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| Pat Flannery... |
 Posted: Fri Sep 18, 2009 12:33 pm |
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Rick Jones wrote:
Quote: In sci.space.history Pat Flannery <flanner at (no spam) daktel.com> wrote:
Unfortunately, the actual bombs had such a good aerodynamic form that
they would sail hundreds of feet forward from the drop point, and were
almost impossible to accurately aim at a target on the ground.
Para-fragmentation... no dive bomber required.
K.I.S.S. Tried that concept and with my luck the chute would have opened
while the bomb was still on the plane, it would have crashed from the
drag, and I'd be in a hell of a lot of trouble with the guy who built it.
Except for one that really did have a sizable explosive charge in it,
all the other bombs used a very small charge (a shotgun shell primer
actually) to eject flour from the back end on impact, for safety's sake.
It hadn't occurred to me at the time that what I had designed had the
potential to be a fuel-air bomb if the flour ignited after it was ejected.
The bombs were very light (around two ounces) and I really didn't expect
them to fly that far forward after release.
The aircraft used to carry the bombs was a old design called a
"Powerhouse" that was quite large and actually covered with real silk.
It had a very big low rpm engine on it that actually used a sparkplug
instead of a glowplug, and it sounded like a small lawnmower in flight.
Pat |
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| Pat Flannery... |
| Posted: Fri Sep 18, 2009 3:47 pm |
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David Spain wrote:
Quote: 'Fast' explosives aka superexplosives, allow the reaction to progress
at the theoretical maximum speed, the speed of sound through the
material.
The only thing I can think of in this regard is Primacord, a super
fast burning detonating cord used for high explosives that burns at a
rate of 7,000-8,000 m/s: http://en.wikipedia.org/wiki/Detonating_cord
....which seems a lot higher than the speed of sound in the material it's
made from, which is a variable that depends on density.
That would mean it's burning at around 16,000 mph, which seems high for
sound, even going through solid lead.
Pat |
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| David Spain... |
| Posted: Wed Sep 23, 2009 9:43 pm |
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Peter Fairbrother <zenadsl6186 at (no spam) zen.co.uk> writes:
Hi Peter,
All true. However, you could have saved yourself a good deal of
typing if you had read my follow-on posting where I corrected
myself.
It would appear that the shock-wave for RDX detonation proceeds
through the material at about 2.65x the speed of sound in
the material, based on what I could find quickly on the net.
Brisance is key. It super-explosives the molecular configuration
seems (to me at least) key in allowing the shock-wave to propagate.
If I read the paper by Eckhardt et al. correctly, the speed of
sound in RDX crystal is also somewhat dependent on the orientation
of the molecules wrt to the sound stimulus. To properly detonate
I'm speculating that the shock-wave must initiate in the proper 3d
direction to which the molecular lattice is most susceptible to
brisance. Since most detonators are probably pretty crude in this
regard, they probably expend enough energy to force it, but I
wonder if you couldn't have extremely efficient ones as well,
that like a diamond cutter that taps it with an edge along the
correct axis, could set it off with very little energy expended.
Do you know physical principle is behind ZND theory?
Brisance is a description of the phenomena, but I don't find it
a very satisfying explanation of physically what is happening.
Since the shock-wave is propagating at supersonic speed, I have
to believe the physical force at work is electrical. Do you
know if this is the case?
Dave |
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| David Spain... |
| Posted: Wed Sep 23, 2009 11:07 pm |
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David Spain <nospam at (no spam) 127.0.0.1> writes:
Quote: Peter Fairbrother <zenadsl6186 at (no spam) zen.co.uk> writes:
(etc, see below)
Quote: Do you know physical principle is behind ZND theory?
Brisance is a description of the phenomena, but I don't find it
a very satisfying explanation of physically what is happening.
Since the shock-wave is propagating at supersonic speed, I have
to believe the physical force at work is electrical. Do you
know if this is the case?
Well, you addressed this question someone in your footnote #2
where you talk about 'opacifiers' being added to explosives to
change chemical propagation by 'light'. I'll leave it at that.
The rest of your descriptions fall pretty much in line with
what I understand is called ZND theory.
So is it fair to say that brisance determines the material's
ability to change to gaseous state *before* the chemical reaction
which is necessary for the supersonic propagation of the shockwave
relative to the solid material?
And if enormous pressures are generated in the shockwave, what
about the temperature within the shockwave? Since temperature
can effect the speed of sound in a gas and according to your
footnote #3 the pressure is variable why not the temperature?
And if so, wouldn't that make the shockwave speed also variable?
Dave |
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| Peter Fairbrother... |
| Posted: Thu Sep 24, 2009 1:22 am |
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David Spain wrote:
Quote: Peter Fairbrother <zenadsl6186 at (no spam) zen.co.uk> writes:
Hi Peter,
All true. However, you could have saved yourself a good deal of
typing if you had read my follow-on posting where I corrected
myself.
It would appear that the shock-wave for RDX detonation proceeds
through the material at about 2.65x the speed of sound in
the material, based on what I could find quickly on the net.
Sounds about right.
Quote:
Brisance is key. It super-explosives the molecular configuration
seems (to me at least) key in allowing the shock-wave to propagate.
Shockwaves will propagate through any material - the normal behaviour is
for them to disperse their energy in the material, and die out.
In a detonation shockwaves are fed by chemical energy and grow rather
than die out.
Brisance is one way the energy of a shockwave is dissipated, by
shattering material, especially if they are powerful high pressure waves.
Brisance however has little or nothing to do with the detonation process
itself.
Quote:
If I read the paper by Eckhardt et al. correctly, the speed of
sound in RDX crystal is also somewhat dependent on the orientation
of the molecules wrt to the sound stimulus. To properly detonate
I'm speculating that the shock-wave must initiate in the proper 3d
direction to which the molecular lattice is most susceptible to
brisance. Since most detonators are probably pretty crude in this
regard, they probably expend enough energy to force it, but I
wonder if you couldn't have extremely efficient ones as well,
that like a diamond cutter that taps it with an edge along the
correct axis, could set it off with very little energy expended.
Mostly RDX is used in polycrystalline form, or plastic bonded single
crystals.
Perhaps someone has investigated the detonation of single crystal RDX,
but in practice it is of little or no significance.
Quote:
Do you know physical principle is behind ZND theory?
Yes, it's just like CJ (Chapman-Jouguet) theory, except the reaction
takes time and stages, whereas in CJ theory we simply ignore those
details of the reaction.
But I wouldn't worry about ZND theory, start with CJ theory.
ZND theory can give predictions for some details which CJ theory can't,
for instance the thickness of the reaction zone, detonation limits etc -
but the results aren't very accurate, unlike CJ theory, you need
computers to do the calculations, and this is far more advanced that
just a physical interpretation of what is going on in a detonation.
Quote: Brisance is a description of the phenomena, but I don't find it
a very satisfying explanation of physically what is happening.
Okay, there are several physical explanations for CJ theory (all of
which are actually the same explanation, but seen from different
viewpoints). I'll try again:
Suppose an explosive reacts in a strong completely sealed container
which no energy can pass through. It will turn to gas at some high
pressure and temperature, say 4000K and 4000 bar, known as the CJ
conditions. The speed of sound in the product gas at this pressure and
temperature is known as the CJ velocity.
The CJ conditions do not depend on the path of the reaction, how long it
took, or whether a detonation occurred or not; only on the constituents
of the explosive and the available chemical energy.
Now imagine a plane shockwave is travelling through a block of some
non-explosive solid.
Material at the front of the shockwave is subject to high pressure from
behind and low pressure in front, and it wants to and does accelerate
forward. It presses on the next layer, and this next layer resists quite
well, becoming compressed in turn and thereby slowing the previous layer
to a stop. This is how a shockwave normally [1] propagates in a solid.
In a detonating explosive, when the shockwave reaches a new layer of
explosive, the layer is compressed and accelerated forward at a speed S,
where S is approximately the speed of the shockwave.
The layer turns to gas, and expands behind the front edge of the
shockwave, starting at the very high pressure of the shockwave and
ending at the still-high CJ pressure and temperature.
Now unless a converging-diverging nozzle is used an expanding gas can't
reach a velocity faster than the speed of sound, and in this case it
expands at (very close to) that value.
In a detonating explosive the shock/detonation wave passes through the
explosive quickly, before the bulk of the explosive has time to move
anywhere. The velocity of the gas when the post-shockwave expansion is
finished is therefore zero, because overall the gases from the explosion
haven't had time to go anywhere [2].
The layer of explosive/expanding gas was moving forward at speed S, but
it has expanded backwards until stationary at the speed of sound - and
thus S, which is the speed of detonation, is equal to the speed of sound
(in the product gas, at CJ conditions).
I hope this is clearer.
Typically, the speed of sound at CJ conditions, and thus the speed of
detonation, is 2-3 times faster than the speed of sound in the solid
explosive. The increased temperature is the main factor (the speed of
sound varies with the square root of temperature, so going from say 300K
to 4000K will give an increase of 3.65 times), but the stiffness of the
solid will decrease that, to about 2-3 times.
[1] it is of course a bit more complicated than that, for instance some
of the energy is changed to heat or sound etc, and shockwaves tend to
break things too!
[2] the gas will then be at the CJ conditions, and will normally then
expand again from there, of course. This expansion is subsonic, but the
speed of sound in the gas is high, so it can happen fast.
Quote: Since the shock-wave is propagating at supersonic speed, I have
to believe the physical force at work is electrical. Do you
know if this is the case?
It's just atoms bouncing off each other, plus a bit of chemical energy,
that's all.
-- Peter Fairbrother
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