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

1. Aug 8, 2011

### Urvabara

Is it possible to build a large mirror array (around 10 km²) with adjustable focal length (from 10 meters to around 400 kilometers)?

Last edited: Aug 8, 2011
2. Aug 8, 2011

### Staff: Mentor

What's the application? Summer daytime missle defense?

3. Aug 8, 2011

### MATLABdude

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!

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):

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):

So you might still be able to slag an aircraft carrier, but it'll probably take a while.

Last edited: Aug 8, 2011
4. Aug 8, 2011

Top secret.

5. Aug 8, 2011

### Urvabara

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?

Last edited: Aug 8, 2011
6. Aug 8, 2011

### MATLABdude

Who's "I"? Is he/she a nefarious Blofeld-esque super villain? :tongue: (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

Last edited: Aug 8, 2011
7. Aug 9, 2011

### Urvabara

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

Thanks! Seems interesting...

8. Aug 9, 2011

### Staff: Mentor

Well it's not top secret anymore. You just posted about it on the Internet. :tongue2:

9. Aug 9, 2011

### RocketSci5KN

The application is clearly a solar driven rocket. Much like laser propulsion, but with mirrors.
Sure, lets 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. Aug 10, 2011

### Mech_Engineer

More importantly- it wouldn't work for missile, aircraft, or boat missile defense because you could just fly around it!

11. Aug 11, 2011

### HowlerMonkey

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. Aug 11, 2011

### Mech_Engineer

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. Aug 13, 2011

### MATLABdude

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

As per the Tim Taylor philosophy!

14. Aug 20, 2011

### Lsos

1.21 gigawatts by any chance?

[PLAIN]http://abduzeedo.com/files/posts/back-future/back-future-6.jpg [Broken]

Last edited by a moderator: May 5, 2017
15. Sep 17, 2011

### danielandpenn

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