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.
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.
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%).
- Each team is to build one rocket car using the supplies on the activity table or
any material on your table.
- Each team can use 1-3 balloons to power their car.
- The car must use wheels and two must be touching the ground at all times.
- Each team will choose a payload weight; this weight may be changed for each run.
- The payload weight must stay on the car during the judged run but the weight must
be removable so another team can use it.
- 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.
- 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.
- 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.
- The team score will be calculated using a formula which is described in the
“Design Evaluation Information” section above.
This activity is taken from the following resources below:
Home Science Tools
SAE A World in Motion (AWIM) JetToy