Pressure, velocity, and OAL

[Steve Anderson]

Does pressure = velocity?

For example,

If load A uses 3 grains of Supablast powder @ 1.15 and gets 1100 fps

And load B uses 4 grains of Supablast powder @ 1.25 and gets 1100 fps...

Do they have the same pressure level?

(BTW, same bullet for this hypothetical)

(Edited by Steve Anderson at 2:23 pm on May 28, 2002)

[ErikW]

That's one for the armchair ballisticians. I don't think even someone experienced in internal ballistics would answer that one without clarifying the question and what exactly you mean by "pressure." Pressure builds, peaks, and diminishes over time.

[Steve Anderson]

Ok, if we know that decreasing OAL increases pressure, I'm wondering if the same velocity at a longer OAL has less pressure, or if the pressure makes the velocity, so they are the same.

Less powder in smaller space = 1100 fps

more powder in larger space = 1100 fps

Is the pressure similar?

Does that help me clarify my question?

Or do we need actual math and stuff....

[Flexmoney]

Steve,

In general...I'd say, no.

It would be subjective though.

Different powders deliver their pressure over a curve.  Some are quick, some are slow.  Some hi pressure, some low.  

Many variables, I think.

(Edited by Flexmoney at 5:38 pm on May 28, 2002)

[Chriss Grube]

It comes down to MAP (Mean average pressure) Some loads reach peak pressure faster. The case capacity has a lot to due with the pressure.  If you seat a bullet shorter it will raise pressure (sometimes a lot) if you load longer it gives more space thus lowering the pressure. Basic physics is that the force required to move an object at a given speed is equal. In reloading it is the same. What matters is the pressure curve for that load in the gun / case combination. There are too many variables involved to give a blanket load. That is why they say to work up the load in your gun. Generally the slower powders reach peak pressure slower. The bullet is already moving thus increasing the space which contains the explosion of the igniting powder.

[jmaass]

There are examples in the VihtaVuori Reloading Manual (even the freebie pamplets) showing the effect in varying OAL on both pressure and velocity, as well as the effect of temperature on pressure and velocity.

Seating the bullet deeper in the case has a much larger change in pressure than in bullet velocity (4% vs. 0.7% for a -1% change in case volume for the example they show).

Actually, I'll just reproduce the table of the example:

                                      CHANGE IN      CHANGE IN

VARIABLE      CHANGE      VELOCITY      MAX PRESSURE

Bullet Weight   +1%               -0.4%            +0.8%

Charge Wt       +1%               +0.8%           +2.0%

Temperature    +1 degF         +0.04%         +0.08%

Cart. Case Vol  +1%               -0.7%           -4.0%

Lots of very interesting reading in the VV loading manuals!

[Steve Anderson]

Jeff,

That's exactly what I wanted to know. Can we assume that's relatively consistent for all powders? Probly not...

[shred]

Good stuff Jeff-- I'll have to go dig out my VV pamphlets.

I'm thinking that if my open loads follow those rates (which they probably don't), varying the powder +/-0.1gr means velocity will wander ~10 fps either way, and pressure (WAG here..) will be +/- ~800psi.

And a 20-25' F temperature change is roughly equivalent to 0.1gr powder.. hmm.

Time for some testing (as usual, DO NOT assume random math is equivalent to the real world.. especially when reloading)

(Edited by shred at 9:30 am on May 29, 2002)

[Clark]

The article is called "extremely interesting" by Richard Lee in his book "Modern reloading"

Pistols do not obey the 1% change in powder casues .8% change in velocity. They act more like 1% causes 1%.

It is interesting that the later Speer manuals apear to have been "dry labbed" with the VV formula to determine the starting load velocities for pistol cartridges.

[Patrick Sweeney]

Let's really muddy the waters with some extra physics.  If you plot the curve of the pressure over time, you end up with a sharp peak and then a taper down to the muzzle, when pressure drops to equal the atmosphere.

Work is equal to the area under the curve.  Work is what moves bullets.  Slower powders keep the taper higher, thus generating more work without increasing the peak height.

If you lengthen the loaded round, you increase the volume of the combustion chamber, thus requiring more powder to pressurize the case and cause explusion of the bullet.

Simple so far.

However, each powder has its own burning peculiarities.  Simply increasing the powder mass may change its reaction to combustion, the rate of onset, the top of the peak, the taper of the curve and thus work done.

So, by "pressure" do you mean the average of the peak?  Area under the curve?  Rate of onset to the peak? (The steepness of the initial upward curve.)

Generally, equal velocities with a given powder mean equal pressures.  Luckily, with most powders used in pistols, the difference between "generally" and "specifically" isn't very great.

[Clark]

Pat, you can write very well.

You should write a book:)

[Tapper]

Pat,

I'd disagree just a bit, if you'll forgive me.

>If you lengthen the loaded round, you increase the >volume of the combustion chamber, thus requiring >more powder to pressurize the case and cause >explusion of the bullet.

What happens when the OAL is shortened in pistols burning a fast powder, relates more to travel time.

Consider two rounds, one with a "normal" OAL, and another with a shorter length. If we assume the same charge in both cases, and that the distance from the breechface to the leade is the same, then the reason we see higher chamber pressures in shorter rounds is a fairly simple matter of travel time.

Bullet 1 travels distance X during the initial period before encountering the leade, and Bullet 2 travels distance X+(difference in seating depth).

Pressure is proportional to the amount of gas produced (a function of burn rate) and the volume the gas occupies.  

So - Bullet 2 travels a slightly longer distance, and in time it takes to travel that distance more gas is generated for the same volume, - and therefore, pressure is higher.  Same absolute volume, more gas = higher pressures.