Beginner's Guide to Rockets
Gas Pressure

Answers

1. Before you begin: Observation of pop can at room temperature.
   Students may measure the can and note that it is a normal "empty" pop can.
   
   
2. Observation of pop can with boiling water.
   Students should note the moisture leaving the can and that its size and shape remain unchanged.
   Also, the air inside the can has increased in temperature.
   
   
3. Observation of pop can after inverting it in ice water.
   The can implodes very quickly.
   Students' descriptions will vary but stress for detail.
   Allow discussion within the lab group.
   
   
4.  Sketch diagrams of the pop can as it appeared: at room temp , with boiling water, immersed in ice water.
   At room temperature and with boiling water the force arrows are in balance.
   When immersed in ice water the atmospheric air pressure is much larger than the air pressure inside the can.
   
   
5. Explain how air pressure changed the pop can during the procedure.
   Both the can at room temperature and the can with boiling water were at equilibrium, forces in balance. Although the can with the boiling water had fewer gas molecules per unit volume, it did not implode because those heated molecules were moving much faster and equalizing the force of atmospheric pressure. Once the can was put in the cold water, the heated gas molecules lost kinetic energy to the ice water and slowed down. Now unable to equal the force of atmospheric pressure the can implodes. NOTE: Many students will say that the can was "sucked in." Use this as an opportunity to inform them that "Science doesn't suck!" You may even make them write that here.
   Is the can empty?
   No, it contains gas molecules (mostly nitrogen and oxygen and water vapor).
   
   
6. 
   A. What happens to gas pressure as exhaust travels through a rocket nozzle?
   There is an overall decrease in gas pressure. The energy present in the hot exhaust in the combustion chamber is converted to high velocity at the nozzle exit. The resulting exhaust is cooled and the pressure decreases.
   
   B. How does molecular motion change through the nozzle?
   The high random molecular motion (temperature and pressure) is converted to lower random motion (lower temperature and pressure) plus higher ordered motion (high exit velocity).
   
   C. What is one function of a rocket nozzle?
   To convert random molecular motion into thrust.
   
   
7. After studying gas pressure and rocket engines, list three questions that you and your lab partners now have about this topic.
   Answers will vary.
   The instructor may use this as a springboard to an extension to this lesson. The instructor may opt to explore topic(s) as a class or to divide the class into small groups that will report their findings to the class.
   A good place to start is the Beginner's Guide to Rockets,particularly the section on nozzles and thrust. The great change in pressure through a rocket nozzle presents challenging design problems for the engineer.