From: reidld@ecf.toronto.edu (Lloyd D Reid)
Newsgroups: rec.guns
Subject: Powder Detonation [LONG]
Date: 13 Nov 92 12:29:38 GMT
Organization: University of Toronto, Engineering Computing Facility

Occasionally the topic of powder 'detonation' comes up in this
group.  I find the subject rather interesting, and would like to
offer a theory as to what is happening.

First, I'll describe some of the reported causes of the problem that
I am aware of:
- a lower than normal powder charge sometimes results in abnormally high
pressure increases.  Blown primers, stuck cases, or worse....
- loose powder charges seem to make the problem worse (that is, an increase
in the ammount of 'ullage' - (thats fancy talk for 'empty space'))
- cold weather can sometimes make the problem worse
- happens in the larger rifle cartridges (at least, I haven't heard about
it in shotgun or pistol cartridges)
- ball powders are more susceptible
- slower burning powders are more susceptible
- magnum primers are recommended as a way to reduce the problem

If you are unfamiliar with the interior ballistic cycle, I have attempted
to give an explanation at the end of this posting.

My theory:

I believe that the so-called 'detonation' of powder is caused by incomplete
ignition of progressive burning powders.  I do not think a true
detonation (in the technical sense of the word - supersonically
propogating shock-induced bulk chemical decomposition) is taking place.

First, consider a rifle cartridge using a large progressive powder charge
that is designed to be ignited simultaneously, quickly reach a peak pressure
that is within safe limits, (but as high as possible in order to maximize
velocity), and increase its burn rate after peak pressure is reached,
in order to continue the maxmimum safe acceleration (pressure) of the
projectile.  One can see that if the bullet does not accelerate as planned
(due to bore obstruction obstruction, or a bullet of higher weight was
substituted), the powder charge is far more than adequate to cause a
severe overpressure or even blow up a gun.  The reason a gun does not
blow up under normal use is that the charge is designed to burn at a
relatively slow rate for the first part of the bullet's travel, only
increasing the gas generation rate when the bullet is moving rapidly
enough to expand the combustion chamber at a sufficient rate to
keep the pressure within safe limits.  Faster burning pistol and
shotgun powders produce too much gas too soon; an equivalent mass
(ie, roughly the same energy content) of these will easily blow the rifle.

Now take this safe powder/bullet/primer/case/rifle combination,
and reduce the powder charge substantially (say, 30-40%).  Now lets
say that the powder charge does not receive a uniform ignition, due
to the loose packing of the powder grains.  Let us assume that
initially, only 50% of the powder is ignited.  Now the pressure
begins to build, and the remaining (unlit, not yet burning) powder
begins to rise in temperature, and get ready for ignition.

However, because only a relatively small amount of powder is burning,
it will take longer for the 'shot start' (bullet movement) pressure
to be reached.  This means that, of the grains which are burning, a
greater fraction of their mass is consumed before enough pressure
is generated to begin moving the bullet (and start to increase the
volume available for the gases to occupy).

Let us now assume that just before the bullet begins to move, the
remainder of the powder begins to ignite.  The first half of the
powder charge is now substantially advanced into its burn cycle,
well into its accelerating progressive burn phase.  This means that
each of these grains will be generating gas at a higher rate than
would be normally seen at this early stage of the cycle (little or
no bullet movement).  The second half is burning at more or less
the nominal shot start burn rate.

We now have a situation where we have a slow moving bullet, half the
powder charge burning at the correct rate for this inital part of
the ballistic cycle, but the first half is now burning at a ferocious
rate.  This will result in an excessive gas generation rate,
leading to peak pressures greater than those produced by the heavier
charge (which was uniformly ignited).

As far as I can tell, this theory is consistent will all of the
exacerbating factors I listed above:
- cold weather:  A cold powder charge is more difficult to ignite,
more subject to partial ignition
- larger cases: Given the same primer, a larger case has more powder
that must be ignited.  Also, the larger cases tend to be higher
performance - higher pressure, slower powders (=more difficult
to light)
- rifle vs (shotgun, pistol): Shotguns and pistols are lower performance
(that is, have a lower powder charge/bullet mass ratio), and use
faster burning powders.  These smaller, faster burning charges are
easier to ignite.  Also, such powders do not tend to be as
progressive as the rifle powders.
- ball powders:  Ball powders tend to be extremely regressive burning
because of their spheroidal geometry.  In order to compensate for
this, and produce a progressive burning powder, the outer layer of the
grain is heavily treated with burn rate inhbitors (much more so than
stick propellant).  As a result, ball powders are more diffcult to
ignite, and more susceptible to erratic ignition.
- slower powders: For an equivalent grain size and geometry, slower
powders use more inhibitors, for the same effect as seen with ball
vs. stick powders.
- magnum primers: These deliver a hotter spark, tending to improve the
completeness of powder initiation.


Anyone else have any pet theories on this topic?

Daniel Chisholm  (posting from prof's acct.)


On a semi-relevant note,

Charge designers for artillery-size guns sometimes experience 'pressure
excursion' problems.  There is evidence that attributes this to a poor
(nonuniform) powder ignition, which produces pressure waves inside the
combustion chamber causing an excessive increase in the powder burn
rate.  I think the faster burn time and smaller combustion chambers
found in small arms would severely reduce the effects of such pressure
waves.  Of course, I am open to correction on this...


***************************************************************************

	APPENDIX  "Now, the interior ballistic cycle as I understand it."

I think there are several people on this group who can probably correct
errors I may be about to make, or add to an incomplete description.
Lets hear from you guys!

When the primer fires, it emits a high velocity jet of hot gas and
fine particles, which passes through the flash hole and through the powder
bed.  This raises the temperature of the individual powder grains'
exterior, until the propellant's ignition temperature is reached.  
Due to the size of the powder bed (many flash hole diameters long in
rifle cartridges), the ignition will not be perfectly uniform.  One
would expect the grains in close proximity to be well-lit before the
more remote grains are ready to start.  For now, let's assume
simultaneous ignition of all powder grains, for simplicity.

The bulk mass of a powder grain is cold, and will not be burning.
Only the exposed surface of the grain in contact with the hot
gas will be at a high enough temperature to be chemically decomposing
and producing hot gun gas.  Basically, only the surface of the
grain is burning, at a rate determined by the local chemical
compostion, pressure, and temperature.

At this point, we have a confined space in which the pressure has
been raised somewhat by the primer's gas generation, and a mass
of burning propellant which is emitting hot gas, causing the combustion
chamber's pressure and temperature to continue rising.  The gas
generation rate is usually modelled by
	G = R*A*rho	G = gas generation rate  (mass/sec)
			R = powder grain linear burn rate (this is
			    on the order of several inches/sec - thats all!)
			A = exposed surface are of burning powder
			rho = density of (solid) powder grain

	R = k p^B       k = a fn. of temperature and chemical composition,
			p = pressure, B (beta) = 'pressure exponent'

This means that the pressure intially starts to build exponentially.
Higher pressure leads to a higher gas generation rate, and even more
rapidly growing pressure, until at a certain point the bullet begins
to move (breaks free of the case neck friction, engraves itself in the
rifling), and the volume of the combustion chamber begins to increase.
The artillery engineers call this event 'shot start'.  In their case,
the shell is rammed against the rifling and partly engraved.  The shell
is designed to not begin movment until a fairly large percentage of
maximum pressure is reached.  In small arms, we have a more complex
situation where the bullet motion is resisted by case neck friction,
usually followed by a jump in the freebore before the bullet engages
the rifling at the leade.

#From the point on, the bullet is accelerating, causing the combustion
chamber volume to grow at an ever-increasing rate.  This expansion
causes the pressure and density of the hot gas to fall (or, for the
initial stages, slows down the rate of increase).  After a fairly
short 'shot travel' (couple of inches), pressure has reached its
peak value.  From this point on, even though velocity is increasing,
the pressure (and hence, acceleration) is decreasing.

In order to maximize performance (velocity) from a given combustion
chamber volume and barrel length, we want to tailor the burn
characteristics of the powder to achieve
- a rapid buildup to maximum working pressure
- maintain this safe pressure as long as possible.

This requires matching the gas generation rate 'G' to the (very closely
related) bullet acceleration.  Since the bullet is accelerating and the
combustion chamber is increasing in volume at an ever-expanding rate, the
powder must increase it's burn rate as the cycle progresses.  Such a
powder design is referred to as 'progressive'.  The other two
types are 'Neutral' (no change in burn rate, except that caused
by pressure), and 'Regressive' (burn rate slows down).


	Progressive Powder Design and Characteristics

Progressive powders achieve their characteristics using two methods:
	- grain geometry.
	- inhibitors.

Powder grains can be designed so that their exposed area increases as the
burn progresses.  Hollow sticks (cylinders), coated on their outside
surface with a chemical agent to prevent burning there, have a surface
area that increases as the grain is consumed.  A hollow cylinder which
burns from the outside as well as the inside wall is neutral burning
(the exposed surface area remains constant).  A sphere is regressive;
its surface area reduces as 1/r^2.  Grain gemometry affects the 'A'
term in the gas generation equation; 'A' may be a function of 'x', the
mass fraction consumed.

The other variable we can play with is 'k'.  By diffusing chemical
combustion inhibitors into the powder grain, we can cause the chemical
composition of the grain to vary as a function of 'x', the mass fraction
of the grain.
-- 
reidld@ecf.toronto.edu (Peter Grant)
reidld@ecf.toronto.edu (Peter Grant)