Exploring Design Tradeoffs

An open-ended design challenge like BLAST OFF! has no single best answer. Making the right choices can be difficult, especially if testing the final product is time-consuming, expensive, or dangerous (as in rocket launching).

This activity helps students understand how they can use models and experimentation to evaluate potential choices in a manner that is much quicker and less expensive than actually building and launching the rocket.

This lesson seeks to help answer the questions

How many fins should I place on my rocket?
Should I add mass to my rocket?
How can I test a design idea in the workshop?

Activity 1: How can I determine if a design will fly straight?

The centerpiece of this lesson is what we call the "swing around" rocket flight testing procedure. This first activity introduces the testing procedure, makes theoretical connections, and highlights safety concerns.

The crucial idea is that a rocket design's in-flight stability can be tested in the classroom by securely tying a string to the rocket's center of mass and then swinging the rocket around in circles.  If the rocket flies stably during this swinging test, it will likely behave similarly off the launchpad. This process is illustrated in the photograph below.


To introduce the "swing around" testing model to students, make sure to properly demo the technique as well as explain what the students should be observing.

Ask why they can make a connection between the stability of the circular flight and the stability off the launchpad.

Drag is the force that causes wobbliness and instability.
Drag acts similarly whether the rocket is spinning around or flying up.

Arrange class into small testing groups of four or five and assign each to a "swing around" station.

Each station will allow its group to explore the influence of one parameter on the stability of rocket flight. One station might examine number of fins, fin shape, or fin placement (to reinforce the earlier lesson). At each station, there will be at least three rockets whose design features (fins, nosecones) are similar except for along the key dimension in question. At the fin shape station, for example, there may be one rocket with a semi-circular fins, one rocket with triangular fins, and a rocket with square fins.

For example, at the fin placement station there could be three rockets whose fins are all the same size and shape, but their placement varies as in the photograph below.

At each station, have students first predict the behavior of each model. Will the triangular finned rocket be more stable or less stable than circular fins?  Will the difference be noticable?  Once the group comes to a consensus with good reasoning, let the testing begin!

After predictions are well-documented, allow students to take turns "swinging around" with each model and record on a whiteboard/blackboard the stability of each model. A simple qualitiative classification scheme of "very stable", "a little wobbly" and "wild rotation" should suffice.

After completing the testing at a station, review the performance of the models and ask students to draw general conclusions if possible.  Which rocket was the most/least stable?  Were their predictions correct? Why or why not?

Keep in mind that there may not always be a clear winner (e.g. a perfectly acceptable fin shape conclusion might be that "triangular and rectangular fins are both very stable choices").

Rotate each group through all stations.  Spend about 5-10 minutes at each station.


Although the 2L bottle rockets are ultra light, a fast-moving rocket poses some safety issues.

Make sure all swinging happens in a protected area (which can be created using tables as barriers, as shown in the photo). Additionally, students and instructors should wear safety goggles at all times.

EXAMPLE STATION: How many fins should I place on my rocket?

Here is a video demo that shows the different stability behavior between rockets with 2 fins and 4 fins, respectively.

Note that our available video was only 3-4 seconds for each model, so we've repeated each clip several times to make sure the stability is observable.