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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.

Stability problems (still...)

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
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Sitell
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Stability problems (still...)

Post by Sitell » Tue Aug 03, 2010 3:53 pm

I have constructed and built my first water rocket but it only goes a few meter up in the air before it starts tumbling to the side (in all maximum four meters up) .

I can´t figure out why it does not fly better. I use a Gardena nozzle and guides as shown in the pictures. The bottle does not have fins because I thought the bottle/rocket could fly without them.

Do anyone have any idea what could be the problems?
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WRA2
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Re: Stability problems (still...)

Post by WRA2 » Tue Aug 03, 2010 6:22 pm

You need to add fins for stable flight especially when using a gardenia nozzle as the weight of the nozzle will contribute to the instability.

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air.command
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Re: Stability problems (still...)

Post by air.command » Tue Aug 03, 2010 6:25 pm

Hi Sitell,

A bottle by itself is usually ustable so the behaviour you are describing is very typical. Quite easy to fix though. Add some fins, and a weight on the nose and you should be good to go. This article may help shed some light on the problem:

http://www.aircommandrockets.com/home/_straight.htm

- George


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U.S. Water Rockets1
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Re: Stability problems (still...)

Post by U.S. Water Rockets1 » Sat Aug 07, 2010 10:35 pm

Sitell wrote:I have constructed and built my first water rocket but it only goes a few meter up in the air before it starts tumbling to the side (in all maximum four meters up) .

I can´t figure out why it does not fly better. I use a Gardena nozzle and guides as shown in the pictures. The bottle does not have fins because I thought the bottle/rocket could fly without them.

Do anyone have any idea what could be the problems?
You definitely need to have fins. Even our very long rockets are not stable without fins on them. A short bottle is even less stable than our long thin rockets and it will lose stable flight shortly after liftoff. Try putting some fins on the back of the rocket and you will not have any more problems.


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Sitell
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Re: Stability problems (still...)

Post by Sitell » Sun Aug 08, 2010 3:25 pm

Hello

Thanks a lot for your kind answers. I will try to add some fins. Im glad to hear that you all think that "missing" fins couses the stability problems.

/karl



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Tim Chen
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Re: Stability problems (still...)

Post by Tim Chen » Sun Aug 08, 2010 9:15 pm

Sitell wrote:Hello

Thanks a lot for your kind answers. I will try to add some fins. Im glad to hear that you all think that "missing" fins couses the stability problems.

/karl

I agree with the above. I have proved it experimentally. I tested a launcher I made by using plain bottles and they all flew up and went crazy. I didn't care because they were empty bottles.

Later on I launched my real rocket with fins and it flew perfectly.


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danb
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Re: Stability problems (still...)

Post by danb » Tue Sep 28, 2010 1:56 pm

Definately add fins and some weight at the top of the rocket another idea cut a top off another bottle to make a nose cone to make it more aerodynamic you should see some better results happy rocketing

danb



Noodleman
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Re: Stability problems (still...)

Post by Noodleman » Sat Apr 22, 2017 2:45 am

I also have the same problem. But, it tumbles only when the rocket is filled with water. Whenever i do an all air launch, it flies perfectly straight up without the need of any fins or nose cones. How does this happen?



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anoymous
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Re: Stability problems (still...)

Post by anoymous » Thu May 18, 2017 3:57 am

Just add fins and nose cone
It really will help.
I dont know why the air only launch was more stable, maybe bc it generated less thrust?


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anachronist
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Re: Stability problems (still...)

Post by anachronist » Fri May 19, 2017 4:47 pm

Noodleman wrote:I also have the same problem. But, it tumbles only when the rocket is filled with water. Whenever i do an all air launch, it flies perfectly straight up without the need of any fins or nose cones. How does this happen?
In physics terms, we are dealing with the center of mass versus the center of pressure.

Center of mass is the point where the rocket balances horizontally.

Center of pressure is the point where wind blowing on the rocket in any direction exerts an equal turning force on either side of the point.

For stability, the center of mass must be ahead of the center of pressure.

Think of an arrow with fletching (feathers) on at the tail end. The center of mass will be about the middle of the arrow. You can balance it on your finger at that point in still air. To find the center of pressure, you would hold the arrow horizontally between two fingers on a windy day, and find the point along the length to hold it where the arrow doesn't turn into the wind. Because the feathers exert a turning force, you will have to hold the arrow farther toward the back to find this balance point. The center of mass is in front of the center of pressure.

In an all-air launch, the mass of the bottle is probably about the middle of the bottle, maybe a bit toward the base if the base is a heavier material (it's usually a bit thicker there). The center of pressure may be about the same place, or even slightly behind the center of mass, so the bottle won't have much reason to turn (if the center of mass and center of pressure are in the same place, there's no reason not to turn either, so typically we see finless bottles as unstable).
Similarly, if you remove the fletching from an arrow, it might fly straight, or it might not. A feather-less arrow will fly reasonably straight if you add weight to the nose, because the body of the arrow provides a weak weathervane effect.

Now consider when the bottle is full. Almost all the weight is at the back. Suddenly the only "fin" available (the body of the rocket) is ahead of this mass. So it will try to flip around, just as if you tried to launch a rocket backward, or tried to push a boat through the water rudder-first. This, by the way, is also why it's so hard to keep a canoe on a straight path when you're the only occupant and sitting toward the back: the body of the canoe is acting like a rudder and it's in front of you, causing the canoe to want to turn.

If you add fins (this site has a good tutorial on how to create "box fins" which you can do with cardboard), then you shift the center of pressure backward. Sweeping the fins back (not just sticking straight out) puts the fin area even further back for better stability.



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