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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

Drag Coefficient help

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Astrofox
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Drag Coefficient help

Post by Astrofox » Mon Sep 12, 2016 9:49 pm

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!



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anoymous
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Re: Drag Coefficient help

Post by anoymous » Mon Sep 19, 2016 5:46 am

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


"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former."
- Albert Einstein

Sorry for malconstructions in my sentences, I am Belgian. And I was never good at making easy sentences.

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China_WaterRocket
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Re: Drag Coefficient help

Post by China_WaterRocket » Thu Mar 02, 2017 1:37 am

More hands, less theory.


I come from China, do not understand English, your information I use the translation software to view. I like water rockets, technology is the same, I hope we can become friends.

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China_WaterRocket
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Re: Drag Coefficient help

Post by China_WaterRocket » Thu Mar 02, 2017 1:37 am

More practice, less theory.


I come from China, do not understand English, your information I use the translation software to view. I like water rockets, technology is the same, I hope we can become friends.

bshot89
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Re: Drag Coefficient help

Post by bshot89 » Sat Mar 04, 2017 11:14 am




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anachronist
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Re: Drag Coefficient help

Post by anachronist » Fri Jun 09, 2017 10:55 am

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.



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