Why Not Use Empirical Adjustments for Solar Death Rays?

[DaveC426913]

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: http://www.solardeathray.com/about.html

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 achieve 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...

 

[willib]

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..

 

[cronxeh]

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 that's about 1 sq m of those mirrors

 

[JamesU]

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 achieve 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.

actually, the mythbusters' death ray was flat, that's why it didn't work.

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...

Can you supply mirrors for $1.00?

 

[cronxeh]

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?

 

[cronxeh]

OF COURSE!

Sterling Engine!

Edit: the mirror alignment with the Sun's normal vector is a hard task, I don't see how this could be achieved without a computerised controller

 

[morry]

Wasnt it archimedes who came up with this idea? I have no idea whether it would work or not.

 

[sid_galt]

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?

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.

 

[Integral]

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.

 

[Chi Meson]

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.

 

[willib]

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..

 

[LURCH]

http://www.nrel.gov/documents/solar_energy.html

About halfway down the page.

Also, this company;
http://www.pureenergysystems.com/news/2004/07/09/6900033_Solar_Hydrogen/
has produced temps of over 1500^oF with an 18-inch mirror! Wood lights up at just 450^o

 

[russ_watters]

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?

Sorry, cronxeh - already under development: http://www.solarpaces.org/technology/tower.html

In power tower systems, heliostats reflect and concentrate sunlight onto a central tower-mounted receiver where the energy is transferred to a heat transfer fluid. This is then passed optionally to storage, and finally to power-conversion systems which convert the thermal energy into electricity and supply it to the grid.
Power tower plants are defined by the options chosen for a heat transfer fluid, for the thermal storage medium and for the power-conversion cycle. The heat transfer fluid may be water/steam, molten nitrate salt, liquid metals or air. Thermal storage may be provided by phase changing materials or ceramic bricks. Steam-Rankine power conversion systems are used, with the possible alternative of open-cycle Brayton power-conversion systems.

No patent for you!

 

[cronxeh]

that design is a ridiculous waste of land space

roughly twice as big as it should be really

 

[DaveC426913]

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 that's about 1 sq m of those mirrors

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.

 

[DaveC426913]

actually, the mythbusters' death ray was flat, that's why it didn't work.

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.

 

[cronxeh]

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.

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

 

[DaveC426913]

Ah. Your wording was a bit ambiguous on that point. But yeah.

 

[Integral]

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.

 

[hitssquad]

Power concentrated to what degree

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.

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 that's about 1 sq m of those mirrors"? There seem to be some steps missing from your math equation.

 

[hitssquad]

The sun is not a perfect point

It is not clear to me whether Arch. provided each man with a concave mirror, or if each shield was flat

Why would each man need a concave mirror? The sun is not a perfect point, so a lack of focus would be appropriate, given that each mirror was relatively small. Besides, a fixed concave mirror would only focus at a fixed distance, making it useless for military purposes.

 

[DaveC426913]

You do not need a parabolic shape, nor do you need concave mirrors.

You need x flat mirrors, positioned however is convenient (say, along the shoreline). It does not matter if each mirror is the same distance from the target, which is why there is no restriction on the shape of the macro-structure (i.e it does not need to be a parabola). Each individual flat mirror is positioned so that it reflects the Sun upon the desired target.

See attachment.

No, it does not focus the sun on a point, so it is not as efficient as a true parabolic mirror. It actually focusses on an area that is equal to the area of the mirrors used (eg. 3 foot round mirrors will produce a 3 foot round target).

It can be arbitrarily large. You just add more soldiers with mirrors, up to the limit of how much space they take up. This is not so with a parablic mirror. (Which has to do with why Fresnel invented it).

Note how dynamic this system is. Each mirror operates independently and is self-correcting, the position and movement of both the Sun and the target are easily compensated for. Individual elements can even move around as is convenient for landscape or targeting factors.

 

[Sowhatalot]

You can in fact do it but only to a smaller scale I have a seen it done http://www.solardeathray.com/ i was in fact hoping to make one my self just to see if i could get it to work

 

[Danger]

I haven't heard of this in the past 30 years, and my memory sucks, but I seem to recall that the mirrors were used to set the sails on fire, which then fell to the deck and touched off the stuff with a higher ignition point. I don't know what sails were made of back then, but it for sure had a lower flash point than wood.

 

[DaveC426913]

I haven't heard of this in the past 30 years, and my memory sucks, but I seem to recall that the mirrors were used to set the sails on fire, which then fell to the deck and touched off the stuff with a higher ignition point. I don't know what sails were made of back then, but it for sure had a lower flash point than wood.

Yeah, but they weren't actually trying to set it on fire so much as measure the temperature reached. It was insufficient to set paper afire.

 

[Danger]

Yeah, but they weren't actually trying to set it on fire so much as measure the temperature reached. It was insufficient to set paper afire.

Aha... the peril of hearing only partial accounts. All this time I was thinking that it was something that happened trying to repel an invasion. I didn't realize that it was an experimental set-up.

 

[Random]

Not Archimedes.. Leonardo Da Vinci. Did a lot of drawings that still survive on focus of mirrors. Sneaky fella was working for both sides of an opposing battle.. Check the Leonardo web ring for pics.

 

[morry]

Not Archimedes.. Leonardo Da Vinci. Did a lot of drawings that still survive on focus of mirrors. Sneaky fella was working for both sides of an opposing battle.. Check the Leonardo web ring for pics.

Im pretty sure Archimedes came up with this while trying to defend Syracuse from the Romans in 212 BC.

I don't really know, because I wasnt there, but this is what the internet is suggesting to me.

 

[Francis M]

I've heard it was done or at least thought up by Archimedes at the battle of Syracuse. THis was also the battle where they say he invented a crane weapon to flip over enemy ships when they got close to the seaward walls.
I think all the Mythbusters proved was that it couldn't be done with the type of metal shields the myth said it was done with, so they've actually only busted one accounting of the myth.

On another note I think on a later Mythbusters show or I think on PBS? I saw a group construct (at least how they thought it may have been done) the ship flipping crane and a period ship. That device actually worked, they flipped the ship over.

 

[mr henry]

No, it does not focus the sun on a point, so it is not as efficient as a true parabolic mirror. It actually focusses on an area that is equal to the area of the mirrors used (eg. 3 foot round mirrors will produce a 3 foot round target).

now I'm no physicist, beyond my A-level, but maintain an interest, so feel free to laugh if required!

Wouldn't the size of focal area be dependant on the size of the mirrors and one's accuracy at lining them up? If they were, say, an inch square, and you had them perfectly lined up (a near impossible feat, i understand), would it not be only an inch square, rather than a cumulative total of the area?

Am going to try and build one myself, just to see if i can, so these little nuggets of info could prove useful and determine whether it works or not.

 

[DaveC426913]

now I'm no physicist, beyond my A-level, but maintain an interest, so feel free to laugh if required!

No laughing here...

Wouldn't the size of focal area be dependant on the size of the mirrors and one's accuracy at lining them up? If they were, say, an inch square, and you had them perfectly lined up (a near impossible feat, i understand), would it not be only an inch square, rather than a cumulative total of the area?

Am going to try and build one myself, just to see if i can, so these little nuggets of info could prove useful and determine whether it works or not.

Yep. Three foot mirrors (the size of a soldier's shield) will make a three foot spot.

See http://www.solardeathray.com/about.html (as in the initial post) who did it with 3.5" mirrors.

Personally, I think he did it the hard way - he calcualted the angle of every mirror. Seems to me, all you need to do is set it one place, point a light at it (parallel), and move it until it reflects the light at the focus. Who cares what the numerical angle is.

 

[mr henry]

aaaahhh, sorry, a simple misunderstanding. I imagined a comma ("3, foot round mirrors...") that blatantly wasn't there!

Thanks for the link, i stumbled across that site about a month ago, hence wanting to build one. Fancy the challenge of a longer range one (although my garden wouldn't quite accommodate 100'!) Clearly levels of observation and attention to detail will need to be improved if I am to stand a chance!
I do prefer the idea of doing the mirrors empirically though, the calculations seem a bit heath robinson, and more open to error.

Thanks

 

[DaveC426913]

...the calculations seem a bit heath robinson

Ah, what??

 

[wimms]

well, flat mirrors don't reflect 100% of sun energy, do they? Ordinary metalised mirror is lucky to reflect about 50% of visible light, but shoudn't they hunt for more like infrared side of spectrum?