Subject: Re: C-4
From: glhurst@onr.com (Gerald L. Hurst)
Date: Mar 18 1997
Newsgroups: alt.engr.explosives

In article <332DDB38.1FFE@netcom.ca>, pfrazier@netcom.ca says:

>This just popped into my head, would an exploding block of C-4 (call it
>1 Kg) have enough pressure to detonate a identical block at 2 meters? 5
>meters? 10? 20?  This is probably a simple question but please answer if
>you know.  Thanks.

No, not at two meters.  Two blocks of C-4 would have to be in close
proximity in order for one block to initiate the other.  Dynamite
would be another question.  NG-rich products such as ditching
dynamite can bridge substantial distances.  Using this sensitive
material it is possible to set off a series of charges in a line
of boreholes using only one cap.  Most modern (last 10 years)
commercial explosives are much too insensitive to be used in
this fashion.  There is a cartridge gap test which was routinely 
run on all commercial products.

There is a card gap test which is used for military explosives.
The material to be tested is separated from a 50 g tetryl booster
pellet by a series of plastic cards or wafers.  Typically, the 
exploding booster will fail to initiate the test explosive at
a gap of less than one inch.  In the same test, I have seen
very sensitive old-formulation commercial material initiate through
over a meter of plastic rod and I have seen a one-pound booster
initiate a secondary charge through a 20 foot column of oil.

C-4 is just barely cap-sensitive and is inert to a 30-06 bullet
even when placed against a steel plate.  The pressure of an HE
explosion drops off very, very rapidly with distance from the
charge.  It is only after the exploding material has reached 
a fraction of its detonation pressure by slight expansion of 
the gases that the pressure drop with distance begins to follow
some sort of square root-of-the-distance law. Although NG-based
explosives are susceptible to these lower pressures, military
explosives generally require initiation pressures of comparable
magnitude to the short range output of HE.

One caveat is that the concept of "sensitivity" is not simple.
Circumstances can produce unpredictable results so one should
never take for granted that any given stimulus won't result
in unexpected initiation.  I can cite instances where a rise of 
a few degrees in the ambient temperature can cause a material 
to become cap sensitive.  Also, there are materials which become
cap-sensitive by contamination with as little as 1/10 percent 
of certain impurities.

Jerry (Ico)