Science Buddies: "Ask an Expert"

i have conducted my experiment and I am now writing a conclusion for the information gathered. When I tested my bottle rockets with various amounts of water, I was trying to find out if a greater amount of water would cause the bottle rocket to go higher. However, after conducting the expeiment more water did cause the bottle rocket to go higher, but I am unaware of the reason for it. The water is acting as a propellent, but it isn't a feul because it is not turing into steam, so I don't know why th water is causing the bottle rockets to go higher.

Megan,
Congrats on having finished your experiment. Can you tell us a little more about how the rockets were constructed? Do you have a link to a page that shows the procedure?
My guess is that the additional water propelled the rockets for a longer amount of time, but it's hard to say for sure without knowing the details.
Chris

Thank you for responding to my question.

*There is a link to the procedure i used for my rocket is: http://quest.nasa.gov/space/teachers/rockets/act11.html and I used the bottle rocket construction worksheet provided by the site.
However, I made some adjustments to the procedure by fillinf the tip of the nose cone with hot glue rather that clay and I used three fins.


*For my bottle rocket launcher I used the site: http://www.delta.edu/slime/images/2Launcher.pdf

Megan

Maybe it will help to think about it this way: the water in each rocket could be payload (passive weight) or propellant. If you add a small volume of pressurized air, only a small volume of water will be ejected. The remaining water in the rocket that was not ejected by pressurized air would then become payload. If you added more water for the same amount of air, you would increase the payload without increasing the propellant, and the rocket would become less efficient.

On the other hand, if you add enough pressurized air that *all* of the water is ejected, then all the water will act as propellant. The mass fraction of the rocket (mass of propellant/ total mass) will be close to 1, and the rocket will be very efficient. The more water you add, the more propellant you will have, and the higher the rocket will fly.

When you pumped air into the rocket, did you try to pump an equal volume into each rocket, or did you pump the rocket up to a specified pressure? This could influence the results also.

When we pumped the air into the bottle we tried to add an equal amount of pressure to each volumn

Hi Megan,
have you figured it out? Your rocket gains its velocity from ejecting mass (water) out the back, conservation of momentum provides thrust for your rocket to fly, more water is more material for propulsion. Of course there is a balance you will find, mass vs. air pressure to height.

This site has a nice explanation of the effect:

http://hyperphysics.phy-astr.gsu.edu/hb ... et.html#c2

Jim