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Beginner's Guide to Propulsion
Balloon Rocket Car (Hard) Activity


Students will learn the concepts of Newton’s Law of Motion, friction, jet propulsion, and air resistance by designing and constructing a balloon powered rocket car.


To build a Balloon Rocket Car that can extract the most energy out of the inflated balloon and make the vehicle travel the longest distance.

Background Information

The thrust of a jet engine is similar to the thrust produced in the balloon rocket car. When the balloon is blown up the air is pushing on the balloon skin keeping it inflated. Covering the nozzle of the balloon keeps this high pressure air trapped and at this point all the forces are balanced. Once the nozzle is opened the forces inside the balloon are no longer balanced and the high pressure air wants to escapes through the nozzle which produces thrust and makes the car accelerate. Similarly, in a jet engine the air enters the engine where it is compressed and heated to create a high pressure region which is then accelerated through a nozzle to produce a thrust force. This principle follows Newton’s Second Law of Motion, Force= mass x acceleration. Otherwise stated, “if an object is acted on by an unbalanced force it will undergo an acceleration. The amount of acceleration depends on the force and the mass of the object.”

Engines must provide enough thrust to overcome the forces of drag on the aircraft as shown in the illustration below. Likewise the thrust of the balloon must be more than the forces acting on the car itself.

four forces on an aircraft

This can also follow Newton’s First Law of Motion, “an object at rest will stay at rest and an object in motion will stay motion in a straight line unless acted upon by an unbalanced force.” Therefore the forces pushing the engine and aircraft forward should be stronger than the force of the drag. Likewise the thrust of the balloon rocket car must be more than the forces acting on the car itself.

What forces are acting on the balloon rocket car? There are two main forces acting on the balloon rocket car: Friction and Air resistance. The friction force is the resistance between two objects sliding against each other. While building your car identify the places where objects will be rubbing against each other creating friction. Air resistance is also another form of friction where an object is sliding against air particles. The rocket car has the greatest air resistance when fully inflated and begins moving because there is more area that has to push past the air particles.

Design Evaluation Information

When engineers design an airplane, they often have several goals they want to meet. First, they are trying to carry a certain amount of payload. Second, they are trying to design an airplane that will fly as far as possible with that payload. Lastly, they want to build the lightest airplane to make it easier to fly. Similar to the challenge of designing an airplane, multiple goals need to be considered by each team in the design of their rocket car. The three goals for the design are to have the lowest car weight, the highest payload weight, and the farthest distance traveled.

The car designed by each team will be evaluated for their ability to meet each of these goals. Each goal or design metric is weighted as followed: car weight 0.30, payload/cargo weight 0.30, and distance traveled 0.40. From the score in each metric, a single composite score will be created using a method known as the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS). In this method, the best car is the one which is closest to the “ideal” car and farthest away from the “negative-ideal” car. These notional ideal cars are created by combining the scores for the best and worst performing cars in each category. Therefore each team is being scored against all the other teams competing. The final score for each car is then determined by calculating the Euclidean distance from the positive and negative ideal solutions and adjusting by the importance factors. The goal is to have a score that is closest to the ideal score as possible (100%).


  1. Each team is to build one rocket car using the supplies on the activity table or any material on your table.
  2. Each team can use 1-3 balloons to power their car.
  3. The car must use wheels and two must be touching the ground at all times.
  4. Each team will choose a payload weight; this weight may be changed for each run.
  5. The payload weight must stay on the car during the judged run but the weight must be removable so another team can use it.
  6. Each team will be judged three times by recording the weight of the car, weight of the payload, and the distance traveled down the track.
  7. The distance traveled will be measured from the point the car comes to a stop on the track or at the point the car crosses the out of bounds line.
  8. If your car travels into the area with the planes do not go after your car let one of the volunteers know and have them retrieve the car for you.
  9. The team score will be calculated using a formula which is described in the “Design Evaluation Information” section above.

Internet Resources

This activity is taken from the following resources below:
Home Science Tools
SAE A World in Motion (AWIM) JetToy


Related Pages:
Propulsion Activity Index
Propulsion Index


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Editor: Ashlie Mcvetta
NASA Official: Tom Benson
Last Updated: Thu, Jun 12 04:39:30 PM EDT 2014

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