Archimedes’ Death Ray?

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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.)

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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.

6 thoughts on “Archimedes’ Death Ray?

  1. I’ve seen the mythbuster’s episode and they ran into exactly the problems you describe.
    That said, Adam and Jamie are a great on-screen duo but a lot of what they build seem like a hack. In this particular episode, the beam was made very poorly, IMO and contributed to the poor results. So, mythbusters is a great entertainment show with some scientific merit, but I wouldn’t quote them as a source on a scientific paper.
    Also, there’s a difference between feasible and practical. MIT’s and mythbuster’s contraptions, as built, would have been far from practical back when they were supposedly used (scale and distance). Therefore, I conclude it’s a myth.

  2. Note to Steinbrenner: be very afraid if this contraption is seen on the Major Deegan.

  3. Actually, they address that issue somewhat in the FAQ: http://web.mit.edu/2.009/www/lectures/10_ArchimedesFAQ.html
    While they didn’t test it, they do note that the hot spot their mirrors created was about three feet square, and that a 100 foot long ship anchored in a sheltered bay probably wouldn’t be moving that much.
    Still, the trick does rely somewhat on the Roman troops not understanding enough optics to realize what’s going on. Otherwise I imagine they’d just pour water on the hot spot. Perhaps this could be reduced by targetting some extra mirrors at the *crew*.

  4. I thought MIT had smart people. If the students are not smart enough to add water, you’d think the teacher might have pointed out taht little detail. From the photo, it looks like the students went to a lot of work to build the model…..let’s hope the students don’t get to build the next big thing until they’ve learned to attend to the details.

  5. Target the sails on a moving ship. Then, when they burn, turn the beam on another part of the ship, like the deck. Blinding the crew might be a valid idea, too.

  6. Archimedes’ Death Ray, Take 2

    Posted By Adam Earlier this month, I posted “Archimedes’ Death Ray,” about the MIT team trying to replicate Archimedes’ legendary defense of Syracuse, setting fire to ships with polished mirrors. Now Mythbusters has brought MIT Professor David Wallace …

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