Consistent velocity: Combustion or Pneumatic?

[Solar]

I remember reading somewhere that pneumatic has a more consistent shot. Is this true? Anyone know why or why not?

[Technician1002]

Metering fuel is hit and miss for many combustion cannons. The quality of the air replenishment is a factor. With proper venting and metered fuel, the combustion cannons are quite predictable.

Air cannons only variable that may be difficult to predict is if the valve is a little sticky and opens slow. Other than that, air cannons are about as predictable as releasing a spring.

[psycix]

Very good explanation!

I'd like to add:
The combustion process is very sensitive to factors like temperature and humidity, pneumatics are less dependent on that.

Note that when metering fuel, you have to add just 4,2% of the chamber volume, which is very hard to get precise as this is a very small amount.
When simply filling a chamber up with a certain pressure, you can just keep pumping till the chamber reaches the desired pressure on the gage and you are all set. Much easier to fill precisely if you've got a good gage.

[jackssmirkingrevenge]

I would agree, pneumatics are a lot more consistent, assuming you can reliably fill to the same pressure and your valve is gives mechanically repeatable opening time. Using temperature sensitive gasses like CO₂ though might not be as reliable.

[Ragnarok]

This is something I've been thinking about recently, as I'm currently looking interestedly into velocity consistency with respect to using it for more consistent trajectories - and thus accuracy.

The observation is (generally) true, because pneumatics have far fewer variables affecting their performance.

I'll start with a brief note on the issue of variable projectile masses, which affect both varieties in exactly the same way.
Obviously, a lower mass projectile has a lesser inertia, and will accelerate more freely under a net force, and higher masses vice versa. Lack of consistency in projectile mass is going to affect either type's velocity - you'll often find that serious air gunners will weigh their pellets into batches of the same masses to ensure greater consistency in velocity and trajectory.

Moving on to the issues that affect each variety differently, a combustion's velocity can be affected by many things, such as lack of homogenisation in the gas mixture, the turbulence of flow within the chamber, slight richness or leanness of fuelling, remaining combusted gas mixture on the next fuelling, etc, etc. Compare this to the only major variable in a given pneumatic's velocity, which is the consistency of it's fill pressure.

That is not to say other things do not affect the velocity. Temperature and humidity have minor effect on the effective speed of sound of the pneumatic's propellant gases, but if the same fill pressure is involved, these effects only become particularly notable at velocities where transonic effects are likely to form within the barrel - things like micro supersonic shock fronts as a result of imperfections in the smoothness of the barrel wall. These shock fronts then limit the velocity of the air flow that passes through them.

This does of course assume a consistent valve opening speed, but as long as we're not talking a hand opened ball valve, that should not be a major issue.

To contrast, combustions are largely immune to transonic effects, because of the large chasm between the gas flow velocities within the barrel and the actual speed of sound of the gases, which can be anywhere up to around 1,000 m/s, although it of course varies with the gas mixture involved.

However, although they have an immunity to transonics, temperature, atmospheric pressure and humidity affect them in different ways, by adjusting the actual molar quantity of oxygen within the chamber.
Lower atmospheric pressure, higher temperature, or higher humidity all reduce the available oxygen in the chamber volume.

This can then affect the stoichometry of the mix, and like any of the other factors listed above, this affects the burn rate and flame front speed within the chamber. As a direct consequence of that, the pressures formed within the chamber are changed, forming a different pressure curve (against time) to normal, which obviously, affects the integral of the force acting on the projectile and therefore the energy which it attains within the barrel.
In contrast, a graph of a pneumatic's pressure against time will be essentially identical each time, assuming a fixed fill pressure.

I would point out that although JSR mentions the temperature sensitivity of CO2, for most purposes, as long as there is some margin between the vapour pressure at a given temperature and the pressure used in the chamber, the temperature sensitivity is largely irrelevant.

It applies to CO2 air rifles, because they seldom have any form of regulation, taking gas straight from the internal gas bulb/cylinder at whatever pressure it happens to be at, which as liquid CO2 is in the system, will vary. But, if regulated down to a specific pressure, the results will not show a particularly notable temperature dependency compared to any other gas.

Well, that's something of a wall of text. I sometimes forget how much I can ramble on a point.

[jimmy101]

I would just add that combustions really don't have all that much shot to shot variability as long as the ammo and fueling is consistent. The consistence of a "spray-n-pray" is going to be terrible, especially when shooting veggies.

Take a look at Latke's CB studies. With a fuel meter, chamber fan and the same round the shot to shot variability is in the 1 to 3% range. With spuds the variability is up in the 5 to 10% range.

Anybody got good solid chrono data for a pneumatic to calculate the shot to shot variability?

I would think the typical compressor isn't all that reproducible, especially when being used near it's pressure limit. My shop compressor has about a 15PSI pressure window, it won't start the pump until the pressure drops below about 105 PSIG, it'll then run until it hits ~120 PSIG before it shuts off. If I crank the outlet regulator to it's max setting (~120 PSIG) then the actual pressure in the chamber is somewhere between about 100 and 120 PSIG. I'm generally too lazy to insure that the pump cycles on during chamber pressurization, which is the only way to make sure the final pressure is consistent.

If I was to set the regulator to say 90 PSIG then that would remove some of the uncertainty. Consecutive shots would be pretty consistent. If I was to change the reg setting and then try to set it back to 90 PSIG there would be a change in the pressure since the reg's gauge is hardly a precision instrument. I doubt the pressure can be reset to any better than a percent or two accuracy.

[Technician1002]

Anybody got good solid chrono data for a pneumatic to calculate the shot to shot variability?

Yes. On the t shirt cannon when scaling pressure/ speed and distance, we have lots of in barrel acceleration data. The variability is very little. The shots became about as predictable as shooting the same rock over and over from a slingshot.

T shirt flight dynamics were all over the map, but Oranges become very predictable and repeatable.

[covey12]

This is something I've been thinking about recently, as I'm currently looking interestedly into velocity consistency with respect to using it for more consistent trajectories - and thus accuracy.

The observation is (generally) true, because pneumatics have far fewer variables affecting their performance.

I'll start with a brief note on the issue of variable projectile masses, which affect both varieties in exactly the same way.
Obviously, a lower mass projectile has a lesser inertia, and will accelerate more freely under a net force, and higher masses vice versa. Lack of consistency in projectile mass is going to affect either type's velocity - you'll often find that serious air gunners will weigh their pellets into batches of the same masses to ensure greater consistency in velocity and trajectory.

Moving on to the issues that affect each variety differently, a combustion's velocity can be affected by many things, such as lack of homogenisation in the gas mixture, the turbulence of flow within the chamber, slight richness or leanness of fuelling, remaining combusted gas mixture on the next fuelling, etc, etc. Compare this to the only major variable in a given pneumatic's velocity, which is the consistency of it's fill pressure.

That is not to say other things do not affect the velocity. Temperature and humidity have minor effect on the effective speed of sound of the pneumatic's propellant gases, but if the same fill pressure is involved, these effects only become particularly notable at velocities where transonic effects are likely to form within the barrel - things like micro supersonic shock fronts as a result of imperfections in the smoothness of the barrel wall. These shock fronts then limit the velocity of the air flow that passes through them.

This does of course assume a consistent valve opening speed, but as long as we're not talking a hand opened ball valve, that should not be a major issue.

To contrast, combustions are largely immune to transonic effects, because of the large chasm between the gas flow velocities within the barrel and the actual speed of sound of the gases, which can be anywhere up to around 1,000 m/s, although it of course varies with the gas mixture involved.

However, although they have an immunity to transonics, temperature, atmospheric pressure and humidity affect them in different ways, by adjusting the actual molar quantity of oxygen within the chamber.
Lower atmospheric pressure, higher temperature, or higher humidity all reduce the available oxygen in the chamber volume.

This can then affect the stoichometry of the mix, and like any of the other factors listed above, this affects the burn rate and flame front speed within the chamber. As a direct consequence of that, the pressures formed within the chamber are changed, forming a different pressure curve (against time) to normal, which obviously, affects the integral of the force acting on the projectile and therefore the energy which it attains within the barrel.
In contrast, a graph of a pneumatic's pressure against time will be essentially identical each time, assuming a fixed fill pressure.

I would point out that although JSR mentions the temperature sensitivity of CO2, for most purposes, as long as there is some margin between the vapour pressure at a given temperature and the pressure used in the chamber, the temperature sensitivity is largely irrelevant.

It applies to CO2 air rifles, because they seldom have any form of regulation, taking gas straight from the internal gas bulb/cylinder at whatever pressure it happens to be at, which as liquid CO2 is in the system, will vary. But, if regulated down to a specific pressure, the results will not show a particularly notable temperature dependency compared to any other gas.

Well, that's something of a wall of text. I sometimes forget how much I can ramble on a point.

...holy crap

[Ragnarok]

Was it really necessary to quote my entire 634 word post to then append it with just the words "Holy crap"?

I've written longer posts you know. I think my personal record for longest word count in a single post is just short of 3,500 words.

[jimmy101]

Yes. On the t shirt cannon when scaling pressure/ speed and distance, we have lots of in barrel acceleration data. The variability is very little. The shots became about as predictable as shooting the same rock over and over from a slingshot.

How consistent is it? 1%, 0.1%, 5%?

[Technician1002]

Yes. On the t shirt cannon when scaling pressure/ speed and distance, we have lots of in barrel acceleration data. The variability is very little. The shots became about as predictable as shooting the same rock over and over from a slingshot.

How consistent is it? 1%, 0.1%, 5%?

It was hard to tell with the digital scope. It was within +_ 2 counts on the scope. Measurement margin of error was masking the actual launch margin. It was pretty tight on the foam ball. Our sample size was quite small when we found this and moved on to graphing the pressure/mass/speed and spent less time on repeatability. I would take a rough guess at repeatability of close to 2%. The older scope doesn't have that many horizontal pixels to measure, so the bit to bit measurement error is large. It was fun playing in the reigon of does not launch to just launched. The launch distance in this riegon is highly variable as the vacuum pulled and friction makes this region the most difficult to predict the launch velocity if any. Small parameters make large distance changes. Trying to repeat a 25 foot 45 degree launch is very hard. Friction variables are a large portion of randomness. High pressure is very repeatable.

When plotted, the pressure/speed showed the curve often seen in GGDT. When trying different barrel length/diameter combinations we started testing for just the minimum operating pressure to eject the projectile and a couple top pressure graphs for distance, a couple Rose Garden Arena distance prediction tests and didn't bother to fully fill in the spaces because of the high predictability and my shirt catcher was rapidly disintegrating. With the data, we could pretty much put a shirt anywhere we wanted to at the game including tagging retired numbers above the 300 section and the back row.

We first started testing at 10 PSI increments. Once we had a baseline, we didn't test outside our area of interest and focused on performance in a narrow band. We tuned to launch T shirts into the 300 section at the Rose Garden Arena and for Distance. Launch speed was good. We needed to focus on flight dynamics. Hitting 400+ feet with a rolled up t shirt is not easy. Doubling the pressure changed the landing zone by an average of 25 feet. 30 PSI and 60 PSI gave very close to the same range even though the launch speed was vastly different.

[Solar]

With the Eclipse, pressurizing from the rear of the gun into a chamber on the opposite end, I noticed that with a slower regulator and higher pressure combination I would get heat from compression in the far end of the ballast chamber and when the next burst of air would enter the chamber, regulated to the same pressure I could watch it enter at say 200psi, then once it hit the warm chamber, jump to 225psi. To fix that I just use a faster regulator or feed the air from the opposite end so the same end is heated and cooled.

This is good news for me. Thanks for all the great data. I will post results of my new launching system using 40mm rubber bullets soon. I will also be manufacturing a new 40mm rubber bullet and have considered dimpling like a golf ball, but that should be another thread(I remember seeing that thread in fact).

The weight consistency is a huge factor it shows from our earlier tests with the foam rockets. We have contacted a factory that can replicate the rockets more consistently with metered molds. Hopefully that will give us the improved accuracy we need.

[jook13]

Am I out of line to ask how the Eclipse is selling? it is just curiosity.

[Solar]

I have only sold a few dozen, but at $1300 apiece I am not surprised given the current economy and the fact that I have done little to no advertising since the remainder of the launchers I have are mostly earmarked for r&d. It will be appearing in a video game this year as well as on TV. Once we get rolling with the new machine shop (and hopefully procure the military contracts we are going after) things will change for the recreational market and we will start marketing in earnest. I have had interest from the DOT for avalanche control (portable Avalauncher) but have determined that the detonating mechanisms for the full size avalauncher is not safe enough for use in a portable device. These would consist of 1lb. explosive rounds detonated with a Winchester blank and a blasting cap. I have been distracted by the rubber bullet gun lately, but will be pursuing the Eclipse further once we get the machine shop moved into the new location. I have a few ideas that could reduce the cost quite a bit and I will also offer a starter gun that can be updated to a full blown (Total) Eclipse.

I have had a lot of feedback and only a few minor issues that were easily eliminated.

[King_TaTer]

Well Solar, I can't wait to see what you'll be coming up with next. Sounds very interesting. Especially the rubber bullets and it's gun. Also the fact that your products might be incorporated in to the military is outstanding. I mean if the military is using your products then that means your workmanship must be phenomenal (and think of all that money ). Into a video game and on TV? May I ask what game and which show? I'm impressed, and I had thought I'd seen it all when the eclipse was shown a few years back.

Again, looking forward to your new work. Good luck with everything.

~ King