Archimedes death ray: idea feasibility
testing > MythBusters
2.009 Archimedes Death Ray: Testing with MythBusters
The first showing of this Mythbusters episode was January 25, 2006 on the Discovery channel. You may also want to read about and see video of the original experiment at MIT in 2.009.
On Friday, October 21, 6 students from the current and previous two years of 2.009 and the course instructor went to San Francisco to burn a real boat with the MythBusters. October 21 was used for final mirror fabrication and to setup the stands for the array.
Click on images to see high resolution versions.
On Saturday, October 22, the weather was good. We were able to cause charring and smoldering in a 1-2 foot wide swath along much of the boat's length. After three passes over the boat, the hull was penetrated and a small open flame was achieved. This burning area was self-sustaining and, about 2 hours after the test had ended, it was still burning with intermittent flame and had created a roughly 10 inch diameter hole in the hull of the boat.
We did not, however, achieve a large flash ignition as we did in the MIT test. We believe that this was due to the high moisture content of the boat that was used as a target. In my view, it is an open question as to whether the boat was representative of a seaworthy Roman ship... but at least this boat should not be considered as too easy a target!
A more detailed description of what happened follows.
The test employed 300, 1 foot square, bronze mirror tiles (510 phosphor bronze alloy). The number of mirrors was determined using the first MIT test as a benchmark and accounting for the different mirror material, along with the different conditions in San Francisco.
The tiles were polished using Brasso by the MythBuster and MIT team. The polish was quite high on most mirrors. About 30% of the mirrors had a variety of flatness defects that would reduce their performance to some degree.
The tiles were aimed in a manner similar to the MIT test, with the exception that there were 4 tiers of mirrors instead of just two rows. The mirror array was about 110 feet long, similar to the MIT test. The goal was to make an array that was big enough to start a fire, but small enough to implement within reasonable budgetary and time constraints (my guesstimate is ~3-4 person weeks, mostly spent on polishing the bronze).
If you are interested in what a more authenic ship would be like (both in design and condition) please review this slideshow. For estimates of how both moisture content and wood type would affect ignition time, please see the FAQ.
The boat was positioned so that it was roughly 150 feet from the center of the array. Since the array was in a straight line along the edge of a dock, the furthest mirrors (at the ends of the array) were roughly 160 feet from the boat.
The test (Saturday, October 22)
After letting the beam traverse the length of the boat it was requested that we try to ignite a sail, again using the bronze mirrors. This created a bright spot on the sail and no charring whatsoever. The dominant reason for this is probably that the sea breeze was rising and the thin sail had substantial convective cooling on both sides of the cloth (see FAQ xii). Also, the lighter color of the sail may be an important factor.
During the time that the beam was focused on the sail, charred areas on the side of the boat continued to smolder. After the test with the sail, we decided to switch to silver/glass mirrors to see if they could take us from smoldering embers to open flame. The time was around 1:30 PM (estimating).
The silver mirrors provided a tighter beam that we could more easily focus (~2-3 feet in diameter). We repeated the process, making a third pass along the side of the boat, starting with the aiming beam at the stern of the boat, and letting the bright spot move its way along the side of the boat (again, at roughly 9-10 inches per minute due to the motion of the earth). There was substantial smoke, and at one point near midship, there was a period of near-calm wind. The smoke pattern looked very much like what we saw just prior to flash ignition in the test at MIT. However, no open flame appeared, even though there were substantial glowing embers.
You may want to check the close up image. If we had applied the sunlight to these areas once more, we probably would have achieved a much more substantial flame.
The fire at midship continued to smolder with glowing embers and also intermittently burn with open flame. Open flame appeared to be dependent on fanning by the wind. In the picture on the right you can see a very small flame (you may want to look at the large image).
About 2 hours after ending the test, the fire was still smoldering with intermittent flame. There was a roughly 10 inch diameter hole in the side of the ship. The fire was extinguished with water.
We were able to cause charring and smoldering along the length of the ship. After three passes along the boat, we were able to penetrate the hull in one location and achieve an open flame. This burning area was self-sustaining as a smouldering fire, and about 2 hours after the test had ended it was still burning and had created a roughly 10 inch hole in the boat.
We were not able to achieve the large flash ignition as we did with the 2.009 experiment at MIT. Most likely, the difference was the moisture content of the wood, which was very high. Certainly wood in any boat on the water would have higher moisture content than kiln dried lumber but, given the history of the boat, it may still be an open question as to whether the moisture content (above the water line) was representative of a typical wooden boat (or Roman ship) in good seaworthy condition. In hindsight, it would have been prudent to have taken a moisture content reading so that this issue could have been addressed.
All said, we feel that the power estimates and models used to design the array were quite close to the actual performance, but we did underestimate the effect of moisture content.
For larger distances, one would want to improve our design to allow a fixed spot on the boat to be tracked... and inevitably, some fairly simple ways to do this have already come to mind! Too much fun...