From: ahahma@polaris.utu.fi (Arno Hahma)
Newsgroups: rec.pyrotechnics
Subject: Re: Two powder questions
Message-ID: <1992Nov28.133035.16343@polaris.utu.fi>
Date: 28 Nov 92 13:30:35 GMT

Mika Matti Jalava (mjalava@lk-hp-22.hut.fi) wrote:

: Is there some difference in the chemistry of deflagration and
: detonation for example in flash powder? I do understand that

I wish you could ask something easier ;). Yes, there is a big
difference, how the reactions go. I have to go rather technical in the
following. I will not explain technical terms and abbreviations, or I
would not stop writing today :-).

If the reaction is a deflagration, there is relatively much time for
the reactions. Also, the pressure will not be very high and things
like the covolumes of the molecules do not affect the chemistry much.
The combustion will be quite close to the chemical equilibrium at
deflagration pressure and temperature.  What is this equilibrium
condition, its temperature and composition, can be calculated very
accurately, but not easily.

In case of a detonation, things go difficult. Heat of formation alone
no longer tells, which species will be mostly present in the products.
Covolume becomes increasingly dominant, as the pressure goes up, the
smaller the molecule the more favorable its formation. In addition,
there is very little time for the reactions. This is a headache, when
one tries to estimate mixtures containing metals and heterogeneous
mixtures.

There is experimental evidence, that aluminum does not take part into
the detonation completely, but only a few per cent of it reacts and
contributes to the shock wave. The rest will burn along the isentrope,
heating the expanding gases. This is very hard to model, as one does
not know about the kinetics of the Al-burn. I have assumed a linear
dependence while modelling explosives with metals (certainly wrong but
a way easier to program than other models).

In case of flash powder, the Al is the only fuel and thus it has to
contribute to the detonation. The detonation velocity of flash powder
is around 2000 m/s, which corresponds to only a few percent reaction
by the C-J-plane. Also, the detonation does not reach its temperature
maximum at the C-J-plane but only later on the isentrope. This makes
the overall pressure wave much flatter than with an ideal explosive of
the same energy content. It also explains very well the huge flash and
the boom flash powder causes.

: detonation means that the material decomposes rapidly due to a
: pressure wave, but does the same redox reaction still happen? If not,

You are correct about the pressure wave (shock wave). No, the same
redox reactions do not happen. In a deflagration, you end up with
about the theoretical composition including the close surroundings,
such as air (AlN is a major product, too). In a detonation, the gases
expand so fast, that they will not have any time to react completely.
Therefore, the detonation products will be more or less incompletely
reacted, as can be seen from any high explosive detonation (solid
carbon is a major product even for zero oxygen balance explosives with
a lot of nitrogen).

: does detonation cause a much different visual/sound effect if compared
: to a rapid deflagration (I would expect a louder report)? Is it even

It usually produces a sharper and shorter report. In case of flash
powder, the difference can not be heard, as the whole detonation is a
sort of a deflagration at the same time.

: possible to make such flash that it only deflagrates also when
: confined, like bp?

Possible, but hard. Without perchlorates, it might be successful.

: Then the other powder question. What kind of powder is used in
: artillery blanks, or any other blanks that don't offer much
: confinement (maybe shotgun)?

a) black powder
b) very porous smokeless powder, so called priming powder
c) mixtures of the two above, either separately or incorporated.

The blanks in use here have the option c) with incorporated mixture.
That is, fast smokeless powder with about 50 % of black powder in it.
This kind of a priming powder (used in the artillery ignition trains,
among others) is very fast and needs little confinement.

: is not much left of the "confinement" after the cap ignites (even the
: cap alone gives quite a bang).

There is. The powder is so fast, that the cardboard disk does even not
know it ignited, before it is burnt ;-). More confinement might cause
excessive pressure loading on the tube.

: 	Mika

ArNO
    2

From: ahahma@polaris.utu.fi (Arno Hahma)
Newsgroups: rec.pyrotechnics
Subject: Re: Overpressure wavefront analysis
Message-ID: <1992Nov28.133802.16713@polaris.utu.fi>
Date: 28 Nov 92 13:38:02 GMT

: In article <1ebtjmINNdjg@endeavor.ksu.ksu.edu>
: lyn@endeavor.ksu.ksu.edu (Myself) writes:

: >  Is there anyone out there who can assist me in finding information on
: >determining the shape of an advancing overpressure wave front caused by
: >detonation/rapid deflagration of a chemical/nuclear 'explosive'?

Try books about detonation. For doing the job by computer, I would
recommend looking at: C.L.Mader, "Numerical Modeling of Detonations",
UCLA Press. The book describes algorithms for doing such a calculation
for explosives. The same ideas can be used for any shock wave, just
leaving out the chemical reaction at the shock front. Be prepared to
study the subject for a long time, the book is _really_ technical and
hard to understand. But it does answer your question...

ArNO
    2

Newsgroups: rec.pyrotechnics
From: ahahma@utu.fi (Arno Hahma)
Subject: Re: detonating BP
Message-ID: <Cxor3y.G8J@utu.fi>
Date: Fri, 14 Oct 1994 23:11:58 GMT

billn@hpcvaac.cv.hp.com (bill nelson) writes:
> acs@leland.Stanford.EDU (Alexander Caldwell Schneider) writes:

> : detonation.  As I understand things, black powder, flash powder, and
> : similar mixtures "explode" because they can produce a large quantity of
> : gas in a short time, quickly enough to rupture a container essentially

You are right. Those mixtures simply burn so fast that the gases will
not have time to escape before the container ruptures.

> : (convection).  Good flash powder produces a report without confinement
> : because the "reaction front" propagates faster than sound does in air,
> : thereby making it appear, from an acoustic standpoint, that the entire
> : mass of flash powder has turned into gas "instantaneously."

That is exactly what happens with flash powder.

> of reaction are solids.  For example, the Al2O3 formed quickly condenses.
> This robs heat from the reaction - reducing the energy available to do

In fact, condensing things adds thermal energy to the system. Potential
energy (i.e. pressure, latent heat) is converted to thermal energy, if
materials condense. Therefore, condensing materials are not useful in a
detonation.

> The shock wave creates hot spots - where the material starts to decompose.
> This decomposed material then undergoes the DDT transition. This is a very
> simplified explanation of what I believe is the current theory of detonation.

That explanation works only for heterogeneous or less reactive
explosives (so called 'Forest fire' -model; after the inventor).
Homogeneous explosives detonate with a different mechanism, although
hot spots will still aid the initiation process.

> : ANFO is understood to detonate?  I could possibly understand emulsion ANFO
> : detonating, simply because of the intimacy of mixture, but prill+kerosone?

> For ANFO, the oil provides fuel that the AN by itself is lacking. This makes
> the propogation of the detonation more certain.  Of course, if a massive

In ANFO, the detonation extracts energy mostly from the AN only. The
oil helps in ignition feeding the hot spots with a fuel. The hotter
burning the fuel the more sensitive the mixture becomes, as the hot
spots become hotter. Therefore, aluminum dust is a very effective
sensitizer for AN, additionally, an Al particle can be the hot spot
itself.

The detonation itself hardly can extract any energy from the fuel
component, better vice versa. As oil is shocked, it decomposes, which
is a strongly endothermic reaction. AN can produce energy under a
shockwave influence, as in AN the oxygen and fuel components are close
enough (a few atomic distances apart at most). Even then, only about 50
% of the AN will participate the detonation (i.e. feed energy to the
shock front), the rest will burn while the gases expand or scatter
around. That is also, where the fuel components (oil or whatever)
burn or then not.

> Why wouldn't you expect an answer?  I would much rather discuss the subject
> of detonation/deflagration - rather than how to make a bomb in your bathtub,
> or how to get a worthless piece of literature (just because it is free).

Me too. There was just quite a little response to this thread -
unfortunately.

> Bill



mdelevie@wam.umd.edu (Mark Ysbrand Delevie) writes:

# 1)	am I right to assume that a true HE then does not need confinement?

Yes, you are. A true explosive needs no confinement to detonate.

# 2)	can a FAE, even an energetic one, *ever* be a HE?  It seems to me

It can, if you are thinking of a detonation. A fuel-air mixture is
naturally much less effective than a condensed phase explosive, as far
as pressures and energy densities are concerned.

# oxygen indicates a high-order explosion.  However, as a mixture of gases, it
# is impossible for a shock-wave to travel through faster than the speed of
# sound... because the speed of sound *is* the speed of a wave through a gas

A shock wave will always travel much faster than sound - that is one of
the definitions for a shock wave. In a gas mixture a shock is even easier
to make than in a solid material.

A shock wave needs significant compression ratio (ratio of material
density behind and in front of the wave) to proceed. In a gas this is
easy to accomplish - gases are readily compressible. In a solid or
liquid, you need tremendously more energy to reach even a few tens of
percent compression. This is why air-gas mixtures detonate real easy,
provided the mixture ratio is correct.

# medium.  So is this a super-fast deflagration that is initiated by a flame
# travelling at the shock-wave (flame front, whatever) face?

It can be, but then you don't get any FAE, just a fireball. In a real
FAE, the cloud detonates and the shock propagates at 1500..2000 m/s.

ArNO
    2

Newsgroups: rec.pyrotechnics
From: ahahma@utu.fi (Arno Hahma)
Subject: Re: Fuel Air
Message-ID: <CCoI59.JMr@utu.fi>
Date: Wed, 1 Sep 1993 14:12:45 GMT

In article <1993Aug25.175817.1786@quest1.uucp>,
Timothy Melton <tam@quest1.UUCP> wrote:

>guess.  To initiate a detonation in an unconfined cloud of methane, it
>takes a mass of HE roughly the same as the mass of C1 in the cloud.

Well, not quite. You do need a fair amount of HE to initiate any cloud
of saturated hydrocarbons, but once you get the detonation going, it
will pass through the cloud. With methane, a few kg of TNT or
equivalent should be enough for initiation.

>Also, most pyrotechnics (maybe with the exception of flash, Bill?)
>deflagrate and do not provide suitable initiation for detonation either in
>vapor clouds or HE.

Although they do deflagrate, they may still create a shockwave in the
air. If the deflagration or the expanding gases are considerably faster
than 330 m/s (sound velocity in air), it will cause a shockwave in the
surrounding air. Of course, you will have to use far more material than
you would have to with HE:s.

>Tim Melton                     uokmax!quest1!tam

ArNO
    2