Is it possible to build a mirror array with adjustable focal length?

AI Thread Summary
Building a large mirror array with adjustable focal length, potentially for applications like missile defense, raises questions about feasibility and design. A 10 km² array could theoretically redirect significant solar power, enough to cause severe damage to large naval vessels, but practical challenges exist in achieving effective focus over vast distances. Concentrated solar power techniques, such as heliostats, could offer some variability in focal length, but achieving precision at long ranges remains uncertain. The concept of using such an array as a weapon is criticized due to its stationary nature and the difficulty of targeting moving objects like aircraft carriers. Overall, while the idea is intriguing, it faces significant technical and practical hurdles.
Urvabara
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Is it possible to build a large mirror array (around 10 km²) with adjustable focal length (from 10 meters to around 400 kilometers)?
 
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Urvabara said:
Is it possible to build a large mirror array (around 10 km²) with adjustable focal length (from 10 meters to around 400 kilometers)?

What's the application? Summer daytime missle defense?
 
berkeman said:
What's the application? Summer daytime missle defense?

According to Wikipedia, the Earth receives an average of 680 W/m^2 of power from the sun. If you had a 10 km^2 array, had that average power, and could redirect 100 % of the power with no losses, you'd have 6.8 GW--forget starting fires, you could probably slag an aircraft carrier battle group with that! :bugeye:

Back on topic, however... To the OP, have you heard of concentrated solar power?
http://en.wikipedia.org/wiki/Concentrated_solar_power

EDIT: Hmmm, after some back of the envelope calculations using the Wikipedia page on iron, and the HowStuffWorks page on aircraft carriers...
http://en.wikipedia.org/wiki/Iron
http://science.howstuffworks.com/aircraft-carrier2.htm

  • Nimitz Class aircraft carrier has 54,000 metric tons of structural steel (assume to be all iron)
  • molar mass of iron: 26 g / mol
  • heat of fusion for iron: 13.8 kJ / mol
  • melting point of iron: 1538 °C
  • specific heat capacity of iron: 25.1 J / (mol * K)

To slag (completely melt) the iron at the melting point, you'd need to put in 28.7 x 10^12 J of energy. With the aforementioned 6.8 GW death ray, that'd take about 1.17 hours to do (assuming no losses):
http://www.google.ca/search?q=54000...s+*+(mol+/+26+g)+*+(13.8+kJ+/+mol)+/+(6.8E9+W)

However, since you'd also need to heating it up to the melting point (and by then, the softening of the metal would probably destroy the ship anyway), a difference of, let's say 1500 K, you'd need 78.2 x 10^12 J of energy. About 3 hours to do the deed (assuming no losses):
http://www.google.ca/search?q=54000...+g)+*+(25.1+J+/+(mol+*+K))+*+1500+K+/+(6.8+GW)

So you might still be able to slag an aircraft carrier, but it'll probably take a while.
 
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berkeman said:
What's the application? Summer daytime missle defense?

Top secret.
 
MATLABdude said:
According to Wikipedia, the Earth receives an average of 680 W/m^2 of power from the sun. If you had a 10 km^2 array, had that average power, and could redirect 100 % of the power with no losses, you'd have 6.8 GW--forget starting fires, you could probably slag an aircraft carrier battle group with that! :bugeye:

Yes. "I" want to do things which need lots of power. "I" cannot do them without powers in the gigawatt range.

MATLABdude said:
Back on topic, however... To the OP, have you heard of concentrated solar power?
http://en.wikipedia.org/wiki/Concentrated_solar_power

Yes, I have, but I haven't figured out, if it is easy or even plausible to adjust the focal length of that huge mirror array from a meter range to a hundred kilometers range. So, is it?
 
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Urvabara said:
Yes. "I" want to do things which need lots of power. "I" cannot do them without powers in the gigawatt range.
Yes, I have, but I haven't figured out, if it is easy or even plausible to adjust the focal length of that huge mirror array from a meter range to a hundred kilometers range. So, is it?

Who's "I"? Is he/she a nefarious Blofeld-esque super villain? :-p (Just kidding!)

I'd think you can probably have some amount of variability with a heliostat design. Not sure if you can actually obtain a very good focus at the far end of the distance scale, however.
http://en.wikipedia.org/wiki/Heliostat
 
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MATLABdude said:
Who's "I"? Is he/she a nefarious Blofeld-esque super villain? :-p (Just kidding!)

It's a top secret project and the status and description of the project are classified.

MATLABdude said:
I'd think you can probably have some amount of variability with a heliostat design. Not sure if you can actually obtain a very good focus at the far end of the distance scale, however.
http://en.wikipedia.org/wiki/Heliostat

Thanks! Seems interesting...
 
Urvabara said:
It's a top secret project and the status and description of the project are classified.

Well it's not top secret anymore. You just posted about it on the Internet. :-p

There goes your TS clearance :redface:
 
The application is clearly a solar driven rocket. Much like laser propulsion, but with mirrors.
Sure, let's say each individual mirror in your array is flat, and 1m^2. As long as your desired spot size is 1m^2 or larger, you could in theory steer each mirror to point the sunlight incident on it to the rocket, giving you a crude controllable variable focal length. Obviously, there are many practical problems that need to be solved to actually make this.
 
  • #10
More importantly- it wouldn't work for missile, aircraft, or boat missile defense because you could just fly around it!
 
  • #11
Actually...you would only have to heat a carrier group beyond what the inhabitants can survive.

160 degrees should be enough to defeat current habitation space cooling.
 
  • #12
You couldn't aim the mirror array at an aircraft carrier anyway, carriers stay hudreds of miles off the coast (beyond the horizon).

This is ridiculous as an idea for a weapon since it's stationary and easily avoided.
 
  • #13
Mech_Engineer said:
You couldn't aim the mirror array at an aircraft carrier anyway, carriers stay hudreds of miles off the coast (beyond the horizon).

This is ridiculous as an idea for a weapon since it's stationary and easily avoided.

Well, I initially introduced the idea just because Archimedes established "ship death-ray" as the gold standard in things involving the sun and mirrors! :wink:

As per the Tim Taylor philosophy!
https://www.youtube.com/watch?v=DewDqsszXi8
 
  • #14
Urvabara said:
Yes. "I" want to do things which need lots of power. "I" cannot do them without powers in the gigawatt range.

1.21 gigawatts by any chance? :biggrin:

[PLAIN]http://abduzeedo.com/files/posts/back-future/back-future-6.jpg
 
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  • #15
berkeman said:
What's the application? Summer daytime missle defense?

:) This response Soooooo made me laugh. I think the units were misinterpreted.
 

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