Build this CD Rocket Engine Car and learn about how rockets work!
All you need are a few discarded office supplies.

Step 1: Parts List

1 piece of Styrofoam (5by 12 inches) or shoebox top

4 discarded CDs

I (9 inch) balloon

Hanging folder or light weight cardboard

Scrap paper


Step 2: Cut Out the Wheel Assembly

Using material from a used hanging file folder or cardboard from a cereal box, cut out two pattern pieces as shown on the picture. Do not cut where the dashed lines are indicated. Fold up both ends on the dashed lines and fold in where indicated by the other dashed lines. Tape the second fold so that it looks like the picture. Tape the pattern assemblies to the Styrofoam.

Step 3: Make the Axels and Attach

Using scrap 8 by 11 inch paper, fold it in half and cut it into two pieces. Using a round pencil, roll one piece tightly around the pencil. While holding the paper tube (axle), insert each end into two separate CD holes. If the tube will not fit into the CD hole, rewind it again. Once both ends are inserted in the CDs, release the paper tube. It will then unwind and fit snugly in each CD hole. With a small piece of tape, tape both ends of the paper axle.

Step 4: Install the Wheels

Remove the CDs from the paper axle and then wrap another longer piece of tape (about 3”) around each axle 1¼ inches from each end. Reinsert one CD until it stops at the second tape. Cut the end of the tube axle into four strips and fold these flat against the CD as shown in the picture. Tape the strips to the CD. Install the paper axle with one CD attached into the wheel axle support assembly. Then attach the second CD to the paper axle and secure the same as the first CD. Tape the completed assembly to the Styrofoam as shown in the second picture.

Step 5: Make and Install the Balloon Tube

Wrap another half sheet of paper around a round pencil and tape both ends.

Insert the balloon on one end of the tube and tape around the balloon to secure it. Using clear shipping tape, tape the balloon tube (nozzle) to the top side of the Styrofoam.

Step 6: To Do and Notice

Blow into the nozzle to add air (fuel) to the rocket engine (balloon). After you finish fueling, quickly hold the tip of the balloon tight to keep the fuel in. Place the Rocket Car on a hard flat surface and let go of the rocket engine (balloon). The Rocket Car should blast off! If the Rocket Car will not move it may be too heavy or some of the wheels are stuck (causing friction) or your may need more fuel. Be careful not to add too much fuel or your rocket engine may blow up! The rocket engine balloons are cheap so don’t worry if this happens. How fast and how far the Rocket Car moves depends on how much you have blown up the rocket engine balloon, how heavy the car is, and friction between the wheels and surface. The Rocket Car wheels are very thin so you won’t have much friction caused by the wheels. If you are using the Rocket Car on a carpet, it won’t move as easily because of friction from the carpet.

Step 7: The Science Behind the Activity

The Science behind the activity

Rocket engines are, on the one hand, so simple that you can build and fly your own model rockets very inexpensively. On the other hand, rocket engines (and their fuel systems) are so complicated that only three countries have actually ever put people in orbit. In this article, we will look at rocket engines to understand how they work, as well as to understand some of the complexity surrounding them.

When most people think about motors or engines, they think about rotation. For example, a reciprocating gasoline engine in a car produces rotational energy to drive the wheels. An electric motor produces rotational energy to drive a fan or spin a disk. A steam engine is used to do the same thing, as is a steam turbine and most gas turbines.

Rocket engines are fundamentally different. Rocket engines are reaction engines. The basic principle driving a rocket engine is the famous Newtonian principle that "to every action there is an equal and opposite reaction." A rocket engine is throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result.

This concept of "throwing mass and benefiting from the reaction" can be hard to grasp at first, because that does not seem to be what is happening. Rocket engines seem to be about flames and noise and pressure, not "throwing things." Let's look at a few examples to get a better picture of reality:

  • If you have ever shot a shotgun, especially a big 12-gauge shotgun, then you know that it has a lot of "kick." That is, when you shoot the gun, it "kicks" your shoulder back with a great deal of force. That kick is a reaction. A shotgun is shooting about an ounce of metal in one direction at about 700 miles per hour, and your shoulder gets hit with the reaction. If you were wearing roller skates or standing on a skateboard when you shot the gun, then the gun would be acting like a rocket engine and you would react by rolling in the opposite direction.
  • If you have ever seen a big fire hose spraying water, you may have noticed that it takes a lot of strength to hold the hose (sometimes you will see two or three firefighters holding the hose). The hose is acting like a rocket engine. The hose is throwing water in one direction, and the firefighters are using their strength and weight to counteract the reaction. If they were to let go of the hose, it would thrash around with tremendous force. If the firefighters were all standing on skateboards, the hose would propel them backward at great speed!
  • When you blow up a balloon and let it go so that it flies all over the room before running out of air, you have created a rocket engine. In this case, what are being thrown are the air molecules inside the balloon. Many people believe that air molecules don't weigh anything, but they do. When you throw them out the nozzle of a balloon, the rest of the balloon reacts in the opposite direction.\

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