Air gun problem

I am trying to find a way to create a system that continuously transports Nerf foam balls up a hose to some flywheels. In the photo below you can see the basic setup with the air pump on one end blowing into a Y connector which has a funnel connected to it, then a hose and finally the flywheels at the far end. (The funnel/hopper currently does not have an agitator but that’s going to be engineering problem #2 so I am ignoring it for now).

When I drop a single ball down the funnel and then turn on the pump everything works great. When I turn on the pump first and then try to drop a ball down the funnel, there is significant blowback pushing the ball back out. If I manually push the ball down the Y piece of course it shoots out just fine. If I disconnect the outgoing hose from the Y piece and run the test there is no blowback and again the balls come out as expected. Question: how can I fix this and get a continuous feed of balls down the funnel and up to the flywheels?

Some parameters: the end goal is something like a stationary mini-gun where you just keep dumping balls into a large hopper. I know I could seal off the hopper and pressurize it similar to what Savage did for his ping pong mini-gun, but I want this to be an open system! The hopper needs to be lower than the flywheel - maybe 3 feet or so. That’s not much but it precludes a pure gravity fed approach.

Any ideas?

I started reading this and my first thought was Adam Savage’s ping pong gun. :smile:

I don’t think there is an easy fix and will probably necessitate some mechanical engineering. Seems that you would need a way to mechanically introduce the balls into the air stream. What comes to mind is something like an airsoft rifle magazine. At the bottom of the mag is a sprocket like gear that racks the individual plastic BBs from the mag hooper into a channel and forces them up into firing position.

The mag is spring driven but you could run it with a motor instead.

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Not sure on your exact setup but you could perhaps have this type of system directly feed your flywheels without the air.

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Thank you for your feedback Ned!
I agree that feeding them directly without air seems like a great idea-one less point of failure. The problem is that the balls have high friction and compress; so if one of them gets caught between the teeth of the sprocket the whole thing becomes jammed. I’ll try to think up a design to test that.

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Maybe you could just close the top of the funnel. If it is air tight the ball will not have to fight a counterflow. If you could divert a bit of the flow to go trough the funnel cover, that will help to push the ball in.


This is an impedance problem in fluid dynamics, which is not my specialty, but… To make it work as an open system, you need to balance impedance so that the air path through the hopper is the same impedance or less than the hose that follows. Your design has multiple factors contributing to higher impedance through the nozzle of the gun than through the hopper.

Some factors increasing the impedance of the hose after your hopper:

  • The design of the hose itself; that hose has ridges that impede airflow; smooth internal wall flex hoses exist; they are stiffer and won’t have as tight a bend radius, but:
  • The bend in the hose; if most or all of the bending were before the hopper it’s more likely to work.
  • The length of the hose after the hopper; the resistance is roughly linear by distance within the hose, but also:
  • The shape of the nozzle. If you have laminar flow, you won’t have a turbulent interface, but otherwise you can think of your nozzle as pushing back pretty hard on the air column.

Induce laminar flow upstream of the hopper with a flow straightener and preserve it through the system by smooth walled hose with minimal bending and length after the flow straightener.

No guarantees but it’s more likely to work!


Closing just the funnel did not make a difference. I drilled an 8mm hole at the funnel’s downslope to see if this could vent some of the excess air but oddly enough that also had no impact.

Yesterday I looked at available tubing, but once you get to the 1" reinforced stuff it becomes very stiff (and non-reinforced, non-corrugated tubing kinks easily).

I was also thinking about reversing airflow and sucking up the balls out of the hopper/bucket. The problem with that is getting the balls out of the airflow again and into a non-pressurized area. There’s a Lego vacuum build which gets large pieces into a trap but then again needs to be de-pressurized to access those:

Control the sequence of ball and pump in a manner that matches what you say works above.

That is:

  1. Put an electric valve between the pump and the Y fittings input.
  2. Put a funnel hopper above the Y fitting with a gate that lets one ball fall through its exit opening.

Using a controller implement this firing cycle:

  • Close pump valve
  • Wait
  • Open the gate and drop the ball into the Y.
  • Close the funnel gate
  • Wait
  • Open the pump valve
  • Wait
  • Close the pump valve

You could put a sensor on the output of the hose to close the loop on the firing cycle…


Quick update: I got some 1" pipe insulation foam instead of the corrugated hose and that made a huge difference. As @mcdanlj said the ridges impede airflow a lot. Now I just need to find some kind of hose that’s just as flexible without kinking.


This really sounds like a science museum project… :slight_smile:

Consider flexible schedule 40 pipe, which I expect would be available at any home improvement or hardware store. It’s not as flexible, but it’s probably flexible enough for your case.

Consider going up a size, too. That will substantially reduce the impedance. Going from 1" to 1.25" will give you half again the volume (current) with only a quarter more surface area. If you use a flow straightener (easy experiment with a bundle of straws), you can probably smoothly reduce diameter to 1" for enough nozzle to keep the shots repeatable. You might not even need to reduce it at all, either.

Note that laminar flow will have another benefit: it will make your shots more consistent. If you are pushing air into a turbulent mass that forms outside a nozzle with a poor impedance match, you will basically be “throwing a knuckle ball” with each shot; if you use a flow straightener before the hopper, you are likely to be able to shoot straighter.

As long as you are blowing air around, you could use a stream of air to agitate the nerf balls in the hopper, too…

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