Idea for better pellet speeds

[JDGriz]

It is reasonable to assume that the limiting factor for pellet speeds in air guns is the speed of sound in the barrel. If you just let compressed air expand behind the pellet, then the pellet would only accelerate until its speed matches the speed of the expanding air, and the speed of the expanding air can't be faster than the speed of sound.
But, if you impart velocity onto the expanding air itself, then this might be able to compensate for this effect.
A bit like what they are doing in this video:

So, creating a nozzle behind the pellet might actually make it go faster, even if it would be a bit of a choke point. This could actually be quite easy to do on most air guns, by incorporating a choke into the probe (to create a nozzle with the barrel walls), assuming much less air resistance all the way through the port, valve and to the air tank (i.e. higher cross-sectional area).

like this:

(the second choke version is intended to be like the ports on some springers, which have stepped changes in diameter to induce eddies and reduce air resistance)

 

[Pale_Rider]

This sounds like an idea you come up with after smoking some good stuff

Seriously though, this would be annoying having to replace the seals for every shot to create the vacuum effect.

 

[JDGriz]

@Pale_Rider It should work without the vacuum, if it isn't a ping-pong ball and if it isn't just several bar.

 

[Pale_Rider]

I fail to see how this would have any effect seeing that the vacuum is necessary to remove the air in the tube making ping pong ball go faster. Without the vacuum you’re just adding a choke to to an airway no different than a transfer port restriction which will SLOW the pellet down.

 

[JDGriz]

I fail to see how this would have any effect seeing that the vacuum is necessary to remove the air in the tube making ping pong ball go faster. Without the vacuum you’re just adding a choke to to an airway no different than a transfer port restriction which will SLOW the pellet down.

But maybe you do fail to see?

 

[Pale_Rider]

Yes that’s what I said

Please enlighten me

 

[JDGriz]

If you shot any existing airgun in a vacuum, everything else being the same, you are still not going to reach pellet speeds above mach one. The impromptu nozzle created by "a transfer port which is too small" is directing accelerated high pressure air towards the barrel wall, and not towards the back of a pellet which is speeding away.

 

[Pale_Rider]

The nozzle is not needed to create the effect you see in the video. It perhaps enhances the pressure when the seal is popped letting in atmospheric pressure into the vacuum. You don’t need the nozzle or any added air source to make a ping pong ball go really fast. You just need a… vacuum

 

[JDGriz]

@Pale_Rider "Really fast" isn't the objective here - we know that this is possible. The objective is "above Mach 1", or "more easily to Mach 1".

I do, of course, assume, that the people in the first video did try without a nozzle first, then added a nozzle when they didn't reach Mach 1 despite the vacuum.

 

[Pale_Rider]

@Pale_Rider "Really fast" isn't the objective here - we know that this is possible. The objective is "above Mach 1", or "more easily to Mach 1".

I do, of course, assume, that the people in the first video did try without a nozzle first, then added a nozzle when they didn't reach Mach 1 despite the vacuum.

I see why you’re thinking a nozzle would work. But it’s only working in the video because it’s enhancing the atmospheric pressure when the seal is popped. Without creating the effect of atmospheric pressure meeting no pressure you’re back at square one. Also, with out the vacuum you still have air in front of the pellet that needs to be imparted. The vacuum is working two fold in the video you posted.

 

[jarmstrong]

it is the vacuum that creates a greater pressure differential,,, we use it in steam turbines to make more power

the Question is, how you gonna hold a vacuum in ure barrel, for practical use?

 

[Peashooter]

It is pretty cool to see atmospheric pressure, totally crush a oil drum just cuz you cool hot air inside it.

It get even more OMG when its a tanker truck or a rail tank car that get squashed as someone forgot to open a valve.

I cant be the only one contemplating making a vacuum golf ball cannon ?

 

[JDGriz]

@Pale_Rider Mnno. Vacuum is needed to lighten up the projectile. Without vacuum, you'd need accelerate the ping-pong ball _and_ the column of air which is sitting in front of it, plus overcome the friction of that 2 meter gas plug with the barrel walls. None of this is a problem in air guns, relative to the weight of the projectile. The other thing is that vacuum does is increase the pressure differential (in front of and behind the projectile) by exactly 1 bar, which is insignificant - you can just add one more bar to the tank.

That they likely didn't reach Mach 1, with vacuum, and without the nozzle, is what is most suggestive for me.

 

[Pale_Rider]

@Pale_Rider Mnno. Vacuum is needed to lighten up the projectile. Without vacuum, you'd need accelerate the ping-pong ball _and_ the column of air which is sitting in front of it, plus overcome the friction of that 2 meter gas plug with the barrel walls. None of this is a problem in air guns, relative to the weight of the projectile. The other thing is that vacuum does is increase the pressure differential (in front of and behind the projectile) by exactly 1 bar, which is insignificant - you can just add one more bar to the tank.

That they likely didn't reach Mach 1, with vacuum, and without the nozzle, is what is most suggestive for me.

The vacuum doesn’t “increase” pressure differential; it CREATES it. Without the vacuum you have a ping pong ball just sitting in the tube. I think you’re missing the point of the video. Without the vacuum you have a regular airgun lol. What’s going to make it go faster? The nozzle will only limit it considering the air pressure behind the pellet is limited and not constant like atmospheric pressure rushing into a vacuumed barrel.

 

[nervoustrig]

I suspect complexity renders it impractical for anything but a novelty or a bench demonstration.

Not only that, the vacuum is effective with a ping pong ball because its diameter is large and its weight is very low. A perfect vacuum only produces a 14.5psi differential. To a dense projectile like a lead pellet, it is an insignificant amount...akin to changing a regulator from 1500psi to 1515psi.

 

[beerthief]

very interesting , can't wait to see what the rigs look like @ EFT and EBR

 

[Just Zack 2]

Just use helium...

As I understand it, the problem isn't breaking past the speed of sound (mach1), it's getting velocities up well past mach2 to be useful. Also barrel twist rates and projectile BC's... It all gets pretty deep.

 

[JDGriz]

@nervoustrig vacuum isn't needed for for an airgun application. All this could need is just a better AirForce/Evanix Rex valve, that doesn't have a T-junction channel on the tank side (or a much wider T-channel). And a nozzle on the inside, where it connects to the back of the barrel (for example).

 

[JDGriz]

@Just Zack 2

Well, sure, the point of this would be more like increasing the shot count per fill, or shorter barrels; no one really needs more than Mach 1.

The ultimate goal is to increase efficiency, which drops towards Mach 1.

 

[Pale_Rider]

I suspect complexity renders it impractical for anything but a novelty or a bench demonstration.

Not only that, the vacuum is effective with a ping pong ball because its diameter is large and its weight is very low. A perfect vacuum only produces a 14.5psi differential. To a dense projectile like a lead pellet, it is an insignificant amount...akin to changing a regulator from 1500psi to 1515psi.

thanks for the technical explanation. I was waiting for someone smarter than me to explain. I got hung up on the video used in the OP to illustrate his point. I don’t know much about physics but my basic knowledge tells me adding a nozzle used in the video wouldn’t help increase pellet speed.