The study of rockets is an excellent way for students
to learn the basics of forces and
the response of an object to external forces.
The easiest rocket to build and fly is the
compressed air rocket.
The system uses an air pump to
launch
the rocket and the rocket coasts throughout the
rest of the
flight.
Air rockets have no engine to produce continuous
thrust, so the
resulting flight is similar to the flight of shell
from a cannon, or a bullet from a gun. This type of
flight is called ballistic flight and on this
page we present the equations that describe ballistic flight.

Strictly speaking, ballistic flight only occurs under the ideal
conditions that
weight
is the only force acting on the object.
There is no
thrust
and no
aerodynamic drag
acting on an object in ballistic flight.
Such flight conditions would occur on the
Moon, where there is no
atmosphere to produce drag. On
Earth,
air rockets generate a moderate amount of aerodynamic drag
and are not strictly ballistic. Ballistic flight is, however, a
good first approximation to the flight of an air rocket. The
flight equations
including drag are much more complex because the drag is constantly
changing throughout the flight. Drag depends on the
square of the velocity
and the velocity changes during the flight.

To simplify our analysis, we assume a perfectly vertical
launch. If the launch is inclined at some angle, we can
resolve
the initial velocity into a vertical and horizontal component.
The horizontal motion is uniform because there is no external
force in the horizontal direction. Weight is the only force
acting on the object and weight is always vertical.
Because the weight of the object is a constant, we can use the
simple form of Newton's second law to solve for the vertical motion:

-W = F = m a = m dV/dt

where W is the weight, m is the mass, V is
the velocity, t is the time, a is the acceleration,
and F is the net external force.
The positive direction is upwards, so the weight is preceded by a
negative sign. Solving the equation:

dV/dt = - W/m = -g

V = Vo - g t

where g is the gravitational acceleration which is equal to 32.2 ft/sec^2
or 9.8 m/sec^2 on the surface of the
Earth.
The value of the gravitational acceleration is different on the
Moon and
Mars.
Vo is the initial velocity leaving the
launcher. The location at any time is found by integrating the
velocity equation:

dy/dt = V = Vo - g t

y = Vo t - .5 g t^2

where y is the vertical coordinate.
With this general description of the motion of a ballistic object,
we can derive some interesting conclusions.

Notice that the flight equation includes no information about the object's
size, shape, or mass. All objects fly the same in purely ballistic flight.
This is similar to
Galileo's principle
that all objects fall at the same rate in a vacuum. If drag can be ignored,
the flight of the object depends only on the initial velocity and the
gravitational acceleration.

At the highest point
in the flight, the vertical velocity is zero. From the velocity
equation we can determine the time at which this happens:

V = 0

t = Vo / g

The time to maximum altitude varies linearly with the launch velocity.
Plugging this time into the altitude equation we obtain:

y = Vo (Vo / g) - .5 g (Vo / g)^2

y = .5 * Vo ^2 / g

The maximum altitude changes as the square of the launch velocity. Doubling
the launch velocity produces four times the maximum altitude.

Here's a Java calculator which will solve the
equations presented on this page:

Due to IT
security concerns, many users are currently experiencing problems running NASA Glenn
educational applets. There are
security settings that you can adjust that may correct
this problem.

To operate the calculator, you first select the planet using the choice button
at the top left.
For ballistic flight, select the "Ignore Drag" option
with the middle choice button. On another page
we develop the equations for
flight with drag.
You can perform the calculations in English (Imperial) or metric units.
Enter the initial velocity,
then press the red "Compute" button to compute the maximum height and
the time to maximum height.
Notice that entering a different value for the weight or the
area does not change the computed maximum height.

We provide an on-line web page that contains only this
calculator.
You can also download your own copy of the calculator for use off-line.
The program
is provided as Fltcalc.zip. You must save this file on your hard drive
and "Extract" the necessary files from Fltcalc.zip. Click on "Fltcalc.html"
to launch your browser and load the program.

Now consider the impact with the ground at the end of the flight. At
impact the altitude is zero. Using the altitude equation:

y = 0

Vo t = .5 g t^2

t = 2 Vo / g

The total flight time varies linearly with the launch velocity. The
total flight time is twice the time to reach maximum altitude. So a
ballistic shell takes as long coming down as it does going up. If we
substitute this time into the velocity equation:

V = Vo - g (2 Vo / g)

V = - Vo

The velocity at impact has the same magnitude but opposite direction
as the velocity at launch.

You can study ballistic flight characteristics by using the
RocketModeler III
simulation program.