Roleplayer #29, November 1992

Stellar Windjammers

The Magnetic Sail Space Drive

by Sean Barrett

[Note: Some formatting in this article has been adjusted from the printed version, to render equations more clearly in this medium. In addition, a scan of the original "We're Professionals" formula and legend has been preserved at the end. –]


In late TL7 (1988) a slower-than-light space drive was designed that could maneuver freely within the Solar system – or any other stellar system – while consuming no reaction mass and requiring very little fuel.

The Andrews-Zubrin magsail uses a superconductor loop, miles across, to create a magnetic field that catches and diverts the stellar wind of plasma, very much like a cloth sail uses an atmospheric wind. Because the magnetic field completely surrounds the ship, any payload is completely protected from all charged-particle radiation – alpha and beta. It is not protected from x-rays or gamma radiation, nor from neutrons.

Magsail Metadesign

Magsails are very simple devices. At TL7, superconductors which could carry the required current density existed, as did superconductors which could operate at high temperatures. Once superconducting cable becomes available at early TL8 that can both carry the current and operate at the required temperatures, magsails can be built.

Like the starship chapter in GURPS Space, this article is a design meta-system. The GM will decide how rapidly superconductor technology will improve after TL8. There is a single optimum design of magsail for a particular current-capacity superconductor. A sufficiently strong magnetic field must be generated to form the requisite magnetospheric boundary, but an unnecessarily strong field is inefficient. Therefore, the only question facing a ship designer is how much he can afford. That and the current capacity of the available superconductor will determine all other parameters of the system.

The equation gives performance as a function of superconductor current capacity. The lowest capacity listed will be achieved very early in TL8. Improvements are at the GM's discretion. Once the GM decides what capacity superconductor is available in his world, that mass-to-thrust ratio is used to design all magsails in that world.

Magsail Mass-to-Thrust Ratio

Current capacity
Mass in tons per
ton of thrust
1 × 10¹º500
3 × 10¹º160
1 × 10¹¹50
3 × 10¹¹16
1 × 10¹²5

Mass-to-Thrust Ratio Equation

Mass-to-thrust ratio is current capacity divided by 20,000,000.

Magsail Design and Construction

To calculate the mass of the magsail system, multiply the mass-to-thrust ratio (chosen by the GM, above) by the desired thrust. The magsail system (the superconducting wire, shrouds and motors) costs $1 million per ton.

A magsail must be charged to operate. Energy requirements are given in the table below. Divide the requirement by the power plant's capacity in megawatts (MW) to determine the number of hours it will take to charge the magsail. Once the superconductor loop is carrying its current, it will operate theoretically forever. In actuality, however, every system has losses. The GM must decide how rapidly mag-sails lose their charge. One percent of the activation energy per hour would be very inefficient. If power is cheap, the GM may simply ignore losses.

Because both the radius and the energy formulae involve a square root, the significant digits in the radius and energy table repeat every two orders of magnitude. For thrusts not listed, multiply (or divide) the thrust by one hundred, then divide (or multiply) the radius by 10 and the energy by 1,000.

For example, a TL8 magsail producing a thrust of 0.01 tons masses 5 tons. The thrust is looked up in the table as 0.01 × 100 = 1. The radius is then 200/10 = 20 miles, and the charging energy is 0.026 MW-hrs = 94 MW-secs. A single standard 1 MW solar panel will do the job in a little over a minute and a half. One percent per hour, 0.26 kW, will maintain the charge.

Magsails are stored on huge reels attached to the outside of the payload's hull. To deploy, the reels are simply freed to turn, and current applied to the wire. The magnetic field created will unfurl it automatically. The time required to deploy the magsail is the time necessary for the power plant to fully charge the wire, or one minute per mile of radius, whichever is greater. To retract a magsail takes one minute per mile of radius. Magsails take no hull volume even when furled, and can enter the atmosphere of a planet that has a usable magnetosphere, at very low speed.

Magsails may not be armored. The payload can be, and that armor will protect the magsail when it is furled.

Magsail Radius and Energy Requirement

Energy to activate

Radius Equation

Radius is 200√thrust in tons

Energy Equation

Energy is 25.7√thrust in tons³

Magsail Operation in a Plasma Wind

Thrust obtained from a plasma wind decreases as the four-thirds power of the distance from the source. The figure obtained above is standardized at 1 AU – Earth's orbit about Sol. Elsewhere, thrust can be calculated or interpolated from the thrust and gravitational acceleration table.

In addition to this outward (radial) acceleration, a magsail can also generate a sideways (tangential) thrust of up to 30% of the value calculated above. By changing the angular momentum around the sun, very sophisticated interplanetary orbits can be used that would be impractically expensive for reaction engines, and utterly impossible for solar sails.

A magsail cannot thrust inward, toward the sun. It can "tack," however, using tangential thrust to lower its orbital speed, and fall sunward. The acceleration of Sol's gravity at various distances is given in the table.

Magsail Operation in a Magnetosphere

Another, equally useful mode of operation involves interaction with a magnetosphere.

From a magnetic point of view, a planet and a magsail can both be thought of as simple bar magnets. The magsail in a polar orbit can be oriented so that it is attracted to the pole it is approaching, thus increasing its orbital velocity. If it then switches off as it passes over the pole (so it won't he slowed by the attraction), it will move to a higher orbit. Alternately, it can orient so as to be repelled by the pole, directly levitating itself upward. This second technique is necessary to ground-launch a magsail, but it is very difficult, because the magsail must be maintained in an unstable orientation to do so. An exact analogy is balancing one bar magnet over another. If their poles are oriented so they attract, they will stay that way, and be pulled together. If the upper one is turned so that they repel, it will feel a lift, but it will also try to flip around to the attractive position. Similarly, a magsail repelled by a planet's magnetic field is extremely unstable, trying to flip over.

Magsailing in the Solar System

The stellar plasma wind is no gentle zephyr. It is a flux of several million protons and electrons per cubic yard traveling at velocities gusting from 250 to 350 miles per second.

Then there are the hazardous environments. Some planets have strong magnetospheres, and some of those planets produce plasma winds of their own. The eddies and currents of these flows can hurl a magsail to tremendous velocities or twist its course sharply. A properly piloted magsail can execute amazing maneuvers while in these volumes. A poorly piloted magsail can be reduced in seconds to a tangle of junk tumbling helplessly out of control. Individual planets vary wildly in their magnetic characteristics, making Area Knowledge (Planet's Magnetosphere) very useful to a magsail ship pilot. Some examples from the Solar system are given in the following sections.

Sol's Acceleration Equation

g = 0.6 ⁄ d²


Earth has a remarkably strong magnetosphere that extends outward about 60,000 miles from Earth's surface (11 planetary radii). It is the only terrestrial world in the Solar system with an appreciable magnetic field. Magsails that can accelerate at greater than 60 milligees in the Solar wind at 1 AU can land on or take off from Earth's magnetic poles. (One is located on Bathhurst Island, Northwest Territories, Canada; the other off the Adèlie Coast of Antarctica.)


Jupiter's magnetosphere is a particularly exciting volume to sail. Unlike Earth's teardrop-shaped magnetosphere, Jupiter's is much flatter, more like a flounder. It extends about 50 planetary radii in the plane of Jupiter's orbit, but only about half that in the vertical direction. The magnetotail (the portion of the magnetic field drawn out by the Solar wind) periodically engulfs Saturn, a third of a billion miles away. Jupiter and its moons also produce a tremendous plasma wind. The four Galilean satellites together produce so much plasma that it escapes both on the sunward side of the magnetosphere and down the magnetotail as a very fast and very hot "planetary wind," substantially faster than the Solar wind.

Io in particular is very active geologically, spewing out an enormous disk of gas radiating outward from its orbit. The bulging shape of Jupiter's magnetosphere is caused by a billion-amp electric current in that sheet, which also causes the cloud to incandesce so energetically that the glow of its inner 20 million miles is visible from Earth. It is the largest permanently visible feature of the Solar system.

Magsails that can accelerate at greater than 5 milligees at 1 AU can maneuver as close to Jupiter's magnetic poles as desired, even entering the atmosphere – if the pilot's skill and nerves are up to the challenge.


Saturn's magnetosphere is considerably smaller and less dynamic than Jupiter's. As noted above, Saturn and its magnetic shroud are periodically engulfed by Jupiter's magnetotail. The rings have major effects on the planet's magnetosphere, limiting the inward extent of the plasma surrounding the planet by absorbing any charged particles that reach them. Titan contributes strongly to the magnetospheric plasma. Magsails that can accelerate at greater than 50 milligees at 1 AU can maneuver as close to Saturn's magnetic poles as desired.


Uranus' plasma density is not very high, but its magnetic axis is offset by a third of Uranus' radius and tilted approximately 59° to the spin axis, which in turn is tipped at 98° to its orbital plane. Thus, the magnetic field is nearly perpendicular to the Solar wind at all times, though the whole configuration rotates daily about the planet-sun line. This fairly smooth helical twisting of the magnetotail is unique in the Solar system. Magsails that can accelerate at greater than 80 milligees at 1 AU can maneuver as close to Uranus' magnetic poles as desired.


Neptune has strong similarities to Uranus, magnetically. The magnetic axis is tilted 47° from the rotational axis and is offset by over half of Neptune's radius. Because Neptune's rotational axis is only tipped at 23° to its orbit, the magnetosphere does not smoothly twist as does Uranus'; it flops dramatically as the planet rotates, alternating daily between being nearly pole-on to the Solar wind, and nearly upright like all other planets. Magsails that can accelerate at greater than 16 milligees at 1 AU can maneuver as close to Neptune's southern magnetic pole (rotationally speaking; a compass would point to it as north) as desired. An acceleration of 310 milligees at 1 AU is required to maneuver near the other pole. The difference is caused by the offset of the magnetic axis.

Thrust and Gravitational Acceleration at Various Distances from Sol

Thrust multiplierSol's acceleration

High Accelerations

Sophisticated magsails can reach velocities that are an appreciable fraction of the speed of the stellar wind. No sail can travel downwind faster than the wind that is blowing it, and a sail's acceleration falls off as its velocity increases relative to wind speed. The ship's velocity could be determined by:

V = W - (W - V0) e-tτ

except that the wind density is also decreasing as the magsail moves away from the source. The combined effect is difficult to describe both accurately and simply.

Compound Magsails

At higher tech levels, magsails consisting of two or more loops connected by a spar along their axes are introduced. Bi- and trisails produce more desirable magnetospheric boundary shapes, yielding much higher tangential thrusts. The analysis of such compound magsails could be the subject of a future article.

Piloting (Magsail)

The skill specialization Piloting (Magsail) is necessary to safely operate a magsail ship. This is a computer-aided skill, so the proper programs will give significant bonuses. Events requiring skill rolls at various penalties include charging a magsail, taking off and landing, crossing the "bow shock" where the stellar wind meets a planet's magnetosphere, dealing with stellar flares, passing through the magnetic turbulence caused by a satellite or ring and executing any maneuver that requires turning the sail.


GMs interested in the details of magsail performance are strongly urged to consult the source literature.

D.G. Andrews and R.M. Zubrin, "Magnetic Sails and Interstellar Travel." Journal of the British Interplanetary Society, 1990. The first paper published, concerned primarily with the cost savings to other propulsion systems from the use of the magsail as an interstellar brake. Poorly edited
(many typos) and of use primarily to GMs running an interstellar campaign without FTL travel.

R.M. Zubrin and D.G. Andrews, "Magnetic Sails and Interplanetary Travel." Journal of Spacecraft and Rockets, April 1991. The technical description and very thorough analysis of the magsail for interplanetary travel. Excellent.

R.M. Zubrin, "The Magnetic Sail." Analog Science Fiction & Fact, May 1992. A version of the above paper edited for a non-technical audience. Useful for general concepts, inadequate for a full understanding.

Various, "" Internet newsgroup. Absolutely invaluable for detailed, precise information on a huge variety of subjects – such as data on planetary magnetospheres.

We're Professionals (Don't Try This at Home)

GMs who like to fiddle with the numbers may use the full formula for a magsail's acceleration:

a = 0.59 0ρ² V4 Rm) ⁄ I   (J ⁄ ρm)


Note that many of these variables are interrelated. An increase in current also raises the current density, unless the wire is made thicker, which increases the total mass of the magsail.

(Back to Roleplayer #29 Table of Contents)

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