A few notes on conventional explosives.

By: Albert van der Sel
Version: 0.5
Date: 28 September, 2021
Subject: Limited overview of conventional explosives.
Status: Ready (for chapters 1 to 4).
Level: just a beginners level, ofcourse.

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I like to write this note, so that anyone who is rather blank on the subject, can get a resonable idea
about explosive substances, in a rather short time.

I consider the note as "ready", since chapters 1 to 4, represent my objective,
as expressed above. Possibly, at a later time, the note will be extended with additional material.
For example, in our society, it sometimes is needed to "detect" high explosives, before
any harm can be done (e.g at airfields etc...).

As you may know, there exists a large array of different types of explosive materials like RDX, C4 (which is mostly RDX), TNT,
or TNT with mixtures like Aluminium (tritonal) etc.., ETN, TATB, TATP, PETN, HMX, Semtex, NTO, black powder, smokeless powder etc.. etc...
Most of the above, are military grade products.

Especially in Mining operations, a few examples are the "family of ANFO products", and Dynamite. However,
many other products are used in the wide domain of mining. Ask an experienced miner, and you will hear
a large amount of information (and quite some interesting stories)....
Indeed, also products like RDX, TNT, HMX, etc.., and some mixtures, are used in civilian work as well.

We are going to find out, what those examples are, exactly. And more types.

There are lots of parameters here. Most folks use certain classification schemes
to order the various types into different catagories.

⇒ One very trivial one, and often used, is the classification low-, and high explosives,
which relates to the expansion of the initial blast, or more exact, the reaction speed of the explosive material.
So, "low" could be several hunderds of m/s, and "high" could be several thousends of m/s.

⇒ Many folks say that the upper classification can be much better defined. There is indeed a lot
to say in favor of that. They say: we have "low explosive" substances, and "primary high" or "secondary high".
So, here we have three classes. It has to do with the internal strain/status of the molucules in those high explosives.
The "primary" ones, are sensitive for either shock, friction and/or other disturbances. The "secondary" ones,
are not so delicate to handle, and are quite insensitive for shocks, friction etc...
This will be better explained later this note.

Notes:

1. The designations "primary high" or "secondary high", are also often called
"initiating" and "noninitiating" explosives.
These names are not strange. Since "initiating" is sensitive (e.g. shock), it can be used to ignite
the noninitiating compound.
Indeed, the "primary high" explosives are usually deployed in initiators/detonators, to "ignite"
the main charge (the "secondary high" explosives). But they can be used "as is" as well.
2. Low explosives, generally need Oxygen (often inline) for their detonation Process. Generally, High explosives
often do not require oxygen, or the oxygen is "built" into the same molecule.
Well, it's not completely "black and white", so in some cases, an extra oxygen agent is sometimes added.
Formally, the term "detonation" is incorrect for low explosives. See point 3 below.
3. Also, some folks tie the term "combustion" to low explosives, and the term "detonation" to high explosives.
However, the latter statement probably is not always appropriate. Opinions may differ ofcourse.
Unless one "ties" the term "detonation" to a high velocity shockwave, as is the case with high explosives.
From such angle, the term "detonation" indeed then only applies with high explosives.

Another key element that distinguishes "low explosives" from "high explosives" is the following:
With low explosives, the decomposition is propagated by a flame front (often called deflagration) which travels more slowly
through the substance, compared to the supersonic shockwave in a high explosive.

⇒ Some authorities make the division of "low explosives", "high explosives", and "blasting agents" (e.g. caps).
We have seen how low- and high explosives are described. Blasting agents are somewhat different, since
often it means a mixture of products, like "fuel" and explosive material, and such a product generally
is very insensitive. It often needs a primer for detonation.
So, blasting agents seem to be quite good for mining operations, but less good for military purposes.
Again, such staments should not be interpreted as "strickt" or "black and white". Exceptions surely exist.

⇒ Some use the "organic" (carbon based) and "inorganic" distinction. Organic molecules are the often
complex molecules like with "RDX" (C₃H₆N₆O₆), and inorganic which are not specifically
those complex molecules with C,N,H,O atoms, but more "simple" like for example "Lead azide" (Pb(N₃)₂).
As you will notice at a later time, the "inorganic" types are often Primary high, while the "organic" types are
often Secondary high. Again..., this is not a fixed rule. Exceptions do exist.

I believe that the above, represents the most important distinctions.

As a collary, we might mention the following too (not so relevant, I think):

_{⇒ Another classification could be the cascade, or train, of various types of materials where each

form the "trigger" for the next one. So, multiple substances are involved, and they form a cascade.

But you might say, that the this classification should better apply for complete devices (e.g. bombs),

instead of the substances itself. Indeed, classifications do not only exists for substances,

but for complete devices as well (obviously).

⇒ Then, we can also investigate if some explosive is more geared towards a function like armour piercing, like for example

"shaped charges" do, contrary to ordinary devices. But this seems again more apt for devices and internal layout.

⇒ Then, you could also characterize by liquids, solids (chrystalline or otherwise), gassuous etc..

⇒ Then, you could also characterize by which means a substance becomes unstable, and might react (like shock, pulse,

spark, electric pulse, temperature etc.. etc..). But that's already done in the Primary-, or Secondary- high explosives.

However, now you really demand, that it has to be electric, or has to be friction etc...

⇒ Then, one could also discriminate on the doping level of some additional substances.

Here, say we have some basis substance "X" which is quite effective. But it has been found that addition of substance "Y"

largely increases the efficiency of the explosion.

The levels (% of "Y") are then the catagories. It's actually quite common to add additional substances to the

basis substance.

⇒ Or, one places attention to a certain (important) element in the compound/substance, like for example non-nitrogenous compounds

versus nitrogenous compounds.

⇒ Or ofcourse, we also have the distinction between "nuclear explosive device" and "conventional explosive device".

Ofcourse, that's rather obvious. However, it's quite instructive to see "where the Energy originates from", as I will

show you later this note.

⇒ Other sorts of classifications like "Military" vs "Commercial" (e.g.mining, demolition) explosives, or "Professional"

vs "Homemade" explosives.}

Indeed, you can see that there are quite a lot of different classifications, each from it's own angle of perspective.

But even for certain conventional devices (e.g. casing, sensors, triggers, stages with different materials), are not so easy
to catagorize. For example, the Russian "father of all bombs" is a convential explosive device which at detonation,
produces rapid various stages, including a highly socalled "thermobaric stage" (later more on this).
This relates to TBX/EBX sorts of explosions. As said, later more on this.
The FOAB exists, but is not so practical to handle (transport, dropping etc..). It should be close to
the equivalent of 44 tons TNT.

So, often we see a classification which really relate to a specific substance, but also quite often as well,
which relate to the "device" as a whole (a potentially complex device, containing the substance).

So, what we have seen up to now, three often used classifications are:

- low explosives, and "primary high-" and "secondary high-" explosives.
- low explosives, and "primary high-" and "secondary high-" explosives, and blasting Agents (explosives for mining).
- organic (usually complex C,H,O,N molecules), and inorganic types of explosives.

Keep in mind that many other classifications go aroud to, like explosives based on certain C atom structures,
or the presence (or %) of Nitro atomic groups in the substance etc..

Let's see what we have, and what we can learn. In short chapters, some basic theory and properties will be explained.

Chapter 1. Nuclear binding-energy vs Chemical binding-energy.

1.1 Chemical binding-energy.

I suppose you know about the different "atoms" which exists. Or, related to that, the different "elements" which exist,
which is almost the same thing.
For example C, H, O, N, P etc.... This can also be seen in the Periodic Table of Elements.
This you may recall from highschool Chemistry.

Anyway, there are many sorts of chemical reactions. However, the atoms themselves, and the number of the particular
atoms, do not change. As an example, we may have a complex molecule, which by a chemical reaction, falls apart into a number
of smaller molecules. It's just an example. Some reactions "cost" energy (must be added), but with a lot of other reactions,
energy is delivered (get's free).

Energy sits in a lot of phenomena, like rotations, vibrations, magnetic momenta etc.., but the main part is formed by
the binding energies of electrons around the nucleus of an atom.
Usually, whenever at a reaction, where energy is freed, many electrons go into a lower potential compared to the situation before.
For example, take a look at a complex TNT molecule. At the reaction, the simple gasses CO, CO₂ and N₂ are formed,
where the electrons have stable covalent bonds (deep in the potential well), and the related energy difference is freed and turned
in to heat radiation, and kinetic energy of the residual gasses.
Chemical energy per atom or molecule, is typically in the order of hundreds of eV or thousends of eV.

1.2 Nuclear binding-energy.

With nuclear explosions, we have two main mechanisms: fission and fusion. Let's stick to fission for now.
Some heavy elements, like ²³⁹Pu and ²³⁵U, might show the behaviour that the nucleus splits into two medium weight elements,
under a neutron flux.
So, with fission, the heavy atom really changes into two lighter isotopes (and heat/energy and 2, 3 neutrons).

Now the comparison with Chemical binding energy: the binding energy of protons or neutrons in an atomic nucleus,
is in the order of MeV. Thus Mega eV, compared to hundreds of eV or thousends of eV when we look at chemical events.
THis simply means: if we have a few kg of military grade ²³⁹Pu or ²³⁵U, it's (in principle) in the order
of thousends times more energetic than a few kg of conventional explosives.

Chapter 2. When does a conventional explosive substance/compound, needs Oxygen?

More often No than Yes, at least with high explosives. This subject can be a tiny bit misty,
since many substances have "inline" Oxygen (Oxygen atoms part of the molecule).
In such case, Oxygen is provided "within", and does not need external Oxygen.

Real High explosives, generally, do not require Oxygen. However, in some cases, an Oxygen agent is added.

Let's try to shed some light on this matter.

2.1 The (ordinary) combustion process or combustion reaction (non explosive):

You can easily set wood or paper in fire. This combustion process needs external Oxygen to continue the process.
If you have a small airtight container, and put some substance in fire, then at a certain moment the combustion stops,
due to lack of Oxygen (from the air around us).
One of the most simple chemical equations, is the "burning" of natural gas, methane (CH₄):

CH₄ + 2O₂ -> CO₂ + 2H₂O

In general you might say that "Fuel + O₂ -> CO₂ + H₂O

2.2 (low) Explosive substance with "inline" Oxygen (Oxygen present in the compound):

Indeed, most "low explosives" consists of both a "fuel" and an oxidizing agent which releases oxygen by a reaction, often when heated.
One famous example is black powder, one of the first implementation of gunpowder.
In black powder, charcoal and sulphur represent the fuel, and postassium nitrate (KNO3) then is the oxidizing agent.

So, black powder does not need Oxygen from the air. It just is present within the compound, and will be used in the
fast combustion (the term detonation is often avoided with gunpowder).

So, if you have a gun cartrige, which is sealed by the bullet, that does not matter because the compound
black powder (the charge), uses inline Oxygen (in the powder itself) for the (fast) combustion.

2.3 High explosive example:

It's amazing how much research has been done, and still going on, about how the exact chemical pathways are,
when a high explosive takes place.
The experimental facts are largely know, like how much Joules energy is released "under such and such" conditions....
These you may see as facts. Also, the correct Physical/Chemical processes are largely known ofcourse.
However, sometimes, they are not always exactly known...
You may have doubts about the upper statements, because wiki pages etc..., shows you clear pathways.
But lots of scientific articles shows a slightly different picture.

But not to worry. Although a good understanding requires many stuff from Thermodynamics (enthalpy for example),
it's still possible to show a general framework on how a high explosive "often" explodes.

Let's take a look at TNT. One such molecule is rather complex. At detonation, one path of decompostion is:

C₆H₂(NO₂)₃CH₃ -> 3 N₂ + 5 H₂ + 12 CO + 2C

The above chemical equation, is only part of the possible pathways.
Ofcourse, one molecule is not a large "bang", but a number of 10²⁴ molecules, will deliver a large punch.

(I will explain the rather typical number of molecules like 10²⁴ , in the next chapter. It's linked to
a mole of a substance and Avogadro's number, which is about 6 x 10²³)

Note the nitro groups in the TNT molecule. Here it is NO₂, of which 3 groups exist. The covalent bonds of these
groups are not strong, and it's corresponding potential energy is high. The "N" atoms are very "eager" to form the strong covalent
bond of N₂, in which the electrons lose energy (they sink in the potential well).

This is an important part of the energy which is released when the TNT molecule is decomposed in smaller molecules
and elements. Due to the energy produced in a large "chunk" of TNT, the gasses obtain an increadable kinetic energy and
the volume is expanded enormously in a extremely short time. This is the detonation of TNT, described in simple words.

Explosive compounds, having significant amount of N, C, O, usually quickly will produce N₂ and CO₂,
and thus having smaller molecules with stronger bonds.

For some explosives, unstable nitrogen-nitrogen bonds exists, which will easily be triggered to compose N₂ gas,
which has much more stable bonds.

Ofcourse, later this note a more exact picture must be provided. Sure.., but one thing at the time...

Somewhere above in this note, I wrote "Low explosives, generally need Oxygen (often inline) for their fast combustion Process.
Generally, High explosives do not require oxygen."

What turns out? Adding oxygen rich compounds to basis TNT, enhances the detonation. So indeed, again things are
never completely black and white, whith sharp boundaries...
So,even high explosive substances, may benefit from adding an oxygen rich agent.

Generally, in many cases, this is done, to "burn" (let react) the Carbon with Oxygen, which produces
highly kinetic CO₂ gas,which adds to the blast.

Other mixtures with TNT and other substances are quite common, as well as with many other explosives.

We need a tiny bit of background info, say from Thermodynamics, Chemistry, general Physics, geared towards
the process of detonation, before we can go into discussions of specific substances.
That will be chapter 4. But first, let's take a look at a simple listing (a reference table), with some
important properties of some well known explosive substances. That listing will form chapter 3.

Chapter 3. Some important properties of some well-known explosives.

This is a table for reference, for the remainer of this note.
About 15 types will be listed with some main characteristics.
I could have listed more, but that is not neccessary for a simple note like this.

All values in the table, should give an "idea of the order" of the real value in a particular setting. That may sound a bit strange,
but take for example the detonation velocity in m/s. This is dependent from a number of parameters, like the density of the substance,
the volume of the substance, and other parameters.

No:	Name:	Type (usually):	Chem. Formula:	D. velocity m/s:	Appearance:	Since:	Remark:
1	Mercury(II) fulminate	Primary High	Hg(CNO)₂ or C₂N₂O₂Hg	around 4200	grey/white solid		Sensitive shock/friction. Used in detonators, caps etc..
2	TATB (not TATP!)	Secondary High (needs detonator)	C₆H₆N₆O₆	around 7200	Brown/yellow christals		Very insensitive. Used for very critical systems, where accidents must be ruled out.
3	TATP usually a mix	Primary High	C₆H₁₂O₄ / C₉H₁₈O₆ / C₁₂H₂₄O₈	around 5000 depending on mix	whitish solid		Very sensitive shock/friction
4	RDX usually a mix expressed in different "compositions". Examples: Semtex, C-4.	Secondary High (needs detonator)	C₃H₆N₆O₆	around 8000 depending on composition.	whitish solid		Secondary High, thus needs a trigger. Very insensitive.
5	C-4 Based on 90% RDX. Additions made to make it moldable (clay like).	Secondary High (needs detonator)	like RDX	around 8000 depending on composition RDX.	whitish clay like		Secondary High, thus needs a trigger. Very insensitive.
6	ANFO, ANNM Ammonium Nitrate+Fuel (Oil), Ammonium Nitrate+Nitro Methane + Fuel. Mixes of AmmoniumNitrate with a large choice of fuels. Very often Custom defined mixes.	Secondary High (needs powerful detonator/primer)	NH₄NO₃ (AN component)	depends on mixture, around 3200	purple chrystalline substance		Very insensitive, needs powerful detonator/primer
7	PETN Can be used "alone", but often also as PETN and trinitrotoluene (TNT) to form "pentolite". Can also be used with RDX, as to form Semtex. Most mixtures are in plastic form.	More of a Primary High due to it's relative sensitivity. Can be a Secondary High too.	C₅H₈N₄O₁₂	around 8000 depending on mixture.	White crystalline solid		Medium sensitive for shock/friction. Can be viewed as a Primary High. A molecule has 4 Nitro NO_2/3 groups.
8	TNT (2,4,6-trinitrotolueen). Used by itself, or also often as a component with other explosive substances. Also, additives like Al are added, for Temp increase. Sometimes, Ammonium nitrate is added.	Secondary High	C₆H₂CH₃(NO2)₃	around 7000	Yellow-brown chrystalline		Lower sensitive for shock/friction. Can be viewed as a Secondary High. A molecule has 3 Nitro NO_2/3 groups.
9	HMX (octogen) One of the most powerful explosives. In some applications mixed with TNT.	Secondary High	C₄H₈N₈O₈	around 9000	White solid, Colorless liquid		Lower sensitive for shock/friction. Can be viewed as a Secondary High. A molecule has 4 Nitro NO_2/3 groups.
10	NTO Nitrotriazolone	Secondary High	C₂H₂N₄O₃	around 7000, But several sources list different values.	white crystalline solid		Low sensitivity. Often seen as follow-up of RDX. It's a better component for Insensitive Ammonition.
11	Composition B	Secondary High	About 60% RDX, 40% TNT, some wax See those for Chem. formula.	around 8000	Yellowish solid, usually pellets.		Relatively Low sensitivity, but not low enough for modern insights. Common in older Shells, Grenades, mortars etc.. Usually getting replaced by alternatives.
12	Semtex See also RDX and PETN, as Semtex is a mixture of both. Several variants exists, determined by the % of RDX and PETN. Wax is added for molding (SBR). Also often called "pentrite". Instead of PETN, BCHMX might be used. Occasionally, instead of RDX, HMX might be used.	Usually Secondary High, (or Primary High (see PETN))	Several % of (usually) RDX, PETN e.g. Semtex 1H: C_6.39H_11.36N_5.34O_7.91	around 7000, depending on composition.	different colours, moldable plastic-like substances		Usually low sensitivity. Sensitivity depends on composition.
13	Lead azide (and other azides like sodium azide).	Primary High Used in detonators, caps. Used in smaller quantities.	Pb(N₃)₂)	around 5000	white powder		Highly sensitive for shock/friction.
14	Aziroazide azide as a nickname for 1-diazidocarbamoyl-5-azidotetrazole". Extremely sensitive. Almost never used.	Primary High.	C₂N₁₄ Substance has many "weak" N bonds.	around 8500		--	Extremely sensitive. Very sensitive for shock, friction, temp. Almost never used, unless for very special applications.
15	ETN	--	--	--	--	--	--

Chapter 4. Some important considerations with High Explosives.

Detonation is actually a supersonic shockwave(s) that pass through the material, thereby causing chemistry
that happens really much faster than "burning", or better, "fast burning" with low explosives.
It's a relatively thin layer that moves extremely fast through the substance, and there, chemistry happens,
like the breakdown of larger molecules, at high temperatures and GPa (Giga Pascal) pressure.

That's really the crux of the matter.

How to describe it in detail, is quite difficult. I am not describing that in any detail at all, in such a simple note.
But, in "Jip and Janneke" language, it's fun to do so.

A real-world detonation is not a perfect "continuum" in which you describe the event. There is probably about 100 years now,
of research and experiments, on what is happening at a detonation. Today, researchers are for example talking about
continuum and mesoscale models. At a mesoscale (say a few 0.01mm³), the effects of burning of microstructures,
like tiny Carbon structures are tried to take into account too. It's just an example.

Also, today, nifty computermodels are in use (and revised), and even so nifty experiments are performed,
at highly equipped laboratories.

The "science" here, is discovering the physics/chemistry at high velocity detonations, which is indeed
ofcourse scientific curiosity. And..., interest from the Military. And..., interest from Mining organisations.
The first two, are obvious. For the latter, as resources get more scarce, it's recognized that sharp, well-defined
charges, gives a yield which fits the current needs in a much more precise way.
Also, environmental awareness get's stronger and stronger all the time. Well.., not by everyone ofcourse..

So, what is studied? Chemistry and fluid dynamics, considerations from Hydrodynamics, Thermodynamics etc..
It's generally reckognized, that we have Chemistry at a thin, very fast expanding layer, the shockwave of detonation.

Directly at/after such shockwave, we have temperatures of about 2000 - 6000K, pressure of about a few hunderd thousends
atmosphere (e.g. 400000 atm) or tens of Giga Pascal, and Chemical reaction times of about 10^-6 to 10^-9 seconds.

One very remarkable fact of many detonation conjectures, is the phenomenon that the shockwave and chemical
actions right behind it (as a thin sheet, so to speak), is a "self-sustaining" phenomenon.
Well, the amount of explosive substance is always "limited" ofcourse, and the whole timespan of the
detonation is very short, so "self-sustaining" is a limited idea anyway, in terms of space and time.

Now, quantifying the process, is a bit involved.

Around 1900, the "CJ Theory" of detonations was formulated, by Chapman and Jouguet, as the first
mathematical attempt, with a physical/chemical basis of reasoning.
However, the CJ Theory is not adequate enough, as was later realized.

Around the 40's of the former century, the "ZND detonation model" was developed.
It does have a mathemathical framework, but it could also be explained (sort of) in plain English.

Short and simple explanation of the ZND model:

The detonation wave is a propagating at supersonic speeds. Most notably, the leading shockwave "moves forward",
and before the front, still untouched substance is present. Then, the wave adiabatically compresses the substance
(thus the reactants which soon will react). At that momen, the pressure rises enormously.

Almost directly after the shockwave (now neglecting an induction zone), we have a "reaction zone".
Now, the reactants are subject to the chemical processes. During these processes, a large amount of heat is released,
increasing the temperature but decreasing the pressure and density.
The latter sentence is actually just Physics, and nothing to worry about.
The endproducts (of the chemical reactions), often gasses, will expand in the reaction zone,
giving the detonation a big forward thrust. So, as long as untouched substance is present, we have a "self-sustaining" phenomenon.
An illustration can be found in the figure below.

It's pretty "cool", but ofcourse, it's a model, and since a few decades, improved by other models.

Figure 1. My own Jip and Janneke illustration. Simple figure to illustrate the above explanation.

For now: Considered to be the End of this note. Hope you found it useful.

Chapter 5. How to detect High Explosives.

This note is an introduction, for people who like a short text on High explosives, with at least "some" informational strength.

As a whole other type of business: how can high explosives be detected, before they can do any harm (airports etc.. etc..) ?
This is a whole other sort of science.