Two-Stage Balloon Rocket

Background Information for Teachers

This section contains a quick review for teachers of the science and concepts covered in this lesson.

A classic "balloon rocket" project uses a single balloon taped to a straw and attached to a piece of string (Figure 1). When you inflate the balloon and then release its nozzle, air is expelled out the back of the balloon. According to Newton's third law of motion (for every action there is an equal and opposite reaction), this means the balloon is pushed forward along the string. Friction between the string and the straw acts opposite the direction of motion. The balloon's acceleration depends on its mass and the sum of these forces (Newton's second law, force = mass × acceleration).

This lesson expands on the traditional activity by adding a second stage to the balloon rocket, modeled after how real rocket launches work (Figure 2). It takes an enormous amount of energy (and therefore, fuel and money) to send a payload into space. In a two-stage rocket launch, the first stage lifts the rocket through the initial phase of its flight. Then, after its fuel is depleted, it breaks away and falls back to earth, while the second stage continues its flight. This saves energy because the depleted first stage is not carried all the way into orbit, allowing the second stage to travel farther with less fuel.

Additional Background Links

• Newton's Laws, The Physics Classroom
• Balloon Staging, NASA

Prep Work (5 minutes)

During the activity, your students will need to run pieces of fishing line across the room (one per group), pull them tight, and tie them securely on both ends. If necessary, rearrange furniture to accommodate this. For example, students could tie the fishing line to the legs of their desks.

Engage (10 minutes)

Show this introductory video to your students:

In this project, the class will break up into teams and build their own two-stage rockets out of balloons. Each team will test how one variable affects the distance the rocket travels. They will then combine their results to see if they can build a rocket that travels as far as possible.

Explore (100 minutes)

Optional: you can use this slideshow to walk your students through building their rockets.

You could also show them this instructional video:

1. Walk each group through building their first balloon rocket, as shown in Figure 3.
   1. Cut a piece of fishing line long enough to stretch across the room, with enough extra to tie it on both ends.
   2. Thread the fishing line through two straws, pull it tightly across the room, then secure it at both ends (for example, tie it to two heavy pieces of furniture like desks).
   3. Cut a small ring (slightly less than one inch long) from the paper towel tube.
   4. Stretch the balloons to loosen them before inflating.
   5. Inflate one balloon about 1/2 to 3/4 full. Do not inflate the balloon so much that it starts to bend significantly. Use a binder clip to pinch the balloon's nozzle shut so it doesn't deflate.
   6. Pull just the balloon's nozzle through the cardboard ring, keeping it clipped shut.
   7. Thread another balloon partially through the cardboard ring. Make sure its nozzle is facing the same direction as the first balloon. Note that the first balloon you inflated will be the second stage of your rocket, and vice versa.
   8. This is the hard part; be patient! Inflate the second balloon such that it presses up against the inside of the cardboard ring, squeezing the nozzle of the other balloon shut. You should be able to remove the binder clip from the first balloon and have it stay inflated. This may take a few tries; if you have problems getting the first balloon to stay inflated, see the Troubleshooting section.
   9. Use a binder clip to pinch the nozzle of the second balloon shut, and tape the balloons to the straws. Make sure the balloons and straws are pointed in a straight line.
   10. Pull the balloons to one end of the fishing line. Remove the binder clip from the nozzle of the second stage (it should remain pinched shut by the cardboard tube. Then, remove the binder clip from the nozzle of the first stage, and closely watch what happens. The first stage should start to deflate, pushing the rocket forward. Eventually it will deflate enough that it stops pinching the nozzle of the second stage shut against the cardboard tube, so the second stage will start to deflate, and continue traveling along the string while leaving the first stage behind. It can be difficult to get this to work perfectly on the first try. If your students run into trouble, see the Troubleshooting section.

   Can anyone explain what we just observed in terms of Newton's laws of motion?

   When we release the nozzle of the balloon, air is pushed out the back of the balloon. According to Newton's third law of motion, for every action there is an equal and opposite reaction, meaning the balloon is pushed forward along the string.

   Image Credit: Ben Finio, Science Buddies / Science Buddies

   Diagram of a two-stage balloon rocket. Two balloons are taped to straws that rest on the same piece of string. The ballons are then oriented the same direction and connected together using a ring made of cardboard. The cardboard ring fits snugly onto the first balloon and the mouth piece of the second balloon is stuffed between the ring and the first balloon in order to create a temporary seal. The seal on the second balloons mouth piece will become loose once the first balloon deflates enough to create space between it and the cardboard ring.

   
   Figure 3. Two-stage balloon rocket setup.
2. As a class, discuss what variables in the setup you could change to alter the distance the final stage of the rocket travels.

   What variables could we change in this setup that might affect how far the rocket travels?

   Possible answers include (but are not limited to): the amount of air in the balloons, where the straws are taped to the balloons, how many pieces of tape are used to attach the straws to the balloons, and where the cardboard ring is placed along the length of each stage.

3. Assign one variable to each group to test. You may need to have some groups test the same variable, or eliminate some variables, depending on the number of groups.

   If you run out of time, this is a good place to pause the activity until the next day.

4. Let each group test the effect of their variable on the distance the rocket travels (which they can measure using the tape measure). Emphasize that it is important that they keep all the other variables constant and only change their assigned variable. Each group can record their results on the student worksheet.
5. After each group has completed testing, have them report their results to the class. It may help to let students draw their data tables or graphs on posterboard so they are easier for the rest of the class to see. Did their assigned variable have an effect on the distance the rocket traveled? If so, what value of the variable resulted in the maximum distance?
6. Now, have the class combine their results to build a single "optimized" rocket. If they use the values of the variables that maximized distance for each individual group, can they build a rocket that travels even farther than any of the previous tests?

Reflect (10 minutes)

Discuss the following questions as a class.

Assess

You can use this quiz to assess student learning after the activity:

• Online quiz, assignable in any LMS
• Quiz (pdf) and answer key (pdf)

Lesson Plan Variations

• Try hanging the string from the ceiling in your classroom. Use a heavy weight or tie it to something on the floor so it stays taut. Can you get your rockets to reach the ceiling?
• Try making a balloon rocket with three or more stages. How far can you get the rocket to travel?
• Try making a balloon rocket with balloons in parallel (they deflate at the same time) instead of in series (they deflate one after the other), or some combination of both (for example, two stages each consisting of two balloons). Given a fixed number of balloons, can you find a certain configuration that makes the rocket travel the farthest?