This section explains what happens during the flight of a water rocket. The diagram below shows a
graphical representation of a typical water rocket flight profile. From launch to landing (in this case:
The pink line is thrust. The water is pushed out of the rocket by the expansion of the compressed air
inside it. As the air expands, the pressure drops, and thus the thrust wears off. When all the water
has been expelled, the remaining air bursts from the rocket at close to the speed of sound, and the rocket
is depressurerized in milliseconds. After this, there is no more thrust for the rest of the flight.
As thrust is produced, the rocket accelerates upwards (blue line) due to Newton's 3rd law of motion.
Even though the thrust decreases, acceleration actually increases. This is because the rocket
looses weight as the water is lost, and according to Newton's 2nd law, acceleration equals force divided
This acceleration obviously causes the rocket to pick up speed (green line). Water rockets can reach very
high speeds very fast. In this example, about 45 m/s (162 Km/h) in 0.7 seconds - and this is a pretty slow
rocket compared to so many others!
As the speed increases, the rocket climbs off the launchpad and gains altitude into the air (red line).
"Burnout" is a term from pyrotechnic rockets that actually burns their fuel. In water rockets, the
term is used for the point in time where the rocket is emptied, and thrust goes to zero. In the diagram,
this happens at 0.7 seconds.
At this point the rocket is at it's fastest upwards speed, and biggest upwards acceleration, but it has only
climbed about 10 meters. As the burnout happens, there is a discontinuity in the acceleration (blue line,
dashed). The acceleration does not go to zero, but becomes negative. This negative acceleration is the
combined effect of gravity(-1g; -9.8m/s2; 32ft/s2) and drag caused by air-resistance on the rocket. Therefore,
the rocket begins to slow down (green line).
Upwards Coast Phase
In this phase, there is only two forces acting on the rocket: drag and gravity. Gravity is constant and acts
downwards from the center of mass of the rocket. Drag is proportional to the square of the airpeed of the
rocket, and acts towards the rear from the aerodynamic center of the rocket. See the
stability section for in-depth explanations of these terms.
Apart from the square of the airspeed, drag is also proportional to the size of the rocket - namely its
cross-sectional area - and its coefficient of drag (Cd), which is a measure of the aerodynamic efficiency
of the rocket. The better the streamlining, the smaller the Cd, and the less speed it will loose.
As the rocket's vertical velocity (green line) goes to zero, the rocket reaches its highest point - the
apogee. Now, if the rocket was going perfectly vertical, it would come to a stop before falling down again.
But no rocket i have ever seen has been going vertically - it is almost impossible that some small gust of
wind, assymmetry or other factor has not caused a slight veer in the rocket's flightpath. Therefore, the
rocket will still have some forward speed (dark green line).
At this point, the rocket's speed is at its minimum, and and acceleration crosses the 1g-mark because the
drag now begins to slow the rocket's descent, not its ascent.
Downwards Coast Phase
This is very similar to the upwards coast phase: The only forces are drag and gravity. But now, drag acts
upwards, and not downwards, because the rocket is travelling downwards. As the rocket picks up speed in
its fall towards the ground, the drag-force increases, and starts offsetting the force of gravity.
(blue line goes towards zero, green line curves). If allowed to fall for enough time, these forces
would equalize and the rocket would reach its terminal velocity.
In this case, it does not reach terminal velocity, because it touches down before it has the chance.
The downwards coast phase ends when the rocket reaches the ground. In the rocket used in the diagram,
there is nothing to slow it, so it crashes into the ground at about 34m/s (122Km/h) *AUCH*
Usually some contraption - mainly a parachute - is used to slow down the rocket, so it does not
destroy itself and anything it hits during touchdown.