The following is a conversion of the rules from the Magsails article in 
Roleplayer #29, into a form compatable with Vehicles, Second Edition.  Many 
thanks to Sean Barrett for writing the original article and providing the 
initial translation of this drive into game terms.  

Magsails (TL8)
   The magsail, first proposed by D. G. Andrews  and R. M. Zubrin in
1988,  is a superconducting loop of wire,  miles long;  the magnetic
field generated  interacts with the sun's plasma wind,  allowing the
vehicle  to maneuver in the solar system  with no reaction mass  and
very little power consumption.   This lets it perform maneuvers that
would  be impossible for Lightsails,  and unfeasable for traditional
reaction drives.  
   The magnetic field  used by the magsail  provides total radiation
shielding   against  charged-particle  radiation,   but  is  totally
ineffective against x-rays, gamma radiation, and neutrons.  

TL:     Magsails first become available at TL 8.  
Weight: the Magsail's mass  is measured in pounds.  
Radius: the Magsail's  maximum deployable radius  is  0.02 x mass in 
        pounds x (TL - 6), in miles.  
Volume: Volume (when not deployed) is equal to mass/100.  
Cost:   Cost is $2000 per pound of magsail.  
Power:  The Magsail reuires a certain  amount of energy to be dumped
        into it  to expand  to a given radius; this energy is stored
        in  the magsail  until it is retracted,  at which point  the
        energy is recovered for use elsewhere.  The amount of energy
        needed  to expand  the magsail  to  a  given radius  is 11 *
        (target radius)^3 kilowatt-seconds.  
Thrust: At 1 AU, the magsail  can generate 0.05 * (current radius)^2
        lbs thrust  directly away  from the Sun,  and up to  0.015 *
        (current radius)^2 lbs thrust  at  right angles  to the Sun.
        To handle distances  other than 1 AU,  divide the thrust  by
        the cube root  of  (distance in AUs)^4.   Note  that this is
        less of a drop-off than lightsails experience.  
PF:     MagSails  provide a PF of  1.21 million x (current radius)^4
        against charged particle radiation, and a PF of zero against
        neutral-particle  radiation.   Against  mixed-type  hazards, 
        assume a PF of 10.  
PD:     A Magsail  with  a  current radius  of  1  mile  or  greater
        provides PD 6  against charged-particle/anti-particle beams,
        flamers, and fusion beams.  

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AUTHOR'S NOTES:

Weight
The original article suggests that early TL8 should use a mass-to-thrust
ratio of 500, and leaves the placement of the further improvements to the
GM's discretion; the formula I provide here is based on the observation
that a MagSail is in many ways an energy storage device; and GURPS assumes
that such devices gain 50% storage capability for a given mass for every
TL of improvement.  

Volume
I chose mass/100 because, IMHO, the Magsail is very much like a Power
Cell, and Power Cells have a volume of weight/100.  

Price
At Early TL8, a magsail that produces 5 lbs of thrust weighs 2500 lbs and
is 10 miles in diameter.  Even with a price of $1000 per lb, it is
smaller, lighter, and cheaper than a TL8 Lightsail.  In fact, it seems to
be totally superior to the equivelent Lightsail.  So, to keep Lightsails
viable in a game, I kicked the price up, so that Lightsails become the
less expensive option.  

PF
In MA Lloyd's excellent Vehicles Additions notes, he suggests that a
magnetic shield should require 0.01 kWs x sqrt(PF) to run;
reverse-engineering this, we find that the PF provided by a given energy
in Magnetic Shielding would be 10,000 x stored energy^2.  However, the
magnetic field formed by a Magsail is miles away from the ship, and thus
substantially weakened by the range.  Lacking any hard numbers, I've
assumed that the actual PF protecting the ship would vary inversely with
the square of the radius of the Magsail.  From these assumptions, the PF
that I give was derived.