Roleplayer #29, November 1992
Stellar Windjammers
The Magnetic Sail Space Drive
By Sean Barrett
Introduction
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, supercon-ductors 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 super-conductor 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 (amps/m^2) -- Mass in tons per ton of thrust
1 x 10^10 -- 500
3 x 10^10 -- 160
1 x 10^11 -- 50
3 x 10^11 -- 16
1 x 10^12 -- 5
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 x 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
Thrust (tons)..Radius (miles)..Energy to activate (MW-hrs)
1.........................200.....................26
2.........................280.....................73
3.........................350....................134
4.........................400....................210
5.........................450....................290
6.........................490....................389
7.........................530....................472
8.........................570....................580
9.........................600....................690
10.......................630....................810
20.......................890..................2,300
30....................1,100..................4,200
40....................1,300..................6,500
50....................1,400..................9,100
60....................1,500................12,000
70....................1,700................15,000
80....................1,800................18,000
90....................1,900................22,000
Radius Equation
Radius is 200<square root of:> thrust in
tons
Energy Equation
Energy is 25.7<Square root of:> thrust in tons^3
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/d^2
Earth
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
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
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
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
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
Planet Distance (AU) Thrust multiplier Sol's acceleration
(milligees)
Mercury .39 3.5 4.0
Venus .72 1.5 1.2
Earth 1 1 0.6
Mars 1.5 0.57 0.3
Ceres 2.8 0.26 0.08
Jupiter 5.2 0.11 0.02
Saturn 9.5 0.049 0.007
Uranus 19 0.019 0.002
Neptune 30 0.011 0.0007
Pluto 40 0.0074 0.0004
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^(-t/T), 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.
Bibliography
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, "sci.space." 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 <cube root of: (u0) (p^2)(V^4)Rm/I)>(J/Pm)
where
a is acceleration in m/s^2
u0 is 4<pi>10^-7 N/amp^2
V is the speed of the stellar wind (5 x 10^5 m/s for Sol)
p is the density of the stellar wind ((8.35 x 10^-21 kg/m^3
)/Rs^2 for Sol, where Rs is the distance from Sol in AU)
Rm is the radius of the magsail in meters
I is the current in the magsail in kiloamps
J is the current density in amp/m^2
Pm is the density of the magsail's wire in kg/m^3
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.
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