# Could a squirt gun be built to shoot like an archer fish (see details)?

Years ago I saw a small interactive exhibit at the San Diego zoo. It contained an archer fish. If you pushed a button, an object that looked vaguely like an insect hovered over the water and the archer fish squirted water at it. That image has stayed with me, and today I did a google search to learn more.

The fish really are amazing. It is not just that they can accurately target an insect up to six feet away. The really extraordinary thing is that they accelerate the stream as they emit it so that the end of the stream eventually catches up to the front just as the target is reached. The result is that the insect is walloped by a large drop of water. This gives an explanation.

Could a squirt gun be built with an adjustable mechanism to accelerate the stream the way that an archer fish does? I can’t think of any use for it, other than to show what an archer fish does, but if it could be used safely, it might make for a fun toy.

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Practical application could be potentially improving the water guns of firefighters.

ragingloli (52161)

yes it can, the key is to have a mechanism that narrows the squirt gun nozzle linearly as it is firing.

I have not done the math yet but it seems that a tube filled with water and force out by a pistion powered by a constant pressure might work that way. As the slug of water begins to leave the tube the mass still in the tube is reduced but the force on it is constant so the acceleration is higher. That would make the end of the slug move faster than the beginning of the the slug. .
This is such an interesting problem.

LuckyGuy (43843)

That is an interesting idea. If you do the math, let me know what you get. I will attempt to do if, but I always stayed away from differential equations in favor of more theoretical (and less useful) math.

You have to reduce the diameter of the nozzle continually as the water is being pushed out at a rate that increases the velocity of the stream to match the distance that the first part of the stream has traveled. The result is a big blob of water. It’s actually an easy problem.

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@ARE_you_kidding_me If the pressure is constant doesn’t that mean the smaller diameter only restricts flow.but travels at the same velocity. F=MA
Somehow we need to increase the acceleration of the later portion of the stream. Since water is virtually incompressible we need to ramp up the force or reduce the mass.

This is actually a very clever problem.

LuckyGuy (43843)

You maintain a constant volume discharge. When you do that across a narrowing diameter the only quantity left to change is velocity. Imagine a big pipe feeding the narrowing orafice. The big pipe has a plunger that pushes the volume from the big pipe into the orafice at a constant rate. If the orafice gets smaller velocity must increase since water is incompressible.

@ARE_you_kidding_me So your idea is to push a constant volume. A powerful piston pushing through a smaller orifice. That would work.
I was thinking of a different approach. Determine the kinetic energy needed to have all the mass delivered at the target at the same time. That tells you the size of fuel charge you need. (Double it for efficiency loss)
Now slow the energy charge down so it fully expends itself in 6 times the total flight time. Then allow the pressure to accelerate whatever load is in the tube.
At the beginning of the shot there is a lot of mass so the acceleration rate is slower, Toward the end of the shot the mass is lower so the acceleration rate is greater. Size the tube so the beginning and end of the shot arrive at the same time.

LuckyGuy (43843)

@ARE_you_kidding_me , That seems very doable. I wonder if the archer fish does something similar, maybe by changing the size of its mouth openiing. The fish, not being an engineer, must have some relatively simple mechanism.

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