How to build two stage balloon rockets

Materials

• Two modeling balloons (These are the long, skinny kind used to make balloon animals, not the round kind.)
• Two straws
• Two large binder clips
• Paper towel tube
• Fishing line or string (A fishing line will generally work better because it has lower friction.)
• Scissors
• Clear or masking tape
• One other person to help set up the rocket
• Balloon pump, to make it easier to inflate the balloon (optional)
• Two sturdy pieces of furniture across the room from each other (to which you can tie your fishing line or string)—The farther, the better!
• Open space where you can set up the activity

• Prep Work

  1. Fishing line can be hard to see—make sure you warn other people that you are doing this project, so they don’t walk into the line.
  2. Thread the fishing line through the two straws.
  3. Tie the ends of the fishing line to two sturdy pieces of furniture and make sure it is pulled tight. The longer you can make the line, the better.
  4. Cut a small ring (less than one inch long) from the cardboard tube.
  5. Stretch the balloons to loosen them before inflating.
  6. 

  Procedure

  1. Inflate the first balloon about three quarters full. Have your helper pinch the balloon’s nozzle shut, but do not tie it. Optionally, you can use a binder clip to pinch the nozzle and prevent the balloon from deflating.
  2. Pull the first balloon’s nozzle through the cardboard ring and press it up against the side. Make sure you do not let the balloon deflate.
  3. Thread the second balloon partially through the cardboard ring, so its nozzle is facing the same direction as the first balloon.
  4. Carefully inflate the second balloon about three quarters full. Your goal is to inflate the balloon such that it presses up against the inside of the cardboard ring, and squeezes the nozzle of the first balloon shut. This can take some practice—be patient! It will be much easier if you have one person hold on to the cardboard ring and the first balloon while another person inflates the second balloon. If you do this perfectly, you should be able to release the nozzle of the first balloon without deflating it. If you have trouble, you can keep the first balloon’s nozzle pinched shut for now.
     
  5. Keep the nozzle of the second balloon pinched shut, either with your fingers or a binder clip.
     
  6. Tape the balloons to the drinking straws, with the balloons pointing along the fishing line. Do your best to make sure the balloons and straws are in a straight line. If the balloons are very curved and the straws are twisted at an angle, this will cause extra friction along the fishing line and slow your rocket down.
     
  7. Make a prediction.

     What do you think is going to happen when you release both nozzles of your balloons?

  8. Pull the balloons to one end of the line and release both nozzles.

     What happened? How are your two balloons similar to the two stages of a rocket? How much farther did your two-stage rocket go than it would have gone if you had had just one balloon?

  Cleanup

  Take down your fishing line as soon as you are finished with the activity; reuse other materials if possible or throw them away.