Lunar Sample Return via Tether

Henry Spencer wrote:

Assuming you can make a solar-sail-based system work at all. Don't
forget that solar sails impose some troublesome constraints, like the
need for quite large surface areas and some sharp limitations on what
direction you can thrust in.

On the other hand, they don't need any fuel at all. And I'm not
sure the thrust direction problems cannot be handled - you "just"
need more complex sail configurations.

--
Sander

+++ Out of cheese error +++

In article ,
Sander Vesik wrote:
Assuming you can make a solar-sail-based system work at all. Don't
forget that solar sails impose some troublesome constraints, like the
need for quite large surface areas and some sharp limitations on what
direction you can thrust in.

On the other hand, they don't need any fuel at all. And I'm not
sure the thrust direction problems cannot be handled - you "just"
need more complex sail configurations.

There are concepts like Forward's "solar photon thruster" which are
essentially solar sails that can thrust in any direction. However, you
pay for that flexibility with much more demanding requirements: you still
need huge surface areas, but now their shape has to be controlled quite
precisely.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

In article ,
Sander Vesik wrote:
Assuming you can make a solar-sail-based system work at all. Don't
forget that solar sails impose some troublesome constraints, like the
need for quite large surface areas and some sharp limitations on what
direction you can thrust in.

On the other hand, they don't need any fuel at all. And I'm not
sure the thrust direction problems cannot be handled - you "just"
need more complex sail configurations.

There are concepts like Forward's "solar photon thruster" which are
essentially solar sails that can thrust in any direction. However, you
pay for that flexibility with much more demanding requirements: you still
need huge surface areas, but now their shape has to be controlled quite
precisely.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Sander Vesik wrote:
Henry Spencer wrote:

Assuming you can make a solar-sail-based system work at all. Don't
forget that solar sails impose some troublesome constraints, like the
need for quite large surface areas and some sharp limitations on what
direction you can thrust in.

On the other hand, they don't need any fuel at all. And I'm not
sure the thrust direction problems cannot be handled - you "just"
need more complex sail configurations.

Nope.
Solar sails can be adequately modeled by a fixed paddle reflecting a stream
of balls.
You can make the balls bounce in any direction, but the resultant thrust
can never have any component in the direction of the source.
You are strictly limited to thrusting to a direction at least 90 degrees
away from the light source.
And near 90 degrees, the thrust is vanishingly small.

To get thrust towards the sun, you need either a reflector outside
your position (impractical due to fundamental optics for more
than a hundred times the width of the reflector) or a solar
powered laser of some sort pushing on the sail.

Sander Vesik wrote:
Henry Spencer wrote:

Assuming you can make a solar-sail-based system work at all. Don't
forget that solar sails impose some troublesome constraints, like the
need for quite large surface areas and some sharp limitations on what
direction you can thrust in.

On the other hand, they don't need any fuel at all. And I'm not
sure the thrust direction problems cannot be handled - you "just"
need more complex sail configurations.

Nope.
Solar sails can be adequately modeled by a fixed paddle reflecting a stream
of balls.
You can make the balls bounce in any direction, but the resultant thrust
can never have any component in the direction of the source.
You are strictly limited to thrusting to a direction at least 90 degrees
away from the light source.
And near 90 degrees, the thrust is vanishingly small.

To get thrust towards the sun, you need either a reflector outside
your position (impractical due to fundamental optics for more
than a hundred times the width of the reflector) or a solar
powered laser of some sort pushing on the sail.

Henry Spencer wrote:
In article ,
Sander Vesik wrote:
Assuming you can make a solar-sail-based system work at all. Don't
forget that solar sails impose some troublesome constraints, like the
need for quite large surface areas and some sharp limitations on what
direction you can thrust in.

On the other hand, they don't need any fuel at all. And I'm not
sure the thrust direction problems cannot be handled - you "just"
need more complex sail configurations.

There are concepts like Forward's "solar photon thruster" which are
essentially solar sails that can thrust in any direction. However, you
pay for that flexibility with much more demanding requirements: you still
need huge surface areas, but now their shape has to be controlled quite
precisely.

Yes. Thats more or less what I mean by "complex sail configurations" - sorry,
i'm horribly bad at remembering names and attributions, so it usualy easier
to omit them.

Ultimately, I think solar sail based systems and very fuel efficent hybrids
(that is less than 1/2000 of fuel/payload) are workable.
--
Sander

+++ Out of cheese error +++

Ian Stirling wrote:
Sander Vesik wrote:
Henry Spencer wrote:

Assuming you can make a solar-sail-based system work at all. Don't
forget that solar sails impose some troublesome constraints, like the
need for quite large surface areas and some sharp limitations on what
direction you can thrust in.

On the other hand, they don't need any fuel at all. And I'm not
sure the thrust direction problems cannot be handled - you "just"
need more complex sail configurations.

Nope.

Only for some values of nope.

Solar sails can be adequately modeled by a fixed paddle reflecting a stream
of balls.

But only of you have a single, fixed geometry sail.

You can make the balls bounce in any direction, but the resultant thrust
can never have any component in the direction of the source.
You are strictly limited to thrusting to a direction at least 90 degrees
away from the light source.
And near 90 degrees, the thrust is vanishingly small.

To get thrust towards the sun, you need either a reflector outside
your position (impractical due to fundamental optics for more
than a hundred times the width of the reflector) or a solar
powered laser of some sort pushing on the sail.


Not at all. If you make the sail be slightly bowl shaped instead of flat,
then you should be able to make both "newtonian" and "cassegrain" sails
which will allow you to both thrust normaly, at odd angles to the sail
and even directly towards sun by moving of and changsing the angle of the
"secondary sail".

Sure, its not free.

--
Sander

+++ Out of cheese error +++

Sander Vesik wrote:
Ian Stirling wrote:
Sander Vesik wrote:
Henry Spencer wrote:

Assuming you can make a solar-sail-based system work at all. Don't
forget that solar sails impose some troublesome constraints, like the
need for quite large surface areas and some sharp limitations on what
direction you can thrust in.
snip
Nope.

Only for some values of nope.

Solar sails can be adequately modeled by a fixed paddle reflecting a stream
of balls.

But only of you have a single, fixed geometry sail.

You can make the balls bounce in any direction, but the resultant thrust
can never have any component in the direction of the source.
You are strictly limited to thrusting to a direction at least 90 degrees
away from the light source.
And near 90 degrees, the thrust is vanishingly small.

To get thrust towards the sun, you need either a reflector outside
your position (impractical due to fundamental optics for more
than a hundred times the width of the reflector) or a solar
powered laser of some sort pushing on the sail.

Not at all. If you make the sail be slightly bowl shaped instead of flat,
then you should be able to make both "newtonian" and "cassegrain" sails
which will allow you to both thrust normaly, at odd angles to the sail
and even directly towards sun by moving of and changsing the angle of the
"secondary sail".

Sure, its not free.

But the overall thrust on the sail system can never be towards the
sun.
And the seperation between them can never be larger than some hundred times
the diameter of the primary sail, sharply limiting the use.

(Henry Spencer) wrote in message ...

There are concepts like Forward's "solar photon thruster" which are
essentially solar sails that can thrust in any direction. However, you
pay for that flexibility with much more demanding requirements: you still
need huge surface areas, but now their shape has to be controlled quite
precisely.

I have had a google for the solar photon thruster and have come up
blank. Also i can't find any refs in the solar sail litrature. I can't
see how it can thrust in the direction of the light (ie toward the
sun). You wouldn't have some refs handy would you?

Thanks

Greg

(Greg) wrote:
I have had a google for the solar photon thruster and have come up
blank. Also i can't find any refs in the solar sail litrature. I can't
see how it can thrust in the direction of the light (ie toward the
sun). You wouldn't have some refs handy would you?

Pages 91 to 95 of "Solar Sailing - Technology, Dynamics
and Mission Applications" by Colin R. McInnes.

It is not that it can thrust toward the Sun. It is just that
"Since the main collector always faces the Sun, the solar
photon thruster is not subjected to the reduction in projected
area that a flat solar sail suffers when directing the solar radiation
force away from the Sun-line." (page 92,93)

-- Vince