10. ROCKET NOZZLES

from Come Fly With Me – Exploring Science through aviation and aerospace concepts.

SUBJECT: Science
GRADE: 7,8,9
GROUP SIZE: Small TIME; 2 45-minute periods
TYPE OF ACTIVITY: Student Investigation or Teacher Demo
TEACHING STRATEGY: Expository Guided Discovery
CONCEPTS: Thrust Velocity RPM Venturi
SKILLS: Construction Collecting and Interpreting Data

Objective: To have students investigate, using a funnel, the effects of a decreasing diameter in a nozzle on the velocity of air passing through it.

Materials: 4 – 7 oz. Styrofoam cups; 3 – 8 1/2″ x 11″ sheets of paper; scissors; masking tape; single edge razor blade or utility knife; 2 – aluminum pie plates, the throw away kind; ruler; pencil; magic marker; 1 small finishing nail; 1 T pin; 1 coat hanger; 1 stick 1/4 x 1 x 10 – a paint stirrer works well; 1 or 2 small buttons; an air source – a vacuum that can be hooked up to blow is fine.

rocket nozzles

Procedure:

Construct four funnels and two pinwheels using the above materials, as follows:

  1. Cut the bottoms out of the Styrofoam cups using the razor blade.
  2. Roll the sheets of paper into funnels; tape them securely with masking tape and insert into each of three of the cups; tape the funnels to the cups.
  3. Cut each funnel in turn to produce openings of 1 cm, 2 cm, and 4 cm respectively. The fourth cup should have an opening of 5 cm and will be used without a paper funnel.
  4. Measure two squares on the aluminum pie pans, one 6 cm x 6 cm, the other 12 cm x 12 cm. Using the ruler, draw pencil lines corner to corner on each square. Make a small hole in the center where the lines cross. Cut in from the corners, to within 1 cm of the center hole.
  5. Make small holes in the corners as shown in the diagram. Bend the corner with the hole into the center and pin the pinwheel to the stick making sure to put a small button between the pinwheel and the stick. (This will allow the wheel to turn freely). Repeat this procedure with the large pinwheel and mount it on the coat hanger. (You can tape the pin to the coat hanger).
  6. Paint one fin of each pinwheel with the magic marker to better keep track of rotations.
  7. Insert the air hose into each nozzle from the wide end and direct the airflow toward each pinwheel.
  8. Have the students observe the rotation speed of each pinwheel when acted upon by each funnel nozzle. Have the students try different distances from the pinwheels but make sure they control this variable for their final data collection.

Discuss the data collected with the students. What happened to the RPM’s of the pinwheel when the funnel openings got smaller? Were these results the same when the pinwheel size changed? What was the difference, if any, between the pinwheels? How did a change in distance from the nozzle to the pinwheel effect the RPM’s?
Note: Nozzles are thermodynamic devices for converting pressure into velocity according to Bernoulli’s equation:

pressure energy + velocity energy = constant.

Therefore as pressure increases, velocity must also increase. The smaller nozzle creates higher pressure inside the funnel and therefore higher velocity of the air flowing through it. Remember your air source is constant. Of course, this is how great velocities are achieved in a rocket engine nozzle.
Extension: Have the students compute the size ratio of the pinwheels and the nozzles from largest to smallest. (5 to 1 for the nozzles) (2 to 1 for the pinwheels). Can the students predict from their data what the next size of pinwheel or nozzle will produce in terms of RPM?

References: Ned Hannum, NASA, Cleveland, Ohio Bruce Wilms, Chesterland, Ohio

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