Hero’s Engine

Introduction

Demonstrate one of the first steam jet propulsion systems ever invented. Use the Hero’s engine to illustrate phase changes, conservation of energy and Newton’s third law of motion.

Concepts

• Newton’s third law of motion
• Phase changes
• Steam engine
• Conservation of energy
• Rocket propulsion

Background

The “Hero’s engine” is the invention of the Greek mathematician and inventor Hero (or Heron) of Alexandria. Many historians believe he lived between 10 and 70 AD, but due to the lack of surviving records of the time, some scholars place his life anywhere between 150 BC and 300 AD. The Hero’s engine, also know as an aeolipile, was the first rudimentary steam engine that used the interaction of heat and water to generate mechanical motion in the form of jet propulsion.

This demonstration illustrates the conversion of chemical energy to heat energy (the burning of a petroleum gas in oxygen), which is then converted to mechanical energy in the form of a rotating flask. This mechanical energy could then be used to generate electricity, or be used to perform work, such as being used as a spool to raise heavy equipment. When the water inside the flask is heated, a phase change occurs, and liquid water is converted into water vapor. Since water vapor takes up more space than liquid water, the pressure inside the flask increases. The exhaust tubes allow for a pressure release, but since the tubes are thin, only a small fraction of the pressure can be released so the inside of the flask remains under higher pressure. As a result of Bernoulli’s principle, the smaller chamber of the exhaust tubes increases the speed of the moving steam. The fast-moving steam then exits the exhaust tubes in equal and opposite directions because the positions of the tubes on top of the flask make them point in opposite directions. The force of the fast-moving steam as it escapes causes a reaction force in the opposite direction on the exhaust tubes. This principle is known as Newton’s third law of motion—for every action force there is an equal and opposite reaction force. The escaping steam pushes the Hero’s engine in the opposite direction like a rocket engine and it begins to rotate.

Materials

Safety Precautions

Wear chemical splash goggles and heat-resistant gloves when performing this demonstration. Steam will shoot out the ends of the exhaust pipes. The Erlenmeyer flask will get hot during this demonstration. Do not touch the Erlenmeyer flask until it has cooled for at least five minutes.

Disposal

The water may go down the drain. Save the Hero’s Engine materials for future demonstrations.

Procedure

1. Fill the Erlenmeyer flask with about 50 mL of tap water.
2. Place the rubber stopper with exhaust pipes onto the opening of the Erlenmeyer flask. Press the rubber stopper firmly into the flask, and make sure the exhaust pipes are parallel to the ground.
3. Hang the flask on a suspension hook from the swivel attached to the hanger. The flask should hang freely and have enough room to rotate.
4. Raise or lower the Hero’s engine on the support stand until the bottom of the flask is about 10 cm above a Bunsen burner. Allow the Hero’s engine to stop swinging and come to rest (see Figure 1).
5. Remove the Bunsen burner from below the flask.
6. Light the Bunsen burner and adjust the height of the flame so that the top of the flame reaches the bottom of the flask. Caution: Anyone seated or standing near the demonstration should wear chemical splash goggles.
7. Slide the Bunsen burner under the Hero’s engine and allow the water to heat and boil.
8. It will take about a minute for enough steam to generate and flow out the exhaust pipes.
9. Watch the steam shoot out the exhaust pipes as the Hero’s engine begins to rotate and pick up speed.
10. Remove the flame after the Hero’s engine has rotated for about 30 seconds, or if the Hero’s engine begins to spin too fast. Note: The Hero’s engine may not be perfectly balanced so at high speeds the Hero’s engine may begin to wobble and precess. If this occurs, remove the flame to allow the Hero’s engine to cool and slow down.
11. Allow the Hero’s engine to continue spinning until it comes to rest. Then discuss the appropriate concepts such as steam engines and Newton’s third law of motion with the class.

Student Worksheet PDF

12558_Student1.pdf

Teacher Tips

• This kit contains enough reusable materials for one Hero’s Engine.

Further Extensions

Vinyl caps are included to perform experiments with exhaust-hole size. Use a pushpin or heat a nail to poke a hole at the end of the cap. Create various sized openings with the six caps provided. Place the caps on the ends of the exhaust pipes and perform the Procedure again. How do the smaller holes affect the speed of the exiting steam and the resulting rotation of the Hero’s engine? This is a great way to discuss topics on Bernoulli’s principle. Caution: Before performing this extension in front of students, test the vinyl caps to make sure the holes are large enough. If the holes are too small, and the Hero’s engine is heated too quickly, steam may not be able to escape and the rubber stopper could pop off the Erlenmeyer flask, resulting in boiling hot water splashing out of the flask.

Answers to Questions

1. At what temperature does water turn into steam?
   

   100 °C (or 212 °F)

2. Which of Newton’s laws of motion are demonstrated with the Hero’s engine? Write the definition for each of Newton’s laws that is demonstrated with the Hero’s engine.
   

   All three laws can be demonstrated with the Hero’s engine.

   Newton’s third law—for every action force there is equal and opposite reaction force.

   Newton’s second law—a force produces acceleration (in the form of rotation for the Hero’s engine).

   Newton’s first law—law of inertia, after the heat is removed, the Hero’s engine continues to rotate.

3. If the exhaust tubes pointed in the same direction, would the Hero’s engine rotate? Why or why not?
   

   No, the Hero’s engine will not rotate because the force from the two exhaust ports would counteract each other.

4. Explain how the energy of the Hero’s engine could be made useful or converted into other forms of energy.
   

   Student answers will vary, but could include conversion to electrical energy, or mechanical motion, such as the Hero’s engine being used as wheels. Accept any reasonable response.