# Bottle Rocket

1. Oct 9, 2006

### Loismustdie

I was wondering if there was anyway for me to calculate pressure, velocity, momentum and more on a bottle rocket made by filling a 2 liter bottle full of water and pressurized and launched. If anyone could give me a link or explain it to me that would be very much appreciated.

2. Oct 9, 2006

### daniel_i_l

Lets assume that the rocket reaches max speed in the first second and slows down after that. So if you shoot the rocket and mesure the time it takes to get to the ground then you can calculate the initial time using:
0 = V_i * t - 0.5*g*t^2
after finding V_i multiply by the mass of the rocket to get the momentum at the launch time.

3. Oct 9, 2006

### marcusl

You can calculate the initial thrust at liftoff since you know the initial air pressure and area of the nozzle. Things get more complicated right away, because the pressure drops as water is expelled and the air volume expands, so the thrust begins to drop. The thrust is lifting lighter load as this happens, so the whole system is described by a set of coupled differential equations. The most accurate solution requires computer simulation.

There's a lot you can do without equations; here's one example
http://www.ast.leeds.ac.uk/~knapp/rockets/" [Broken]

NASA has a complete self-guided course on water rockets that gives you principles, has you answer questions to help you understand, and includes animations along the way.
http://exploration.grc.nasa.gov/education/rocket/BottleRocket/index.htm" [Broken]
You'll know a lot about operation and design of bottle rockets if you "Start Your Journey" and take courses 101-103.

Do a Google search on "water rocket equations" to find more technical discussions and simulations.

NASA's has a fairly advanced one that includes interactive simulations where you enter parameters for your rocket
http://exploration.grc.nasa.gov/education/rocket/rktbot.html" [Broken]
and continue through the relevant equations by clicking on "Water Rocket--Next" on the bottom of each page.

Have fun!

Last edited by a moderator: May 2, 2017