Lesson 4 Grades 3–5

Fins and balance: why rockets fly straight

Timeabout 45 minutes
WorksheetE4 Swing test
Rocket Builderon the projector
Optional instructor demoa 3D-printed rocket (no motor)

On this page

Standards
I can…
Words to know
Materials
Steps
Check for understanding
Teacher notes

Standards

Code	In plain words
3-5-ETS1-3	Run fair tests to find where a model fails and what to improve.
3-PS2-1	Test how balanced and unbalanced pushes change the way something moves.

Practice: planning and carrying out investigations. Crosscutting concept: structure and function (the shape of the fins does a job).

I can…

I can find a rocket's balance point.
I can test a rocket with no fins, with fins, and with weight in the nose, and say which flies straight.
I can explain the rule: the balance point goes in front of the push point.

Words to know

Fins the flat wings at the back of a rocket. They work like the feathers on an arrow.
Balance point where the rocket balances on your finger. Rocket Builder shows it as a blue dot.
Push point where the air pushes on the rocket. Rocket Builder shows it as a red dot.
Stable flies straight, nose first.

Materials

For each group:

A paper-towel tube, a cardstock cone for a nose, cardstock for 3 fins (about 6 cm × 5 cm), tape
A walnut-sized lump of modeling clay
About 1.5 m of string
Worksheet E4

For the class: Rocket Builder on the projector. Optional: a printed Rocket Builder rocket (no motor) for the instructor to show.

45 minutes · learn plan hands-on wrap up

1Hook5 min

Show an arrow, a dart or a badminton shuttle (or a picture). Ask: Why does it fly point-first? What would happen without the feathers?

2Learn10 min

Read aloud
Fins are a rocket's feathers. They keep the nose pointing forward.

Every rocket has a balance point, where it balances on your finger. And it has a push point, where the air pushes on it. Here's the rule: the balance point must be in front of the push point. Then if the rocket tips, the air pushes the tail back in line, like a weathervane.

Fins at the back move the push point back. Clay in the nose moves the balance point forward. Both help.

Find the balance point on a finger. The blue dot (balance point) must be in front of the red dot (push point).

Why the rule works: if the push point is behind the balance point, the air swings the tail back in line, like a weathervane.

On the projector: build a rocket in Rocket Builder. Find the blue dot (balance point) and the red dot (push point). Change the fin shape and build again. Watch the red dot move. If the score is green, it's stable.

3Do: the swing test20 min

The swing test: no fins tumbles; with fins the nose leads; clay in the nose flies straighter.

Go outside or to a big open room. Nobody stands within 3 m of the swinger.

The swing test: string tied at the balance point, swing it in a circle, watch the nose.

The four test rockets.

Build the rocket with no fins. Balance it on a finger to find the balance point. Tie the string there and tape it.
Swing it in a circle beside you. Does the nose lead? Tick straight, wobbly or tumbles on E4.
Tape on 3 fins. Find the new balance point, move the string there, and swing again.
Press clay inside the nose. Find the balance point again and swing.
Bonus: move the clay to the tail. What happens?

4Wrap up10 min

Which version flew straight? Which one failed?
Why did clay in the tail make it worse? (It pulled the balance point back, behind the push point.)

Check for understanding

What do fins do for a rocket?
Which dot must be in front: the blue balance point or the red push point?
Your rocket tumbles. Name one thing you could change.

Answer key

They keep it flying straight, nose first (like the feathers on an arrow).
The blue balance point.
Add fins or make them bigger; add a little clay to the nose.

Teacher notes

What to expect: no fins usually tumbles; fins help; fins plus nose clay flies straight; tail clay wobbles or tumbles.
Grade 3: test only "no fins" and "fins plus nose clay". Grade 5: measure the balance point from the nose tip each time and write it down.
Swing safety: a short string (about 1 m), a clear space, one swinger at a time.
Optional instructor demo, a 3D-printed rocket: show a printed Rocket Builder rocket with no motor in it. Balance it on one finger to find its balance point, point at the fins, and compare it with the blue and red dots on the screen. Students can hold and look at it. Motors are rated 12+ and only adults handle them, so the demo never includes a motor or igniter.
Further resources: Apogee Components' free Rocketry Reservoir (apogeerockets.com/downloads/PDFs/Rocketry_Education_Pack.pdf) has printable pages on the parts of a rocket and on centre of gravity and centre of pressure that you can project or hand out.
Clay in the nose, on screen: in Rocket Builder's Advanced mode, Nose weight (clay) adds grams to the nose. Add some, build again, and the class watches the blue dot move forward and the score go up: the same thing their clay did.