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In order to justify my Idea, I will first give some explanation of how solar sail craft navigate. Solar sail craft that want to travel to the outer solar system will not use their sails to reflect the light directly back at the sun. If a sailcaft did this, it would do the equivalent of reducing the sun's gravity slightly. Unless the sail is able to completely counteract the Sun's gravity (and this seems to be beyond the technology of the near future) the sailcraft will travel in an elliptical orbit that would bring it right back where it started. In order to travel outward, the sailcraft must increase its rotational momentum.
All sailcraft designs that I have seen would increase rotational momentum by angling the sail so that reflected light has momentum in the rotational direction. The amount of momentum that a photon imparts to the sail in the rotational direction is proportional to the sin(2x) where x is the angle between the photon's path and the normal to the sail. However, as the sail is angled, it also looses angular size as seen from the sun. The angular size of the sail is proportional to cos(x). The maximum value for sin(2x)*cos(x) is approximately .77 and occurs at about x=36.26 degrease. Therefor the maximum thrust of a traditional sail in the rotational direction is .77*sail area*pressure of light.
This thrust can also be used to decrease the orbital momentum. Decreasing the orbital momentum decreases the centrifugal affects of the rotation and allows gravity to pull the ship closer to the sun. This affect has been called tacking, though the concept is completely different from the way a sailboat tacks.
My Idea for a solar sail design would be to have the primary solar sail stay perpendicular to the path and reflect the light onto a smaller secondary sail that is between the ship and the sun. The secondary sail could then either reflect the light in the orbital direction, or (probably easier) reflect the light back at the ship where tertiary sails or mirrors would direct the light in the proper direction.
I realize that the secondary sail would make the sailcraft more complicated, and that it might be difficult to keep the light focused correctly, but those seem to be simple engineering problems that can probably be overcome. If this idea works correctly, the craft would generate 30 percent more useful thrust than it could with the primary sail alone. The increased power output seems to me to be enough that the idea should at least be investigated.
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Can you draw us a picture? And post it somewhere and give us a link? Sounds like an interesting idea.
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