Water Rocket Forum

So, as I return to my ventures in bottle rocketry, I must ask the question:

What are the drag coefficients of certain configurations of bottle rockets?

More specifically, what is the (estimated) drag coefficient of a bottle rocket built for backslider recovery? I need to know it so that I may be able to find the optimum amount of water for my rocket.

My bottle rocket is an extended backslider that has a 4-ft fluorescent light cover with the base of a (tiny) Easter Egg on top I cannot give much more info here, as it is a mostly secret project...

Oh, and it is a 2-liter rocket, that uses the top end of a bottle as the adapter to the FLC-tube. You would know, a basic backslider design.

And of course, I would love it if there was a table/spreadsheet/pdf/word-doc, that had a table of values and images to correspond to those values.

Thank You!

I've searched for this , too. But i never found a good one.
I'm startingvto think the only way tofind the drag coefficient is by doing experiments

More hands, less theory.

More practice, less theory.

i think this will help. https://en.wikipedia.org/wiki/Drag_coefficient

Astrofox wrote:What are the drag coefficients of certain configurations of bottle rockets?

More specifically, what is the (estimated) drag coefficient of a bottle rocket built for backslider recovery? I need to know it so that I may be able to find the optimum amount of water for my rocket.

The only practical way to find drag coefficient is by experiment. Less practical, requiring a steep learning curve, is by using a computer fluid dynamics (CFD) simulation. There are some free ones available, like EasyCFD (which has a free trial), and the free version of simFlow, which requires the open-source CFD engine called OpenFoam.

Experimentally, if you have a good water rocket simulator, and you've instrumented your rocket to gather flight data, you can run a test flight of the rocket and then compare the flight profile to the simulator's results, and adjust the drag coefficient in the simulator until the flight profiles match. You could even do this with a peak altimeter, just adjust the drag coefficient until the peak altitude matches. If the simulator is reasonably good, the resulting drag coefficient will be close to the actual drag coefficient of your rocket.

http://www.seeds2lrn.com/rocketSoftware.html is a pretty good one, but I've written my own that accounts for a bit of additional physics.