Archimedes' Death Ray?
Boingboing directs us to “Archimedes Death Ray: Idea Feasibility Testing,” in which an MIT class decides to test Archimedes’ ray: The use of mirrors to set warships on fire.
Mythbusters claimed it was a myth, that the idea couldn’t be made to work. Well, the MIT class gave it a shot, and it turns out that, as pictured, you can light a bunch of wooden planks on fire with sunlight at a good distance (100 feet or so).
That’s pretty darned cool.
Somethings nagging at me.
So…that’s a fascinating feasibility study, and was probably a lot of fun to do. But something struck me as I looked at the mocked-up boat, sitting high and dry on the MIT lawn, and then on the roof of a parking garage.
There’s no water.
Now, that might be an acceptable oversight if this was a class at say, the University of the Sahara. But as people who have visited MIT are aware, MIT happens to be close to water. (I provide a map for easy reference. The first experiment, on the lawn, appears to have taken place across the street from the red push-pin at 77 Mass Ave.)
Now, you might think that the lack of water in the set up isn’t a very big deal. After all, as the concentrated light hit the wood, the water would simply steam off. Of course, that steaming off would slow the process, and warn the sailors that pouring more water down the side on their boat would be a fine idea. Old sailors tend to be quite aware of the dangers of fire, and have a variety of methods for fighting it.
But more important than the lack of water as a fire retardant is that water is a liquid, and things floating on water are rarely stable. They bob. They bounce. They drift (even at anchor.) Each of those increases the difficulty of keeping the light focused on one spot. It also introduces a targeting trade-off: If you ignite near the water line, so that the fire rises through as large an area as possible, then you’re igniting wetter wood, and have to heat the adjacent wood as well. (The MIT guys targeted right above the water line.) Your hot spot may also dip into the water, cooling it off. So your aim point needs to be higher up, causing less damage, and increasing the ability of those on the ship to fling water at it.
And so, when we ask ourselves, could Archimedes’ Ray have worked, we need to take into account the water, the movement of the ship and the ability of the soldiers to keep light focused on a single spot.