Tuesday, July 13, 2010

Archimedes' Steam Cannon

Think that a half-cup of water is harmless?

Think again.

Watch this video, and don’t even consider trying to do the same thing at home.

A one-pound projectile traveling at the speed of sound with 10 times the kinetic energy of an AK-47 bullet is not a toy. This experiment was performed under carefully designed and controlled conditions.

Curious? Read on.

In episode 54 the MythBusters took on the myth of Archimedes’s steam cannon, and after a gentle lob of a few feet, the myth was busted. A large boiler-driven cannon was used for the show’s finale.

Being frustrated with the outcome, having a visit to MIT in their schedule, and remembering last year’s Archimedes’s death ray collaboration, the MythBusters asked us to take a look. They wondered if there was a simple, plausible solution compatible with da Vinci’s vague drawing and explanation of Archimedes's steam cannon.
The drawing has been interpreted as a direct injection concept, where water rapidly sprays into a hot breech and the resulting flash of steam launches the projectile.

But, some back-of-the-envelope calculations suggest otherwise. A 540 °C (1000 °F) copper breech can provide a heat flux of roughly 100,000 kiloWatts per square meter to water that is in direct contact with its surface. And if water can be sprayed onto the breech in a uniform 1mm thick layer, 2,000 kiloJoules of energy per square meter of surface will produce 400 PSI steam—a pressure that would generate a reasonable projectile velocity.

The pressurized steam must be generated quickly, before the projectile exits the cannon, perhaps in 0.005 seconds. But at 100,000 kiloWatts per square meter, the breech can only provide around 500 kiloJoules per square meter in such a short time, not nearly enough energy to produce high-pressure steam.

Our conclusion? Direct injection is not likely to produce much more than a burp (.mov, 0.23 Mb).

In 1981, Sakkas (see figures 23-25) recreated Archimedes’s steam cannon and incorporated a wood board into the concept to hold the projectile in place while some pressure developed, although da Vinci’s drawing does not show a similar feature. Even so, this produced a fairly low velocity projectile (perhaps 70 mph) that traveled only 50 meters.

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