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Mythbusters Death Ray

  1. May 22, 2005 #1

    DaveC426913

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    Just saw the Mythbuster's ep where they build a fresnel type reflector that focusses the sun's rays. There's a guy online who built a small version of it: SolarDeathRay

    Now, there's two things weird about both the construction mechanisms here.

    1] In both cases, they calculated the angles of the mirrors, an arduous and finicky task at best. I don't understand why that's necessary at all. Do it empirically! Shine a light on each mirror one at a time and adjust it manually until its beam is at the focal point. Why is that so hard??? Heck, you don't even need fixed parts. I'm thinkin' (at least in the case of the smaller one) a wad of bubble gum will form your base - infinitely adjustable.

    2] The Mythbusters were exploring an ancient myth that has a warring race on the Nile or something using a death ray on a Phoenician ship at sea. They declard the myth busted because they couldn't get the apparatus to concentrate enough light to acheive ignition. But their reasons were all purely logistical - all to do with stability and adaptability of the apparatus as well as the problems in aiming at a moving target.

    It seems to me, that you don't need an apparatus at all. All you need is 300 soldiers holding 300 mirrors. They line up on the shore (creating a 2 dimensional fresnel-type reflector), and they each handle their own focussing. This eliminates all the logistical problems. Every lens is now dynamically and intelligently controlled.

    OK, so I need 300 volunteers to meet me in the parking lot at the corner of Lakeshore and Brown's Line. Please bring a mirror...
     
  2. jcsd
  3. May 22, 2005 #2
    i agree that 300 individally controllrd mirrors would probably catch an old sailing ship on fire..
    i suppose that the distance from the mirrors to the ship might have a lot to do with it though..
    and mirror size also..
     
  4. May 22, 2005 #3

    cronxeh

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    well max you'll ever get is 1.4 kW/m^2

    now this number might go down due to air convection and mirror conduction to maybe 1.0-1.2 kW/m^2

    so its not how many mirrors you got, its the total area and the type of material you are trying to ignite. For the case of wood you would probably need approximately 300 degrees C constant, so thats about 1 sq m of those mirrors
     
  5. May 22, 2005 #4

    JamesU

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    actually, the mythbusters' death ray was flat, that's why it didn't work.
    Can you supply mirrors for $1.00?
     
  6. May 22, 2005 #5

    cronxeh

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    pretty good idea came up when i was fiddling around with this on my notepad..

    if one can harness 1.0 kW/m^2 of heat and transform that into steam and then run through power turbines and generate electricity

    as compared to pvc or solar cells, how much more or less efficient the electro-mechanical method is?
     
  7. May 22, 2005 #6

    cronxeh

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    OF COURSE!

    Sterling Engine!

    Edit: the mirror alignment with the Sun's normal vector is a hard task, I dont see how this could be achieved without a computerised controller
     
    Last edited: May 22, 2005
  8. May 24, 2005 #7
    Wasnt it archimedes who came up with this idea? I have no idea whether it would work or not.
     
  9. May 24, 2005 #8
    I don't think you can harness 1 kW. It's more like .66 kW and that too on perfect sunny days. Secondly, the initial investment is high as you need concentrators to concentrate the sunlight in one place. The heating is slower compared to say coal. You cannot use it on cloudy days. The storage methods for electricity in case of night are expensive. That said, I think there is a solar furnace plant somewhere in Europe in the mountains.
     
  10. May 24, 2005 #9

    Integral

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    Let's get the history right, it was Archemedes at the siege of Syracuse (Sicily) during the 2nd Punic war.

    There is much that is unknown about this, so unless the guys running the Myth Busters show have a mind the quality of Archimedes designing their devices, I do not think that they can honestly say they have busted the myth, only that they were unable to duplicate the feat.

    Several hundred trained men with mirrors of the appropriate focal length could, conceivably, put a lot of energy in a small area.
     
  11. May 24, 2005 #10

    Chi Meson

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    I tried this with one of my honrs classes a few years back. Can't say much about ancient ships, but 20 mirrors (about 3 x 5 inches each, and approximately 20 feet away) could not set a dried corn stalk on fire. It burned a few retinas though.
     
  12. May 24, 2005 #11
    I was googling Sterling Engine once and came across a company that used the sun to power a Sterling Engine.
    dont have the link offhand, it wasnt cheap, but it was very cool..
     
  13. May 24, 2005 #12

    LURCH

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  14. May 24, 2005 #13

    russ_watters

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    Sorry, cronxeh - already under development: http://www.solarpaces.org/technology/tower.html
    No patent for you!
     
  15. May 24, 2005 #14

    cronxeh

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    that design is a ridiculous waste of land space

    roughly twice as big as it should be really
     
  16. May 24, 2005 #15

    DaveC426913

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    The total area of material you are trying to ignite is, ideally, zero. The whole point is to concentrate the light on as small an area as practically possible.
     
  17. May 24, 2005 #16

    DaveC426913

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    No, it's supposed to be flat. See the construction of the Death Ray in the link in my initial post.

    It's a fresnel lens. The flatness of the superstructure is accounted for by the angles of the mirrors (that's what a fresnel lens does.)

    The reason why it didn't work is becasue it was too flimsy, and they couldn't get all lenses focussed on one spot. You can see when they're moving it, that the superstructure is quite flexible and unstable.
     
  18. May 24, 2005 #17

    cronxeh

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    no no I meant the area of a focusing mirror. From physics, we know that the maximum amount of solar power we can harness is 1.4 kW/m^2. The area of your telescopic or parabolic mirror is thus the upper limit on how much you can get out of the Sun. The lower limit of course is the clouds and other environmental conditions, plus the Sun's relative angle
     
  19. May 24, 2005 #18

    DaveC426913

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    Ah. Your wording was a bit ambiguous on that point. But yeah.
     
  20. May 24, 2005 #19

    Integral

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    It is not clear to me whether Arch. provided each man with a concave mirror, or if each shield was flat and he arrayed them in a large parabola, with the target ship (or point) at the focal point. Since this was a siege, perhaps the besieging ships had been at anchor for some time, this would have enabled him to locate a target ship (the Roman leaders of course) and know exactly where it would be at any given time (ships at anchor move with the tide). Thus he could have laid out a parabola on the over looking cliffs. For this to work the sun would have to be at a specific angle above the horizon and other conditions would have to be just right.

    Seems to me that the technology of the day would have leaned toward Arch. laying out a large parabola, rather then making hundreds of concave shields with a very specific radius of curvature.

    The myth cannot be busted until someone comes up with several hundred highly polished bronze shields (could he have used silver? ) and uses ancient methods to survey a parabola on the cliffs above the bay of Syracuse.
     
  21. May 24, 2005 #20
    Power concentrated to what degree

    And if you focus any amount of power on a perfect point, the resulting temperature will be infinitely high. How did you come up with the figures "300 degrees C constant, so thats about 1 sq m of those mirrors"? There seem to be some steps missing from your math equation.
     
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