Saving energy is silly; the bottom line is money (was Forklifts tip

Keith Henson schrieb:
> Just ignore the power collecting function. Can you station keep in
> GEO with a solar sail?
>
I am pretty sure that the orbit solar sail specialists could do
something that comes near station keeping through slowing the craft
down and speeding it up once a day, so that the craft is on the same
position relative to Earth after 24 hours.

If you want to verify that, just ask one of them.
Or even better, help to get the funding to prove it in space.
> Normal solar sail use has the light source and the orbital center in
> the same place, the sun. By angling the sail you can get velocity
> change either speeding up or slowing down. If the sail slows down,
> gravity will pull it into a lower orbit.
>

That is correct.
> In GEO the earth is the object being orbited around and the light
> pressure is not radial from the earth.
>
Even a sailboat on the water cannot bank on the wind coming from
the same direction, the sailor has to steer the boat accordingly and the
wind
angle is not radial either.

> Consider a very light sail in GEO. If it is anything but edge on to
> the sun, it would be blown out of GEO and away from the earth like
> dandelion fluff.
>

A solar power satellite is no dandelion fluff, - you are speaking of
several tons
of mass.

Even a solar sail design with our best todays technology (that is mainly
a sail
thickness of about 2 microns) is far away of the
lightness number of one, which is the point were the solar power pressure
and the gravity of Sun would cancel each other out when the sail is
fully facing
Sun's surface, so that the solar sail could hover on a point over the
Sun without
having to orbit it.

To blow the craft outwards, a lightness number under 1 would be necessary.

The point were the solar cell arrays including the holding structure
reach a
lightness number of 1 seems very, very far away for me :-) .

> Heavy objects are accelerated one way and then 6 months later the
> other way.
I think this is because you have hight thrust propulsion and you want
to save fuel in this case.

Solar sailing is low thrust propulsion and changing the orbital speed does
not need fuel at all, no matter how often you do that.
So I would prefer 24 hours as a cycle.
The benefit is that the craft could be above the same place on Earth every
24 hours.

> Light power sats pick up nearly 1000 m/sec velocity over 3
> months compared to orbital velocity of 1500 m/sec.
>

> Perhaps there is a way, but I just don't see how to couple the force
> vectors you can get out of a light sail into keeping a sail in GEO.
>
> If you can, please explain how to do it.
>

I don't see your problem. You already can imagine, that slowing the
craft down or speeding it up by turning the force vector is possible.
The rest does the gravity of Earth.

Best wishes

Frank