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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.

Nozzles for WRA2 Class D Rockets

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Inq
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Joined: Tue Feb 19, 2019 1:12 am

Nozzles for WRA2 Class D Rockets

Post by Inq » Tue Feb 19, 2019 2:06 am

Side story - Many years ago a form of car racing in the United States had a fuel tank maximum size rule. Some inventive team got around this by having 2 inch fuel lines running from rear fuel tank to the front engine. This gave them a significant fuel advantage. They won, were discovered by another team and instantly disqualified. Creative cheating. :yikes:

Why the story? We’re thinking about 3D printing a nozzle. As it is screwed into the 2-liter bottle threads and below said threads, it will contain some extra volume (fuel). How much extra volume can a nozzle contain before it is creative cheating?
Let’s take several cases that are related.
1) The rules can’t be ABSOLUTELY ZERO or even the off-the-shelf Gardena quick-disconnect with its extra few milliliters would be unacceptable.
2) The rules can’t be wide-open as someone might create a half-liter chamber that they tout simply “smooths” the flow. While the rest of us say, “clearly that’s a honk’n 2 inch fuel line!”
3) As a reality check and similar issue... the standard launch system using a ½ inch PVC pipe up the center of the 2-liter bottle that is used as the air supply and a launch guide tube has a variable volume depending on where the air shutoff value is located. (A) If the valve is right next to the rocket, the air pressure inside the rocket is dropping as the rocket slides up the tube. However (B) if the tube bore is large and the volume large between the rocket and the valve, the pressure remains nearly constant until the rocket clears the tube. Yes, this is a small advantage, but someone might argue an unfair advantage.

We’re not looking to bend or break the rules. However, we really don’t even want the perception of bending the rules. As this will be performed as part of an 8th grade science class, it would be quite embarrassing to subsequently be labeled a cheat and disqualified... Not quite the example we want to present to the students.

Thanks



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WRA2
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Re: Nozzles for WRA2 Class D Rockets

Post by WRA2 » Wed Feb 20, 2019 7:50 pm

Inq wrote:
Tue Feb 19, 2019 2:06 am
Side story - Many years ago a form of car racing in the United States had a fuel tank maximum size rule. Some inventive team got around this by having 2 inch fuel lines running from rear fuel tank to the front engine. This gave them a significant fuel advantage. They won, were discovered by another team and instantly disqualified. Creative cheating. :yikes:

Why the story? We’re thinking about 3D printing a nozzle. As it is screwed into the 2-liter bottle threads and below said threads, it will contain some extra volume (fuel). How much extra volume can a nozzle contain before it is creative cheating?
Let’s take several cases that are related.
1) The rules can’t be ABSOLUTELY ZERO or even the off-the-shelf Gardena quick-disconnect with its extra few milliliters would be unacceptable.
2) The rules can’t be wide-open as someone might create a half-liter chamber that they tout simply “smooths” the flow. While the rest of us say, “clearly that’s a honk’n 2 inch fuel line!”
3) As a reality check and similar issue... the standard launch system using a ½ inch PVC pipe up the center of the 2-liter bottle that is used as the air supply and a launch guide tube has a variable volume depending on where the air shutoff value is located. (A) If the valve is right next to the rocket, the air pressure inside the rocket is dropping as the rocket slides up the tube. However (B) if the tube bore is large and the volume large between the rocket and the valve, the pressure remains nearly constant until the rocket clears the tube. Yes, this is a small advantage, but someone might argue an unfair advantage.

We’re not looking to bend or break the rules. However, we really don’t even want the perception of bending the rules. As this will be performed as part of an 8th grade science class, it would be quite embarrassing to subsequently be labeled a cheat and disqualified... Not quite the example we want to present to the students.

Thanks
Hi,

Probably the safest bet would be to share the design of your nozzle with the admin staff if you are in doubt if it would be legal to use. We can give a ruling before you make the effort to build it.

You do bring up an interesting idea that someone could "enhance" the volume of their rocket with a clever nozzle design. I think that for the most part any advantage would be negligible due to the added weight. Long "pipe" type nozzles would create lots of friction and slow the thrust. 3d printed parts by their very nature are not good "pressure vessels" due to the process that they are printed in layers. Usually a 3d printed part must be much thicker than a bottle and thus much heavier. Probably the reason no one has tried it yet.


Lisa Walker,
:WRA2: Forum Administrator. :WRA2:
:WRA2:The Water Rocket Achievement World Record Association :WRA2:

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