Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Near light speed solar sail?

  1. Oct 11, 2011 #1
    What if you had a laser solar sail and in front
    of the sail was a small thruster (either chemical or itself a
    laser)...thrusting constantly at a right angle to the forward direction of
    the craft so that the sail's trajectory would bring it back towards the
    first (main/stationary) laser that would then provide another push. you
    could keep doing this until the sail's velocity was near c.

    So let me just explain it another way. Laser pushed solar sails are a well
    discussed and thought out idea, the problem is that the further you go away
    from the laser the less push from the laser you will get...so you need a big
    laser and a tough material to withstand the high initial temperature...but
    if you could bring the sail back to the laser by circling it's trajectory
    you could keep pushing the sail faster and faster. Just like you would spin
    a wheel faster and faster by constantly providing pushes.

    If you used a small thruster at the front of the solar sail (really where
    ever you want but to keep it simple put it at the front) and you had it
    pushing at a 90 degree angle to the forward trajectory of the sail you could
    have the craft return obliquely back towards the first most powerful laser
    which could push it again and increase the velocity. You could do this as
    many times as needed or until the small front thruster runs out.

    Now I anticipate three objections. The first is that the front thruster
    would cancel out the velocity provided by the main laser. But this is not so
    because the velocity of the front thruster pushes the craft at 90 degrees
    the actual velocity would be changed and increased (look up a basic 101
    physics book about addition of velocities). The second is that as the craft
    was accelerated faster by the main laser the small front thruster would not
    return it in a 'circle'. The return need not be in a perfect or near circle
    so long as the angle is constantly applied and/or controlled (on/off) at 90
    degrees you can return the sail back towards the location of the laser. In
    other words it is not necessary that the sail return travelling in the same
    direction as it was previously pushed since you could rotate the main laser. The
    last objection could be that the energy needed to rotate the sail would be
    very large, but this is also not so since the energy isn't against the
    forward trajectory. In fact, by my calculations, a craft travelling at .1c
    (3*10^7 m/s) would require 134 days of a constant 10 m/s impulse to return
    back to the main laser. And this is without the addition of the impulse from the front thruster which would shorten the time.

    So...what would be the problem of using a laser push sail and having the craft 'do laps' to be accelerated again by the laser?
  2. jcsd
  3. Oct 11, 2011 #2


    User Avatar
    Gold Member

    Despite your claims, if you want to bring the spaceship back into the high power range of the laser, you will have to expend the energy to do so - cancelling any gain in velocity you made from the laser. There's no way around this.

    Either the craft is closer to the laser (in which case, your thruster moved it there, by whatever circuitous path you want to try), or it is not closer to the laser, in which case you don't get the benefit.

    You can't "sneak up" on getting closer to the laser.
    Last edited: Oct 11, 2011
  4. Oct 11, 2011 #3
    I respectfully disagree. Moving the sail away from the laser isn't like moving it against an energy gradient (gravity or electromagnetic field). In theory you could put the sail on a -very large- wheel and achieve the affect I described. Something you couldn't do if the force was an energy gradient.

    Here is how I approached it. I draw a force with magnitude and direction of a 100 Newtons. Now draw another force with a magnitude of a 100 Newtons and at 90 degrees to the first force. The resultant force will be at 45 degrees and 141 Newtons.

    Now repeat, but this time use 141 Newtons for the force to be applied at 90 degrees. The resultant force is 199 Newtons.

    We are now traveling at 90 degrees from our starting trajectory with a total force of 199 Newtons.

    If you repeat this 6 more times (45 degrees each time) the result is to have the craft with a resulting trajectory of 360 degrees from its initial trajectory- or equal to it's initial trajectory. But now the resultant magnitude force is 796 Newtons or a 1 kg craft that has been accellerated to 796 m/s.

    Or is my logic faulty?
  5. Oct 11, 2011 #4


    User Avatar
    Gold Member

    Yes you could. In fact, I initially posted as a suggestion that you could tie a long string to it to bring it around for another pass. I deleted it before pressing Post because I didn't want to send you on another goose chase.

    However, a long string is not the same as a thruster.

    OK, you've used 200 units of energy, but are now only 141 units of distance form home. 59 units have been split between
    - spinning your trajectory around the solar system
    - moving you faster away from home

    Unfortunately, only one of of those accomplishes anything; the other is completely wasted.

    Same thing. Your lateral thrust wastes energy. Some goes into increasing your speed from home, the rest accomplishes nothing at all.

    Also, don't forget you are now 141 units from home so you laser is at half power.

    With every turn, you put some oblique thrust from your thruster into useful motion away from home, and you waste the rest, uselessly rotating 45 degrees around the solar system.

    At the same time, with every turn, your laser power is dropping in proportion to the square of your distance from home.

    Note that this last part has nothing to do with your technique. Even if going in a straight line, for every sqrt(2) increase in your distance, your last power drops to half. No way around that.

    You would do better to simply point your thruster in line with your motion and stop wasting it.
    Last edited: Oct 11, 2011
  6. Oct 11, 2011 #5


    User Avatar
    Gold Member

    Have a look at this. It's a little oversimplified, in that it is mixing velocity and displacement, but the principle is there.

    No matter how you cut it, two things happen:
    1] the farther you are from the laser, the less power it provides
    2] Any energy from any source that is not devoted to moving directly away from home to our destination is energy wasted. (In this case, it is the oblique angle of the thruster.)

    Attached Files:

    Last edited: Oct 11, 2011
  7. Oct 12, 2011 #6
    DaveC426913 already told you why this wouldn't work. But you could do it without the thruster by using more than one laser.
  8. Oct 12, 2011 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    Perhaps I'm being dense here but what's the need for the thruster? It's my understanding that the sail can just sit in the beam. Is the thruster for in case it drifts?
  9. Oct 12, 2011 #8


    User Avatar
    Gold Member

    He wants to use the thruster to keep the sail closer to the laser, where it has more power. His idea is to give the ship as much speed as possible before it leaves the effective range of the laser.

    He doesn't see that you don't get anything for free.

    I imagine after he examines the diagram I drew, he'll see that his geometry is off. He thought each 45 degree leg of the journey would be identical in length. He did not account for the fact that each leg is not as effective as the previous leg because the ship is getting farther from the laser.
  10. Oct 13, 2011 #9
    Take 4+ Laser arranged in a Square and Shoot the ship in a perfect circle inside the Square.
    You need to adjust the laserpower every round to keep it in line.

    Laser Macroparticle Accelerator :)
  11. Oct 13, 2011 #10


    User Avatar
    Gold Member

    It's an interesting idea. I was thinking something similar. You get maximum power from the lasers and maximum velocity.

    The downside is that you are bleeding off a lot of energy into turning the ship through 90 degrees. That is energy not going into distance-made-good.

    The question to be answered is: for a given total output of power from the lasers, do you get a higher top velocity this way than if you simply used that amount of power in a standard setup?

    I suspect that the standard setup is the most efficient but I don't have the numbers to back it up.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook