Green Room

A potential weapon of war?

posted at 9:20 pm on May 28, 2011 by

This is the next step in a minor debate with Instapundit. This response is quite long, so I figured I’d post it here instead of mailing it to him.

It’s been said that a lot of great ideas can be killed off just by running the numbers. So let’s run a few, shall we? The question is whether a steerable 13 terawatt beam can be used as a weapon. A lot of that depends on how wide the beam is when it reaches the planet. So let’s try some examples, and see what kind of power density you’d get, OK? Just to make sure everyone is on the same page, a "terawatt" is a trillion watts. 13 terawatts is 13,000,000,000,000 watts.

If the beam is half a kilometer squared (250,000 square meters), then the power density is 52 million watts per square meter.
If the beam is a square kilometer (a million square meters), then it’s 13 million watts per square meter.
If the beam is 100 square kilometers (a hundred million square meters), then it’s 130,000 watts per square meter.

And if the beam is 100 square kilometers, then the system is impractical because the receiving stations would be infeasible.

My microwave oven has an internal area of about a quarter of a square meter, and it’s 1100 watts. It does a really good job of cooking food, and if anything alive was put in there, it wouldn’t survive long. That’s a power density of about 4000 watts per square meter (rounded). The 100 square kilometer beam described above would have about 30 times that power density. The quarter square kilometer one would have 13,000 times the power density.

Glenn says,

Well, on the old O’Neill/JPL solar power satellite studies, the beam density was very low — low enough that birds could fly over the receiver without being harmed. But that plan involved satellites in geosynchronous orbit. I don’t know if it applies to lunar power-beaming.

The location doesn’t matter. What does matter is that those studies assumed power generation on the order of hundreds of megawatts. This hypothetical 13 terawatt beam is 4 orders of magnitude stronger.

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The feasibility of a 100-sq km receiver depends in no small part on the wavelength of the incoming beam. The receivers Reynolds mentions were designed as simple wire grids, with the wires spaced several inches apart. If you’re talking about a regular parabolic dish, then a 100-sq km receiver is an engineering “challenge.” But a wire grid 10 km on a side should be simple enough to manufacture. The problem then becomes one of expense (materials and land used, and hushing up the NIMBY BANANA crowd).

That’s not to say that this variety of space-based solar power is necessarily the best way to generate power, but it’s not as technically impossible as you’re making it out to be.

Blacksmith on May 28, 2011 at 11:10 PM

Blacksmith, the point isn’t whether it’s possible. The point is that it’s too dangerous to build even if it is possible.

Who do you trust with the controls?

Steven Den Beste on May 29, 2011 at 2:02 AM

I took Reynolds’ point to be that it should be possible to build in such a manner, with a low enough energy density, that I could trust anyone alive with the controls – including those who would attempt to use it for ill. Through the design of the device, they must necessarily fail at using it for ill, before I’d approve of its construction. Humans are too prone to inattentiveness to be trusted with a 13 TW beam without such overlapping controls, period.

This will drive up the cost of the device even further of course, which is part of why I really wouldn’t push for building it as-described (I’d be greatly amused if the Japanese said MW originally, and the reporter put in TW by accident). But that doesn’t make it necessarily too great a danger to contemplate its existence, period.

Digression/Hypothetical examples:
In the case of an electrical circuit, are there not several components that can be designed to fail (and just as important, be very difficult to replace) should specific thresholds be exceeded?

Also, tracking can be physically limited to certain swept paths. Those paths are something the designer can set to exclude current areas of risk.

Blacksmith on May 29, 2011 at 2:34 AM

But what if a fork (accidentally) was in the path of the waves? Where would it arc?

ericdijon on May 29, 2011 at 8:45 AM

“I’m Nicolai Tesla, and I approved this message.”

The_Livewire on May 29, 2011 at 9:04 AM

It does a really good job of cooking food, and if anything alive was put in there, it wouldn’t survive long. That’s a power density of about 4000 watts per square meter (rounded). The 100 square kilometer beam described above would have about 30 times that power density. The quarter square kilometer one would have 13,000 times the power density.

You need to factor in the abortion. Microwaves work because they are tuned to hit a resonant frequency of water, and thus are absorbed very readily by it. You clearly aren’t going to be using that frequency, so you would have to see what fraction of it was actually absorbed by objects on the ground besides the antennas.

Count to 10 on May 29, 2011 at 12:09 PM

I wonder if maintenance issues have been worked out?

MSimon on May 30, 2011 at 12:53 PM

Microwaves work because they are tuned to hit a resonant frequency of water, and thus are absorbed very readily by it.

Count to 10 on May 29, 2011 at 12:09 PM

That’s a misconception. See Wikipedia article and references:

Microwave heating is sometimes explained as a resonance of water molecules, but this is incorrect: such resonance only occurs in water vapor at much higher frequencies, at about 20 GHz.

JimC on May 30, 2011 at 3:56 PM

Are they working through the earthquake and tsunami scenarios?

Alden Pyle on June 1, 2011 at 10:16 AM