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

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Discussion Overview

The discussion revolves around the feasibility of constructing a large mirror array, approximately 10 km² in size, with an adjustable focal length ranging from 10 meters to 400 kilometers. Participants explore potential applications, including missile defense and solar propulsion, while considering the technical challenges involved in achieving such a design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question the practical application of a large mirror array, suggesting it could be used for missile defense or solar-driven propulsion.
  • One participant calculates the potential energy output of a 10 km² mirror array, proposing that it could generate 6.8 GW of power if 100% efficient, raising concerns about its capability to damage large naval vessels.
  • There is a suggestion that a heliostat design might allow for some variability in focal length, although doubts are expressed about achieving a good focus at extreme distances.
  • Another participant argues that using such a system for missile defense is impractical due to the mobility of targets like aircraft carriers.
  • Some participants humorously speculate about the motivations behind the project and its classification status, while others emphasize the theoretical nature of the proposed applications.
  • Concerns are raised about the practicality of aiming the mirror array at distant targets, with one participant noting that aircraft carriers operate far beyond the effective range of such a system.

Areas of Agreement / Disagreement

Participants express a range of views on the feasibility and practicality of the proposed mirror array, with no consensus reached on its potential applications or effectiveness as a weapon. Disagreement exists regarding the technical challenges and the viability of using such a system for missile defense or other purposes.

Contextual Notes

Participants acknowledge various assumptions and limitations in their calculations and proposals, including the efficiency of energy redirection and the practical challenges of maintaining focus over large distances.

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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