Henry’s Place: Bottle Rockets!
How a Rocket Flies
A rocket is subjected to the forces of thrust, lift, drag, and gravity just like airplanes
(which we discussed last time).
Do you know what thrust, lift, drag and gravity are?
(This part will be review for those of you who did Henry’s Place last session.
See if you can remember the definitions of thrust, lift, drag, and gravity!)
Thrust & Lift
Thrust is the force that causes something to move in the direction it is going. For
example, for the paper airplane, thrust is forward movement and you give the paper
airplane thrust when you throw it forward.
”Lift” is exactly what it sounds like. It’s the force causes things to go up. In an
airplane, lift comes when the air below the airplane wing is pushing up harder than
the air above the wing is pushing down. This difference in pressure keeps the plane
from dropping strait to the ground right away – it allows the plane to fly.
Drag & Gravity
Objects that push a lot of air are said to have a lot of "drag," or resistance, to moving
through the air. You want to decrease drag to make things go farther in the air.
Gravity is the force that causes things to fall down. It’s actually trying to pull you and
me to the center of the Earth! Decreasing the weight of an object decreases the
strength of gravity’s pull.
For a model rocket, the aerodynamic forces (or forces that deal with moving in the
air) are generated by both the body and the fins. Also, rockets use thrust to
overcome their weight to get into the air, and use lift to stabilize and control the
direction of flight. Finally, while the effects of the forces remain about the same for
an airplane, they change dramatically during a typical rocket flight. Because of these
facts, there are four stages to a rockets flight: liftoff, powered flight, coasting flight,
Liftoff: At liftoff, the thrust of the rocket engine is greater than the weight of the
rocket, lifting the rocket into the air. In rockets, Thrust is the force that pushes the
rocket up. Unlike full scale rockets, however, our model rockets rely only on
aerodynamics for stability.
Powered flight: During powered flight, thrust dominates over the other forces
(weight and drag) propelling the rocket vertically into the air.
Coasting flight: When the engine runs out of fuel there is no more thrust force. From
here the model rocket glides up to its maximum altitude. During the upward coast,
the rocket is constantly slowing down. Eventually, a point is reached where the
rocket can go no higher. Because gravity is still acting on the rocket, the rocket will
then begin to fall back to earth. The rocket gains speed as it falls. Since the speed
increases, the drag increases and a point is soon reached where the drag is equal
and opposite to the weight. When the drag force equals the weight force, the rocket
falls at a constant speed, also called the terminal velocity.
Recovery: Usually a parachute will open which increases the drag force on the
rocket, making it slow down. The rocket then lands softly on Earth. But since our
rockets will not have parachutes, they will hit the ground harder than normal. So
make sure you don’t stand in way of a falling rocket!
Bottle Rocket Blast-Off!1
How to make your rockets:
Make a paper rocket and see how high it flies.
One sheet of 8 1/2" x 11" paper (can be printed on one side)
30 cm length of PVC pipe or any tube with a 1/2" diameter
Card stock (or any stiff paper, such as used file folders or 3" x 5"
Roll a sheet of 8 1/2" x 11" paper into a cylinder that will fit over the tube. The
paper should not be tight around the PVC pipe, but should be able to slide off
easily. Tape your paper tube so it stays rolled up, and slip it off the PVC pipe. Put
the PVC pipe aside.
You can roll your sheet of paper the long way or the short way.
With scissors, clip the end of the tube to make it pointed. Use tape to seal the
point so it’s airtight. This will be the "nose" of the rocket.
Rocket fins will help your rocket fly straight. Cut fins from a 3" x 5" card or
some other stiff paper. Play around with your wing designs. Will you add
any? How many will you have? What shape will they be? How big will they
If you have fins, tape them to the sides of the rocket, how high is up to
you. Be sure to tape both sides of the fin to the rocket.
Use a marker to write the name of your rocket on the side.
When you think you have a good design, set it up on your launcher. One
person should aim the rocket and the other person can stomp on the pad.
Then let it fly! If you want to make your rocket fly higher or straighter,
maybe think about the design of your wings or even the way you stomp the
launcher. Once everyone has gotten a design they like, you will all line up
and see whose rocket flies the highest!
1) What are the four stages of a rocket’s flight?
2) What is the force that pushes the rocket straight up?
3) What is it called when the rocket falls at a constant speed?
4) What modifications made your rocket fly better? Why?