I have. As an inexperienced young grad student, I was handed a copper-vapor laser with virtually no safety training. This was a pulsed laser, operating at 20 KHz, and had a time-averaged power output (not input) of 20 watts with a beam diameter of about one inch. My beam “dump” was a cardboard box painted flat black. First time I did this, the box started smoldering after a minute or so; that was my clue to get a better beam dump. And some laser safety goggles. And a door interlock.
For some reason I didn’t really recall this incident until I read your post. And of course I went and did the math. 20 watts on a one-inch circle works out to 40.7 kW/m^2. Turns out that’s a pretty good match for my OP estimate of 50 kW/m^2.
That probably helped. I didn’t experiment with all the ways to specifically start a fire with the laser cutter (I think the makerspace might have been displeased at that…). Regardless, it was easily enough to ignite wood under many conditions. Maybe a sustained fire with natural wood would have required some kind of kindling to help.
It gets even worse-- There’s no hard-and-fast figure for “high enough to orbit without significant drag”: It depends on the area-to-mass ratio of your satellite. Any figures you find online will be assuming some sort of typical values for a satellite, but this mirror will be anything but typical: It’d be a huge area, and probably made mostly of a thin film of mylar or something similar, meaning that it’ll have a huge amount of drag for its mass.
In the long run, the math still eventually works out: The required size for the mirror is only quadratic with the distance, while the atmosphere thins out exponentially. But it still ends up being a ludicrous size.
@CalMeacham, one thing going for us if we use an extremely brief pulse, like from an SDI single-shot nuke laser, is that we don’t need anything like precise targeting for this project. Our target is a dry forest covering many acres: Get the required energy density onto any spot in the forest, and we’ll start a fire. And with the kind of pulse duration you’d get from SDI, there wouldn’t be time for the beam and ground to move significantly relative to each other.
And then of course you also have the effect that you just built a solar sail. So even if, as you say, it’s been built high enough to overcome the atmospheric drag effect you’ll have to deal with the Sun’s radiation pressure moving the sail in ways that don’t help the orbit.
Not to mention the slight challenge of aiming a thing 20 or 40 miles across. That needs somehow to be micro-inch flat across that span while being torqued to hold the heat spot on a single point on the surface while the Earth is rotating and the reflector is moving along its orbital path.
This whole idea is a long way past “not even wrong”.
But it is fun to take potshots at. Because as big as it is, it’s hard to miss.
Just a little envelope math about blooming. Air pressure on the ground is 101,000 Pa. Divide by g and we get 10,300 kg/m^2. Heat capacity of air is 1000 J/kg-K. Multiply to get 10,300,000 J/K-m^2.
Our hypothetical laser is 50000 J/s-m^2. Assume about 20% absorption to get 10000 J/s-m^2. Divide by the above figure to get 0.00097 K/s.
That’s the heating rate in the beam column. It’s totally negligible. Blooming isn’t a factor here.
On the burning-glass idea, it might be easier to launch a whole bunch of small, independently-aimable mirrors, rather than one big one. Make each one small enough to subtend an angle of less than half a degree from the target, and you can make them flat, instead of a parabolic curvature. Make them much smaller than that, and you can tolerate up to a half-degree deviation away from planar. And slewing a bunch of small mirrors is a much simpler problem than a single one miles across.
For several years, the group I was in was in the same organization (and building) as the group the designed and build really, really big lasers. Some facts to anchor the conversation:
From Space, once you are outside the atmosphere, spreading and loss are either negligible or can be rendered negligible (beam shaping, adaptive optics, etc.). The best spot for laser beaming of power is geo.
A space laser for power beaming (or world domination!) would be an “electric” laser built up by combining multiple individual lower power lasers (think kw lasers combined to create a megawatt laser).
A space based laser that puts about a MW on the ground can be shown to be feasible (only engineering advances required), but the sheer size and power required put the on-orbit cost in the 10’s of billions of dollars (but the good news is its not more than 100 billion).
That was a megawatt class laser in the 747. The Wikipedia article claims (program cancelation) that they’d need 10 to 20 times the power to shoot down missiles in the boost phase.
Yeah, I’m well aware. It was also the last great gasp of the chemical laser. The plane mostly held large tanks of chemicals. I was just answering the overall question about whether you could put a weapons grade laser on a plane. The program was cancelled before the demo, but the demo would have worked.
The focus shifted to large “slab” semiconductor lasers, and is now settled on combining high power kW fibers into weapons grade lasers. There’s a lot of cool advanced technology and small scale demos have been done.
The nice thing about naval use is that they usually have power to spare (sometimes a nuclear generator) and a large source for heat exchange.
When I lived on Guam, power outages were frequent, due to a poor electrical infrastructure. It wasn’t uncommon to connect to US Navy ships to supplement the power provided to the island. (My mom at the time worked for Guam public works and would sometimes help facilitate that kind of thing.)
Spoke too soon. Accuracy of the bat bombs is pretty poor with a target area of 20 to 40 mile radius. In keeping with the spirit of using lasers, if you could train the bats to roost where you are aiming a laser, you would have a Laser Aimed Bat Incendiary Armament. Nah, I still got nothing.
