Post by David Spain Post by Pat Flannery Post by Scott M. Kozel
Wear Kevlar clothing and a full helmet ?
...the H-Bomb of fireworks. ;-)
[...]The scientific community defines
Post by David Spain
a high explosive as one that detonates when unconfined.
Not so. Though the definition isn't actually standardised afaik, a high
explosive is "brisant", which is French for shattering. I'll explain
this term later.
Moreover, it's better to refer to high and low explosions than high and
low explosives, as a particular explosive may go off in either high or
low mode depending on size, conditions etc.
However some explosives can't be brisant, and can be called low. I don't
know offhand of any explosive which is always brisant, but if it existed
it would be called high.
(when confined flash most definitely _can_ go high, as can even confined
gunpowder in very large quantities, though the latter is rare).
A low explosive is
Post by David Spain
defined as one that deflagrates - not detonates - whether confined or
A low explosion is one which is not brisant. Confinement per se is
irrelevant to the definition of an explosive, except maybe for legal
reasons , though it can turn a non-brisant explosion into a brisant one.
The distinction between "detonate" and "deflagrate" is the key
Not necessarily. All high explosions are detonations, but not all
detonations are high explosions.
A low explosive, that deflagrates, generates pressure waves
Post by David Spain
in the air that are slower than the speed of sound, while a high explosive,
which detonates, generates pressure waves that are higher than the speed of
Deflagration and detonation refer to the speed of reaction through the
explosive itself, not the blast effect through the air.
Indeed. The crucial difference between a deflagration and a detonation
is the mechanism of propagation. In a deflagration the mechanisms by
which energy is transferred to unreacted material are varied, including
thermal transfer by conduction, radiation, hot gases getting between the
cracks or gaps in gunpowder, etc.
In a detonation the major mechanism of propagation is by supersonic
shockwave. For a high explosive this occurs at typically 2-3 times the
speed of sound in the unreacted material.
In a shockwave the pressure can be very high indeed.
Imagine you have some explosive in a container, and you set it off. If
the container is really strong and doesn't conduct heat (a force-field?)
then the eventual conditions will depend solely on the chemistry of the
explosive, and the methods and paths the reaction takes won't change that.
For an imaginary-but-typical explosive XO-nite the final temperature
will be maybe 3,000C, the pressure maybe 3,000 bars.
If we now detonate that same explosive, the temporary maximum pressure
in the shockwave might be 50,000 bars, or even more.
When a 50,000 bar shockwave hits something it tends to shatter it,
rather than break it up - this shattering is known as brisance, as is
the ability to cause shattering, and the adjective is brisant.
A high explosion is one where a significant portion of the energy is
generated as brisance.
Thus a high explosion must be a detonation, as only shockwaves cause
brisance, and shockwaves only happen in detonations, not deflagrations -
but if a detonation only produces weak brisance, it's still a low
explosion, the line between high and low is not the same as between a
detonation and a deflagration.
There isn't a strict line which says how much brisance is needed to make
an explosion high however, just a significant amount.
Post by David Spain
'Slow' explosives deflagrate, the reaction progresses through the
material at a speed below the speed of sound through that material.
'Fast' explosives aka superexplosives, allow the reaction to progress
at the theoretical maximum speed, the speed of sound through the
IIRC, black power is an example of a slow explosive, (well explosive
Nitroglycerin, PETN and RDX fall in the super-explosive class.
'Generates pressure waves that are higher than the speed of sound?'
Detonation is caused by, and causes, supersonic shockwaves.
Imagine a block of explosive which is detonating. Part of it has
detonated, part of it is in the reaction zone, and part unreacted.
At the front of the reaction zone the shockwave hits a new untouched 
bit of explosive, compressing the bit of explosive to high pressure and
accelerating it forward.
The compressed and accelerated bit of explosive then turns to gas, which
expands, producing force. This force is exerted on the forward
shockwave, and also against an expanding reverse shock at the back end.
The expansion takes place at the speed of sound of the product gases
(which is what causes the reverse shock).
So, how fast is our shockwave? The explosive as a whole is staying
pretty much where it is, as it hasn't had time to move anywhere yet in
bulk - but the reacting bit we are concerned with is already moving
forward and expanding at it's speed of sound.
The reverse shock is therefore stationary with respect to the bulk of
the explosive, as the bulk of the explosive isn't moving anywhere yet;
and the bit is expanding apart between the forward and reverse shocks at
the speed of sound in the product gases; so the front end of the bit, ie
the forward shockwave, is moving at the speed of sound in the product gases.
For our XO-nite, the speed of sound in the solid explosive is 3,000 m/s.
The product gases are at about  3,000 C and 3,000 bar. The speed of
sound in these gases is 8,000 m/s, and that's the speed the shockwave
The pressure of the shock wave is variable, see  below.
 legal definitions of explosives are typically unrelated to their
properties. For instance in the UK if something is on a list, it's
explosive even if it can't go bang, and if it isn't on the list it isn't
legally an explosive, even if it can go bang.
 untouched because everything else that has happened so far in the
explosion is bound by the speed of sound in the unreacted explosive -
only shock waves and light can travel faster than this. In fact chemical
propagation by light can change detonation properties, and opacifiers
are often added thigh explosives.
 actually slightly less, as some energy goes into the shockwave. The
shockwave has to grow in strength or else it dies out, and the expanding
gases give some energy to the shockwave. The speed of the shockwave
doesn't change when it grows in energy, what happens is that the
pressure in the shockwave increases, sometimes to extreme levels.
-- Peter Fairbrother