Sonic Choke? Maybe not. 2162 FPS

[Scotchmo]

Scott,
I think I can assure that a Buna-N o-ring is not going to be in any kind of condition to work for a second shot.  That amount of friction will wear the surface of the o-ring off to a degree that it won't seal for a second shot.  Whether or not the amount of o-ring that gets scrubbed off actually burns at all or burns fast enough to contribute any force, is anybody's guess, and I find it rather a stretch to think that it would contribute to the velocity.  I don't have nitrogen to use, so that change is not a possibility.  About all I can change might be to use a dry barrel and a Teflon seal. 
Lloyd

A single Nitrogen test that matched the air test would seal the deal. If it gave approximately the same result, than I would have more confidence when using air for further data collection.

Outside of that, there will always be a question of combustion, even if minor. You would be surprised at what can act as fuels under the right conditions. I'm not sure if Teflon would be safer from a combustion standpoint.

Scott, like I said, I can not test with nitrogen.  You loan me the gauge set and I will see what I can do.
 
Regarding the Teflon, Teflon does not support combustion and is mandated as wire insulation in most aircraft and naval installations requiring maximum fire resistance.  I am not aware of any seal material that is more non-combustible than Teflon.  A few tests done with a Teflon seal, in my opinion, should eliminate the argument about combustion.
That's as good as I can do with what I have.
Lloyd

If we get the same results with no lube and various seal materials, we are probably good (but still a small doubt). I was thinking that we could weigh the seal before and after, but a rough calc shows that it only takes about .02gr of combustion (rubber) to yield 30fpe. Too small of a difference to measure accurately.

For nitrogen, would this do the job?:
http://www.ebay.com/itm/Paintball-airgun-fill-station-6000-psi-CGA677-/222056548928

Do you have tank access?

[39M]

I might try graphite as a lube for the bore. It's very slick and very difficult to ignite.

Of course with a teflon seal no lubricant would be needed. Maybe something as simple as a .278 teflon tube with a hole just large enough for an aluminum rivet, to give it some weight(fpe), would be the ideal projectile.

[39M]

To get to the top of the mountain might require a barrel with a super slick coating inside the bore.

[Buldawg76]

My two cents worth tell me that if indeed the o ring is burning at all even very slightly and contributing to the velocity by the time that minuscule amount of 30 fps had a chance to have any effect at all on the projectiles velocity it would be out of the barrel far ahead of that combustion and therefore the combustion would have little if any effect on its velocity. That little bit of suspected combustion would not propagate any where fast enough to create any added pressure or push to the pellet since it is already moving far faster then the combustion can propagate behind it and it has to be behind it to have any effect.

The fuel/air combustion in an automotive engine is a slow moving flame front in comparison to the milliseconds the projectile is in the barrel so I see no reason to believe that the heating of the o ring as it moves down the barrel could ever be sufficient to add to the velocity at all as it would be in front of the projectile not behind it where it would need to be to have any effect.

I see it as a non issue IMO.

Mike   

[lloyd-ss]

Scott,
I think I can assure that a Buna-N o-ring is not going to be in any kind of condition to work for a second shot.  That amount of friction will wear the surface of the o-ring off to a degree that it won't seal for a second shot.  Whether or not the amount of o-ring that gets scrubbed off actually burns at all or burns fast enough to contribute any force, is anybody's guess, and I find it rather a stretch to think that it would contribute to the velocity.  I don't have nitrogen to use, so that change is not a possibility.  About all I can change might be to use a dry barrel and a Teflon seal. 
Lloyd

A single Nitrogen test that matched the air test would seal the deal. If it gave approximately the same result, than I would have more confidence when using air for further data collection.

Outside of that, there will always be a question of combustion, even if minor. You would be surprised at what can act as fuels under the right conditions. I'm not sure if Teflon would be safer from a combustion standpoint.

Scott, like I said, I can not test with nitrogen.  You loan me the gauge set and I will see what I can do.
 
Regarding the Teflon, Teflon does not support combustion and is mandated as wire insulation in most aircraft and naval installations requiring maximum fire resistance.  I am not aware of any seal material that is more non-combustible than Teflon.  A few tests done with a Teflon seal, in my opinion, should eliminate the argument about combustion.
That's as good as I can do with what I have.
Lloyd

If we get the same results with no lube and various seal materials, we are probably good (but still a small doubt). I was thinking that we could weigh the seal before and after, but a rough calc shows that it only takes about .02gr of combustion (rubber) to yield 30fpe. Too small of a difference to measure accurately.

For nitrogen, would this do the job?:
http://www.ebay.com/itm/Paintball-airgun-fill-station-6000-psi-CGA677-/222056548928

Do you have tank access?

Scott, 
Good find on the nitrogen fill set-up.  Last time I looked for something like that it was several hundred dollars.  That set-up is very reasonably priced and takes away my resistance to trying the shots with nitrogen.  I had checked with one local welding supplier a few months ago and they only had 2500 psi bottles, but I will call the other welding places this morning and see what they have.  Worst case scenario if I can only get a 2500 psi tank, is to reg it down to 125psi and then pump it back up to 4500 with the Shoebox.  I have a brand new 77cuin CF bottle that has been waiting to be put to use and this could be it.  Of course, a 6k psi nitrogen tank from a local house would be fantastic.
This could work.
Lloyd

[lloyd-ss]

Buldawg, I agree with you that the o-ring probably isn't contributing to the energy more than just the tiniest bit, if at all, but if the nitrogen set-up becomes a reality, all concerns are eliminated, and honestly, that would be the best way to go.

39M, A Teflon seal is in the works!
Lloyd

[Buldawg76]

Lloyd
Yea if the nitrogen can be acquired at a reasonable cost and supplied at the pressures to equal the air it will definitely eliminate that variable from the equation of combustion adding to the velocity.
I hope it is easily attainable as I know I have checked here at my local welding shop several years ago and they can get 120 CF tanks of nitrogen at 6000 psi but it has to be ordered in advance and takes a few weeks to get but thats because it is sourced from Atlanta and trucked to my local shop and it was to costly for me to use as a fill medium for everyday use.

I don't remember the cost now but it was out of my budget and turn around time for a refilled tank was not acceptable either. I would have had to reorder a month in advance and pay at the time of ordering.

Hope the Teflon seal will work also as that will eliminate the possibility of any combustion as well.

Mike

[Buldawg76]

Lloyd
Just an FYI on the nitrogen fill setup as all you need is the CGA677 fitting and you can use your existing HPA bleed valve , gauge and fill hose you have now.

http://www.regulatortorchrepair.com/store.php/products/cga-677-nut-and-nipple

This is only 33 bucks and will attach your fill hose assy to a nitrogen tank.

Mike

[lloyd-ss]

Mike, thanks, another good lead on the nitrogen fittings.

I am tracking down the N tanks.  I can get 2500 psi easy and cheap locally but I have to drive an hour and a half to get 6000 psi.  I am waiting on a call back for pricing on 6000, because around here the rental on the 6k tanks is in dollars a day, not dollars a month.  But like I said, I can use the shoebox to get the nitrogen up to 4500 if it comes to that.
Lloyd   

[Buldawg76]

Lloyd
Keep us informed and that what I ran into here with the nitrogen at 6000 psi as well. Never thought about pumping up with the shoebox but then did not have one then either.

Mike

[rsterne]

I wonder what the intermediate pressure of the ShoeBox is?.... could you drill and tap into the block between the cylinders, install a male Foster, and feed it with, say, 500 psi Nitrogen (or Helium) directly into the 2nd stage?....

Bob

[Buldawg76]

Bob
That's a good question as the standard shoebox get fed 85 psi ( 100 max ) and the freedom 8 shoebox get fed 125 psi ( 135 max) so what the second stage outputs would determine what you would have to feed the third stage since the second stage is fed by either the 85 or 125 psi from the oilless compressor.

You would have to bypass the second stage and feed the third what the second stage outputs to the third with nitrogen to speed up the pressure step up of the nitrogen if I am figuring on how the three stages work correctly. 

Mike

[rsterne]

I meant the 2nd stage on the ShoeBox, which of course is the 3rd stage in the system.... input directly into the block after the larger cylinder on the Shoebox, and only use the smaller, HP cylinder to boost the pressure to 4500.... Should be very little load on the motor that way, you may even be able to run a bit higher pressure than the peak during operation (which would occur at 4500 psi output).... subject to the strength of the lines, fittings, and check valve between the stages, of course.... If the two stages both have about the same compression ratio, and we are going from 125 psi to 4500, which is 36:1, it is reasonable to assume each stage is 6:1, which could mean you might be able to feed the 3rd stage directly with 750 psi.... possibly just use an 850 psi paintball regulator with a shim removed?....

Of course dropping the pressure down to 125 psi would allow you to use more of the pressure in the N2 tank that you are paying for.... so I guess that would be more cost effective....

Bob

[lloyd-ss]

 Bob and Mike,

What happens if you overpressurize stage one in a freedom 8?  It doesn't have that funky cutoff switch, and normally in a compressor, if you overfeed the first stage it will just blow through the check valves till it reaches an equillibrium point.  I will study that with caution in mind and see what it looks like. Would still want a regulator on the input to keep the input pressure reasonable, say, less than 800-1000 psi.

Also, If you try to feed the outside pressure directly into the second stage you'll have to put a check in the input line so it can build pressure and won't just fight with the tank pressure.

Or am I missing something?

Lloyd


 

[lloyd-ss]

OK, got a quote on the 6000 psi nitrogen.   $125 per year rental for the cylinder, and then  $140 to get the cylinder filled.  That's about 75 cents a day.  Heck, I can spend that much on toilet paper on a bad day, LOL.
That's for a size 300 cylinder which is 49 liters water volume.  Bob how much nitrogen is that?  I need a more accurate measurement than a "butt load." 

With the reasonably priced adapters, this might be something that  I "need." 

Lloyd

[rsterne]

I don't know what happens if you overpressurize the first (2nd) stage.... but I don't think the air can blow through to the second (3rd) stage because when the pistons are (both) at the compression end of the stroke the high pressure piston is closing off the air passage on it's inlet side (which runs through the block, no?).... I think the ports inside the block effectively work like an active inlet valve for each stage, open only when the pistons withdraw to between the two O-rings, with check valves only on the outlet side.... Therefore, I don't think you need a check valve on the line (regulator) feeding from the tank.... as it would feed the space between the two O-rings from inside the block.... The check valve on the low pressure cylinder would prevent it from disappearing backwards through there.... (I think)....

Still, makes more sense to use something like a welding regulator to drop the N2 tank pressure down to 125 psi, you get to nearly empty your N2 tank that way.... so forget everything I suggested.... *LOL*....

Bob

[rsterne]

Nitrogen density at 6000 psi is 376 kg/m^3 at 20*C.... so 49 litres would hold (49/1000) x 376 = 18.424 kg of N2.... At 1 atmosphere is it 1.166, so you would have 18.424 x (1000 / 1.166) = 15,801 litres, or 558 CF of Nitrogen.... However, at 4500 psi, the density is 310 kg/m^3, which means if you are filling to that pressure, so you would still have (49/1000) x 310 = 15.19 kg. of Nitrogen left in the tank, and would have only (18.424-15.19) = 3.234 kg. that you would actually be able to use.... That works out to 3.234 x (1000/1.166) = 2774 litres, or 98 CF you can actually use if filling to 4500 psi.... and that assumes that the tank is actually at 6000 psi when you start....

Looking at it from the point of filling your 55cc chamber to 4500 psi, to a density of 310 kg/m^3, you need (55/1000/1000) x 310 = 0.01705 kg per fill, and you have 3.234 kg. available, so that works out to 190 fills.... minus the waste bled from the hoses and fittings.... Is that closer than "a bucket load" ? ? ?

Call it a buck a fill?.... Might be way cheaper to use the lower pressure tank, regulate it down to 125 psi, and run it through your Shoebox?....

Bob

[Scotchmo]

I'm in for $50 for the Nitrogen experiment.

Lloyd, check your PayPal account. Now you are committed.   

[lloyd-ss]

I'm in for $50 for the Nitrogen experiment.

Lloyd, check your PayPal account. Now you are committed.   

Does that mean the men in the white coats will be showing up at my door? 

Ok, game on!  I think I will probably go with the 6k bottle.  If I went with a 2500 bottle, the tank rental is almost as much, but the nitrogen fill is cheaper, but then I've got the wear and tear on the shoebox, and the noise and delay of continuously topping up. 

Bob, that is a big bucket load of fills from the 6k tank and I think I will do that.  That is a big tank to hide from the wife so I've to figure that one out.  Luckily the 6k tank  can be delivered to a local welding supply place so I don't have to make the long drive to pick it up.  That's actually what tipped the balance. Like the Brown truck showing up, LOL.
Lloyd

[lloyd-ss]

I'm in for $50 for the Nitrogen experiment.

Lloyd, check your PayPal account. Now you are committed.   

Scott, Thanks again. Do you offer beer to alcoholics, too?  Seriously though, thank you.  And it does lead to another dilemma.  If you have 6000 psi available, wouldn't it be a shame not to shoot something at 6000 psi?  The problem is the current Version 2 block was designed for 4500 psi. 
Its always something.
Lloyd