on the order of. The magsail design also described [|modes of operation] for interplanetary transfers, thrusting against a [|planetary ionosphere] or magnetosphere, escape from low Earth orbit as well as deceleration of an interstellar craft over decades after being initially accelerated by other means, for example. a fusion rocket, to a significant fraction of light speed, with a more detailed design published in 2000. In 2015, Freeland validated most of the initial magsail analysis, but determined that thrust predictions were optimistic by a factor of 3.1 due to a numerical integration error.Subsequent designs proposed and analyzed means to significantly reduce mass. These designs require little to modest amounts of exhausted propellant and can thrust for years. All proposed designs describe thrust from solar wind outwards from the Sun. In 2000, Winglee and Slough proposed a Mini-Magnetospheric Plasma Propulsion design that injected low energy plasma into a much smaller coil with much lower mass that required low power. Simulations predicted impressive performance relative to mass and required power; however, a number of critiques raised issues: that the assumed magnetic field falloff rate was optimistic and that thrust was dramatically overestimated.
Starting in 2003, Funaki and others published a series of theoretical, simulation and experimental investigations at JAXA in collaboration with Japanese universities addressing some of the issues from criticisms of M2P2 and named their approach the MagnetoPlasma Sail. In 2011, Funaki and Yamakawa authored a chapter in a book that is a good reference for magnetic sail theory and concepts. MPS research resulted in many published papers that advanced the understanding of [|physical principles for magnetic sails]. Best performance occurred when the injected plasma had a lower density and velocity than considered in M2P2. Thrust gain was computed as compared with performance with a magnetic field only in 2013 and 2014. Investigations and experiments continued reporting increased thrust experimentally and numerically considering use of a Magnetoplasmadynamic thruster in 2015, multiple antenna coils in 2019, and a multi-pole MPD thruster in 2020.
Slough published in 2004 and 2006 a method to generate the static magnetic dipole for a magnetic sail in a design called the Plasma magnet that was described as an AC induction motor turned inside out. A pair of small perpendicularly oriented coils acted as the stator powered by an alternating current to generate a rotating magnetic field that analysis predicted and laboratory experiments demonstrated that a current disc formed as the rotor outside the stator. The current disk formed from electrons captured from the plasma wind, therefore requiring little to no plasma injection. Predictions of substantial improvements in terms of reduced coil size and markedly lower power requirements for significant thrust hypothesized the same optimistic magnetic field falloff rate as assumed for M2P2. In 2022, a spaceflight trial dubbed Jupiter Observing Velocity Experiment proposed using a plasma magnet based sail for a spacecraft named Wind Rider using the solar wind to accelerate away from a point near Earth and decelerate against the magnetosphere of Jupiter.
A 2012, study by Kirtley and Slough investigated using the plasma magnet technology to use plasma in a planetary ionosphere as a braking mechanism and was called the Plasma Magnetoshell. This paper restated the magnetic field falloff rate to the value suggested in the critiques of M2P2 that dramatically reduces analytical predicted performance. Initial missions targeted deceleration in the ionosphere of Mars. Kelly and Little in 2019 published simulation results using a multi-turn coil and not the plasma magnet showed that the magnetoshell was viable for orbital insertion around Mars, Jupiter, Neptune and Uranus and in 2021 showed that it was more efficient than aerocapture for Neptune.
In 2021, Zhenyu Yang and others published an analysis, numerical calculations and experimental verification for a propulsion system that was a combination of the magnetic sail and the electric sail called an electromagnetic sail. A superconducting magsail coil augmented by an electron gun at the coil's center generates an electric field as in an electric sail that deflects positive ions in the plasma wind thereby providing additional thrust, which could reduce overall system mass.
Overview
The [|Modes of operation] section describes the important parameters of [|plasma particle density and wind velocity] in conjunction with a use case for:- Operation in a stellar wind.
- Deceleration in the interstellar medium.
- Operation in a planetary ionosphere or [|planetary magnetosphere].
Charged particles such as electrons, protons and ions travel in straight lines in a vacuum in the absence of a magnetic field. As shown in the illustration in the presence of a magnetic field shown in green, charged particles gyrate in circular arcs with blue indicating positively charged particles and red indicating electrons. The particle's gyroradius is proportional to the ratio of the particle's momentum over the magnetic field. At 1 Astronomical Unit, the distance from the Sun to the Earth, the gyroradius of a proton is ~72 km and since a proton is ~1,836 times the mass of an electron, the gyroradius of an electron is ~40 m with the illustration not drawn to scale. For the magsail deceleration in the interstellar medium mode of operation the velocity is a significant fraction of light speed, for example 5% c, the gyroradius is ~ 500 km for protons and ~280 m for electrons. When the magsail magnetopause radius is much less than the proton gyroradius the [|magsail kinematic model] by Gros in 2017, which considered only protons, predicts a marked reduction in thrust force for initial ship velocity greater than 10% c prior to deceleration.
When the magnetosphere radius is much greater than the spacecraft's magnetic field source radius, all proposed designs, except for the magsail, use a magnetic dipole approximation for an Amperian loop shown in the center of the illustration with the X indicating current flowing into the page and the dot indicating current flowing out of the page. The illustration shows the resulting magnetic field lines and their direction, where the closer spacing of lines indicates a stronger field. Since the magsail uses a large superconducting coil that has a radius on the same order as the magnetosphere the details of that design use the [|magsail MHD model] employing the Biot–Savart law that predicts stronger magnetic fields near and inside the coil than the dipole model. A Lorentz force occurs only for the portion of a charged particle's velocity at a right angle to the magnetic field lines and this constitutes the magnetic force depicted in the summary animation. Electrically neutral particles, such as neutrons, atoms and molecules are unaffected by a magnetic field.
A condition for applicability of magnetohydrodynamic theory, which models charged particles as fluid flows, is that to achieve maximum force the radius of the artificial magnetosphere be on the same order as the ion gyroradius for the plasma environment for a [|particular mode of operation]. Another important condition is how the proposed design affects the magnetic field falloff rate inside the magnetosphere, which impacts the field source mass and power requirements. For a radial distance r from the spacecraft's magnetic field source in a vacuum the magnetic field falls off as, where is the falloff rate. Classic magnetic dipole theory covers the case of =3 as used in the magsail design. When plasma is injected and/or captured near the field source, the magnetic field falls off at a rate of, a topic that has been a subject of much research, criticism and differs between designs and has changed over time for the plasma magnet. The M2P2 and plasma magnet designs initially assumed =1 that as shown in numerical examples summarized at the end of the corresponding design sections predicted a very large performance gain. Several researchers independently created a [|magnetic field model] where and asserted that an =2 falloff rate was the best achievable. In 2011 the plasma magnet author changed the falloff rate from 1 to 2 and that is the value used for the plasma magnet for performance comparison in this article. The magnetoplasma sail design is an evolution of the M2P2 concept that has been extensively documented, numerically analyzed and simulated and reported a falloff rate between 1.5 and 2.
The falloff rate has a significant impact on performance or the mode of operation [|accelerating away from the Sun] where the mass density of ions in the plasma decreases according to an Inverse-square law with distance from the Sun increases. The illustration shows in a semi-log plot the impact of falloff rate on relative force F from Equation versus distance from the Sun ranging from 1 to 20 AU, the approximate distance of Neptune. The distance to Jupiter is approximately 5 AU. Constant force independent of distance from the Sun for =1 is stated in several plasma magnet references, for example Slough and Freeze and results from the effective increase in sail blocking area to exactly offset reduced plasma mass density as a magnetic sail spacecraft accelerates in response to the plasma wind force away from the Sun. As seen from the illustration the impact of falloff rate on force, and therefore acceleration, becomes grerater as distance from the Sun increases.
At scales where the artificial magnetospheric object radius is much less than the ion gyroradius but greater than the electron gyroradius, the realized force is markedly reduced and electrons create force in proportion much greater than their relative mass with respect to ions as detailed in the [|General kinematic model] section where researchers report results from a compute intensive method that simulates individual particle interactions with the magnetic field source.