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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.

2.5 liter bottles

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Braaainz
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2.5 liter bottles

Post by Braaainz » Mon Aug 20, 2018 8:58 pm

In the United States, there is a national chain of stores called, Dollar Tree. They carry Shasta soda in a variety of flavors, with the bottles being 2.5 liter in size. They sell for $1 USD. Nozzles appear to be regular bottle size.

Didn't see any mention of them on the forum when I searched, thought I should mention them.



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anachronist
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Re: 2.5 liter bottles

Post by anachronist » Mon Aug 20, 2018 9:41 pm

Interesting. According to this picture, they look about as tall as a regular 2L bottle but fatter. That means, the additional energy you get from another 1/2 liter of compressed air may be cancelled by the increased drag from the larger cross-sectional area.

I've always wanted to try an optimization problem to see what sort of bottle is best for maximum altitude. The current class D record uses a 1.5L bottle.

I think I'll see what I can come up with.



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anachronist
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Re: 2.5 liter bottles

Post by anachronist » Mon Aug 20, 2018 10:04 pm

As I suspected, my simulations show that the drag of the larger-diameter 2.5L bottle causes it to achieve a few meters less altitude than the smaller-diameter 2L bottle for the optimal flight. I assumed both bottles are the same height.

I compared simulated flights using the optimal values of water fill and empty weight for each bottle type, for 100psi internal pressure. My simulator takes into account all the physics possible (effect of moisture on adiabatic expansion, air resistance, choked and unchoked flow of residual air pressure, contribution of acceleration to mass flow rate, and lots of other stuff).

The advantage to the 2.5L bottle is that it can hold 20g more payload than the 2L bottle for the best possible altitude for each. The 2L bottle has an optimal payload weight typically much less than what you actually put in the rocket (altimeter, battery, timer/servo, camera, etc.), so you might actually have an advantage with the larger bottle.

Now if the 25L bottle were the same diameter as a 2L bottle, but taller, it would really perform well due to the lower drag!



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