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Steve VanDevender wrote:
> L. Parker writes:
>  > I am not going to get into the math, but you CAN tack a solar sail to sail
>  > towards the sun, so what is the problem with using TC? You might have to
>  > have the Pathfinder tack around TC several times in a decreasing spiral, but
>  > it should be possible to decelerate with JUST the energy from TC.
> No, please, get into the math.  The only way I know of to get a solar
> sail to move towards a star is to let it fall in the star's gravity.
> The thrust from a solar sail _always_ has an outwards component, never
> inwards.  You can direct the sideways component to do things like drop
> your orbit to a lower radius, but you can't accelerate towards the star
> any faster than you would accelerate by falling.

This is from 

= It might seem at first that the optimal configuration for a
= solar sail is one in which the light hits the sail at normal
= incidence (perpendicular to the surface).  This doesn't turn
= out to be the case, though.  A sail oriented this way exerts
= all its thrust along the line away from the sun.  Because the
= intensity of the light from the sun falls off as the square
= of the distance, the magnitude of this outward thrust must
= fall off also as the square of the distance.  In this way it
= is exactly like gravity.  In fact, putting the sail at normal
= incidence to the sun has the same effect as would have
= reducing the mass of the sun.  It places the sail into an
= elliptical orbit which moves farther away from the sun for a
= while, but must return to its starting point after one
= complete revolution about the sun.  This is not a
= particularly useful configuration.  The only way to avoid
= this with a sail at normal incidence is for the solar pressure
= to exceed the force of gravity, so that the sail goes into a
= hyperbolic escape from the solar system.  In order to do this,
= for the power output and mass of our sun, the sail would have
= to mass no more than one kilogram for every 600 square metres
= of sail area, including the mass of payload and electronics.


= So, putting the sail at normal incidence to the sun is not the
= best configuration.  It is better to angle the sail in such a
= way as to maximize the component of the thrust which is
= parallel to the direction of travel. This turns out to be when
= the angle between the sun and the perpendicular to the sail is
= about 35.3 degrees.  In this configuration the spacecraft is
= being pushed along the direction of travel, and so it climbs
= the gravity well.  In the counter-intuitive realm of orbital
= mechanics, the spacecraft slows down the whole time it is
= climbing the well. 
= Well, if the only important thing is the component of the
= thrust along the velocity vector, it can clearly be aligned
= the other way to oppose the velocity vector.  This pushes
= against the direction of travel, dropping the sail down the
= gravity well, causing it to speed up the whole time.  A
= solar sail, contrary to popular belief, can travel sunward
= just as easily as it can travel anti-sunward.