Sonic Choke? Maybe not. 2162 FPS

[lloyd-ss]

Lloyd , regarding gas expansion what equation of state is being used?
There are at least a half a dozen cubic versions, van der Waals being
the most notable.


                                                                                      Ron

Ron,
Nothing very sophisticated.  My spreadsheet model calculates in .00001 second increments, outputting a delta V for the increment, after accounting for pellet break-away and dynamic friction, volume increase, T-port volume, accelerating the mass of air that is behind the pellet, closing point of the valve, etc. 
The new volume for each increment is calculated from the delta d of the barrel length, which is derived from the delta V of the previous increment.  Then the new pressure in the tank is calculated.  After the valve closes, the volume in the calculation changes to just the volume inside the barrel and T-port and the continuing expansion within the barrel.
My original spreadsheet used a simple P2= P1*(V1/V2).   Later, at Bob's suggestion, I incorporated an exponent into the equation to account for isothermal vs adiabatic expansion. The new equation is P2=P1*((V1/V2)^exp) , with the ability of the spreadsheet user to choose the exp, i.e., 1.0 for isothermal, and 1.4 for adiabatic, or anything in between. Different exponents can be chosen for the expansion period when the valve is open, and for the period after the valve has closed. Honestly, the changes attributable to changing the exponent are rather modest.   For the 1745fps shot in question, the calculated velocities are 1745 vs 1732, with all other parameters left the same. (Edit- it seems like the difference ought to greater than that, doesn't it? Lloyd)

Here is a chart of the calculated internal ballistics of the 1745 shot.
The X axis is the length along the barrel.  Y axis on the left is the FPE, shown in red. Y axis on the right PSI in the barrel, shown in blue,
and the velocity of the projectile, shown in green.



And here is a chart of a more typical PCP shot where the valve closes very early in the cycle. The .65 efficiency in the chart title is for the system efficiency of the gun.  The efficiency of the system for the 1745 shot was .70.


I hope this helps clarify things a bit.
Lloyd

[Buldawg76]

I'm home from the cruise. I'm looking forward to reading about the next test:

.245 cal
6.5 gn
4500 psi
36cc dump shot
47"

My spreadsheet shows about 2060fps using the .22 caliber fluid friction formula and 1 lb. pellet friction. My best guess is that fluid friction will also be proportional to caliber. Doing so yields about 2020fps in the spreadsheet. If the .245 caliber is later tested at different barrel lengths and pressures, we can see if it holds true. Either way, if we see a little over 2000fps, we should be able to predict about what's needed for 2200fps, 2500fps, or 3000fps. And it won't be pretty.

As far as efficiency, there are many ways to calculate it. Instead of using a "fudge factor", I'm trying to account for all losses.
Overall efficiency (pellet energy/air energy) is very low.

Scott

How was the burger and Mai Tai you had for us on the cruise.

Mike

[Scotchmo]

I'm home from the cruise. I'm looking forward to reading about the next test:

.245 cal
6.5 gn
4500 psi
36cc dump shot
47"

My spreadsheet shows about 2060fps using the .22 caliber fluid friction formula and 1 lb. pellet friction. My best guess is that fluid friction will also be proportional to caliber. Doing so yields about 2020fps in the spreadsheet. If the .245 caliber is later tested at different barrel lengths and pressures, we can see if it holds true. Either way, if we see a little over 2000fps, we should be able to predict about what's needed for 2200fps, 2500fps, or 3000fps. And it won't be pretty.

As far as efficiency, there are many ways to calculate it. Instead of using a "fudge factor", I'm trying to account for all losses.
Overall efficiency (pellet energy/air energy) is very low.

Scott

How was the burger and Mai Tai you had for us on the cruise.

Mike

The burger and drink that I had for you was very good. A week of indulging and I gained 5 lbs. Now I have to go into subsistence mode to lose it.

[Tomg]

I was thinking about lightweight bullets, instead of o-ring over metal projectile. Why not machine a delrin jacket that fits over a metal core? Cheap, repeatable and easy to make.

 

[rsterne]

I think releasing a Delrin bullet would be the problem.... Lloyd's test "gun" has full pressure against the bullet prior to release, so the loads are very high.... See post #156 on page 8....

Bob

[rgb1]

Lloyd, thanks for the explanation, that's what I was looking for, and more.

                                                                                      Ron

[Buldawg76]

Scott
Yea that's the problem with vacations we always over eat but at least you did not get a sunburn Right ?

I have never had any luck in subsistence modes, no will power I guess. LOL

Mike

[rsterne]

Lloyd, I think the reason for the small difference between Isothermal and Adiabatic expansion for your 4500 psi dump shot is that the expansion is not very great.... Your residual muzzle pressure is 63% of the starting pressure.... so you get very little cooling.... It makes a much bigger difference in a PCP where the valve closes early with a much lower residual muzzle pressure.... I am currently using Isothermal while the valve is open (assuming a large reservoir), and then Adiabatic while the valve is closed....

Bob

[lloyd-ss]

Bob, that makes sense.
Lloyd

[phoebeisis]

Is the 1745 fps a smooth bore?
I seem to remember seeing a comment that mentioned rifling marks on the o-ring.
Thanks
Charlie

[rsterne]

No, the .22 cal barrel was rifled.... the new .25 cal will be a smoothbore....

Bob

[phoebeisis]

Bob
Thanks

[lloyd-ss]

The material for the new Version 2 barrel and "pellets" arrived today.  Thanks guys!

One piece of 5/8" O.D. x .25 I.D. x 48" long DOM A513 steel.  I need to prep the ends and take a close look inside and try pushing a few lead pellets thru to see how consistent the I.D. feels and figure how much work it will need.  Just peeking thru the sawed ends, it doesn't look too bad.

The other piece is the 1/4" 7075 Aluminum rod to make the pellets out of.

And, I had to get this in to show what I do during my free time  .  The airgun parts are setting on a 13 foot long dining table I am in the midst of making.  Still have to make the base and put a Danish oil finish on it.  The outer frame  is 2" white oak cut from our property and the field is heart pine from my mother-in-laws 1910 house.  I found them in the attic and it looks like the plasterers used them for walk boards.  Even with a lot of hand scraping ahead of time, they were death on the planer blades.


Here is the "before" of the pine boards.


If you look at the pictures in reply # 133 you'll see that I am also putting that pine and oak to good airgun usage, LOL.

Lloyd

[rsterne]

Hey, Lloyd.... slide me the gravy, will ya!.... Nice job on the table....

You have a new goal to beat.... 2043 fps with a .22 cal tinfoil bullet.... http://www.gatewaytoairguns.org/GTA/index.php?topic=104752.20

Bob

[lloyd-ss]

Hey, Lloyd.... slide me the gravy, will ya!.... Nice job on the table....

You have a new goal to beat.... 2043 fps with a .22 cal tinfoil bullet.... http://www.gatewaytoairguns.org/GTA/index.php?topic=104752.20

Bob

Sorry Bob, I don't do tin foil.      Need some FPE to go with the FPS.   
Lloyd

[39M]

I'll always take a good burger over a mediocre steak.   

------------------------------------

I need some help with o-ring material selection, please.   

This is fitting in a smooth bore  (.245 dia) and I plan on using what might be called a "too small" o-ring. I want to stretch it over the over-sized center post of the projectile so that the O.D. of the o-ring is tight and will resist rolling over.  I might have to experiment with fit, but probably about 7 to 9% compression,  which has worked so far in this experiment. Normal compressions are closer to 20%.

Size 005 o-ring .101 I.D. x .241 O.D.  over a .118 center post will give about .0635 gland depth in a .245 bore.  Compression will be approx 8% after accounting for the thinning due to stretching on the over-sized center post.
But the fit is an experimental thing, and I can putz with that.

Now, for the material selection and I am hoping you folks know more than I do about this.  This o-ring needs to do things that o-rings were never intended to do, but these are one-shot usages, so they can get smoked out the barrel. 

The O-Ring material needs to be:
Low friction for sliding along the smooth wall.
Heat resistant.  The friction will probably burn or melt it.
Stretchy. So that it can be stretched over the head of the pellet.
Abrasion/wear resistant so that it will hold a seal all the way to the end of the barrel.
Must be available  in small qtys (100 or so) without a $$$$ minimum order. It doesn't do any good if I can't buy it.
What do you think? Help! 

Edit--
After some research, am leaning toward Aflas (FEPM) or Viton (FKM, Fluorocarbon).  Lloyd

I don't know if it would be too heavy. But if you had a mold that the pellet head would fit in face down, you could put a tiny drop of lead to form a ring.

[39M]

Or I'm not sure if teflon can be melted, but it would seem the harder, slicker materials would be easier to install in a liquid form. Unless you had a tiny roll of teflon tape.

[lloyd-ss]

Or I'm not sure if teflon can be melted, but it would seem the harder, slicker materials would be easier to install in a liquid form. Unless you had a tiny roll of teflon tape.

Thank you for the suggestions.
Whatever I use, it has to able to hold 4500psi in a static state so that full pressure can be built up behind the pellet before it is released.  I will try some conventional o-ring materials first before getting to exotic.  Pat of it also is that will need a lot ( a few hundred) of these and they need to be easy to make.
Lloyd

[39M]

Or another thought.

The pellet really doesn't need the ring around the top to hold the seal in place, only the one behind the seal. So without the forward ring, a teflon seal could be easily pressed on. Plus, it would slightly lower the SD.

[39M]

Or another thought.

The pellet really doesn't need the ring around the top to hold the seal in place, only the one behind the seal. So without the forward ring, a teflon seal could be easily pressed on. Plus, it would slightly lower the SD.

I guess I forgot the pressure from behind would blow the seal off in this case. Disregard.