Calculating chamber pressure

[Lead Freak]

Is there a simple formula for calculating chamber psi, knowing boolit weight and muzzle velocity?

[olafhardt]

No , even with strain gaugess and piezo detectors the best you get is a sort of a guess. I put in some years as a instrument engineer and have never heard of a method that gives any better than a WAG. However, I can't do any better. Follow published data and start low and work up.

[Larry Gibson]

Is there a simple formula for calculating chamber psi, knowing boolit weight and muzzle velocity?

No.

Piezo-transducers and strain gauges are the best actual measuring methods we have so far.

Even the sophisticated programs like QuickLoad are only an educated guess as there are so many variables and the input has to be correct or "garbage in, garbage out".

As mentioned, best to follow manuals data and instructions on working up a load and watching for excessive psi signs in your gun with your components.

Larry Gibson

[popper]

Yes, there is a simple formula but it will NOT give you the correct answer. Don't even try.

[jonp]

Of course there is! However you must do these in order:

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[jonp]

Which will lead you to The Steady State Chamber Pressure:

Attachment 75104

and finally to the Tail Down Phase of Chamber Pressure after the propellant has been completely consumed and The Tail Off Chamber Pressure:

Attachment 75107
Attachment 75108

This is the chamber pressure and burn rate formulas for a rocket engine and if you consider it for a moment a caseing is a rocket motor case with the propellent going the opposite way firing down the chamber and the case itself the rocket motor case transferring its pressure to the surrounding chamber.

[williamwaco]

jonp

I am working on these and have worked through equations one through six but I can't read equation 7 and 9. Could you re-post them?

[popper]

Then calculate the twist & COF & heat transfer to bbl & expansion of bbl and you'll have it. Oh, don't forget the non linear acceleration rate, which a burning gas doesn't have.

[Shiloh]

After looking at the math, think I'll start with published data, start low and work up.
This has served me well for 30+ years.

Shiloh

[MtGun44]

NO.

Bill

[303Guy]

If only rocket scientists were interested enough they would find a way.

[jonp]

jonp

I am working on these and have worked through equations one through six but I can't read equation 7 and 9. Could you re-post them?

My post was tongue in cheek in that there is no "easy" way to determine chamber pressure as others on here have said.
When I loaded something some time ago and had a nice fireball I thought "that looks like a rocket taking off" and a lightbulb went off and I started fooling with this.
Here is a link to a nice article with explanations if you are interested: http://www.nakka-rocketry.net/th_pres.html
I was serious in that I do think a chamber is kinda like a miniature rocket engine but I don't think it is an easy translation or entirely applicable but if you have the time could come pretty close.

[truckjohn]

I suppose a simpler way to explain is that you are talking about Energy - which is the interaction of Weight and Velocity....

You can generate the same quantity of energy several ways.... You can make a high, short spike... Or you can make a lower, longer curve....

With "Slow burning" powders - you tend to generate a lower pressure curve that lasts longer.... Fast burning powders tend to generate a higher pressure curve that dissipates sooner....

THEN... Within this, you can have powders that Start faster or Start slower, End faster or End slower.... and there are powders that burn most efficiently within a certain pressure range.....

All these can generate the same "Velocity".... but will generate FAR different pressures....

Thanks

[popper]

jonp - i kinda thought so. The biggest problem is the rocket motor has a highly controlled burn, guns not so much. My comment was to dissuade someone else from trying to 'do the math', thinking they would get close. The math is interesting though, kinda like gyro math. Now if you know the dynamic velocity down the bbl, you can come close to calculating the dynamic pressure. You can get velocity (close) from olher pressure measurements. You could also get felt recoil by putting a 3 axis accelerometer on the stock. Most of us just deal with it.

[rsrocket1]

Try out Quickload. Yes, it's only an estimation, but it is darn close probably close enough for "gub'ment work" (spoken like a true contractor). Download the demo and buy it if you like it. If you are a propeller head and like developing and testing loads, it's well worth the money. Go over to their site and check it out. While you're at it, follow their link to high speed videos and watch their Youtube video:
http://youtu.be/QfDoQwIAaXg

I dare you to quit watching it in the middle. You'll be hypnotized for the whole 10 minutes.

[gspgundog]

Dang this conversation hurts my head think it is time to go back to Real Housewives of New Jersey

[jonp]

jonp - i kinda thought so. The biggest problem is the rocket motor has a highly controlled burn, guns not so much. My comment was to dissuade someone else from trying to 'do the math', thinking they would get close. The math is interesting though, kinda like gyro math. Now if you know the dynamic velocity down the bbl, you can come close to calculating the dynamic pressure. You can get velocity (close) from olher pressure measurements. You could also get felt recoil by putting a 3 axis accelerometer on the stock. Most of us just deal with it.

Indeed although the difference between a controlled burn and a pressure spike of a chamber with a round being fired is just the amount of time. I think you could work with these equations and come close enough to satisfy curiosity or at the least come within a standard deviation which would keep you under an unsafe level.
Lots of math but interesting.