Idea for better pellet speeds

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Have you considered a two stage air-gun setup, instead of attempting to create a pressure differential by applying a vacuum to a bore? Essentially take the concept of a springer, but instead of compressing air (or gas of choice) at atmospheric pressure via the piston, lightly pressurize the the air within the chamber and then further compress by the piston...
 
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JDGriz said:
@Just Zack 2 This is a probably a solution in search of a problem.
BTW helium works because the speed of sound in helium is 3 times higher than that in air.
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Most likely so. As for everything dealing with helium, it's honestly beyond my scope of understanding. I just know it makes more power (velocity) than compared to hpa. Did hear it's problematic to keep from leaking, having to do with molecular size or something of that nature...

Anyways, if you have a theory of something, build it, prove (or disprove) it's relevance in a live application, then present for others. You just might be on to something, but chatting hypotheticals online usually doesn't get anywhere. But sure is entertaining.

Wish you the best in your endeavors and don't forget to have fun.
-Zack
 
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JDGriz said:
BTW helium works because the speed of sound in helium is 3 times higher than that in air.
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I think speed of sound through the medium (helium) can be considered a secondary effect, with the primary one being helium's molecular velocity is vastly higher than the nitrogen & oxygen composition of atmospheric air.

The following from:
chem.libretexts.org

5.9: Molecular Speed Distribution

chem.libretexts.org chem.libretexts.org

molecular velocity of common gases.png


Observe that the gas with the lowest molar mass (helium) has the highest molecular speeds, while the gas with the highest molar mass (xenon) has the lowest molecular speeds. When we increase the molar mass, the most probable speed decreases (the highest point on the curve shifts to the left). In addition, the entire curve gets narrower and taller: we have a smaller range of speeds, but we have more molecules at the most probable speed.

molecular velocity of common gases2.png
 
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While this topic is interesting, its basically irrelevant ;). A pellet's speed is limited by its own design, and the sound barrier transitions. Even if you could get a pellet out of the muzzle at 4000fps without disrupting its flight, the skirt/parachute and backwards center of balance is still going to slow it down and blow it off target. If you want to push a projectile faster than the sound barrier its better to do it while in the barrel/rifling so it has some control. I agree with the idea that this is a solution in search of a problem tho :giggle:
 
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@nervoustrig The speed of sound is just that: the speeds of individual molecules at a certain temperature, due to temperature itself. A pressure wave cannot propagate faster than molecules bumping into each other, like a relay, and individual molecules can only participate in that relay at their actual speeds.
But because heat (energy) is the sum of the kinetic energies of the individual gas molecules, and KE = (mv^2)/2, a lighter molecule has much higher speed at a given temperature, compared to a heavier molecule.
 
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JDGriz said:
@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.
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Also I dont understand what you mean by the above? Are you saying pellet slingers are not capable of pushing a pellet faster than ~1125fps @Sea Level? Or are you talking about making a stable pellet at speeds of Mach1 and faster?
 
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MongooseV8 said:
While this topic is interesting, its basically irrelevant ;). A pellet's speed is limited by its own design, and the sound barrier transitions. Even if you could get a pellet out of the muzzle at 4000fps without disrupting its flight, the skirt/parachute and backwards center of balance is still going to slow it down and blow it off target. If you want to push a projectile faster than the sound barrier its better to do it while in the barrel/rifling so it has some control. I agree with the idea that this is a solution in search of a problem tho :giggle:
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This was just about the suggested combination of springer & PCP.

The not about breaking mach 1 in flight, this an efficiency problem to do with using gas expansion to push on something moving faster than the molecules of the gas.
Can a sailing ship move faster than the wind, straight downwind? This is what's all about.
 
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MongooseV8 said:
Also I dont understand what you mean by the above? Are you saying pellet slingers are not capable of pushing a pellet faster than ~1125fps @Sea Level? Or are you talking about making a stable pellet at speeds of Mach1 and faster?
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They are, but with difficulty and not very efficiently, when a room temperature gas is used. The effect of the nozzle is not completely nonexistent in current designs, it's just far from optimal.
 
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I think your topic would be better discussed and understood if you used a slug for your example vs pellets. There are dozens of pellet rifles (I have 7) that can easily break the sound barrier with a pellet, which is a virtual guarantee of poor accuracy. But what if compressed air was able to push a slug at 3000fps? or faster? Now you get into the limitations of mechanical design and propulsion choice.
 
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JDGriz said:
The speed of sound is just that: the speeds of individual molecules at a certain temperature...
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Oh yeah I get that these behaviors are inextricably related, I was just making the case that the molecular velocity of helium is the fundamental reason (first principle) why both things occur....propagates sound at a higher velocity and propels a projectile to a higher velocity.

And I must admit I got wrapped up in the technical points and lost the plot. It appeared to me initially that your interest was in seeking a way to produce higher velocities, but later I see the interest is in potentially improving efficiency. I think it's safe to say we are pretty far down the curve of diminishing returns and/or the practical limits of the usual things...valve flow rate, valve dwell, barrel length, bore friction, etc. Whereas things like pulling a vacuum, adding heat, shaping passageways, etc. have been explored along the way...much of which applies more fruitfully to springers than they do PCPs.
 
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nervoustrig said:
Oh yeah I get that these behaviors are inextricably related, I was just making the case that the molecular velocity of helium is the fundamental reason (first principle) why both things occur....propagates sound at a higher velocity and propels a projectile to a higher velocity.

And I must admit I got wrapped up in the technical points and lost the plot. It appeared to me initially that your interest was in seeking a way to produce higher velocities, but later I see the interest is in potentially improving efficiency. I think it's safe to say we are pretty far down the curve of diminishing returns and/or the practical limits of the usual things...valve flow rate, valve dwell, barrel length, bore friction, etc. Whereas things like pulling a vacuum, adding heat, shaping passageways, etc. have been explored along the way...much of which applies more fruitfully to springers than they do PCPs.
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A good nozzle _can be_ a very simple technical solution, and might be easy to implement. At least compared to helium or heat.
 
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It seems to me that evacuating the bore or displacing the air within it with helium would make a noticeable difference in velocities not only due to the change in pressure, but also by reducing the mass of the air in the bore. I ran some quick and dirty calculations of the "weight" of air in a .22 x 30" barrel and came up with around 3.7 grains, (and I definitely could be way off). That's not a whole lot, but it's definitely enough to make a measurable difference.

I doubt there's any practical way to make use of that though.
 
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nervoustrig said:
The pressurized air behind the pellet represents a significant fraction of the overall mass being accelerated during the firing cycle. Not so for the infinitesimal mass of air in front of the pellet, being at atmospheric pressure.
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That's realistic, and if it's the case then that's the same problem as in recurve bows and crossbows. A lot of the energy goes into accelerating the ends of the limbs and string to the speed of the arrow. Pulleys (as in compound bows) make the string go faster, for a lower force, to prevent the limbs from having to move too much. You end up needing a twice larger stroke length, for the same arrow energy/speed, but the efficiency is way higher.
 
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nervoustrig said:
The pressurized air behind the pellet represents a significant fraction of the overall mass being accelerated during the firing cycle. Not so for the infinitesimal mass of air in front of the pellet, being at atmospheric pressure.
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3.7 grains isn't infinitesimal. It's approximately the difference between a 14.3 grain pellet and a 18.1.

It's not enough of a difference to make evacuating the bore or flushing it with helium worthwhile, but it's still significant.
 
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