From: billn@hpcvaac.cv.hp.com (bill nelson)
Subject: Re: Basic questions.
Organization: Hewlett-Packard Company, Corvallis, Oregon USA
Date: Mon, 14 Sep 1992 22:19:30 GMT

Paul.Elkins@bbs.oit.unc.edu (Paul Elkins) writes:

: >They just deflagrate - eg. burn very rapidly. Any propogative burning that
: >occurs does not lead to a shock wave triggered decomposition of the mixture.
: 
: Thanks for the info Bill, but I  have some more questions.
: 
: 
: detonation is decomposition propogated by shock?
: Fire/heat plays little or no role?

Not exactly. There is heat generated at the CJ surface. If the heat released
is not great enough, then the detonation dies out, no matter how strong the
initial shock.  Once the detonation is initiated, the decomposition has to
furnish all the energy to keep it going.

It is possible for some compounds to undergo what is called a deflagration
to detonation transition. This is where the compound starts out burning and
keeps increasing in velocity until it starts to detonate. This increase is
not linear, but has plateaus and quick jumps in the propogation rate. Some
compounds will do this when unconfined - others have to be strongly confined.

If you want more information - I would suggest finding a good textbook on
the physics of energetic materials. There should be several in your local
college library.

: >You could call PETN, NG, Cyclonite, Dynamite and similar substances     
: >"secondary" explosives. They are substances capable of being initiated by
: >primary explosives. 
: 
: I assume such secondary explosives are preferred because they are much
: less dangerous to deal with.

Well, some of them can also be directly initiated. NG with entrained air
bubbles, for example. Also, most of the others will do so, if given a sharp
enough blow.

Yes, in general, they are less dangerous - at least, until you combine them
with a primary explosive. 

: Other than initiation of secondary explosives, are there applications
: where it is desirable to have the entire charge consist of primary explosives?

I suppose there are. However, the danger of handling primary explosives 
increases rapidly with the mass of explosive. Thus, the amount used is
generally kept to a minimum. The relatively safe secondary explosives are
placed, in bulk, and the sensitive primary explosives are only added when  
they have to be.

: >There are also "tertiary" explosives. These explosives are those that are
: >either insensitive to initiation by primary explosives, or are only partly
: >initiated. They require a high (secondary) explosive booster for complete
: >detonation. Examples are ANFO and ammonium picrate.
: >
: 
: I'm not sure I understand the purpose of tertiary explosives.
: They would seem to be of little value.

Well, consider an artillery shell, for example. Here is an explosive shell
that undergoes high g forces when it is fired from the gun. This put high
setback strains on the massive explosive charge. If it is a secondary
explosive, then it may explode from the strain.  However, if a tertiary
explosive is used, then this problem is minimized. Since the mass of primary
and secondary explosives is small, the setback hazards for these components
of the fuze train is also relatively small.  Since the amounts of sensitive
explosives is small, it is possible to block them from the main explosive
until some specific condition has been fulfilled, such as the shell exiting
the gun muzzle. Thus, if either the primary or secondary explosive detonates,
it does not destroy the gun, crew and anything else in the vicinity.

: According to the rough definitions outlined, are caps (from toy pistols)
: primary explosives because they are set off by shock; or did I miss something.

Some mixtures might be primary explosives - I do not know what is currently    
being used. Note that many deflagrating mixtures, and even many show burning
mixtures, can be set off by shock, heat or static - it is not a unique property
of high explosives.

There is still a lot of argument going on over whether some flash powders
can detonate - many claim that they do. The US government now classifies flash
powders as high explosives. (Is this a DOT or BATF classification?)

Bill
Newsgroups: rec.pyrotechnics