Robert W. Mellette
LESSON PLAN
Topic
Hero’s engine
Rationale
Strange as it may seem, the action-reaction engine widely used today in both aviation and spacecraft had its genesis in a toy that served no practical purpose other than entertainment. This activity reproduces at a very modest cost, the famous toy invented by Hero over 2000 years ago.
Objective
To demonstrate the principle of action-reaction using common laboratory equipment.
Materials Required
250 ml Pyrex boiling or erlenmeyer flask
30 ml flint glass tubing
rubber two hole stopper
ball-bearing fishing swivel
heat source (bunsen burner)
water
figure available in print form
Procedure
Obtain a 250ml boiling or erlenmeyer flask. Select a two hole stopper that will fit into the top of the flask. Using flint glass tubing and a bunsen burner or propane torch, fashion the two jet arms pictured in the diagram. The outside ends of the arms should be formed so that they form nozzles that will restrict the flow of the escaping steam. If they are fashioned correctly they should resemble the end of an eye dropper. A pattern to make a strap that will support the device so that it can be hung from a wire or string, is provided in the appendix of this unit. This strap can be made from a tin can or piece of scrap metal. A strap could also be fashioned from coat hanger wire.
CAUTION: When attaching this supporting strap, it ia very important that the strap does not block the stopper from lifting upwards if the steam pressure becomes so great that the flask ia in danger of bursting. The fact that the rubber stopper can pop up if the steam pressure becomes to great is an important safety feature, that must not be interfered with. Next, attach a ball-bearing fishing swivel to the top of the strap. This swivel will allow the fl ask to spin freely. Suspend the device from an overhead support. Place a small quantity of water (25-50 ml) into the flask. Insert the stopper with the two jet arms into the flask. Press the stopper in, but do not force it in to tightly. Use a bunsen burner, alcohol lamp or propane torch under the glass flask. Heat the water under a moderate flame until the water begins to boil. The steam generated from the boiling water will escape out the two jet nozzles. The action of this escaping steam will cause the model to rotate rapidly in the opposite direction due to the reaction force generated, demonstrating in a spectacular fashion Newton’s Third Law of Motion.
Background Information
In this activity, we construct a model of an ancient reaction engine that was first described in writings over 2000 years ago. This device was invented by Hero of Alexandria, a great mathematician and natural philosopher who lived somewhere around 100 or 150 B.C.. Hero seems to have invented a great number of machines and automata, however, he is best known for his creation of this reaction engine.
Its construction was quite simple. Water was poured into a spherical boiler. This boiler was placed over a fire. The boiler was free to rotate on two vertical axes. Steam generated within the boiler escaped through two nozzles that were positioned on opposite sides of the sphere. The force of the escaping steam jets caused the sphere to rotate rapidly in the opposite direction of the escaping jets.
Apparently this device was created by Hero solely for entertainment purposes and served no practical value. In 1750, the Hungarian physicist and mathematician Johann Andreas Von Segner developed a device based on reaction motion produced by escaping water jets. This device known today as The Segner Wheel is often found in physics classrooms.