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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.

My buddy Jim's 1960s water rocket toy

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cprobertson1
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Re: My buddy Jim's 1960s water rocket toy

Post by cprobertson1 » Wed Jul 18, 2018 8:16 am

Whoops! My first rocket used the pressure from the garden hose instead of a pump - the head in my area is around 50 PSI - so by filling a bottle from the tap, it compresses the ullage air at the top as the rocket fills up until an equilibrium is reached between the water main pressure and the bottle/reservoir pressure.

In my case it will quite happily fill up 3/4 of a bottle with water - corresponding to compression of the air to around 1/4 of its original volume (which as per Boyle's law would correspond to somewhere near 4 atm (60 PSI)). At the moment I use it primarily for testing my launcher.

Should give it a go - if my results are anything to go by you should be able to have quite a bit of fun with it ;)



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anachronist
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Re: My buddy Jim's 1960s water rocket toy

Post by anachronist » Wed Jul 18, 2018 3:05 pm

If you fill a large bottle (like 1.5 liters) that full of water, then it likely won't lift off the ground. And if you start filling at ambient pressure, using only the water to build up air pressure as the air volume contracts, you'll still end up with a lot of water weight in the rocket when there's barely any pressure left to push the water out.

It might work for smaller bottles though.

Depending on pressure and rocket geometry, there is a sweet spot of fill volume, somewhere between 20%-40%, where altitude is maximized. Too little water and you don't have enough reaction mass for thrust, too much water and the rocket is too heavy to achieve a good velocity.



cprobertson1
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Re: My buddy Jim's 1960s water rocket toy

Post by cprobertson1 » Fri Jul 20, 2018 3:49 am

Oh, I dunno - I managed to get a good few feet off the ground with mine ;) Which is why I use it for testing the launcher rather than as a working rocket xD

I'm going to test something once the weather improves actually - see, I believe that if I use a filling reservoir (a second bottle that stays on the ground but gets filled as well) once could pressurise a larger volume of air relative to the volume of water in the launch vessel.

The reservoir would stay on the ground of course - one would initially fill the reservoir and let it overflow into the rocket vessel. This would of course require two pipe transits into the reservoir vessel (one for water-in, one for air>water-out).

I'm not entirely sure how the 1960's water rocket is working though - I don't think there was mention of any reservoir tanks on the ground which would leave us with only the pressure from the water main! I do wonder though...

What would happen if we used an expandable tank - say, a rubber tube - so that not only do you have the pressure of the contracted air at the top, but also the pressure from the elasticity of the vessel itself? This would obviously need to be contained in a tube or something to prevent it just ballooning all over the place - I think we still hit the problem of pressure vs weight though ;) Wish we had a picture of the original toy!



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