From: email@example.com (Gerald L. Hurst)
Subject: Air blast, energy & gas release
Date: 26 Mar 1996 07:22:33 GMT
I was corresponding with a friend in the explosives business when
the subject came up concerning the effect the relative amount of
gas released by an explosive on the air blast. The following are
a couple of exerpts from my writings which I have combined
without much editing. The treatment of the subject is informal
and to some extent anecdotal, but it may be of interest to those
with a serious interest in explosives.
If you think you can find exceptions to what I have written, you
are undoubtedly right, but who wants to read a rigorous mathematical
treatise in the midst of all those alt.engr.explosives smoke bombs :)
Years ago, EXCOA frequently published a "figure of merit" for their
explosives based on the product of the gas release and the energy.
This figure was nothing more than commercial hype, but it has left
the lasting impression that the amount of gas release is more
important than it is in reality. If you double the energy output and
halve the amount of gas produced, the performance in terms of peak
pressure and work output in the expansion phase will vary not
because you have less gas, but because the product gases have a
higher heat capacity at higher temperatures. If the difference in
gas release is small then the difference in temperature is also small.
Under these conditions, there is little difference in the heat
capacity and thus the difference in performance attributable to
reduced gas release is mitigated.
As you know, it is dangerous to generalize, but let me go out on a
bit of a limb. If you simply ignore gas volume and concentrate on
plain old energy you'll generally get a reasonable estimate of air
Look at the ideal gas law PV = nRT. Think of PV as the work a gram
of explosive does as air blast. Then n is the gas release per gram
and R is a new form of the gas constant we derive to suit our
Were it not for increasing specific heat at higher temperatures we
could write for a given energy output n x T = K. So if you double
the gas (n) the temperature falls in half and PV work) stays the
Now, in real life, the diatomic and triatomic gases (N2, H20, CO2)
show gradually increasing heat capacities with rising temperature,
so you would think there would be an advantage in an explosive
with high gas release because the lower heat capacity at lower
temperatures should give you a somewhat larger PV (work energy)
However there are two more rubs. Firstly, if you have less gas, PV
may not be quite as big, but the hotter gas cloud expands faster,
and because of its higher heat capacity it cools off more slowly
and thus sustains the pressure peak longer.
You have also to consider the reason for an explosive having a
relatively low gas release. It could be because it produces more
high molecular weight species like CO2 than it does lower MW
gases like water. CO2 also has a high heat capacity so the arguments
in the preceding paragraph apply well to such materials, The
octanitrocubane molecule would be an extreme case as would be the
old carbon/LOX explosive.
Explosives like TNT and its close relatives suffer from a shortage
of oxygen so they spit out a lot of solid carbon. As it turns out,
though, carbon has a quite low heat capacity so it does not compete
that much with the gases for energy. TNT's lack of outstanding
performance is mainly a matter of mediocre energy.
In the extreme cases one might ask about explosives with other
solid products other than carbon. BP is a case in point, and with
its low energy, there is no doubt that solid products compete for
its meagre energy. You don't want to take this analogy too far
though, because with higher energy explosives those materials that
are not normally thought of as gases may in fact become gases.
Look at ordinary flash powder:
8Al + 3KClO4 --> 4Al2O3 + 3KCl
No gases at all by the look of the reaction, but the reaction is
so hot that the KCl acts as a gas and the Al2O3 dissociates into
gaseous Al2O and O2 and it puts out one heck of an air blast.
It is a matter of sheer energy turning a solid pig's ear into a
gaseous silk purse.
Perhaps the classic example is Astrolite G compared with (Aluminized)
Astrolite A. G was a very high gas release product with very good
energy, comparable to good military explosives, and a great deal of
brisance for its density. The high gas release (>> 1000l/kg) was due
to the high proportion of water and hydrogen in the products and the
complete absence of solids or CO2. G was one of the quietest
explosives pound for pound of any you have ever heard.
But when that clear liquid was doped with a very high percentage of
aluminum to make a more energetic material with a hypothetically
lower gas output, the charges ROARED and the airblast was phenomenal.
This was why we chose that particular formulation for LZ clearance.
On a much milder scale, at what later became The Kinepak plant,
we first manufacteured Moleculite, an AN/DNT/Al explosive containing
a mere 6% aluminum, but again this mild composition ROARED by
comparison to G.
The first time I shot G vs. Moleculite on lead cylinders to show
Roger Byers, the owner of the Moleculite formulation, the detonation
pressure of G, he first shot Moleculite charge, KABOOM, and got
a nice mushroom. He grinned when he heard the anemic PING of the
G and made some disparaging remark. You should have seen his face
when he saw the lead cylinder turned into a large thin plate.
There appears to be magic in getting a hot cloud even at the expense
of some volume, at least for air blast.
You know the above treatment is not rigorous, but you may find it
handy as a rule of thumb.