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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.

New to Water Rockets

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
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New to Water Rockets

Post by icyrocket » Thu Dec 06, 2018 3:33 pm

Hi, I do not have much of an idea on water rockets. A competition I've enrolled in requires me to build a simple rocket from a 2L bottle, 45 degree launch angle and 50 psi max pressure. I need suggestions on the nose cone and fin shapes and any other additions that will help the rocket travel the max distance. I am also not sure on what type of launcher to use.

Thanks in advance. x

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Re: New to Water Rockets

Post by anachronist » Thu Dec 06, 2018 6:19 pm

Your main problem is drag and weight.

Nose cone: make it rounded or parabolic. This seems non-intuitive, but a pointy nose at subsonic speed actually has more drag. Ideally your rocket shape should approximate a teardrop or symmetrical airfoil. So use the top end of a 2L bottle, cut off the nozzle, and insert a plastic ball in it such that the ball surface and the bottle surface are the same slope where they meet.

Fins. Make them from a stiff light thin material. Use three fins, not four (more fins = more drag). If you use corriflute plastic, be sure to seal the holes or you'll increase the effective surface area by a large amount.

Weight: If you're too light, air resistance will stop the rocket like putting on brakes. If you're too heavy, you have the inertia to overcome air resistance but you won't be able to accelerate to a good velocity for maximum range. There's a sweet spot of weight that depends on fill volume and initial pressure.

For example (according to my simulator using vertical height as a proxy for downrange distance), at 50psi initial pressure, if your drag coefficient is 0.18 then your maximum range occurs if your empty weight is 88 grams and your fill volume is 0.42 liters.

Worst case your drag coefficient will be 0.295 (the same as a bullet with a round nose and a cylinder body), in which case your maximum range is significantly less, but it occurs at 112 g empty weight and 0.5 L fill volume.

However, if you use that same weight and fill volume for the 0.18 drag rocket, your range reduces only a couple of meters, so there is plenty of wiggle room.
On the other hand, if you use 88 g and 0.42 L with the high drag rocket, your range is pretty drastically reduced.

Therefore, not knowing your drag coefficient, I'd err on the high side, keep the empty weight between 90-115 g and the fill volume 0.5 L.

I suggest making a rocket and test firing it at 50 psi and 0.5 liters, adding weight in the nose each time until you find the optimum weight.

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