April 08, 2005 —
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Students at the preparations table, where they weigh their rockets, measure
the amount of water--propellant--to be used and wait for their turns at the launch
pad. The soft drink cans contain propellant for students, not for their rockets. |
As part of the undergraduate Fluid Mechanics class (AME 309) students worked
in eight groups of four or five on a project to design and analyze a water rocket.
The rocket consisted of a 20 oz. drink bottle partially filled with water and
pressurized up to 4 atmospheres with air. The students were allowed to design
appendages, i.e. fins, a nose cone, etc., to streamline and stabilize the rocket
in flight.
The project required each group to develop equations to analyze the internal
gas expansion and the motion of the liquid propellant to predict the thrust. The
thrust produced during the boost phase of the flight occurred over on a small
time scale compared to the total flight time.
The students were also required to develop the fluid mechanical equations for
the external aerodynamics of the rocket and predict the rocket’s height. The
addition of the drag calculations made the rocket equation non-linear.
The goal of the project was to optimize the amount of internal propellant and
external appendages to attain the maximum height of the vehicle in flight.
Theory met reality on a chllly day when the students flew their rockets on
the Intramural Field west of Heritage Hall.
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The basic water rocket consisting of fins glued onto plastic soft drink bottle.
This rocket sitting atop the laucher is partially filled with green-dyed water.
The black plug seals the rocket exhaust (bottle opening) as air is pumped in.
Releasing the three metal clamps launches the rocket. |
Two different rocket launchers were busy between 12:30 and 3 p.m. testing the
students' designs and analyses. Each group was able to launch their rocket multiple
times, varying the amount of propellant and the pressure. The maximum height
attained by the rockets was measured by triangulation.
Many of the rockets reached heights in excess of 120 feet. After acquiring the
required data towards the end of the test flights, several of the groups increased
the pressure to seven atmospheres and altered the propellant load to explore
the maximum altitude attainable.
The results were interesting; a maximum height of 170 feet was achieved by one
group but one rocket exploded on the launcher due to the higher pressure.
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Angela Shibata, and Richard Taras watch their rocket head skyward immediately
after Angela launched it. Phillip Prejean (on the left) watches the rocket as
Jerry Chen watches from a safe position behind the netting. In the foreground
the photo has captured the water jet consisting of water streaks and lumps left
by the rocket a few milliseconds after it has moved out of sight. |
Each group prepared a written report after launchday, explaining their design
and analysis as well as comparing their flight results with their predictions.
An interesting side note to the testing was that no one got wet (except when
the rocket exploded). Examination of the launch photographs afterward revealed
the exhaust from the rockets produced a very cohesive jet of water but virtually
no spray. The jet consisted of intermittent lumps of water interspersed between
liquid strings as seen in the accompanying photo.
A more advanced project will be to study a similar water jet and explain the
mechanism that produces the lumps and strings of water as observed.
— Ron Blackwelder
