From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: alt.engr.explosives
Subject: Re: Detonation of flash powders ?
Date: 22 Sep 1995 06:56:44 GMT

In article <43rkbp$9ub@waage.rz.uni-ulm.de>, s_gleich@cellar.rz.uni-ulm.de 
(Bernhard Gleich) says:

>Gerald L. Hurst (glhurst@onr.com) wrote:
>
>   ... quoted post deleted ...
>
>: Back to the drawing board. Condensed explosives do not need air to 
>: detonate. HE does just fine in a vacuum. The things you have to 
>: consider with flash explosions are the roles played by such transient 
>: gaseous species as Al2O and KCl as well as that of chemically bound or 
>: adsorbed water.
>
>: Jerry
>
>You didn't understand the the calculation. (Or I didn't explain it
>well enough.)
>
>I made a model for a flash powder and I wanted to determine if that
>model would give a detonation. I calculated the increase of
>temperature if any powder (with a given heat capacity) containing gas
>of any sort is compressed in a reversible way. (This is equivalent to
>compressing the powder slowly if friction can be neglected.) If this
>sort of compression would rise the temperature of the powder above its
>ignition temperature, a detonation would occur. (Detonation defined
>as propagation of the burn front with at least sound speed in the
>detonating media.)
>
>The calculation showed, that this mechanism with the compressed gas
>does not prove a detonation. (At least, if the flash powder is well
>below its ignition temperature.) But I tried to show a way how to
>determine whether a given powder would detonate in large amounts,
>without igniting several tons of it. A (relative) small experiment would be:
>
>Take an amount of flash powder. Determine its maximum burn
>pressure. (Or estimate it by thermodynamical calculations.) Put flash
>powder into some sort of compression mechanism and increase the
>pressure slowly to (known) maximum burn pressure. If the powder
>ignites reliably you can be sure that your powder will detonate in
>large amounts.

I did not misunderstand your adiabatic compression calculations.
You are involving ambient air in your scheme, but it does not play 
any significant role in the detonation. When you calculate the 
adiabatic pressure which will generate enough heat to raise the 
temperature of the entrained air to the same temperature it would 
reach from the thermochemical heat of reaction of flash, all you 
will have is a meaningless number. I will freely admit that I do
not know where you got your "burn pressure" since I have never
heard of the term and I do not know how the ideal gas law could
lead to any pressure calculation unless you know how many moles
of gas you're dealing with in the reaction products.

Without actually running the calculations I would assume that
flash can reach 4-5,000 deg K. It is also true that pressure of 
a few hundred kilobars can cause the adiabatic heating of air
to similar temperatures, but these numbers tell us nothing 
about the detonation of flash. If you like high numbers, run
your compression calculations using argon rather than air
and get a temperature rise to 15,000 deg K. 

>By the way: HE does fine in vacuum, but it also needs gas bubbles (or
>at least voids) for one sort of detonation mechanism. The bubbles are
>compressed in a non reversible (fast) way. The temperature rises
>locally and ignites the surrounding explosive. I don't know the
>temperatures in in HE, but bubbles in the focus of ultrasonic waves
>can reach >10^4 K according to light emission spectra. 

Hm, do I hear an echo? Void volume is the operative phrase. Solids
and liquids are not very compressible and therefore do not readily
allow the conversion of PV energy into heat under the influence of 
even very high pressures. Add some well distributed void and the 
pressure can now do work on the porous mass and reappear as heat
energy within that mass. This enabling of compression explains 
the sensitization of detonating explosives by voids. Note however
that this has nothing to do with the adiabatic compression of 
any gas in those voids. If there is gas in the voids, it can reach
astronomical temperatures, but in condensed explosives it makes 
relatively little difference.

Please be aware that many books refer to the "adiabatic 
compression of air" or "gas" in dealing with bubbles in high
explosives such as NG. .I base my statements on this matter on my 
own experiments in sensitizing both solid and liquid explosive 
formulations. I first used the term "void volume" in a patent I 
wrote 25 years ago.

Jerry

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: alt.engr.explosives
Subject: Re: Booming and Nothingness (Was Explosives vs. flash powder?
Date: 26 Oct 1996 21:40:49 GMT

In article <54tm80$jm7@hptemp1.cc.umr.edu>, brandonb@umr.edu (Seymour) says:

>>Both primary and secondary HE react internally via a shock wave mechanism
>>and thus do not rely on free surface to propagate the reaction. They
>>perform at peak pressure as the void volume is decreased and approaches
>>zero if they are truly ideal HE. One caveat here is that HE generally
>>does benefit from a little dispersed air but not for the same reasons
>>as low explosives.
>>
>>Jerry  
>
>Maybe, I am not understanding you, but if you consider gunpowder an
>explosive, it does not propagate via a "shockwave".  As far as air
>goes for a HE, not necc.  I can detonate PETN underwater, no problem.
>If you have a fuel/oxygen source in the explosive, such as ANFO, there
>is no need for air if the mixture is oxygen balanced.
>
>A high explosive, by Tommy's definition, have a velocity of detonation
>greater than 1000 m/s.  Don't know if this is correct....

Seymour, you are using the word "air" out of the context of my message.
If you will look again, you will see that I am applying the word as the 
equivalent of "void volume" or "gas."  The effect of air on high 
explosives has absolutely nothing to do with the chemistry of air 
(or oxygen).  What is important is the *absence* of solid or liquid, 
i.e., condensed material in dispersed volume areas of the detonating 
mass. In other words, HE benefits from dispersed voids.

LE uses interstitial space (which is usually filled with air on earth)
as room in which to spurt hot gases to neighboring surfaces.  HE, on
the other hand, uses these voids as volumes through which pressure
can be readily exerted such that PV work can be expended and turned
into heat (hot spots).  Some people like to think of this process as 
the adiabatic compression of entrained gas because they know that 
gases get hot when compressed.

In reality, the HE does not care if there is gas present to compress 
or not.  Even an empty void acts as a place where heat can be generated
by impact with the same end effect as if gas had been present.

Insensitive HE tends to benefit most from dispersed void.  Your ANFO is
an excellent example.  A well-balanced mixture with little or no 
interstitial voids is not detonable in any practical amount or 
configuration even with heavy priming.  

TNT is another example of an explosive that needs a teeny bit of space.
The cast material will not shoot with a 50 gram primer, but the ground
and pressed material shoots fine with a cap even if it is pressed to
*near* its crystal density.  With sensitive secondary explosives such
as PETN and RDX, the dispersed void is not so important although these
substances also are more sensitive when they have entrained air (void)
and they do become less sensitive at very high pressures.

PETN does shoot under water in the form of detonating cord, but it is
relying to some extent on the wrapping to keep water out of the
interstices between the particles because the void space is still
helpful to the propagation.

Nearly all commercial explosives rely heavily on entrained gas (void)
for sensitivity and will not shoot without its presence. It sometimes
happens that hydrostatic pressure in boreholes is great enough to 
compress the voids to the point where the material loses its primer
or cap sensitivity.  For such situations there are many formulations
which rely on microballoons of glass or plastic to preserve void volume.
At least in the case of the glass material the void is a pretty good
vacuum.

I was unaware that not all member's servers get alt.eng.explosives as 
well as rec.pyrotechnics. I will include rec.pyro for my purely technical
posts re explosives until such time as someone objects.

Sartre would probably like it.

Jerry (Ico)

From: "Gerald L. Hurst" <GHURST@austin.rr.com>
Newsgroups: alt.engr.explosives
Subject: Re: explosives at great depths
Date: Tue, 19 Oct 1999 08:29:49 GMT

Most explosives lose sensitivity with increasing pressure.
The fact that a pellet is pressed to, say, 50 kpsi does not
mean that  same explosive will remain cap sensitive under
the same pressure.  Remember that the pellet rebounds after
it is pressed.

Take the ideal example of PLX.  This liquid explosive is cap
sensitive at atmospheric pressure but will not shoot with
a #8 cap at 10,000 psi.   Military explosives are much more
tolerant of pressure than most commercial explosives.  Even
nitroglycerine explosives can be easily deadpressed.  The
modern slurry and emulsion explosives are desensitized by
even moderate pressures.  For deeper holes, microballoons
are sometimes added but these formulations too will fail at
about 1500 psi, the crush strength of the MBs.

I do not believe that depth makes much difference to a cutting
charge or that they benefit much from confinement.  The
explosive certainly doesn't have to "lift 100 feet of water" just
because it is at that depth.  The jet from the liner will have cut
the pipe before the water 10 feet up knows there has been a
detonation. It is the brisance of an explosive, not its heaving
power that determines how well it will function in a lined-
cavity shaped charge.  When that charge goes off, a couple
of million psi compress the surrounding water to form a
reverberating bubble space, but the surface has to wait
for the shock pulse to travel 100 feet before it starts to move.

Secondary breaking with surface charges also relies mainly
on shock pressure (brisance).  Mudcapping a charge to
improve confinement helps with nonideal explosives such
as dynamite, but doesn't make a noticeable difference with
ideal (military) explosives such as are used in shaped
charges.

Generally, explosives at great depths can easily be protected from
pressure by metal casings.  Metals under compression are much
stronger than in tension so the rupture strength of a cylinder
from internal pressure is a poor measure of its ability to withstand
external pressure.  For example, the thin shell of a seismic blasting
cap such as the little DuPont SSS can withstand 12,000 psi.
Were this not the case, we would have been unable to test PLX
for cap sensitivity at that pressure.

BTW, Astrolite, unlike PLX, shoots quite nicely at 12 kpsi.  "Why?"
is a really good question.  It is probably an LVD phenomenon.

Neglecting screwball explosives like Astrolite, the desensitization
by pressure appears to be readily understandable.  All explosive
propagation relies on heat generated by adiabatic compression.
The instantaneous heat produced by the shock front is simply
PdV.  Initially the compressibility is great and thus a little P
produces a large dV and therefor a fair amount of heat.  But as
the pressure increases the compressibility drops and the dV
increments become smaller and smaller.

It would be an interesting exercise to calculate the amount of
heat produced in compressing a crystal of, say, RDX from
ambient pressure to 50,000 psi and then compare the value to
the amount of heat produced by going from 50,000  to
1, 000,000 psi.  If the latter turns out to be much larger than the
former then the theory sucks :)  Well, not really. You would then
have to take into account that real pressed charges are not
crystals but agglomerates that contain micropores, which make
their initial compressibility greater than that of single crystals.
The math can still be handled, but you need some empirical
data you won't find in the handbooks.

Who's got a slide rule?

Jerry (Ico)


TheJayhawk <jamesnaismith@engineer.com> wrote in message
news:fqPO3.12709$%62.213780@c01read02-admin.service.talkway.com...
> To the last question, yes, to a certain extent (syrofoam yes, water
> no, etc.), but why try it when an air or vacuum standoff is so easy to
> obtain?
>
> I am a design engineer for an offshore explosives products company, we
> blow up all types of offshore structures, and water depth matters in a
> few instances. 1. If you are severing a well or pile, you are better
> served by having a certain amount of hydrostatic head (+100ft.) to
> help provide confinement to the explosive energy. remember that when
> detonation occurs it has to literally lift that 100ft of water
> immediately. If it can't move it (this is good) it will initialy do
> more work on the tubular than if the column is very short. This has
> been very adequately proven over the tens of thousands of shots we
> have made in company history. Secondly, no, the water depth such as
> around the Titanic would make no difference to the effects of an
> explosive. Basically all underwater shots in direct contact with the
> target steel or metal, may neglect everything about the water except
> for it's inherent density. 10,000psi is even negligible when you are
> looking at blast face pressures of 2,800,000psi to 4,500,000psi.
> As for any effect on the explosive itself, most secondary explosive
> are pressed or cast to a much higher density than 10ksi. detonators
> are normally pressed to 20 to30ksi in the petn or rdx pellet. the
> primary explosive can not always be pressed this high
> --
> Posted via Talkway - http://www.talkway.com
> Exchange ideas on practically anything (tm).