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How Solar Sails Work
Gary writes "You can also learn more about How Solar Sails work.
In mid-April 2001, The Planetary Society, in collaboration with several Russian space organizations, will test launch the deployment mechanism for the first solar-sail mission, Cosmos-1, in a sub-orbital flight.
It is a rounded solar sail that is divided into eight triangular blades with inflatable booms for support.
Maneuvering a solar-sail spacecraft requires balancing two factors: the direction of the solar sail relative to the sun and the orbital speed of the spacecraft."
The solar wind has almost nothing to do with solar sails. They derive power from the impact of photon, not from the impact of the charged particle known as solar wind.
The power from photons is orders of magitude greater. If they relied on the solar wind, they wouldn't go anywhere.
Don Negro
Don Negro
Perl 6 will give you the big knob. -- Larry Wall
They also have a good high-level overview of solar sails, and theirs in particular.
Good to see Carl's people still working to promote his dreams. I got all nostalgic reading the site.
Don Negro
Don Negro
Perl 6 will give you the big knob. -- Larry Wall
You are now leaving the Sol system. Please obey all interstellar traffic laws. Thank you for visiting and come again soon.
Mile Marker 18,000,000,111
One of the interesting things about this particular project is that it's privately funded. (By Cosmos Studios, which is a joint venture of Carl Sagan Productions, Inc. and Intend Change, Inc.) "We" aren't "dumping" any money into it, in this case.
Nod. Using these for interstellar travel would be trickier than just sailing aroung the solar system. Some possible work-arounds have been proposed, though.
2. Travelling towards the sun is difficult and requires some sort of "tacking" process.
Travelling toward the sun is cake. "Tack" to lower any orbital velocity "around" the sun, and the sun's own gravity will do the rest.
3. Travelling in any direction other than away from the sun requires a centerboard and rudder for maintaining a direction.
Oops. No. These are work great in the context of sailing on top of a fluid medium, but there are other ways of maintaining orientation -- gyros, small thrusters, auxilary "guide sails", etc.
4. Sun-spots change the intensity of the solar winds, creating 'gusts'. Gusts will be difficult to control.
"Gusts" of solar wind aren't really closely comparable to gusts of air. With reasonable sensors and feedback systems, a solar sail would manage just fine.
From what I can see, there is only one direction the vessel can travel, and that will be determined by the release time from the earth, and the velocity at releas. After that, the only form of control is to change the "sail area" by either reducing sail size, or by changing orientation. Neither of these processes change direction, only speed.
Changing the orientation of the sail can change the direction and/or speed of the craft, especially when used in combination with gravity fields.
(Thanks, NASA et al, for screwing up the DC-X and generally continuing to smackdown any other chance for cheaper launch systems.)
I think it depends a lot on the mission characteristics. Some propulsions systems are good for some kinds of missions, some for others.
Besides, how are solar sails ever going to become feasible unless someone starts actively working on them?
Um. I think the force imparted by the sunlight is the same. The force imparted just results in a lower acceleration if the mass of the sailcraft is higher.
Real sailboats aren't really "pushed" by the air, really the sail is used as a vertical wing-shape, so that the passage of air over the sail creates "lift" in a forward direction. If sailboats were just pushed by the wind, they wouldn't be able to tack upwind at all.
As far as the solar sail, I wonder if they could alter the direction of the reflection in order to steer. Does the direction in which the light bounces off of the mirror affect the resultant acceleration?
At the very least you could angle the aspect ration of the sail to the sun in order to modify your acceleration; if your sail catches less photons then you won't accelerate as fast.
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Think about it this way. You've got a target free to move in two dimensions sitting on a pool table. Roll pool balls at said target. The direction of travel of the target will vary depending on the angle of the target to the incoming track of the pool ball. The pool ball will rebound off the target (angle of incidence equals angle of reflection) and the target will move away from the impact in the direction of the bisector of the angle of the ball's incoming and outgoing tracks. Cornfused yet? : )
Most sails (unless you're running directly away from the wind with your main on one side and your jib on the other, or using a spinnaker) work by the Bernoulli effect...they're airfoils. Solar sails are pure Newtonian kinematics...f=ma.
Why yes, I AM a rocket scientist!
When you've got a solar sail many kilometers on a side, you're not at all worried about holes from even medium-large (think basketball) sized meteors.
Interacting with something that far away is just not gonna happen. Keep in mind that once the signal comes from the spacecraft and is received by the ground station, the spacecraft is already FAR past the article of interest, and moving at Ludicrous Speed (tm). Turning around for another look is not an option. Imagine trying to make a cannonball go back for another look. : )
Why yes, I AM a rocket scientist!
In case anyone's interested, I worked out some numbers on the force of the light on this thing relative to that of the sun's gravity. Both forces drop as inverse squares of the distance from the sun, so the ratio between them remains constant.
The force due to the light is determined by the area of the sail and the luminosity of the sun (which I looked up), while the force due to gravity is just determined by the mass of the spacecraft and the mass of the sun.
When you work out the numbers using the area and mass quoted in the article, the force of gravity from the sun turns out to be about 200,000 times greater than that of the light.
This means (as the article states) that the best such a spacecraft can do is remain in orbit around the sun, and angle the light in such a way as to gradually spiral out to a higher orbit.
Alas, no interstellar travel with one of these guys, since we can never break out of solar orbit. For a spacecraft with this size solar sail to achieve interstellar travel, it would need to have a mass of less than 0.3 grams! That would require quite an impressive materials breakthrough, I'm afraid.
You can find more information on solar sails here.
Al.--
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It's like this, if i can explain it.
The sail gets its acceleration from the inertia imparted on it by bouncing photons, essentially. So imagine a tennis ball bouncing off a wall. Whatever direction you throw the ball, the net force on the wall is directly perpendicular to it - it changed the ball's velocity only in the perpendicular direction. The ball maintained all of its momentum parallel to the wall, but its momentum perpendicular to it changed. Hence, the wall wants to accelerate perpendicularly to its own face.
The same is the case with the solar sail. It will accelerate perpendicularly to its own sails. The magnitude of the acceleration will be greatest when the sails are perpendicular to the "light beams" or however you want to refer to them - that way, the sail absorbs all of the momentum times two of each photon. when at an angle, the sail will only absorb (total momentum) sin (whatever the angle is) times two.
I don't see how the angle of the sail relative to the sun would have any effect on the direction of the vehicle. It's not a sail boat. The difference between this and a sailboat is that a sail has a rudder which has drag against the water to control direction.
With no atmosphere and no water, a solar sail has nothing to drag against to control direction. It can simply be pushed one way: Away from the sun, and then affected by gravitation of other objects. The angle of the sail to the sun just can't have any effect that I can fathom. Maybe I'm wrong.
On a separate though, I wonder if they could use the negative refractive index composite material in making the solar sails, since they are probably still investigating the material to use for the sail?
From yesterday's linked article:
"``negative refraction'' would make possible the construction of a lens capable of focusing light to limits not currently achievable. "
and a related article on the Economist about how negative refractive index material can possibly help make a "perfect lens"
Perhaps they could use the phenomenon of the negative refractive index to make more efficient or more maneuverable sails?
What Is Interesting is to take look at the series, which was so prescient in so many ways, which also missed the Internet as being a part of world culture. It was really too hot for TV, and took more than a nibble out of the hand that fed it. For those who do not remember it, it was a 1987 TV show based on a world run entirely by the multimedia moguls, and where the Internet did not exist, and TV was mandatory.
Talk about hitting a little close to home.
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but once it got too far from the sun it would not be getting enough photons of light to accelerate any more.
The farther it gets from the sun the less impacted it is from solar gravity, even though the light energy gets weaker, thus acelleration is expected to remain constant.
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Would be really useful if they could find a material that the sails could withstand damage done by getting hit by meteors, maybe people at NASA could actually put them to extreme uses and send them billions of years in space.
It would've also been nice if included in the article, there was a link to show how astronomers at places like NASA control these things when they're so damn far. I know they would pre-program directions, but what if something was spotted close by, how the heck do they manage to change the course of something millions of miles away to investigate it?
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Because you'd need something like 99.9% of the weight of the vehicle to be fuel. That's with chemical rockets. With fusion, you could do considerably better, maybe only 90% fuel. And that fuel should be something like hydrogen or deuterium -- pretty hard to keep it from leaking away in a ten year flight...
It _is_ like blowing into your own sails. But you would get a little thrust by shining the laser backwards. The thrust in newtons is power in watts divided by the speed of light (3E8 m/s), you need a heck of a good source of power, like a big fusion generator or tons of antimatter. And it doesn't have to be visible light -- microwaves, gamma rays, or infrared would do just as well as long as it's directional. Note that all practical energy sources emit more power as waste heat than as usable energy -- so maybe the best light-rocket would simply radiate heat out one end.
However, if we do get fusion powered craft, I expect they would put the usable energy into accelerating the fusion products. You get more thrust from a given power by accelerating helium than by shining a light. For interstellar flight, most of your craft would have to be fusion fuel, so you might as well use the byproducts... Light-propulsion makes sense only as a way to use some power supply you don't have to take along. (Except maybe in a warcraft where the big laser could do double-duty?)
The sails are reflective (mirror-like), so by angling them you can get sideways thrust -- a combination of the incoming momentum of the sunlight and the reaction from the momentum of the reflected light. So angling the sails at 45 degrees gives you thrust at 45 degrees to the sun, but it cuts the thrust by half. (sin(45) squared -- once for the reduction in light intercepted by angling the sail, once for the off-axis thrust.)
You can't tack like a ship tacking into the wind (where the vector sum of forces on the sails, keel, and rudder gives a net force partially into the wind) because you don't have any equivalent to the keel. But you can use an angled sail to reduce the orbital velocity, so the ship drops sunward. And the best way to go outwards is to use sideways thrust to increase the orbital velocity.
The real problem with solar sails is the very tiny force per square meter that is possible. It should be enough for minor orbital adjustments. If you plan way ahead, it might be possible to use light sails to slowly spiral in or out (like weeks or months to the moon, years or decades to Mars). Or you find a way to augment the thrust -- like building a giant battery of lasers on the moon to provide much more intense light. In a couple of Larry Niven stories, they used lasers to launch an interstellar lightsailer, but to brake at the destination, it had to almost dive into the sun...
I can see this as being practical for maybe keeping something at one place in orbit around the sun or for moving very slowly around our solar system, but once it got too far from the sun it would not be getting enough photons of light to accelerate any more. I guess the trick would be to gain enough speed before it gets too far away from the sun. The problem with that would be that, at least it seems logically, you have to increase the size of the sail a large amount to gain even a little acceleration. Oh well, I am sure some viable use will be found for it. Either that or we will dump millions of dollars into it and then decide that it isn't viable.
What good is a used up world, and how could it be worth having? --Sting
As a sailor, I can see some major issues with solar sail travel...
1. The further you are from the sun, the less force is exerted on the sails, until the forces from other stars/suns outweighs the force of our sun. Remembering that our sun is relatively small, this will not necessarily be at the midpoint between solar systems.
2. Travelling towards the sun is difficult and requires some sort of "tacking" process.
3. Travelling in any direction other than away from the sun requires a centerboard and rudder for maintaining a direction. Both these systems rely on resistance. In space there is no resistance. There sill need to be something to exert force away from the sun to keep the correct heading.
4. Sun-spots change the intensity of the solar winds, creating 'gusts'. Gusts will be difficult to control.
My understanding is that solar-wind is best considered as only part of a spacial travel system. Combining solar-winds with other power mecnahinsms. From what I can see, there is only one direction the vessel can travel, and that will be determined by the release time from the earth, and the velocity at releas. After that, the only form of control is to change the "sail area" by either reducing sail size, or by changing orientation. Neither of these processes change direction, only speed.
Thus, for useful travel, an intersection trajectory must be set at the beginning of the flight, and then the speed of the craft must be carefully managed so that the destination point and the craft intersect at the correct time.
.. if only.
Back of the envelope: Let's take a sheet of aluminum foil 1 kilometer square and 5 micrometers thick. The total volume is 5 cubic meters, total mass about 13,500 kg. It receives about 1.35 gigawatts of sunlight at 1 AU. If it has 100% reflectivity, by P=E/c it can develop a best-case thrust of 2.7*10^9/3*10^8 = 9 N (4.5 N at a 45 degree angle, of which 3.2 N would be outward and 3.2 N would be tangential).
4.5 N doesn't sound like much, but it's an acceleration of about 0.33 mm/sec^2. By a = w^2r, the acceleration of Sol's gravity at 1 AU is only about (2*pi/(365.25*86400s))^2*150,000,000,000m = 5.9 mm/s^2. More to the point, Earth's orbital velocity around the Sun is about 30 km/sec. To get to solar escape velocity, you need about 42 km/sec. 12,000m/s/.00033m/s^2 = 36 million seconds = 421 days, ignoring the decrease of sunlight with distance. If you dropped in to the distance of Venus you could cut that in half, if you built your orbit into an ellipse that took you in to about the orbit of Mercury and then boosted like hell on the way out you could shave even more. You wouldn't get to another star in any reasonable amount of time, but you could sent a probe to literally anywhere in the solar system in a few years without any fancy gravity assists (flyby, not rendezvous).
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You can generate thrust in any direction in the half-sphere centered on the Sun-spacecraft line, with the thrust falling off according to cos^2 theta. So long as your trajectory changes can be performed without a thrust vector aiming away from the Sun, you can (theoretically) perform them with a solar sail. In space, gravity (especially from a third body, like Earth) and inertia allow tack-like maneuvers.
Solar sails use the pressure of the photons, not the plasma wind. There may be some small influence from plasma impingement, but it is just that: small. You should look up the "heliogyro" concept for a Comet Halley rendezvous probe (it was never built, of course). The details will show you where your thinking is faulty.--
Give a man a fish and he eats for a day.
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I co-edited the definitive book on the subject, with the listed editors ARTHUR C. CLARKE and DAVID BRIN. The title was "Project Solar Sail", and there are still about 75,000 used copies of the paperback out there in North America... The book, published by the ROC division of New American Library, had a selection of fiction about Solar Sails -- from some famous authors -- plus new nonfiction, illustrations, and short version of a longer poem on the subject which I coauthored with Ray Bradbury. It is listed on Amazon, with rave reviews... The time for Solar Sailing has come... As Clarke says, for most of the long history (including the future) of humanity, "ship" will more often mean vessels in space than vessels in water...