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Thread: How do solar sails work? Can they go on any vector?

  1. #1
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    How do solar sails work? Can they go on any vector?

    Hi folks, I have a couple of questions re solar sails for any kind soul with both the answers and the time...

    Firstly if you angle the sail does the craft feel a force other than directly radial to the sun? Ie can you adjust the thrust vector?

    Also if the sail is directly facing the sun is it the light pressure or the solar wind that exerts the most force? Does that relationship vary with distance?

    Also how does the light pressure work.... presumably absorbed photons give up their momentum to the craft. (I know photons have no rest mass, but they do have mass when travelling??) Assuming thats true... and it may not be... do the reflected photons push on the sail? Do they give a 'double' push? ie when first they hit, then secondly when they are emitted/reflected... or is that a newtonion thing that doesnt apply in this wierdly wibbly wobbly world of quantums?

    Any light on the subject will push me closer to understanding... (sorry... I cant help making bad jokes...)


  2. #2
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    Quote Originally Posted by WalrusLike View Post
    Firstly if you angle the sail does the craft feel a force other than directly radial to the sun? Ie can you adjust the thrust vector?
    Yes if the sail is reflective.

    But what is fundamentally impossible is exerting a force with any component towards the sun.
    Quote Originally Posted by WalrusLike View Post
    Also how does the light pressure work.... presumably absorbed photons give up their momentum to the craft. (I know photons have no rest mass, but they do have mass when travelling??)
    Yes, they do.
    Quote Originally Posted by WalrusLike View Post
    Assuming thats true... and it may not be... do the reflected photons push on the sail? Do they give a 'double' push? ie when first they hit, then secondly when they are emitted/reflected...
    They do give double push.

  3. #3
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    The reflected photons apply thrust normal to the plane of the sail (because the angle of incidence and the angle of reflection are equal). Absorbed photons (and other particles that strike and stick to the sail) apply thrust in the direction of impact, usually radially away from the sun.
    As a reflective sail gets closer to edge-on to the sun, the thrust vector gets closer to right angles to the radial direction, but its magnitude decreases towards zero.

    Grant Hutchison

  4. #4
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    Quote Originally Posted by chornedsnorkack View Post

    But what is fundamentally impossible is exerting a force with any component towards the sun.
    Fortunately, the force of gravity between the Sun and the sail provides a force which is in EXACTLY this direction.
    By adjusting the angle of the sail to modify the direction of the force due to radiation on the sail,
    the captain of the ship can give the spaceship a net force in just about any direction.

  5. #5
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    Does anyone know about the relative magnitudes of the solar wind versus radiation pressure? Now that I know the thrust can be vectored, I wonder about whether the relative ratios of the non-steerable radial wind force, and the steerable radiation pressure, may become factors. And whether that changes with distance as the wind and light pressures die down. (I suspect not because both would be varying with the cube of distance??)

    I have often wondered about this before but never bothered to find out. Now I am just trying to understand the mechanics of this because the Japenes agency is doing a mission to Venus(?) this way.

  6. #6
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    The pressure from the solar wind is about 1/1300 as much as the pressure from sunlight. For solar sails, the force of the solar wind is practically ignorable.

    Both solar sails (which use sunlight pressure) and magnetic sails (which use the solar wind) are steerable. Solar sails are steerable by adjusting the reflecting angle. Magnetic sails are steerable by adjusting the direction of its magnetic field (this changes the direction that the charged particles are deflected in).

    Electric sails use a different principle, and they are not steerable. They essentially drag directly in the direction of the solar wind.

  7. #7
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    Is anyone up to the task of explaining how light energy is transformed into kinetic energy in quantum mechanics?

    I mean I guess the "reflected" photons are of a lower frequency than the incoming photons? and how does it translate into physical motion in a direction?

  8. #8
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    Thanks you guys... that is most helpfull. This is a wonderful site.

    I wonder how long before we have family solar sailboats. Hundreds of years is my guess unfortunately.

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    Saying just a bit more about a point several have made: For solar sails designed so far, the accelerations they provide are tiny compared to the orbital velocity of the craft (or the planet it launched from), and so the movement of the craft is a slow in, or out, or changing of planes of the orbit. E.g. If the reflection is toward the direction of travel in the orbit the craft will slide to a lower orbit.
    Forming opinions as we speak

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    Nice explanation - thanks. So you basically use your existing orbit and make small adjustments within that framework of motion.

    Oh, nice OP question, by the way. 'cept for the joke - no time for those.

  11. #11
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    Question Solar wind or photon momentum powers solar sail? Crookes gives a clue...

    Quote Originally Posted by WalrusLike View Post
    Does anyone know about the relative magnitudes of the solar wind versus radiation pressure? Now that I know the thrust can be vectored, I wonder about whether the relative ratios of the non-steerable radial wind force, and the steerable radiation pressure, may become factors. And whether that changes with distance as the wind and light pressures die down. (I suspect not because both would be varying with the cube of distance??)

    I have often wondered about this before but never bothered to find out. Now I am just trying to understand the mechanics of this because the Japenes agency is doing a mission to Venus(?) this way.
    The JAXA calculations show that IKAROS acceleration matches the photon pressure, as per this illustration: http://www.jaxa.jp/press/2010/07/20100709_ikaros_e.html

    And Physorg.com article clearly states it is NOT from the solar wind: http://www.physorg.com/news195460006.html

    However, the conundrum is that Crookes (intuitively) expected the spinning blades in Crookes radiometer (Wiki) would push on the white side, though the opposite happened, that light pushed on the black side of the blades; which insinuates that 'particle' pressure in the partial vacuum of radiometer has more acceleration than 'photon momentum' as predicted, though the calculated results favor photon pressure. Scientists are still puzzled over this.

    OTOH, this article calls the solar pressure 'solar wind': http://www.greenmuze.com/climate/tra...pacecraft.html
    Last edited by nutant gene 71; 2010-Jul-29 at 04:51 PM. Reason: added link

  12. #12
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    Quote Originally Posted by IsaacKuo View Post
    The pressure from the solar wind is about 1/1300 as much as the pressure from sunlight. For solar sails, the force of the solar wind is practically ignorable.
    And a good way to remember that is to remember the (coincidental) rule of thumb that the solar wind mass flux is about the same as the solar luminosity in mass units, so dM/dt ~ L/c2. The momentum flux in the wind requires multiplying the mass flux dM/dt by the wind velocity v, so v*dM/dt ~ (L/c)*(v/c). But L/c is the momentum flux in sunlight, so you see that the ratio v/c is just what you need to know. Both v and dM/dt vary depending on where and when you look, but an average value of v is about 1/1000 of the speed of light.

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    Quote Originally Posted by antoniseb View Post
    For solar sails designed so far, the accelerations they provide are tiny compared to the orbital velocity of the craft (or the planet it launched from), and so the movement of the craft is a slow in, or out, or changing of planes of the orbit.
    That is saying that the acceleration from the radiation pressure is much less than the acceleration of gravity. That comparison can be made because both fall off like 1/d2, where d is the distance to the Sun. However, it does depend on the ratio of the surface area to the mass. Assuming objects of fixed density, that means it depends on the ratio of surface area to volume. Thus for objects of a given shape, it depends on 1/L, where L is the size scale of the object. Thus to get radiation pressure to rival gravity for objects of given shape and composition, you need to make L quite small. For spherical dust particles, I believe it comes out about 1 micron-- dust grains smaller than that cannot be bound by the Sun's gravity. This also means that a spherical object being used as a solar sail would have a radiative acceleration of g/r, where r is the ratio of its radius to 1 micron (ETA: and g is the local acceleration of the Sun's gravity). That's going to be pretty small-- but then, a spherical object is not a good way to make a sail anyway, so improvements must focus on getting the largest surface area given the mass.
    Last edited by Ken G; 2010-Jul-29 at 06:29 PM.

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    Actually the joke was pretty good...

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    If we angle the sail to the the photon direction doesn't this produce significant torque to rotate the craft, thus changing the angle, unless counter torque is applied? = we need a keel like a sail boat? Neil

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    Quote Originally Posted by neilzero View Post
    If we angle the sail to the the photon direction doesn't this produce significant torque to rotate the craft, thus changing the angle, unless counter torque is applied? = we need a keel like a sail boat?
    Torque has to do with the location of the center of mass, relative to the center of force. A boat sail can only stick up, not down, but a solar sail is easily symmetrized around the spacecraft, so there shouldn't be a need for a keel.

    It's actually a shame that there can't be a keel, because a keel on a sailboat not only keeps it from tipping over (your torque issue), it also is what allows a boat to sail into the wind. That's because when you position the sail between the wind and the keel directions, the drag from the keel cancels the component of the wind force that would otherwise make it impossible to sail into the wind. But since gravity does pretty well anyway for going against the solar wind, the trick here is sailing with the wind!

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