From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: alt.engr.explosives
Subject: Re: Exploding bridgewires...
Date: 26 Jan 1996 05:18:32 GMT

In article <4e8vtu$k47@Hermes.grace.irl.cri.nz>, P.Hagyard@irl.cri.nz
(Paul Hagyard) says:

>I am curious to know more about these. In particular I was wondering if they
>are limited to certain types of explosives, and what sort of discharge
>current/total energy is needed? I am thinking of some small scale experiments
>using a large photo flash capacitor and black powder.
>
>Does anyone have any books they could advise, or better yet, some source of
>info over the internet? Failing that, any personal experiences - particularly
>things to avoid :-)
>
>I have heaps of experience with electrical/electronic setups - and have seen
>what large capacitors do to big screwdrivers... I have also played around
>with black powder for some years.

You do not need an exploding bridgewire for BP. A simple heated
or preferably overheated filament wire will do the job nicely.
Exploding bridgewire caps are intended to generate an explosive
shock similar to a small HE charge in order to initiate a
secondary explosive. BP is a primary low explosive and thus requires
only direct surface heating for ignition.

If you want to build a true exploding bridgewire setup, you should
calculate the electrical energy equivalent to at least a good
fraction of the chemical energy of a conventional blasting cap
and choose the rate of delivery of that total energy to occur
over a comparable timespan.  Start by assuming you'd like to 
generate about 2000 joules within one microsecond and work your
way down to match whatever fraction of this power and energy you 
can afford in terms of a capacitor bank. The bridgewire explosion
can be considerably less powerful if you augment the output
with a small base charge of secondary explosive (e.g. PETN) by 
the same methods used in the detonation trains of some commercial
caps. Now you need only enough oomph to set off the first element
of the secondary charge and the rest will follow. How much power 
do you need now? That is the stuff experiments are made of, but 
I'll bet you can cut the required output by at least a factor of 
10 and probably much, much more if you handle the base charge 
correctly in terms of particle size, density distribution and 
confinement.

You can run your experiments without explosives by the clever 
design of spit-and-bailing wire methods of estimating the
detonation pressure and energy impulse of the exploding wire.
Remember that the minimum initiating energy for common
explosives are in the literature, and you can convert these
figures into the equivalent IR pulse. I'll leave the 
calculations to you. If you can snatch that pebble, you are
ready to go it on your own.

Note that the pits in your screwdriver are a good starting
point in estimating pulse energy, just as the plate dent
test os a indicator of shock energy.

Jerry (Ico) 

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: alt.engr.explosives,rec.pyrotechnics
Subject: Re: Capacitive discharge to initiate ANFO?
Date: 17 Dec 1995 00:15:55 GMT

In article <4avl68$dml@ns2.ptd.net>, xopher@ptd.net (xopher) says:

>Saw a web page about exploding materials like water and Jell-O by
>discharging banks of large capacitors through them...would there be
>enough energy to initiate high explosives too?

Yes, capacitative discharge through an exploding bridgewire
is used in some installations, notably Stanford Research
Institute, for initiating conventional explosives.

ANFO would require too large a capacitor bank to be practical,
but the technique works OK with PETN.

Jerry (Ico)