Actually, now that you mention it, I think that it only appeared as The Wind from the Sun, Clarke was going to title it Sunjammer, but someone had a novel coming out with the same title at about the same time and the publisher asked Clarke to change his title in order to avoid confusion.
Erm, I’m pretty confident that’s not right. Were the shape of a wing only designed to redirect air, the plane would sink, because the wing would tend to redirect air up. To meet the specifications required to achieve what you describe, the wing would be more or less flat, set at a 45° angle such that the front end was higher up than the bottom.
The second part doesn’t work either. When a force is applied to an object, the object will move in the direction of the force, or slightly off it with certain other interferences. It is technically impossible to use force to accelerate toward the force (or more than 90° off of the heading of said force) without some other force of resistance (like a fulcrum). There is no such force in space.
iamthewalrus(:3= may be onto something more. However, I would point out that since the boat is heading more into the wind than away from the wind if it’s tacking, it would absolutely most definitely without a doubt go backwards, not forwards.
I probably shouldn’t have gone into this topic. It’s a whole another thread.
I guess I wasn’t clear.
Tilting the sail changes thrust. If the sail is pointed 45 degrees away from the sun, the thrust is 135 degrees away from the sun:
| / SAIL
|/
SUN ---- /
/ \
/ \force
But the most it can tilt is 90 degrees. There is always some component of force away from the sun. However, that doesn’t matter for a spacecraft in orbit around the sun. If the force “backwards” (opposite to orbital motion) is large compared to force away from the sun, the spacecraft will slow down and go into a lower orbit, i.e. closer to the sun.
More generally, tacking (or analogous maneuvers) in boats depend on the fact that the boat is in two different media which are moving relative to each other, the air and the water. A submarine can’t do anything analogous to tacking, and neither can a surface boat if the water current happens to match the wind. A solar sail only has one medium available, so it can’t perform any maneuver which would properly be called tacking. It can, however, perform various maneuvers which would bring it closer to the Sun, and I suspect that if solar sails ever become commonplace, such maneuvers might come to be called tacking.
Tell me more, because this doesn’t make that much sense to me. Were this the case, a ship should not have up its sails while tacking- it does (if it didn’t, it would lose all steering and go in the direction of the current, regardless of its shape).
For a solar sail, there still has to be a force opposite to the sun to change direction back toward the sun. Without an aerofoil effect available (since the spacecraft probably isn’t travelling through a fluid :)) the spacecraft will have problems accelerating against the force.
A very interesting thread link, scr4. Nonetheless, a sail during tacking is essentially an aerofoil. We just can’t decide how an aerofoil works.
What Chronos means (and even as a long time, if somewhat irregular sailor, I had to stop and think about it a for a few seconds) is that you need two differential flows in order to “play them against each other”, so to speak. If you were on a sailboat in a current going 10 knots, with a 10 knot tailwind, you wouldn’t perceive there to be any wind at all, as your vessel would not be moving with respect to the wind. By having two flows moving at seperate velocities you can adjust your point of sail and trim of your sails in order to add or subtract the velocities to get a resultant vector in any direction. But if you are pointed up too high into the wind, you won’t get any “lift” from the sails (analogous with an airplane wing with an insufficient angle of attack) and will be “in irons”, at mercy to the currents and headwind. (For most boats, ~45 degrees is as far up as they can point, but many larger, less nimble boats can’t even make weigh that high.) Tacking (the zig zag motion that gives you at least some headweigh against the wind) is a result of this limitation. (Note that sailors, confusingly, also refer to their point of sail as a “tack”, which like a lot of nautical terminology is confusing as all heck for a landlubber.)
So, what you get from tacking a boat is the resultant force from summing the forces on the sails (from air flowing over them in the upwind point of sail) and the force of water on the hull, rudder, and the keel or centerboard. All of these sum out to a net forward and sideways component, allowing you to go into the wind. (In theory, you could do this in a submarine, too, if you could somehow contrive to have the water flow at different rates and directions across the hull, so it need not necessarily be two different mediums, though you’d have a hard time making this happen in reality.)
A solar sail craft has no relatively stable fluid medium, like water, to brace against. What it does have is the gravitational field (or for cutting edge speculative particle theorists, the Higgs field, if you like), but as it is a pretty weak source of resistance at any reasonable distance from a mass it doesn’t work in the same way the water does on a sailboat; with a boat, the water provides a significant inertia to lateral motion (thanks to the large lateral aspect of the hull, greatly enhanced by the keel); on a sailcraft, however, the only force opposing the impinging sunlight is gravity, generally coming from the same source, thereby providing no difference in orientation, and no significant way to accelerate laterally. (You can orient the mirror or reconfigure it to direct the “exhaust” to the side, but this gives a greatly reduced efficiency and is dynamically unstable, so you have to apply additional rotational thrust to remain in the same orientation.) As has already been noted, the orbital velocity of the craft and its resulting inertia is more effective component of the resultant force, allowing you to spiral inward or outward, albeit in very indirect, swooping paths.
Solar sails are a great, if somewhat convoluted, method of propulsion for payloads that you don’t mind spending a lot of time en route; say, deliverating ore from the asteroids to an Earth-Moon Lagrange point, or sending a scientific probe to another planet on a liesurely schedule. I think after spending a few days in the inky blackness of space on the way to Ganymede with the kids in the aft compartment unrelentingly screeching, “When are we going to be there?”, I’d opt for something a little quicker.
Stranger
What Chronos means (and even as a long time, if somewhat irregular sailor, I had to stop and think about it a for a few seconds) is that you need two differential flows in order to “play them against each other”, so to speak. If you were on a sailboat in a current going 10 knots, with a 10 knot tailwind, you wouldn’t perceive there to be any wind at all, as your vessel would not be moving with respect to the wind. By having two flows moving at seperate velocities you can adjust your point of sail and trim of your sails in order to add or subtract the velocities to get a resultant vector in any direction. But if you are pointed up too high into the wind, you won’t get any “lift” from the sails (analogous with an airplane wing with an insufficient angle of attack) and will be “in irons”, at mercy to the currents and headwind. (For most boats, ~45 degrees is as far up as they can point, but many larger, less nimble boats can’t even make weigh that high.) Tacking (the zig zag motion that gives you at least some headweigh against the wind) is a result of this limitation. (Note that sailors, confusingly, also refer to their point of sail as a “tack”, which like a lot of nautical terminology is confusing as all heck for a landlubber.)
So, what you get from tacking a boat is the resultant force from summing the forces on the sails (from air flowing over them in the upwind point of sail) and the force of water on the hull, rudder, and the keel or centerboard. All of these sum out to a net forward and sideways component, allowing you to go into the wind. (In theory, you could do this in a submarine, too, if you could somehow contrive to have the water flow at different rates and directions across the hull, so it need not necessarily be two different mediums, though you’d have a hard time making this happen in reality.)
A solar sail craft has no relatively stable fluid medium, like water, to brace against. What it does have is the gravitational field (or for cutting edge speculative particle theorists, the Higgs field, if you like), but as it is a pretty weak source of resistance at any reasonable distance from a mass it doesn’t work in the same way the water does on a sailboat; with a boat, the water provides a significant inertia to lateral motion (thanks to the large lateral aspect of the hull, greatly enhanced by the keel); on a sailcraft, however, the only force opposing the impinging sunlight is gravity, generally coming from the same source, thereby providing no difference in orientation, and no significant way to accelerate laterally. (You can orient the mirror or reconfigure it to direct the “exhaust” to the side, but this gives a greatly reduced efficiency and is dynamically unstable, so you have to apply additional rotational thrust to remain in the same orientation.) As has already been noted, the orbital velocity of the craft and its resulting inertia is more effective component of the resultant force, allowing you to spiral inward or outward, albeit in very indirect, swooping paths.
Solar sails are a great, if somewhat convoluted, method of propulsion for payloads that you don’t mind spending a lot of time en route; say, deliverating ore from the asteroids to an Earth-Moon Lagrange point, or sending a scientific probe to another planet on a liesurely schedule. I think after spending a few days in the inky blackness of space on the way to Ganymede with the kids in the aft compartment unrelentingly screeching, “When are we going to be there?”, I’d opt for something a little quicker.
Stranger
I’m not sure what you’re trying to say here - tilting a solar sail gives you plenty of lateral thrust, and no inherent instability. Tilting the sail 45 degrees from normal reduces thrust by 1/sqrt(2), and 1/sqrt(2) of that is the lateral component. So the lateral component is 1/2 that of a sail normal to the sunlight. And for a flat, balanced sail (center of mass at the center of geometric area), there’s no torque on the sail regardless of incident angle.
Solar sails are most useful for sending spacecraft to the inner solar system, where photon pressure is greatest. It would be ideal for sending probes to Mercury or even close to the sun. Unfortunately solar sails are still considered unproven technology, so most probes to the inner solar system will use electric propulsion (ion drive) for now.