Level: Middle School to H.S.
Time involvement: 15 -20 minutes.
Materials/procedure: See photo above. Built from a copper toilet float ball modified by the addition of two small curved tubes soldered into holes at the “equator”, water is injected and a propane torch heats the ball forcing steam out the curved jets. The steam acts like rocket fuel. As the torch boils the water steam becomes visible and the ball spins rapidly. The effect ends quickly as the water is used up. Kids will ask to try this demo themselves but good judgment may require denial or limiting it to one or two “mature” students.
Discussion: The copper ball is very hot and stays hot for some time and must not be touched! The underlying principle is Newton’s third law of action and reaction. Same principle as rocket ships and jet airplanes! Not easily repeatable by students but fun to watch. This is a simplified version of the ancient Classical “Hero’s Engine”, often described in Physics books.
In the creation of this “Do-It-Yourself” copy of Hero’s engine the first test run failed. Although the water boiled and visible steam came out of the “jets”, the boiler did not turn! A basic principle of all rocket engines is that the thrust is directly proportional to the exhaust velocity. Rocket engineers have to carefully balance the size of the nozzle with the power of the burning fuel. Too large an opening yields too small a thrust. Too small an opening and the rocket explodes! Getting the size right on this engine was a process of trial and error. The size of the nozzles was slowly adjusted by gradually crimping the openings. Eventually the right size was reached, however, the openings were so small that a small syringe was necessary to inject the water into the boiler!
While this demonstration bears the title “Rocket steam engine”, which is truthful, students probably have visions of a railroad locomotive when they hear the term “steam engine”. The invention of the “modern” steam engine in 1711 by an Englishman, Thomas Newcomen was a game changer. Although used then as a power source for pumps to clear water out of coal mines and eventually to power the factories of the Industrial Revolution, it lead to the internal combustion engine, today used in trucks, cars, planes, etc.
Notes: Close adult supervision required. Construction of a duplicate model best left to a plumber friend or someone with tools and skills. Silver solder required because lead solder will fail. One eighth inch diameter brass tubing was used. Opening at ends must be carefully crimped smaller but not closed to increase velocity of escaping steam.
Learn More: Go to http://en.wikipedia.org/wiki/Hero_of_Alexandria for an interesting story about Hero of Alexandria c. 10 – c. 70 AD, Greek mathematician and engineer.
For information about “modern” steam engines go to: http://www.livescience.com/44186-who-invented-the-steam-engine.html