2031 FPS on air . Latest shot, 2162 FPS

[MicErs]

Yes, the Delrin pellet would be great, but unfortunately, the pellet is a structural element in the set-up, held from the rear, and the o-ring actually pulls the pellet from the front.  I tried the Delrin initially and it pulled apart.  The lightest projectile in the .278 cal so far has been 6.1 gns, but I need to go quite a bit lighter to get more velocity.  More design work to do.
Lloyd

Um sorry.  Pellet is structural.  Maybe you could change that a bit but I get it.  It is hard.

You got me to rummaging around in my "plastic materials".  I normally keep everything well marked with exactly what material it is, but I did find a piece of unidentified composite that I think is G9 or G10 Garolite.  If it is, it has about 3 times the tensile and compression strength of Delrin, and the calcs show that it has the strength needed for the task.  It is 60% of the weight of aluminum, so the potential for even higher velocities is very good.
So even though the Delrin didn't work out, you got me to thinking, and sometimes that does the trick.
Lloyd

Glad to have poked your noodle.     What about carbon fiber?  I am not an ME so excuse my dumb questions.

[pneumero uno]

Just wanted to add my thanks for a fascinating read. I have never heard of anyone using the projectile as the exhaust valve. A greater part of the velocity must be down to the extremely high pressure and long smoothbore barrel. Using the projectile as a dump valve does limit the velocity the system is capable of, especially when using 4500 psi but there is no way around that. An efficient non-dumping straight line valve would be the way to go for beating your current already splendid velocity achievement. That is just my opinion .

[pneumero uno]

My post was not meant to offend anyone in any way. I did add an expression at the end to the effect that I wasn't trying to teach anyone something they are already aware of but it did not appear .

[lloyd-ss]

My post was not meant to offend anyone in any way. I did add an expression at the end to the effect that I wasn't trying to teach anyone something they are already aware of but it did not appear .

John,
Definitely no concern whatsoever, constructive ideas are always welcome, whether or not they are acted upon.    As you know ideas that  very good can often be extremely difficult to implement. 
For a shot where maximum velocity is the goal, I don't understand how using a non-dumping valve could achieve higher velocities, given the same cc reservoir.  However, if you mean using a larger reservoir, metered with a valve such that the exhaust pressure would remain constant, I agree.  But I have always found that closing the valve in a precisely timed manner is much more difficult than opening it.
Lloyd

[pneumero uno]

My post was not meant to offend anyone in any way. I did add an expression at the end to the effect that I wasn't trying to teach anyone something they are already aware of but it did not appear .

John,
Definitely no concern whatsoever, constructive ideas are always welcome, whether or not they are acted upon.    As you know ideas that  very good can often be extremely difficult to implement. 
For a shot where maximum velocity is the goal, I don't understand how using a non-dumping valve could achieve higher velocities, given the same cc reservoir.  However, if you mean using a larger reservoir, metered with a valve such that the exhaust pressure would remain constant, I agree.  But I have always found that closing the valve in a precisely timed manner is much more difficult than opening it.
Lloyd

I meant using the same size reservoir, pressure etc but with a valve that closes after releasing part of the contents of reservoir. The closed valve has an effect. Give it a try you could be surprised. Be adventurous my friend

[lloyd-ss]

My post was not meant to offend anyone in any way. I did add an expression at the end to the effect that I wasn't trying to teach anyone something they are already aware of but it did not appear .

John,
Definitely no concern whatsoever, constructive ideas are always welcome, whether or not they are acted upon.    As you know ideas that  very good can often be extremely difficult to implement. 
For a shot where maximum velocity is the goal, I don't understand how using a non-dumping valve could achieve higher velocities, given the same cc reservoir.  However, if you mean using a larger reservoir, metered with a valve such that the exhaust pressure would remain constant, I agree.  But I have always found that closing the valve in a precisely timed manner is much more difficult than opening it.
Lloyd

I meant using the same size reservoir, pressure etc but with a valve that closes after releasing part of the contents of reservoir. The closed valve has an effect. Give it a try you could be surprised. Be adventurous my friend

John,
If you have an appropriate valve, and data to show that what you are suggesting is true at these performance levels, please be so kind as to share it.
Lloyd

[I_like_Irons]

We keep hearing about the vacuum in the barrel idea.... works great for ping-pong balls which are large in area and are being driven by very low pressure, where removing the 14.7 psi in front of the ball is the same as adding the same pressure behind it.... With what we are doing, adding 14.7 psi to 4500 is hardly worth the effort....Would you get a bit more velocity?.... yes but not in proportion to the amount of work involved, IMO.... I have heard Lloyd express the same opinion....

Bob

I'm not so sure that the atmospheric air pressure is relatively insignificant.  It certainly is at the beginning where the velocity is well below mach 0.8 for the air in front.  However, when that threshold is reached, the gas in front becomes a semi compressible fluid that has no where to go except directly ahead.  The pressure at the nose then becomes, I think, significant. 

Modeling the air pressure in front of the pellet/bullet is just as problematic as it is behind.  You have some that is escaping out the muzzle into a large (~infinite) reservoir, but some is still trapped in the barrel.  In the case of an infinitely long barrel, the gas cannot escape.  Thus there would be some point at which the pressure in front (plus friction) equals the pressure in the rear.   Here, the peak velocity may be before this occurs, and then drops off to a steady state. 

Now, if one can get a bunch of consistent shots, then put a thin cap over the muzzle with a vacuum hose fitting, the effect of the atmospheric air pressure could be measured at discrete points when the barrel is shortened incrementally. 

In my collection of stuff, I have a .38 i.d. by .625 o.d. length of DOM tubing, with which I am considering instrumenting, and make a similar dump design gun.  (The tubing will be used later for a scale version of my howitzer idea.)  I have not yet found all the equipment needed that is within my budget.  (Multi-channel  A/D converters that can handle the speed and simultaneous measurements are kind of pricey.  ) Also, I do not have the means to get pressures much above 3000 psi.

[rsterne]

I agree that the effect would definitely be more than just 14.7 psi.... and once the bullet exceeds 1650 fps (in fact once it exceeds Mach 0.8 ) the air in the barrel will be compressing.... However, even if no air escaped from the barrel, with the 46" barrel we are using, the pressure would only be 10 bar (145 psi) when the bullet is only 4.6" from the muzzle.... which is less than 7% of the residual pressure still driving the pellet at that point.... Most of the acceleration of the bullet occurs in the first half of the barrel or less (in Lloyd's 2162 fps shot, the spreadsheet says it is going 2000 fps at 26" down the barrel).... If the barrel was corked, the average pressure inside at that point would be less than 34 psi at that point (2 lbs. bore drag).... I realize that there would be a pressure gradient, but also, much of the air would have already exited the muzzle, so perhaps such a simple assessment isn't that far off....

As I said, pulling a vacuum would make a difference.... but nowhere near of the magnitude it makes with ping pong ball cannons that operate at low pressures....

Bob

[Bent]

But you would be consuming energy to compress and accellerate the air molecules in the barrel. Energy that should have been used for accellerating the pellet.

[rsterne]

100% correct.... It is equivalent to overcoming additional barrel friction, as I explained above.... Here is yet another way of looking at it.... There is 2.8 CI of air inside the barrel.... At STP, air is 1.204 kg/m^3, so that air weighs....

2.8 CI of air is 0.000046 m^3, so it would weigh 0.000046 x 1.204 = 0.000055 kg. or 0.85 grains.... In Lloyd's 2162 fps shot, the pellet weighed 7.7 grains, so if we add that to the weight of the pellet (therefore assuming that not even one molecule of air was pushed out the muzzle before compression of the air commenced, which is not the case).... we now have a pellet weight of 8.55 grains....

If we substitute a pellet of 8.55 grains for the one that was 7.7 grains, in Lloyd's J4 Spreadsheet, the predicted velocity drops from 2162 fps to 2127 fps.... Therefore that would be the maximum loss in velocity from the mass of air in the barrel.... less than 2%....

By comparison, in a ping pong ball cannon, if the barrel is only 2 metres long (they are usually much more), the air in the barrel weighs MORE than the ping pong ball.... and the 14.7 psi atmospheric pressure is a MUCH larger percentage of the driving pressure (one I saw was 300 psi)....

Bob

[pneumero uno]

My post was not meant to offend anyone in any way. I did add an expression at the end to the effect that I wasn't trying to teach anyone something they are already aware of but it did not appear .

John,
Definitely no concern whatsoever, constructive ideas are always welcome, whether or not they are acted upon.    As you know ideas that  very good can often be extremely difficult to implement. 
For a shot where maximum velocity is the goal, I don't understand how using a non-dumping valve could achieve higher velocities, given the same cc reservoir.  However, if you mean using a larger reservoir, metered with a valve such that the exhaust pressure would remain constant, I agree.  But I have always found that closing the valve in a precisely timed manner is much more difficult than opening it.
Lloyd

I meant using the same size reservoir, pressure etc but with a valve that closes after releasing part of the contents of reservoir. The closed valve has an effect. Give it a try you could be surprised. Be adventurous my friend

John,
If you have an appropriate valve, and data to show that what you are suggesting is true at these performance levels, please be so kind as to share it.
Lloyd

I will see what I can do Lloyd. I don't know about at your performance levels though .

[pneumero uno]

I am part way to giving you some data Lloyd. A quick update. In amongst my junk I have an old pistol that should provide a base for some figures. It is running at just below 6 ft/lbs energy with a 7" barrel and gives a shed full of shots. The internal valve control device was adjusted  for maximum release and  a 28"  barrel fixed, from an  rimfire match rifle. As it was a pistol the reservoir is small. Internally just over 3/4" diameter x 1 3/4". My maths are not good enough for me to work out the capacity allowing for the valve and control device. Fill pressure was 200 bars as I only had 220 left in the tank. Using JSB 15.89 grain pellets 3 shots were fired  before refilling. I repeated the exercise several times and the figures were approx. the same each time. 1164 fps, 1171 fps, 1161 fps. Next step is to remove the controlling device to convert it to a dump valve. I will let you know if velocity goes up or down.

[lloyd-ss]

pneumero uno,
Very good.  I am glad to see that the experiment is progressing.  The 3/4 x 1-3/4 volume calculates out to 12.7cc.  You filled to chamber to 200 bar, but do you have a guess as to what the pressure was after the 3rd shot?
Thanks,
Lloyd

[pneumero uno]

pneumero uno,
Very good.  I am glad to see that the experiment is progressing.  The 3/4 x 1-3/4 volume calculates out to 12.7cc.  You filled to chamber to 200 bar, but do you have a guess as to what the pressure was after the 3rd shot?
Thanks,
Lloyd

I have just tried it before taking out the control device. It is difficult to slowly open the valve when refilling. It jumps to about 100 bars. Filling to 140 bars gives 1165 fps and 110 bars 1135 fps if that helps.

[lloyd-ss]

pneumero uno,
If you start shooting at 220 bar (3190 psi) and finish at 100 bar (1450psi) and get 3 shots 15.89 gn averaging 1165 fps, that gives an air efficiency of about 1.6 fpe/cuin.  That is very good.  Here is the calculator, but you have to convert the pressure to psi
http://calc.sikes.us/1/index.php
 
The 1135fps at 110 bar is also very nice!
Lloyd

[dwalk]

Am I the only one who thinks our airgun community should proceed with caution before implementing This further?

Otherwise the government will restrict air rifles even more.

But congrats and thanks for pushing the envelope!

^^^YES!

air guns are NOT considered as being firearms in California (yet) but...they do have a generic term to cover things they deem as being 'unacceptable: "Destructive device"

[lloyd-ss]

Am I the only one who thinks our airgun community should proceed with caution before implementing This further?

Otherwise the government will restrict air rifles even more.

But congrats and thanks for pushing the envelope!

^^^YES!

air guns are NOT considered as being firearms in California (yet) but...they do have a generic term to cover things they deem as being 'unacceptable: "Destructive device"

dwalk,
Please go back and read my reply # 16 to the post you quoted.  I certainly respect your and Tuckeron's thoughts on that particular subject, and I know you are not alone, but that is not what this thread is about.
Thank you,
Lloyd

[pneumero uno]

pneumero uno,
If you start shooting at 220 bar (3190 psi) and finish at 100 bar (1450psi) and get 3 shots 15.89 gn averaging 1165 fps, that gives an air efficiency of about 1.6 fpe/cuin.  That is very good.  Here is the calculator, but you have to convert the pressure to psi

 
The 1135fps at 110 bar is also very nice!
Lloyd

That was certainly  quite a blast . I was wrong. The fill pressure is 200 bar. I removed the internal controlling device so that all the air was dumped on one shot. The volume was slightly increased as a result. Velocity was 1363 fps with 15.89 JSBs.

[lloyd-ss]

pneumero uno,
If you start shooting at 220 bar (3190 psi) and finish at 100 bar (1450psi) and get 3 shots 15.89 gn averaging 1165 fps, that gives an air efficiency of about 1.6 fpe/cuin.  That is very good.  Here is the calculator, but you have to convert the pressure to psi

 
The 1135fps at 110 bar is also very nice!
Lloyd

That was certainly  quite a blast . I was wrong. The fill pressure is 200 bar. I removed the internal controlling device so that all the air was dumped on one shot. The volume was slightly increased as a result. Velocity was 1363 fps with 15.89 JSBs.

That is one fast JSB!   
Lloyd

[MJP]

Thats a nice one Loyd, can't believe I have missed this thread. That is quite clever on holding the pellet and not using a valve.
I shot some 8gr plastic sabots out of my 357 with helium and we clocked them around 845m/s at the muzzle. Just to see how fast they can fly. Have a pic of the a chrony session somewhere hafto try and find it.
I do like these experiment threads, keep them coming.

Marko