From: fernee@newton.physics.uq.oz.au (Mark Fernee)
Subject: Re: New firecracker formula
Organization: Physics Dept. The University of Queensland
Date: Tue, 15 Sep 1992 12:08:07 GMT

In article <1992Sep14.134430.8830@samba.oit.unc.edu>, Paul.Elkins@bbs.oit.unc.edu (Paul Elkins) writes:
|> In article <BuI72n.HtK@bunyip.cc.uq.oz.au> fernee@newton.physics.uq.oz.au (Mark Fernee) writes:
|> >In article <spiegl.716228254@lead15>, spiegl@rtsg.mot.com (Mark Spiegl) writes:
|> >|> 
|> >|> 
|> >|> Recently, we came across the following firecracker formula:
|> >                                          ^^^^^^^^^^^^^^^^^^^^
|> >  ************************************************************************
|> >  *                                                                      *
|> >  *    THIS IS NO FIRECRACKER FORMULA IT IS A PRIMARY EXPLOSIVE!!!!!!    *
|> >  *                                                                      *
|> >  ************************************************************************
|> >
|> 
|> What is meant by the term "Primary Explosive"?
|> how do applications differ for "firecracker formula"'s and "primary
|> explosives".
|> Are there such things as "Secondary Explosives"?  What are they used for?
|> 
|> Is this stuff covered in a FAQ?  (if not, maybe it should be)
|> 
|> Again, thanks in advance for the education of a new reader,
|> 
|> 
|> --
|>    The opinions expressed are not necessarily those of the University of
|>      North Carolina at Chapel Hill, the Campus Office for Information
|>         Technology, or the Experimental Bulletin Board Service.
|>            internet:  bbs.oit.unc.edu or 152.2.22.80

Although this point has been comprehensively covered, I would like to add
the following, somewhat simplistic, interpretation:

Explosives.

Brissance; The shattering effect of an explosion. High brissance 
explosives provide a large impulsive shock which tends to shatter and
destroy any neighbouring materials. This is a characteristic of most
military explosives. Low brissance explosives tend to shove, rather 
than shatter, and thus are useful in mining where large amounts of
rock and earth may need to be shifted safely.

Power; The amount of work a given quantity of explosive can do. This
is often measured by the relative expansion a given sized charge can
induce in a lead block (measured as an increase in volume of the block).
Generally, the higher the power the better, as this increases the
brissance and the efficiency of use.

Detonate; Explosive decomposition caused by the propagation of a shock
wave within the charge. The velocity of the shock wave is termed the
detonation velocity and this generally increases with increasing density
of the charge. The detonation velocity is directly related to the
brissance of an explosive. There is generally a minimum charge diameter
which will allow detonation to be initiated or permit detonation to
proceed.

Low explosives; These compounds are often mixtures of fuel and oxidiser 
which simply burn (deflagrate) when ignited and unconfined. They often 
need substantial confinement in order to explode (eg a black powder 
firecracker). With some compositions the rate of burning increases
dramatically with pressure, thus improving the performance (this 
seems to be the case with any chlorate powder). However, low explosives
do not detonate, they require a fst rate of burning to achieve their
performance and are relatively low power and low brissance.

High explosives; All high explosives can be made to detonate. They fall into
two main categories: 

a) Primary explosives; These are compounds that are highly sensitive to any
of the following:- SHOCK, FRICTION, STATIC ELECTRICITY, FLAME,TEMPERATURE,
PRESSURE, CHEMICAL ACTIVATION. These explosives are easily detonated by one
of the above methods. They are usually highly flammable where the rate of
burning rapidly increases to detonation. Often primary explosives are
highly unstable and unpredictable.
A special subclass of the primary explosives are the initiating explosives.
These are particularily high power, high brissance explosives which are
capable transferring the detonation to (initiating) a less sensitive high
explosive. Initiating explosives need to be sensitive to FLAME and
insensitive to PRESSURE. 
Due to their inherent sensitivity, only very small quantities of primary
explosive are ever used.

b) Secondary explosives; These are compounds that are sensitive to 
detonation by SHOCK, but insensitive to most other factors. Consequently
most secondary high explosives will hardly burn and must be detonated by
the use of a detonator or blasting cap, which contains a primary explosive.
Thus secondary explosives are much safer to work with due to their 
increased stability.
A subclass of the secondary explosives are the booster explosives. These
are generally very high power, high brissant explosives which are highly
sensitive to SHOCK and thus easily detonated with very small amounts
of initiating explosives. These explosives are used as the main filler
of compound detonators and are used as booster charges for extremely
insensitive high explosives (such as ANFO and ammonium picrate). While
they are highly sensitive to SHOCK, they are still about a factor of
ten less sensitive than initiating explosives. However they are still
considered too sensitive to be used alone as the main explosive filler.
Examples of booster explosives are PETN, RDX, HMX, picric acid, and 
tetryl. These have all found use as military explosives, however, due
to its tendency to form highly sensitive primary explosive metal 
picrates, picric acid is rarely used. These explosives can all be 
desensitised by the addition of other less sensitive explosives or
inert buffers.

The use of Aluminium; Powdered aluminium is commonly added to high
explosive mixtures to increase the power of the mixture. It is 
commonly mixed with those explosives which have an excess of oxygen,
but this need not be the case. An increase in the power of an 
explosive of the order of 20% is easily obtained, however this is
accompanied by a drastic loss of detonation velocity and consequently
brissance in almost all cases (a natable exception is ammonium nitrate
-aluminium mixtures (ammonal)).
This loss of brissance with an increase of power is beneficial for
commercial and mining applications, but is not too useful for 
destructive military uses.

General notes; These are not rigid classifications, and many compounds
and compositions may fall into more than one category. Such an exception
is the low explosive chlorate mixtures which can be made to detonate. 
Nitroglycerin, methyl nitrate, ethylene glycol dinitrate etc. are 
all extremely SHOCK and TEMPERATURE sensitive but can be made to burn
in a slow and controlled manner. DON'T TRY THIS OR YOU MAY DIE!. They
may be considered primary explosives, but they are mostly desensitised
to form secondary explosives (dynamite etc.) by the addition of various
materials. 

The above categories are simply used as a guideline of an explosives
general behavior. Each compound or mixture has its own ideosyncracies
which MUST be understood BEFORE attempting to use it. Explosives are
NOT safe (that includes low explosives).

To answer the original question. Low explosives are used in fire crackers.
A fire cracker made with flash powder (67% KClO4 + 33% Al powder), which
is considered a low explosive, explodes with little confinement, gives
a loud report, a bright flash and rips a paper container into little 
pieces. In contrast, a 2.5cm long 2mm dia paper tube filled with a
primary high explosive such as HMTD detonates with a very highpitched
bang which hurts the ears at a distance of 3m in the open. There is
no bright flash and the paper tube is reduced to a fine pulp. This is
quite a considerable difference. Such a small innocuous device is
surprisingly destructive and VERY DANGEROUS. If it pulps the paper, 
think of what it can do to your hand! That is why fire crackers are
not made of high brissance high explosives!!

Mark.