After this activity, students should be able to:

• Give an example of a real-world constraint on engineering projects, such as budgets, deadlines, supplies or safety.
• Explain that engineering design has several steps, that a project is not usually perfect after the first design, and that it may take several redesigns to get a project right.
• Use and fill out a budget sheet to create a specific design using imitation money.

Dreaming about how to build something is completely free. Engineers, however, are actually paid to think about how to build something. Doesn't that sound like fun? Being imaginative, therefore, is an important skill for an engineer. Building something also takes materials, building skills and space in which to build. Materials cost money, gaining skills costs money (either education or trial and error with materials), and enough available space in which to build a project usually costs money. This is why engineers strive to build things that are useful and can be sold at a profit or used to save people money in the long run.

Building a rocket and using whatever it takes for a successful launch is fun, but the real challenge comes when the resources are limited. (Note: NASA essentially had unlimited freedom during the race to the moon, as the government gave them whatever resources they needed!) This is when an engineer's creative thinking skills are very important! To reach a goal with limited resources, choices must be made balancing cost, value and performance. We call these decisions which affect our actions tradeoffs. Tradeoffs are when we give up one thing in return for another. Buying the best engine for a rocket may mean there is not enough money for proper structural materials, and subsequently, the rocket may fail. Using the strongest materials may mean the rocket is too heavy and cannot lift off. Using Fuel A may improve thrust over Fuel B, but Fuel A costs twice as much as Fuel B (not a good value). It is only worth using the more expensive fuel if the extra money means the difference between success and failure. Part of being an engineer is about deciding how best to compromise on these issues before actually building something.

When designing a real rocket, engineers have budgets, deadlines and requirements that limit what they can build and how they can build it. An engineer must work within these limits. Spacewoman Tess and Spaceman Rohan have needs, such as getting their satellites and spacecraft into space. They have a required timeline: they need to get up to space quickly in order for the satellites to be orbiting before Maya starts her journey. Also, they do not have much money with which to work. A good rocket design is a careful compromise between speed, strength, weight, cost and safety. While we may have the technology to build better rockets, we may not have enough money or time to actually build one. We also may change the way we build a rocket because of safety or environmental concerns. It may take many designs and tests before an engineer has a design that satisfies all the requirements. Today, we are going to work within a budget. We are going to be given a certain amount of money to buy materials to build our best pop-rocket. Do you think you can do it? Let's try!

Background Information

Pop Action (Review from Lesson 3)

Rockets move by expelling fuel in one direction to move in the opposite direction (Newton's third law of motion). For our pop-rockets, we will be getting the thrust force from a pressure build-up caused by a chemical reaction. When the antacid tablet is placed in water, many little bubbles of gas are created. What exactly is going on?

Antacid tablets contain aspirin, sodium bicarbonate (NaHCO₃) and citric acid (H⁺). Bicarbonate compounds react with acids to form carbon dioxide and water.

HCO_3- + H⁺  H₂O (liquid) + CO₂ (gas)

Figure 2. The basics of a pop-rocket click for copyright

In an antacid tablet, the bicarbonate and citric acid are solids and so the H+ and CO3-2 ions are not free to move, collide and react. When plopped into water, the citric acid and sodium bicarbonate dissolve, freeing the ions to react. This results in the formation of carbon dioxide gas.

The bubbles go up, instead of down, because they weigh less than water. When the bubbles get to the surface of the water, they break open. All that gas that has escaped from the bubbles pushes on the sides of the canister. Eventually, something has to give ─ the canister literally pops its top (which is really its bottom, since it is upside down in this activity). All the water and gas rush down and out, pushing the canister up and away, along with the rocket attached to it.

The rocket travels upward with a force that is equal and opposite to the downward force propelling the water, gas, and lid (Newton's third law of motion). The amount of force is directly proportional to the mass of water and gas expelled from the canister and how fast it is expelled (Newton's second law of motion).

In order to effectively use half tablets of antacid in powder form, the following method of preparation can be used:

• Use scissors to cut a two-tablet packet down the middle (between the two whole tablets).
• Carefully, tear open each of the foil antacid packets and remove both tablets. Break them in half as evenly as possible.
• Place ½ of each tablet back into its foil packet.
• Fold over the open end of one of the packets, hold it shut and use a blunt object to crush the half tablet in the packet (this takes some practice but works well). Repeat for the other tablet.
• Now you should have two crushed half tablets nicely contained in their packets and two solid half tablets set aside.

Common student problems when building rockets:

• Forgetting to tape the rocket body to the film canister.
• Failing to mount the canister with the lid down.
• Not extending the canister far enough from the paper tube to ensure the lid can be snapped on easily.

It may be easier for the students to build the rockets and then allow the teacher to launch them. The chemical reaction of the Alka Seltzer® and water sometimes happens too fast for small hands.

Remember to have the students stand back when the rockets are launching so that they do not get hit with flying rocket parts.

To extend the activity, give the students more time the first day to complete and launch their first rocket. Talk to them about redesign and why it is important for engineers to learn from their mistakes. On the second day reissue Blast-Off Bucks, and have them design and build a new rocket with the lessons they learned from day one.

Have students create a bar graph (X axis: group names and trial numbers, Y axis: height that the rocket reached) representing all of the group's rocket data.

Fisher, Diane. National Aeronautics and Space Administration. Space Place, "Build a Bubble-Powered Rocket," September 8, 2005, http://spaceplace.jpl.nasa.gov/en/kids/rocket.shtml - accessed February 24, 2006.

Speake, Vicki, Science is Here: E-SET, "What Puts the Fizz in Alka-Seltzer ™," September 2002, Iowa State University, University Extension, http://www.extension.iastate.edu/e-set/science_is_here/alkaseltzer.html - accessed February 13, 2006.

The Society for International Space Cooperation. Space Xpress, International Space Station Curriculum and Activities, "Film Canister Rockets," http://www.spacesociety.org/spaceexpress/Curriculum/film_canisters.html - accessed February 24, 2006.