Alternative to Rockets

In article , (Greg) writes:
wrote in message
and using this, the light generated force on the sail is simly

F = K*a/r^2

Note that this has the same form as gravity, only repulsive instead of
attractive, so you can associate with it an 1/r potential energy (with
positive, instead of negative, sign). Thus, startin tracking the sail
from some moment when it has a velocity v and is at distance r from
the sun, you can write for its energy

E = m( v^2/2 - GM/r + K/r*(a/m))

where m is the mass of the sail (and all that's attached to it).

This is not correct because the *direction* of the force is *not* away
from the sun in general. In fact its *not* constant in general.

Indeed, and when it is not exactly away from the Sun, the force
*decreases*.

It's not a conservative "feild",

Again, the "conservative field" part is a charitable assumption. You
can always make things worse. You cannot make them much better.

that is the path the sail takes and its
relative angle history all effect the final velocity.

The only extra gain you can have beyond the numbers I provided is by
getting the sail tilted around the closest approach point, to get a
force component along the trajectory. This only yields an advantage
along a short fraction of the trajectory in the case described. The
overall result doesn't chage appreciably. You're welcome to try and
calculate.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"