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Q & A: Air/water pressure rocket bottles

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Most recent answer: 10/22/2007
Q:
would more air pressure and less water pressure shoot a bottle faster or would more water pressure and less air pressure?
- andy
A:
I hope this is the question you are asking about.. Feel free to ask again if it isn't.

I used to play with little plastic rocket toys when I was small. They worked by putting some water in them and then pumping air as hard as you could into them with a pump that was sold along with them. You got it all pressured up, pointed it up in the sky (outside please! and don't point it at anyone!) slid a plastic catch, and the water would spray everywhere and the plastic rocket would go some distance into the air. This is most fun on a hot, summer day. So when we say "bottle", I am thinking of my old toy water rockets.

To optimize how far it would rise into the air, it is good to experiment with different amounts of water in the bottle. The pressure of the air and the water in the bottle will be the same (except for a small hydrostatic difference caused by the weight of the water). What matters is how much air and how much water there is. Water does not compress very well, so if you fill the bottle entirely up with water, it will not store very much energy when you pressurize it. The water comes out of the bottle because the air expands when the end is released, and if there is no air, then the water won't spray anywhere and it won't be fun.

On the other hand, if you don't put any water in it at all, you can squeeze lots of air in there which will all come rushing out when you release the end, but air is not very dense, and it will not push the bottle as hard as the same volume of water coming out.

So....

What you need is some mixture of the two. I would suggest to experiment!!! It's fun!

Tom

(published on 10/22/2007)

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