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Bottle Rocketeers

Fluid Dynamics Flies Out of the Textbook and into the Air

April 08, 2005 —
 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.
 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.

 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