1300 Foot Pounds large bore airgun

[rsterne]

My reference that it might be considered a firearm relates solely to the BATF definition.... I agree it's an airgun, with the possibility it is getting a boost in power from some form of combustion or heating initiated by the spark in a 4000 psi container.... The fuel could be an oil mist, tiny particles of metal, or even vapourized metal, I don't know, and don't really care....

The bottom line is that the shots in the video show more power than I would expect from that pressure, bore and barrel length.... It is interesting that when Bill did a test where he simply let the disc fail from increasing pressure, he only got about 470 FPE at 3500 psi.... well within "normal" parameters for a .438 cal with a 32" barrel.... In fact, I wouldn't bat an eye until the FPE for that pressure went over 700 FPE, which is the 50% of maximum level we sometimes see in top notch PCPs....

Bob

[mikeyb]

420 V
4.20E-04 F
=
37.044J

E=1/2*C*V^2

420uF at 420V stores 37 Joules of energy. Most of that discharge becomes heat inside the airgun. Maybe 1/4 of that could be added to the projectile velocity? I estimate +7 ft-lbs max, not more and probably less. Not enough to cause a significant difference.

One issue that concerns me is how well the discharge circuit is constructed and shielded. I used to design Pulse Power circuits and the EMI/EMP from pulsed discharges usually disrupted any measurement devices within 20'-30'. Special grounding and shielding of both the discharge circuit and the measurement devices was necessary to get any valid event data. If a computer was part of the data collection, a fully shielded room (Faraday Cage) was needed or the PC would lockup or reboot from the impulse.

I wonder if the Chronograph is getting EMI'd showing a false faster velocity during the electronic trigger and a more realistic slower velocity during the passive (not electrically triggered) burst disc test?

[rsterne]

I don't know enough to comment on whether an EMI could affect a Chrony.... but it's as good a theory as any other, IMO....

Bob

[billzweig]

It seems likely that the higher energy is related to the spark mode of triggering. I did not consider any side effects like this and the spark simply seemed like an elegant way to trigger the gun. In the actual gun design the discharge is entirely confined in the bolt and thus fully shielded. The cross sectional drawing is showing the 30KV trigger electrode (12) concentric with the main electrode (13). I did bench test this configuration and it works well.
When I shall return to this project (certainly hoping...) I'll try the following:
 1. New, commercial barrel. Possibly 9mm.
 2. Comprehensive testing of the electrically triggered vs mechanical or pressure buildup
 3. Intentional introduction of combustible particles (oils and similar) into the air charge
 4. A new "receiver" with adjustable pressurized air volume to determine the optimum one.

Again, thank you all for the comments and suggestions. Much appreciated.

[billzweig]

if it spiked to 8000psi. a camera placed next to the built in gauge should be able to catch the spike.

The pressure spike and release would occur so fast that the (damped) pressure gauge would not have time to react; at least not all the way to 8000 PSI.  If you mean that any upward needle twitch on firing would be sufficient evidence, then maybe. 

Remember, the (probable) ignition source involves rupturing the burst disc by partially melting it.  So, both the ignition trigger for a pressure rise and the escape path that drops the pressure would occur at the same time.  A second unheated burst disc, that vents to the outside, would react to such a pressure event, if it were engineered to burst at just over 4000 PSI.

The indicating gauge was installed only to verify the initial charge pressure. Not only it peeks at 4000, but it is highly damped (by a small orifice in the gauge air passage: a set screw with a 0.3mm hole drilled through it) to prevent gauge damage during the burst.

[lloyd-ss]

.............................
.............................
I wonder if the Chronograph is getting EMI'd showing a false faster velocity during the electronic trigger and a more realistic slower velocity during the passive (not electrically triggered) burst disc test?

Michael, Looks like your comments got everybody thinking.
I happened to have a DIY electric fence charger handy (don't laugh, it is not pretty, but it works) that can easily make a 10mm spark. Bringing the Chrony's control unit or its connection wire anywhere close to the spark gap does something. Bill, I realize that this chrony and setup are quite different from yours, but something might be going on.

Bill, if you need a barrel blank, TJ's in Alexandria KY has an extensive selection. PM me if you need his barrel specs list or contact information.
Lloyd
 

[rsterne]

This comes under the heading of "Leave it to Lloyd" to figure out how to test something nobody ever though of.... 

Well done, buddy.... 

Bob

[rsterne]

Bill, looking at your nicely detailed drawing, I don't understand the air path.... You are probably aware of the necessity of having a plenum (firing chamber) of large enough volume to provide the necessary air for the shot.... In a conventional, regulated PCP, we find that about 1 cc of HPA per FPE you intend the gun to produce is a good value to keep the pressure during the shot from dropping too much.... Decreasing the plenum to 1/2 cc per FPE will require about a 10% increase in regulator setpoint to make up for the additional pressure drop.... This means that high powered PCPs need a very large plenum.... This is why you don't see regulators on many Big Bore PCPs....

Dump valves, such as you are building, are slightly different, in that we usually look at the valve volume (HPA to be dumped) as a percentage of barrel volume.... Generally speaking, once the valve volume exceeds half the barrel volume the efficiency goes into the dumper.... You are running a dump chamber about the same as the barrel volume (0.438 x 0.438 x PI/4 x 32) = 4.82 CI = 79 cc.... Here is what happens to the pressure during the shot for a few valve volume percentages, starting with 4000 psi:

Valve is twice barrel volume.... pressure at bullet exit = 2667 psi.... average pressure = 3333 psi....
Valve equals barrel volume.... pressure at bullet exit = 2000 psi.... average pressure = 3000 psi....
Valve 50% of barrel volume.... pressure at bullet exit = 1333 psi.... average pressure = 2667 psi....
Valve 25% of barrel volume.... pressure at bullet exit = 800 psi.... average pressure = 2400 psi....

If we assume that the average pressure, times the bore area (which is the average force), times the barrel length in feet, divided by 2 (to allow for losses), is the approximate FPE.... we get the potential FPE = (0.438 x 0.438 x PI/4 x 32 / 12 / 2) x average pressure.... which is the barrel volume (in CI) / 24 x avg. press.... Your barrel volume is 4.82 CI, so that divided by 24 is 0.201.... If you had an unlimited plenum, and therefore a constant 4000 psi, that works out to (0.201 x 4000) = 804 FPE.... As the average pressure decreases because you have a limited volume to dump, you get the following....

Valve is twice barrel volume, FPE = (0.201 x 3333) = 670 FPE
Valve equals barrel volume, FPE = (0.201 x 3000) = 603 FPE
Valve 50% of barrel volume, FPE = (0.201 x 2667) = 536 FPE
Valve 25% of barrel volume, FPE = (0.201 x 2400) = 482 FPE

To determine the approximate efficiency, we divide the potential FPE by the volume of air used, in standard CI, which is the pressure in bar times the valve volume.... We would get the following....

Valve is twice barrel volume (9.64 CI x 276 bar) = 2660 CI.... Efficiency is (670 / 2660) = 0.25 FPE/CI
Valve equals barrel volume (4.82 CI x 276 bar) = 1330 CI.... Efficiency is (603 / 1330) = 0.45 FPE/CI
Valve 50% of barrel volume (2.41 CI x 276 bar) = 665 CI.... Efficiency is (536 / 665) = 0.81 FPE/CI
Valve 25% of barrel volume (1.20 CI x 276 bar) = 332 CI.... Efficiency is (482 / 332) = 1.45 FPE/CI

Now these are only vague approximations, but it lets you see what happens to the FPE and the efficiency as you change the valve volume as a percentage of barrel volume.... I hope that gives you some insight into the volume of the dump reservoir you need, and how changing it will potentially affect the FPE and the efficiency....You can see the reason that dump valves seldom exceed 50% of the barrel volume.... the small gain in FPE is usually not worth the extra air used to produce it....

Bob

[lloyd-ss]

This comes under the heading of "Leave it to Lloyd" to figure out how to test something nobody ever though of.... 

Well done, buddy.... 

Bob

Thanks Bob,
But ya know, that really shows how well shielded my plain vanilla Samsung android phone is. It was able to record video and audio with no noticeable effects from the spark. But the chrony processor box, or the cable going to it, certainly picked up some EMI.

[subscriber]

Decreasing the plenum to 1/2 cc per FPE will require about a 10% increase in regulator setpoint to make up for the additional pressure drop....

You can see the reason that dump valves seldom exceed 50% of the barrel volume.... the small gain in FPE is usually not worth the extra air used to produce it....

So, rather than doubling the plenum volume, double the starting pressure (if possible)?  Or, is a higher starting pressure so much more efficient that you only need to increase it by 10% to have the same effect as doubling the plenum volume, dumped at the lower starting pressure?

[subscriber]

Is EMI related to the spark voltage or quantity of charge dumped?  The spark generating coil must be exceeding 10kV, while the capacitor dumped to initiate the burst disk is operating only at 400 volts.  Perhaps the 30 kV trigger is interfering more than Lloyds transformer.

The electrical discharge (400V, 400uF) is triggered by 30KV spark gap.

[mikeyb]

Is EMI related to the spark voltage or quantity of charge dumped?  The spark generating coil must be exceeding 10kV, while the capacitor dumped to initiate the burst disk is operating only at 400 volts.  Perhaps the 30 kV trigger is interfering more than Lloyds transformer.

The electrical discharge (400V, 400uF) is triggered by 30KV spark gap.

Speed of a discharge, the rate of current change over time = di/dt, is related to how much EMI is generated. That means the sparkgap circuit IS more likely to generate interference. The ignition coil in the video uses a fast di/dt to generate a high voltage impulse. The resulting spark radiates significant EMI which is nicely shown messin' with the Chrony. Cell phones are designed to send and receive very high frequency EM waves. While they are not bullet-proof to EMI (don't arc directly TO the phone!), they will usually function closer to EMI sources that shut down other less robust electronic devices.

I see a similar problem when TIG welding on equipment with electronics attached or nearby. The TIG high frequency arc starter function (basically a small Tesla Coil) has to be disabled or I run the risk of frying some critical electrical controls.

[billzweig]

[/quote]


Bill, if you need a barrel blank, TJ's in Alexandria KY has an extensive selection. PM me if you need his barrel specs list or contact information.
Lloyd
 
Thanks. But I am not sure how easy it is importing a gun barrel to Canada.

[subscriber]

I am not sure how easy it is importing a gun barrel to Canada.

Contact Bob Sterne via PM.  As a fellow Canadian, and a custom airgun builder, he probably knows all pertinent laws and channels:  https://www.gatewaytoairguns.org/GTA/index.php?action=pm;sa=send;u=2569

[jackssmirkingrevenge]

My reference that it might be considered a firearm relates solely to the BATF definition.... I agree it's an airgun, with the possibility it is getting a boost in power from some form of combustion or heating initiated by the spark in a 4000 psi container.... The fuel could be an oil mist, tiny particles of metal, or even vapourized metal, I don't know, and don't really care....

I'm reminded of an "Electrothermal gun" that had been posted to the spudfiles forum:

Some time ago i started building a fairly large capacitor bank that aim planing to use for my rail-gun project.
Its consist of 24, 15000uF 400v capacitors for a total energy of 28800 Joules.
The capacitors are arranged in 3 parallel banks that are wired in series for 1200v and 40000uF.
To test the capacitor bank i decided to build a ElectroThermal Gun.
The ETG (ElectroThermal Gun) works by discharging the capacitor bank threw fine aluminium powder and foil inside a chamber.
The electric current then heats the aluminium enough to make it evaporate and turn into plasma.
When the aluminium turns into superheated plasma it expands and the pressure in the chamber increases, the plasma also heats the air so it expands.
The expanding gases then force the projectile down the barrel like a conventional (spud)gun.

The ETH have a 12mm smooth-bore barrel that is screwed into a steel chamber with a electrode that goes threw a insulator in the end, the power is then feed into that electrode and threw the aluminium-powder.
Some videos.

Test shot 1:

Test shot 1, Second view:

Test shot 2:

[billzweig]

Bill, looking at your nicely detailed drawing, I don't understand the air path.... You are probably aware of the necessity of having a plenum (firing chamber) of large enough volume to provide the necessary air for the shot.... In a conventional, regulated PCP, we find that about 1 cc of HPA per FPE you intend the gun to produce is a good value to keep the pressure during the shot from dropping too much.... Decreasing the plenum to 1/2 cc per FPE will require about a 10% increase in regulator setpoint to make up for the additional pressure drop.... This means that high powered PCPs need a very large plenum.... This is why you don't see regulators on many Big Bore PCPs....

Dump valves, such as you are building, are slightly different, in that we usually look at the valve volume (HPA to be dumped) as a percentage of barrel volume.... Generally speaking, once the valve volume exceeds half the barrel volume the efficiency goes into the dumper.... You are running a dump chamber about the same as the barrel volume (0.438 x 0.438 x PI/4 x 32) = 4.82 CI = 79 cc.... Here is what happens to the pressure during the shot for a few valve volume percentages, starting with 4000 psi:

Valve is twice barrel volume.... pressure at bullet exit = 2667 psi.... average pressure = 3333 psi....
Valve equals barrel volume.... pressure at bullet exit = 2000 psi.... average pressure = 3000 psi....
Valve 50% of barrel volume.... pressure at bullet exit = 1333 psi.... average pressure = 2667 psi....
Valve 25% of barrel volume.... pressure at bullet exit = 800 psi.... average pressure = 2400 psi....

If we assume that the average pressure, times the bore area (which is the average force), times the barrel length in feet, divided by 2 (to allow for losses), is the approximate FPE.... we get the potential FPE = (0.438 x 0.438 x PI/4 x 32 / 12 / 2) x average pressure.... which is the barrel volume (in CI) / 24 x avg. press.... Your barrel volume is 4.82 CI, so that divided by 24 is 0.201.... If you had an unlimited plenum, and therefore a constant 4000 psi, that works out to (0.201 x 4000) = 804 FPE.... As the average pressure decreases because you have a limited volume to dump, you get the following....

Valve is twice barrel volume, FPE = (0.201 x 3333) = 670 FPE
Valve equals barrel volume, FPE = (0.201 x 3000) = 603 FPE
Valve 50% of barrel volume, FPE = (0.201 x 2667) = 536 FPE
Valve 25% of barrel volume, FPE = (0.201 x 2400) = 482 FPE

To determine the approximate efficiency, we divide the potential FPE by the volume of air used, in standard CI, which is the pressure in bar times the valve volume.... We would get the following....

Valve is twice barrel volume (9.64 CI x 276 bar) = 2660 CI.... Efficiency is (670 / 2660) = 0.25 FPE/CI
Valve equals barrel volume (4.82 CI x 276 bar) = 1330 CI.... Efficiency is (603 / 1330) = 0.45 FPE/CI
Valve 50% of barrel volume (2.41 CI x 276 bar) = 665 CI.... Efficiency is (536 / 665) = 0.81 FPE/CI
Valve 25% of barrel volume (1.20 CI x 276 bar) = 332 CI.... Efficiency is (482 / 332) = 1.45 FPE/CI

Now these are only vague approximations, but it lets you see what happens to the FPE and the efficiency as you change the valve volume as a percentage of barrel volume.... I hope that gives you some insight into the volume of the dump reservoir you need, and how changing it will potentially affect the FPE and the efficiency....You can see the reason that dump valves seldom exceed 50% of the barrel volume.... the small gain in FPE is usually not worth the extra air used to produce it....

Bob

Thank you, Bob, for the really nice calculation. In the test setup I have used an air volume about equal to the volume of the barrel. The volume behind the projectile was charged to 4000 psi ( directly from the air tank using the tank valve) and then fired.

The gun drawings are really jut conceptual. Item 25 is the pressurized air admitting valve and in this concept a manual valve. It can be a needle valve for controlled filling to the desired pressure, of a larger orifice valve allowing quick fill by, say, pressing a 1/4 turn spring return lever. The air (communicating with the air bottle  by passage 26) passes through the circumferential gap 24 confined by seals 16 and 17 and fills the volume 14. In the drawing none of this is the final size or volume. The barrel will probably be (if ever built...) smaller calibre and the bolt volume adjusted accordingly.
In this concept I am assuming that either the user fills the bolt volume manually to his/her desired pressure, or the bottle is equipped with a pressure regulator for constant, one touch fill.
The way I see the operation of the gun is:
1. Load the "belted" bullet
2. Close the bolt
3. Fill the bolt chamber  (slow needle valve of "instantaneous" one touch)
4. Start the capacitor charging by the holding the back trigger on the pistol grip or optional other switch. This take about 5 seconds on fully charged NiCad battery and once the voltage is reached the circuit cycles to keep the charge.
5. Fire by closing the trigger switch

[billzweig]

I am not sure how easy it is importing a gun barrel to Canada.

Contact Bob Sterne via PM.  As a fellow Canadian, and a custom airgun builder, he probably knows all pertinent laws and channels:  https://www.gatewaytoairguns.org/GTA/index.php?action=pm;sa=send;u=2569

Thanks!

[billzweig]

Is EMI related to the spark voltage or quantity of charge dumped?  The spark generating coil must be exceeding 10kV, while the capacitor dumped to initiate the burst disk is operating only at 400 volts.  Perhaps the 30 kV trigger is interfering more than Lloyds transformer.

The electrical discharge (400V, 400uF) is triggered by 30KV spark gap.

Speed of a discharge, the rate of current change over time = di/dt, is related to how much EMI is generated. That means the sparkgap circuit IS more likely to generate interference. The ignition coil in the video uses a fast di/dt to generate a high voltage impulse. The resulting spark radiates significant EMI which is nicely shown messin' with the Chrony. Cell phones are designed to send and receive very high frequency EM waves. While they are not bullet-proof to EMI (don't arc directly TO the phone!), they will usually function closer to EMI sources that shut down other less robust electronic devices.

I see a similar problem when TIG welding on equipment with electronics attached or nearby. The TIG high frequency arc starter function (basically a small Tesla Coil) has to be disabled or I run the risk of frying some critical electrical controls.

I am familiar with electromagnetic pulse effect on electronics, but in my tests the chrono did not respond at all to repeater pulses. The air gap is only about 1mm and most the discharge confined inside the gun. But thanks for the comments and thanks for the ignition coil video. I will investigate it again.

[billzweig]

I'm reminded of an "Electrothermal gun" that had been posted to the spudfiles forum
[/quote]

Impressive, but hardly portable.

[billzweig]

I want to thank you all for this lively discussion. Once I get back to this project, hopefully some of the unknowns shall become clear.
But today I've arrived to town (Vancouver, BC), to a hospital tomorrow morning... and a hope of a quick return to my rural home. I am not much of a city person.