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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.

Bottle caving in after launch

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anachronist
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Bottle caving in after launch

Post by anachronist » Tue Sep 05, 2017 12:32 am

Occasionally I've noticed, once in a while, the soda bottle pressure vessel will cave in after the water is all ejected, and continue flying with a giant dent or fold in the side of the rocket. Has anyone else seen this?



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Blenderite
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Re: Bottle caving in after launch

Post by Blenderite » Tue Feb 13, 2018 8:33 pm

I have noticed this too. The only thing that makes sense to me is that the compressed air leaves the pressure chamber so quickly that it creates a vacuum in the pressure chamber for a second afterward. Of course 2L bottles are definitely known for having weak sides.

My best guess.


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anachronist
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Re: Bottle caving in after launch

Post by anachronist » Tue Feb 13, 2018 9:10 pm

In the past, 2 liter bottles were manufactured with an indentation around the circumference, in the center of the bottle cylinder. Like this:
2L-ridgebottle.jpg
Those won't cave in. But as far as I can tell, they aren't made anymore. Or are they? Do you recall running across one of these recently? I'm gonna have to prowl the local grocery store and fondle the bottles to see if I can feel a circumferential indentation under the label. Every soda bottle I have, though, is smooth.

Any other solution I can think of to reinforce a bottle is too heavy or non-aerodynamic. The pre-manufactured indentation is lightweight, strong, and aerodynamic when covered by Scotch tape.
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