Flying model rockets is a relatively
and inexpensive way for students
to learn the basics of forces and the
of a vehicle to external forces.
A model rocket is subjected to
four forces in flight;
thrust, and the
lift and drag.
The relative magnitude and direction of the forces
determines the flight trajectory of the rocket.
On this page we show the events in the flight of a
Water rockets are among the
simplest type of rocket that
a student encounters. The body of the rocket is an
empty, plastic, two-liter soda bottle. Cardboard or balsa
fins are attached to the bottom of the bottle for
and a fairing and nose cone are added to the top as a
Prior to launch, the body of the rocket is filled with water
to some desired amount, typically about 40% of the volume.
The rocket is then mounted on a launch tube which is
quite similar to that used by a
compressed air rocket.
Air is pumped into the bottle rocket to pressurize
the bottle and thrust is generated
when the water is expelled from the rocket through the
nozzle at the bottom.
full scale rocket,
the weight of the bottle rocket is constantly changing
because the water is leaving the rocket.
As the water leaves the rocket, the volume occupied by the
pressurized air increases. The increasing air volume
the pressure of the air, which decreases the mass flow
rate of water through the nozzle, and decreases the
amount of thrust being produced.
thrust are constantly changing during the powered portion
of the flight.
When all of the water has been expelled, there may be a difference
in pressure between the air inside the bottle and the external,
free stream pressure. The difference in pressure produces an
additional small amount of thrust as the pressure inside the bottle
decreases to ambient pressure. When the pressures equalize, there
is no longer any thrust produced by the rocket, and the rocket
The remainder of the flight is quite similar to the flight of a
ballistic shell, or a bullet fired from a gun,
except that aerodynamic drag
alters the flight trajectory.
The vehicle slows down under the action of the weight and drag
and eventually reaches some
maximum altitude which you can
using some simple length and angle measurements and
The rocket then begins to
to earth under the power of gravity.
Bottle rockets may include a recovery system like
a parachute, or a simple detachment of the payload section,
as shown in the figure.
After recovering the rocket, you can fly again.
You can study the flight characteristics of a water rocket by
On the graphic, we show the flight path as a large arc through the sky.
Ideally, the flight path would be straight up and down; this provides the
largest maximum altitude. But water rockets often turn into
the wind during flight because of an effect called
weather cocking. The effect is the result
of aerodynamic forces on the rocket and cause the maximum altitude to be
than the optimum.
The parabolic arc trajectory also occurs if the launch platform is
tilted and the rocket is launched at an angle from the vertical.
Types of Rockets:
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