My "not so" homemade watergun

[Ben]

I've moved this thread to the Notable Homemades forum because it's the most interesting project to come here in a while.

One thing I don't think anyone mentioned is nozzle size. You should read in one of the articles I linked to about optimizing the nozzle diameter through testing. At pressures this high, the velocity of the water coming from the nozzle will be extremely high. The velocity should be about 39 m/s in the nozzle from the math I've done (likely lower). That applies for all nozzles in this calculation, which isn't completely realistic because it doesn't take into account loses in pressure at the nozzle.

Supercannon II performs at about 22 m/s for comparison. That velocity was high enough to make the 3/8 inch nozzle I tried reduce range when compared against the 1/2 inch nozzle. That indicates that the velocity was too high for the stream diameter (larger diameter streams have more mass and are affected less by drag). I would imagine that a 1/4 inch nozzle would have a very broken stream. Any nozzle smaller than that would make a very broken stream if not straight mist.

Judging by what else you've told me, the PC capacity is about 700 mL. So at any reasonable nozzle diameter, the shot time will be less than 1/3 of a second. The math I'm doing says you could get 90X on the Super Soaker scale with a 3/8 inch nozzle. I would suggest a nozzle at least 3/8 inches big.

Again, all what I've said is approximate and not completely realistic, but it should be helpful nonetheless. I'm still working on more accurate approximations.

[Silence]

Ben, are you integrating the output as a function of time, or as a constant? I'm guessing you're calling it a function.

I think 6061 said he may use a larger bore. But if it's an airsoft one, it'll still be only 6 mm wide, which is about 1/4". And 6061, how are you cutting the barrel? Make sure there are neither dents nor burrs or it'll have been pointless to use a smooth barrel - perhaps a short pistol barrel, uncut, would work. Alternatively, you could just use metal pipe nipples if you're will to settle for less length.

[Ben]

I used the CPS approximation, so it's as a constant. I'm assuming that given the short shot time this isn't too inaccurate. There's no real choice anyway because I need to redo the air pressure Bernoulli equation and the spring one would be slightly different.

[6061]

Yeah I figured that the water speed might be a little high for such a small diameter bore, but I was hoping that the increased lenght might the flow down to a more resonable level.

As far as cutting, we carefully parted it off on the lathe then chamfered the inner edge to match the mating parts, there will be almost nothing to impede the water flow.

The ball valve is 3/4npt so I will definatly be trying a few more nozzle sizes. I was also wondering what kind of recoil I can expect?


Deburring the worst part of the project


A crappy poster we had to make for our open house next week

Oh yeah, I didn't mean to rag on super soakers, we just have to try to "sell" our product.

[Ben]

I merged your two posts. In the future if you want to add a little something within a day, just use the edit button.

Aluminum should have less friction than PVC pipe. I recently did the math to figure out pressure loss to friction in a water gun with PVC pipe. It's negligible unless you have a very very large (hundreds of feet) water gun with 1/2 inch or smaller pipe. Though, the high velocity here might reduce flow more, but I doubt it would slow substantially. Since you'll probably be experimenting with different bores and barrel lengths you could figure out what's works best.

Recoil-wise you should feel a short burst. You should be getting at least 90X from the 3/8 inch nozzle, and since momentum is mass times velocity and the velocity is higher than normal for that kind of water gun, it should be pretty high. But it will be short, almost like a true gun. Water guns recoil weird because the shots have duration. This might feel more like a gun, but with a tenth of a second duration.

Don't worry about putting down the Super Soakers. At the other board they might get angry at you for that, but here we're all trying to improve upon the Super Soakers and saying what's wrong with them is part of that.

I'd write more but I have to go to class.

[Silence]

I don't see why aluminum would have less friction than PVC since there's zero velocity right at the surface anyway.

Then again, I presume there has to be a reason for friction in straight pipes.

[Drenchenator]

I don't see why aluminum would have less friction than PVC since there's zero velocity right at the surface anyway.

Then again, I presume there has to be a reason for friction in straight pipes.

As always the no-slip condition applies, so there's no velocity at a stationary surface. But friction more or less is heat caused by motion, which the rest of the flow will contribute to and the material enclosing the flow will mess with. It's there; it's just not what we normally think of. In this case it would be the head lost due to kinetic energy turning into heat energy, or something like that I think.

[akumabito]

Just out of curiosity; with a high velocity gun such as this, would there be any merit at all in using a rifled barrel instead of a nozzle? I do not know if it is at all possible to give a stream of water a 'twist'.. you think it would help in forming a more coherent stream, or would it just massively increase the drag and fling the water all over the place at the end?

[WaterWolf]

One Word:
Incredible.

I don't have access to the right machinery to work with the materials necessary, but I had done a little designing for what a water-guns might be like if they were to become a sport on the level of Airsoft and Paint-ball.

It is greatly encouraging to see someone build a homemade such as this, showing what water-guns could one day be like.

We eagerly await your results.

On your poster, it says:

Being employed to bring these to a store near you.

Are you really planning on selling these?

[Drenchenator]

Just out of curiosity; with a high velocity gun such as this, would there be any merit at all in using a rifled barrel instead of a nozzle? I do not know if it is at all possible to give a stream of water a 'twist'.. you think it would help in forming a more coherent stream, or would it just massively increase the drag and fling the water all over the place at the end?

Rifling only works when you are shooting solids. In fact, if you rifle the barrel of a water gun, all that you'll do is make the stream more turbulent. The rifling has sharp corners and and twists around the barrel--a disaster destroying what laminar flow the stream had.

[Ben]

Just out of curiosity; with a high velocity gun such as this, would there be any merit at all in using a rifled barrel instead of a nozzle? I do not know if it is at all possible to give a stream of water a 'twist'.. you think it would help in forming a more coherent stream, or would it just massively increase the drag and fling the water all over the place at the end?

What Drenchenator said is right. Read this: http://www.sscentral.org/physics/nozzles.html

People tried rifling with long screws and it reduced range. I don't even know if they were successful in getting the water to spin.

Are you really planning on selling these?

I doubt he's going to sell these because the current design obviously takes a lot of manual labor and aluminum is expensive compared against plastic. But the result is highly impressive. He's just saying that on his poster likely because it's part of the project.

[Drenchenator]

How's this going? Any more progress so far?

[6061]

There is progress. I just finished the pumping handle and the secondary handle, but it's slow going. The handle alone was just under 4 hours of work, and I have to make 4 of them. I guess that I forgot to mention that I'm actually making two of these things. I have a partner who gets the second one, but only one set of commercial components was purchased to verify that the design works before he spends three hundred bucks on the second set.

I'll post more pictures as they become available.

Oh yeah, no they are not for sale unless you have about $10,000 laying around. The cost of aluminum for just one gun is around the $1000 mark, add the components (about $300) and the hundreds of labor hours... no not for sale...yet.

[Ben]

Any more progress? It's been over a week. You logged in yesterday so I assume you've at least checked here.

[Ben]

It's been about 3 weeks since your last post... any progress?