Discussion:
Why not "Combat" Model Rocketry?
(too old to reply)
jonathan
2009-09-17 03:22:00 UTC
Permalink
Back in fifth grade or so, a buddy and I use to be into
model rockets. Well one day, while searching my older
brother's room for his playboys, we found instead a
great big box of cherry bombs. And with great delight
we stole a big handful of them. Then built a bunch of
rocket/bombs by simply gluing fins and a launch tube
directly to the Estes rocket motor and taping the cherry
bomb to the top. And let the ejection charge light the fuse.

We soon realized what fun it would be for us to go to the
opposite sides of the field and see who could come closest
to hitting the other with our little rocket bombs. It was
a blast!

By the time our parents were alerted, we were getting the
range down to about fifty feet or so, enough to make it
exciting. I wonder, how could we turn that into a sport?


Jonathan

s
David Spain
2009-09-17 03:50:09 UTC
Permalink
Post by jonathan
I wonder, how could we turn that into a sport?
I dunno, but I may have found you a sponsor:

http://www.aopanet.org/

Dave
Scott M. Kozel
2009-09-18 01:14:17 UTC
Permalink
Post by David Spain
 I wonder, how could we turn that into a sport?
http://www.aopanet.org/
Wear Kevlar clothing and a full helmet ?
Jim
2009-09-18 02:17:01 UTC
Permalink
Post by Scott M. Kozel
Post by David Spain
Post by jonathan
I wonder, how could we turn that into a sport?
http://www.aopanet.org/
Wear Kevlar clothing and a full helmet ?
Sounds like a Kevlar straight jacket might be appropriate
Pat Flannery
2009-09-18 05:52:34 UTC
Permalink
Post by Jim
Sounds like a Kevlar straight jacket might be appropriate
Well, I'll bet you never got into a bottle rocket fight with the girl
across the street when you were both around 15, and saw her jump
straight into the air when one went between her legs at around knee
height. :-D

Pat
Jim
2009-09-18 12:48:55 UTC
Permalink
Post by Pat Flannery
Post by Jim
Sounds like a Kevlar straight jacket might be appropriate
Well, I'll bet you never got into a bottle rocket fight with the girl
across the street when you were both around 15, and saw her jump
straight into the air when one went between her legs at around knee
height. :-D
Pat
I actually lived in a state where fireworks were illegal so had no
access to them, that is a funny concept though. Now I live in Oklahoma
where you can buy fireworks and see the amount of damage done every
summer via grass fires and pop bottle rockets on wood shingle roofs in
100 degree weather in early July and have developed a bad taste for them
unless handled professionally. The model rocket part is great though,
have flown my share of them when I was a kid.

Jim
jonathan
2009-09-19 00:47:54 UTC
Permalink
Post by Jim
Sounds like a Kevlar straight jacket might be appropriate
Well, I'll bet you never got into a bottle rocket fight with the girl across
the street when you were both around 15, and saw her jump straight into the
air when one went between her legs at around knee height. :-D
Pat
I actually lived in a state where fireworks were illegal so had no access to
them, that is a funny concept though. Now I live in Oklahoma where you can
buy fireworks and see the amount of damage done every summer via grass fires
and pop bottle rockets on wood shingle roofs in 100 degree weather in early
July and have developed a bad taste for them unless handled professionally.
The model rocket part is great though, have flown my share of them when I was
a kid.
In Florida you have to sign a waiver that says you own either a fishery
or a train company, then you can buy just about anything.

Gotta love this country!

PYROTECHNIC MOTHERLODE
Item #: G-042
. Considered the King of the 500-gram fireworks
http://www.fireworks.com/fireworks_gallery/photo.asp?pid=527


25 SHOT WOLF PACK MISSILE BASE
Item #: L-017
The ultimate missile base! 25 powerful launches that erupt in color and crackle.
http://www.fireworks.com/fireworks_gallery/photo.asp?pid=853
Jim
David Spain
2009-09-18 12:58:39 UTC
Permalink
Well, I'll bet you never got into a bottle rocket fight with the girl across
the street when you were both around 15, and saw her jump straight into the
air when one went between her legs at around knee height. :-D
Does anyone else besides me notice how strange the mating rituals get the
further west one lives?

;-)

Dave
Pat Flannery
2009-09-18 17:56:24 UTC
Permalink
Post by David Spain
Well, I'll bet you never got into a bottle rocket fight with the girl across
the street when you were both around 15, and saw her jump straight into the
air when one went between her legs at around knee height. :-D
Does anyone else besides me notice how strange the mating rituals get the
further west one lives?
The Freudian implications of the incident didn't escape me at the time. :-)

Pat
jonathan
2009-09-19 00:17:01 UTC
Permalink
Post by David Spain
Well, I'll bet you never got into a bottle rocket fight with the girl across
the street when you were both around 15, and saw her jump straight into the
air when one went between her legs at around knee height. :-D
Does anyone else besides me notice how strange the mating rituals get the
further west one lives?
Being from the midwest, I wouldn't know. But the first time
I slept with a first cousin I felt real bad about it. Until my buddy
told me the way he got over it was to stop counting!
Post by David Spain
;-)
Dave
Eric Chomko
2009-09-22 16:50:23 UTC
Permalink
Post by jonathan
Post by David Spain
Well, I'll bet you never got into a bottle rocket fight with the girl across
the street when you were both around 15, and saw her jump straight into the
air when one went between her legs at around knee height. :-D
Does anyone else besides me notice how strange the mating rituals get the
further west one lives?
Being from the midwest, I wouldn't know. But the first time
I slept with a first cousin I felt real bad about it. Until my buddy
told me the way he got over it was to stop counting!
You might be a redneck....


if you attend family reunions in order to find new dates.
Pat Flannery
2009-09-18 05:46:25 UTC
Permalink
Scott M. Kozel wrote:>
Post by Scott M. Kozel
Wear Kevlar clothing and a full helmet ?
I never did shoot one at anyone or anything, but I have to 'fess up to
putting a explosive impact-fused warhead on a model rocket to try out
the detonation system for bombs to be carried on a large RC aircraft at
a fly-in.
...worked like a charm. :-)
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.
Really needed a RC dive bomber for this concept to work, although I did
get production cost down to around twenty-five cents per bomb circa 1976.

Pat
David Spain
2009-09-18 12:55:12 UTC
Permalink
I never did shoot one at anyone or anything, but I have to 'fess up to putting
a explosive impact-fused warhead on a model rocket to try out the detonation
system for bombs to be carried on a large RC aircraft at a fly-in.
...worked like a charm. :-)
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.
Really needed a RC dive bomber for this concept to work, although I did get
production cost down to around twenty-five cents per bomb circa 1976.
I can't get my wife to attend airshows because she's convinced she'll get
killed and now I've got to make sure she never reads this or it will be
curtains for RC fly-ins as well. (Not that she'd *voluntarily* want to
go to an RC fly-in anyway)...

;-)

Dave
Pat Flannery
2009-09-18 17:54:29 UTC
Permalink
Post by David Spain
I can't get my wife to attend airshows because she's convinced she'll get
killed and now I've got to make sure she never reads this or it will be
curtains for RC fly-ins as well. (Not that she'd *voluntarily* want to
go to an RC fly-in anyway)...
I've been to a lot of RC fly-ins, and they are a lot more dangerous than
any airshow (unless the Russians show up of course; then it seems you
can count on a MiG or Sukhoi crashing at some point during the display).
The problem is when something goes wrong with the radio, as then you can
end up with a aircraft coming out of the sky at over 50 mph with a
buzz-saw and chunk of metal at the front.
I've had one crash around five feet from me, and another one would have
hit my father if he hadn't used the bottom of his shoe to deflect it as
it came at him at around two feet in the air.

Pat
Rick Jones
2009-09-18 17:07:05 UTC
Permalink
Post by Pat Flannery
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.

rick jones
--
the road to hell is paved with business decisions...
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
Pat Flannery
2009-09-18 18:33:45 UTC
Permalink
Post by Rick Jones
Post by Pat Flannery
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
Pat Flannery
2009-09-18 06:28:27 UTC
Permalink
Post by Scott M. Kozel
Wear Kevlar clothing and a full helmet ?
Not for Cherry Bombs...those accouterments must be reserved for M-80s:
http://www.fireworksland.com/html/m80.html
...the H-Bomb of fireworks. ;-)

Pat
David Spain
2009-09-18 12:21:39 UTC
Permalink
Post by Pat Flannery
Post by Scott M. Kozel
Wear Kevlar clothing and a full helmet ?
http://www.fireworksland.com/html/m80.html
...the H-Bomb of fireworks. ;-)
Pat
I'm beg to differ with this link on two points:

/quote
Before you send me an e-mail message arguing that flash powder is a high
explosive, here is further discussion of that subject. By flash powder, I mean
the chemical composition inside an M-80, which is a mixture of various
substances, including potassium perchlorate. The scientific community defines
a high explosive as one that detonates when unconfined. A low explosive is
defined as one that deflagrates - not detonates - whether confined or
unconfined. The distinction between "detonate" and "deflagrate" is the key
difference here. A low explosive, that deflagrates, generates pressure waves
in the air that are slower than the speed of sound, while a high explosive,
which detonates, generates pressure waves that are higher than the speed of
sound
/endquote

1st point:

Deflagration and detonation refer to the speed of reaction through the
explosive itself, not the blast effect through the air.

'Slow' explosives deflagrate, the reaction progresses through the
material at a speed below the speed of sound through that material.

'Fast' explosives aka superexplosives, allow the reaction to progress
at the theoretical maximum speed, the speed of sound through the
material.

IIRC, black power is an example of a slow explosive, (well explosive
when confined).

Nitroglycerin, PETN and RDX fall in the super-explosive class.

2nd point:
'Generates pressure waves that are higher than the speed of sound?'

Eh?

Dave
Pat Flannery
2009-09-18 21:47:10 UTC
Permalink
Post by David Spain
'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
David Spain
2009-09-20 05:33:40 UTC
Permalink
Post by David Spain
'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
I gotta learn to stop posting based on recollection.

I WAS WRONG. Well sort of...

I quick review of what's available on the Internet delineates between
between the shock wave that initiates the chemical reaction vs the
chemical reaction itself.

The 'detonation wave' can proceed through the material at supersonic
speed (relative to the material). It physically displaces (compresses)
which heats the reactant which then reacts sonically after the
'shock discontinuity' wavefront passes. [1]

The speed of the detonation wave is aided by an increase in the
density of the material. According to US Patent 4913053 Primacord uses a
process of heating and high pressure to boost the detonation velocity
of the fusing by 15-20% [5].

Technically its not 'burning' or reacting at that speed, and again
taking a risk IIRC, that is why there's no discernible flame front
in a detonation as opposed to a deflagration. The chemical reaction
happens after the supersonic shock wave passes through the material
which would make it appear to be 'burning' (aka reacting) all at once.

To pick this apart a bit I focused on one type of explosive, RDX
and came up with this:

Explosive velocity: 8750 m/s [2]
Speed of sound in RDX: ~3300 m/s [3], [4]

Thus the shock wave propagates through the material at roughly
2.65x the speed of sound in the material.

Sources:

[1]'Toward Detonation Theory' by Anatolii Nikolaevich Dremin page 4 para 3
a description of ZND theory.

http://books.google.com/books?id=pZLdfT-NZ-wC&source=gbs_navlinks_s

[2] http://en.wikipedia.org/wiki/RDX

[3] Molecular Dynamic Simulation of Nanoindentation of
Cyclotrimethylenetrintramine (RDX) Crystal

http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=8752&DID=201139&action=detail

Google search of 'speed of sound in RDX crystals' yields a reference to this
paper with the quote 'the indentation speed is 200 m/s which is 6% of the
sound speed in RDX' this calculates to 3,333 and 1/3 m/s.

[4] The elastic constants and related properties of the energetic
material cyclotrimethylene trinitramine (RDX) determined by
Brillouin scattering by Haycraft, Stevens and Eckhardt.

http://digitalcommons.unl.edu/chemistryeckhardt/2

See the sound velocity diagrams in Fig 3. I noted the logitudinal mode curves,
esp. the ones from the ultrasonic works of Scwartz and Hassul which are in
close agreement at around 3300 m/s.

[5] US Patent No. 4,913,053 McPhee for Western Atlas International Houston TX.
'Method of increasing the detonation velocity of detonating fuse'

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&co1=AND&d=PTXT&s1=4913053.PN.&OS=PN/4913053&RS=PN/4913053


----
Sorry,
Dave

PS: And boy, if this post doesn't end up on a NSA server somewhere,
somebody is asleep at the switch....
Peter Fairbrother
2009-09-23 01:45:57 UTC
Permalink
Post by David Spain
Post by Pat Flannery
Post by Scott M. Kozel
Wear Kevlar clothing and a full helmet ?
http://www.fireworksland.com/html/m80.html
...the H-Bomb of fireworks. ;-)
Pat
/quote
[...]The scientific community defines
Post by David Spain
a high explosive as one that detonates when unconfined.
Not so. Though the definition isn't actually standardised afaik, a high
explosive is "brisant", which is French for shattering. I'll explain
this term later.

Moreover, it's better to refer to high and low explosions than high and
low explosives, as a particular explosive may go off in either high or
low mode depending on size, conditions etc.

However some explosives can't be brisant, and can be called low. I don't
know offhand of any explosive which is always brisant, but if it existed
it would be called high.


(when confined flash most definitely _can_ go high, as can even confined
gunpowder in very large quantities, though the latter is rare).

A low explosive is
Post by David Spain
defined as one that deflagrates - not detonates - whether confined or
unconfined.
A low explosion is one which is not brisant. Confinement per se is
irrelevant to the definition of an explosive, except maybe for legal
reasons [1], though it can turn a non-brisant explosion into a brisant one.

The distinction between "detonate" and "deflagrate" is the key
Post by David Spain
difference here.
Not necessarily. All high explosions are detonations, but not all
detonations are high explosions.

A low explosive, that deflagrates, generates pressure waves
Post by David Spain
in the air that are slower than the speed of sound, while a high explosive,
which detonates, generates pressure waves that are higher than the speed of
sound
/endquote
Deflagration and detonation refer to the speed of reaction through the
explosive itself, not the blast effect through the air.
Indeed. The crucial difference between a deflagration and a detonation
is the mechanism of propagation. In a deflagration the mechanisms by
which energy is transferred to unreacted material are varied, including
thermal transfer by conduction, radiation, hot gases getting between the
cracks or gaps in gunpowder, etc.

In a detonation the major mechanism of propagation is by supersonic
shockwave. For a high explosive this occurs at typically 2-3 times the
speed of sound in the unreacted material.


In a shockwave the pressure can be very high indeed.

Imagine you have some explosive in a container, and you set it off. If
the container is really strong and doesn't conduct heat (a force-field?)
then the eventual conditions will depend solely on the chemistry of the
explosive, and the methods and paths the reaction takes won't change that.

For an imaginary-but-typical explosive XO-nite the final temperature
will be maybe 3,000C, the pressure maybe 3,000 bars.

If we now detonate that same explosive, the temporary maximum pressure
in the shockwave might be 50,000 bars, or even more.



When a 50,000 bar shockwave hits something it tends to shatter it,
rather than break it up - this shattering is known as brisance, as is
the ability to cause shattering, and the adjective is brisant.



A high explosion is one where a significant portion of the energy is
generated as brisance.

Thus a high explosion must be a detonation, as only shockwaves cause
brisance, and shockwaves only happen in detonations, not deflagrations -
but if a detonation only produces weak brisance, it's still a low
explosion, the line between high and low is not the same as between a
detonation and a deflagration.

There isn't a strict line which says how much brisance is needed to make
an explosion high however, just a significant amount.
Post by David Spain
'Slow' explosives deflagrate, the reaction progresses through the
material at a speed below the speed of sound through that material.
'Fast' explosives aka superexplosives, allow the reaction to progress
at the theoretical maximum speed, the speed of sound through the
material.
IIRC, black power is an example of a slow explosive, (well explosive
when confined).
Nitroglycerin, PETN and RDX fall in the super-explosive class.
'Generates pressure waves that are higher than the speed of sound?'
Faster?

Detonation is caused by, and causes, supersonic shockwaves.

Imagine a block of explosive which is detonating. Part of it has
detonated, part of it is in the reaction zone, and part unreacted.

At the front of the reaction zone the shockwave hits a new untouched [2]
bit of explosive, compressing the bit of explosive to high pressure and
accelerating it forward.

The compressed and accelerated bit of explosive then turns to gas, which
expands, producing force. This force is exerted on the forward
shockwave, and also against an expanding reverse shock at the back end.

The expansion takes place at the speed of sound of the product gases
(which is what causes the reverse shock).


So, how fast is our shockwave? The explosive as a whole is staying
pretty much where it is, as it hasn't had time to move anywhere yet in
bulk - but the reacting bit we are concerned with is already moving
forward and expanding at it's speed of sound.

The reverse shock is therefore stationary with respect to the bulk of
the explosive, as the bulk of the explosive isn't moving anywhere yet;
and the bit is expanding apart between the forward and reverse shocks at
the speed of sound in the product gases; so the front end of the bit, ie
the forward shockwave, is moving at the speed of sound in the product gases.



For our XO-nite, the speed of sound in the solid explosive is 3,000 m/s.
The product gases are at about [3] 3,000 C and 3,000 bar. The speed of
sound in these gases is 8,000 m/s, and that's the speed the shockwave
travels at.

The pressure of the shock wave is variable, see [3] below.



[1] legal definitions of explosives are typically unrelated to their
properties. For instance in the UK if something is on a list, it's
explosive even if it can't go bang, and if it isn't on the list it isn't
legally an explosive, even if it can go bang.

[2] untouched because everything else that has happened so far in the
explosion is bound by the speed of sound in the unreacted explosive -
only shock waves and light can travel faster than this. In fact chemical
propagation by light can change detonation properties, and opacifiers
are often added thigh explosives.

[3] actually slightly less, as some energy goes into the shockwave. The
shockwave has to grow in strength or else it dies out, and the expanding
gases give some energy to the shockwave. The speed of the shockwave
doesn't change when it grows in energy, what happens is that the
pressure in the shockwave increases, sometimes to extreme levels.


-- Peter Fairbrother
David Spain
2009-09-24 03:43:06 UTC
Permalink
Peter Fairbrother <***@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
David Spain
2009-09-24 05:07:03 UTC
Permalink
(etc, see below)
Post by David Spain
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
Peter Fairbrother
2009-09-24 21:34:35 UTC
Permalink
Post by David Spain
(etc, see below)
Post by David Spain
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?
No, it's just atoms or molecules bumping into each other. If they get
hot enough (= bump fast enough) they can give off light when they bump,
but that's mostly incidental.

Compared with a sound wave, the mechanism of transmission of a shockwave
through a material is similar except in that in the case of a sound wave
the transfer is almost perfectly elastic (and energy conserving),
whereas in the case of a shockwave the transfer is more inelastic - this
is because the elastic limits in the material have been overcome by the
high pressure in the shockwave.

The speed-of-sound limitation no longer applies (the "sound barrier" has
been "broken" because of the high energy levels involved), and the
actual speed of transmission depends not only on the material condition,
but also the maximum pressure, the energy, and the detailed shape of the
shockwave.

Because the transfer is inelastic some of the energy in the shockwave is
inevitably lost, converted to heat (or sound, or shattering of solids).

An unusual example of this can be seen sometimes when a shockwave from a
powerful explosion meets the surrounding air (and no actual material
from the explosion has reached that far yet) - the air glows briefly in
a wave, because the energy lost from the shockwave heats it up to
several thousand degrees. You need high-speed photography to see the
wave progress though, usually it's just a glow.
Post by David Spain
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.
Opacifiers are used mostly to ensure the energetic coupling between
nearby parts of an exploding material is good - if opacifiers were not
used and if a lot of the energy was given off as light then it might
spread out and not reach the next bit of explosive efficiently enough to
cause it to detonate/deflagrate.

[...]
Post by David Spain
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?
Wow, that's a hard question, like "have you stopped beating your wife?"
- it assumes many things which ain't necessarily so.

First off, the chemical reaction is not necessary for the supersonic
propagation of a shockwave. Shockwaves can propagate through any kind of
material, whether BEC, solid, liquid, gas, sparse or dense plasma. Hope
you have got that part now.

In a high explosion the chemical reaction does however drive the
detonation shockwave so it doesn't lose energy and fade out, in fact it
makes it stronger (constant strength shockwaves in explosions are
unstable, and don't happen - this is how the firework guys make whistle
noises BTW).

[> for most high explosives an initial shockwave is necessary to cause
detonation, otherwise if ignited many (eg TNT) will merely deflagrate,
while some others will undergo a deflagration-to-detonation transition -
which is a whole entire different subject, and it's verra complicaaated
indeed, Capt'n. <]


Second, in CJ theory, the fine details of the reaction - how long it
takes,etc, - are not relevant, we are only interested in the outcome.
They may be relevant in ZND theory, but we are nowhere near that
detailed in our understanding as yet.


Third, in a way you might be considered to be partly right, in that
perhaps the high energy shockwave [256] causes the material to turn to
gas before the reaction properly ends - but as above, for simple CJ
theory purposes it doesn't matter how long the reaction takes, within
reason (ie as long as it has happened before the expansion reaches the
speed of sound).

[256] though not the brisance, which is just the shattering effect a
shockwave can have - in this case it's the heating, not the shattering,
effect of the shockwave which causes the transformation to gas.
Post by David Spain
And if enormous pressures are generated in the shockwave, what
about the temperature within the shockwave?
It can get very high indeed, see the example about air glowing above. In
an explosion it then decreases rapidly to the CJ temperature (which is
still high, usually about 3,000-4,000K).

Since temperature
Post by David Spain
can effect the speed of sound in a gas and according to your
footnote #3 the pressure is variable why not the temperature?
The temperature is variable during the reaction, and during the
expansion to CJ conditions when the speed of sound matches the speed of
the shockwave - but the CJ temperature is fixed (actually it's not
exactly fixed, but the variation caused by the amount of energy given to
the shockwave is small).
Post by David Spain
And if so, wouldn't that make the shockwave speed also variable?
Yes, but only to a small extent, a percent or two, as above - and there
are usually other factors, like shape and confinement, which can cause
it to vary by 20% or more.

Also the small change above is usually offset by another change I'm too
lazy to go into just now, and the two changes almost exactly cancel out.


-- Peter Fairbrother
Post by David Spain
Dave
Peter Fairbrother
2009-09-24 07:22:45 UTC
Permalink
Post by David Spain
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.
Post by David Spain
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.
Post by David Spain
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.
Post by David Spain
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.
Post by David Spain
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.
Post by David Spain
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
Post by David Spain
Dave
Jeff Findley
2009-09-17 12:07:45 UTC
Permalink
Post by jonathan
Back in fifth grade or so, a buddy and I use to be into
model rockets. Well one day, while searching my older
brother's room for his playboys, we found instead a
great big box of cherry bombs. And with great delight
we stole a big handful of them. Then built a bunch of
rocket/bombs by simply gluing fins and a launch tube
directly to the Estes rocket motor and taping the cherry
bomb to the top. And let the ejection charge light the fuse.
We soon realized what fun it would be for us to go to the
opposite sides of the field and see who could come closest
to hitting the other with our little rocket bombs. It was
a blast!
By the time our parents were alerted, we were getting the
range down to about fifty feet or so, enough to make it
exciting. I wonder, how could we turn that into a sport?
I'm not sure how many NAR rules you're violating here. Beyond that, you may
have been violating one or more laws...

File this one under stupid things that almost got you killed as a kid and
don't mention it again.

Jeff
--
"Take heart amid the deepening gloom
that your dog is finally getting enough cheese" - Deteriorata - National
Lampoon
jonathan
2009-09-18 00:54:12 UTC
Permalink
Post by Jeff Findley
I'm not sure how many NAR rules you're violating here. Beyond that, you may
have been violating one or more laws...
Oh we had the safety issues handled, you bet!

His folks were loaded, and had a great big back yard with a field
surrounded by woods. All the night launches were from there.

With the strategy being that the higher they went up, the less likely
anyone would figure out where they came from. A cherry bomb on
top of a C6-5 makes a rather nice red fireball and bang at night.
We quickly learned however, like during the first launch, that it's
a very good idea to shorten the fuse on the cherry bomb as much
as possible. Out of concern for the neighbors of course.
Post by Jeff Findley
File this one under stupid things that almost got you killed as a kid and
don't mention it again.
I was the sane one, after we used up the cherry bombs, he started
emptying out a half dozen rocket motors or so, putting all the fuel
into a plastic bag and stuff it on top of the ejection charge.
He'd launch 'em like it was the Fourth of July.

One day he came over with his greatest creation to date.
The fuel from twenty motors stuffed on top of a two stage
rocket, and he wanted to launch it from my folks back yard
..of course.Which was normal sized, not like his.

I chickened out and suggested he just light the bag off
on the ground, to be..eh hum..safe. So when he was ready
to light the fuse, I walked over and peered just over his right
shoulder, and he said something about not having enough fuse
and was using a sparkler instead. I distinctly remember asking myself...
"he's gonna use a sparkler for what?" ....AND BANG!!!

Just as soon as he lit the sparkler it went off in his face.
A nice 6 foot fireball I would guess, enough so we were
inside of it, I know that for sure. Left nice burn marks
outlining his geeky glasses, and his v-neck shirt, and I only
lost the right side of my hair, due to my cautiously peeking
over his shoulder.


s
Post by Jeff Findley
Jeff
--
"Take heart amid the deepening gloom
that your dog is finally getting enough cheese" - Deteriorata - National
Lampoon
Joe Pfeiffer
2009-09-18 03:51:55 UTC
Permalink
Trollin' trollin' trollin'
--
As we enjoy great advantages from the inventions of others, we should
be glad of an opportunity to serve others by any invention of ours;
and this we should do freely and generously. (Benjamin Franklin)
jonathan
2009-09-18 05:01:14 UTC
Permalink
Post by Joe Pfeiffer
Trollin' trollin' trollin'
More like reminiscing, didn't everyone like blowing things up
when they were kids?
Post by Joe Pfeiffer
--
As we enjoy great advantages from the inventions of others, we should
be glad of an opportunity to serve others by any invention of ours;
and this we should do freely and generously. (Benjamin Franklin)
Polyp
2009-09-18 05:06:31 UTC
Permalink
Post by jonathan
Post by Joe Pfeiffer
Trollin' trollin' trollin'
More like reminiscing, didn't everyone like blowing things up
when they were kids?
Post by Joe Pfeiffer
--
Which is why Mythbusters is so popular with kids & older kids alike.
giveitawhirl2008
2009-09-26 01:59:57 UTC
Permalink
Post by Polyp
Post by jonathan
Post by Joe Pfeiffer
Trollin' trollin' trollin'
More like reminiscing, didn't everyone like blowing things up
when they were kids?
Post by Joe Pfeiffer
--
Which is why Mythbusters is so popular with kids & older kids alike.
Yes, and seeing things fly. I loved model rockets and managed to get
one candles-in-a-dry-cleaner-bag balloon airborne one time, only one
out of several tries. I never got those CANnons to work: aluminum cans
lined up, taped together, alcohol ignited at one end and a mango at
the other end. Think I had too many holes for the pressure to build
up. But CATAPULTS REALLY intrigued me and still do. All your power is
on the ground so no propellant wasted lifting other propellant. Never
built any.

One day, driving down a road in High Point-Greensboro area, NC, a
white object, maybe the size and appearnce of a washcloth or small
bathtowel, came out of the sky, crossing the road and landing in some
woods to my left. (it had the aerodynamics of a somewhat weighty
object.) I came close to going back to look for it but decided not
to.. I had learned that Southerners have an obsession with catapults
and that this probably has something to do with the War Between the
States. (Dave Barry). So I pretty well figured what it was I saw and
how it got airborne.
Pat Flannery
2009-09-26 05:29:03 UTC
Permalink
Post by giveitawhirl2008
Yes, and seeing things fly. I loved model rockets and managed to get
one candles-in-a-dry-cleaner-bag balloon airborne one time, only one
out of several tries. I never got those CANnons to work: aluminum cans
lined up, taped together, alcohol ignited at one end and a mango at
the other end. Think I had too many holes for the pressure to build
up.
I made a rocket based on that principle once.
One empty beer can with the top down, taped on top of three others with
their bottoms down, with another three under that.
You pumped propane or spray paint into it and applied a lighter to a
small hole on the side of the top can, the fire propagated down from
the top one into the three under it via holes in their bottoms, and from
there into the bottom three the same way.
Highest flight was around six feet, from the top of a table, allowing it
to slam into the ceiling of the apartment and leaving a nice circular
indentation there from the force of the impact.
If you really want to surprise the model rocket club, fire one from
underwater sometime; I fired one from around three feet down once via
putting in into a piece of PVC pipe with a cap at the bottom end, and a
sheet of aluminum foil the rocket pierced over the top.
I used a extremly pointed nose cone to pierce the foil and cut down drag
at it traveled to the surface, and it went around 200 feet up after
coming out of the water.
Stabilization was by four sticks like those on skyrockets that centered
it in the launch tube... primitive, but it worked just fine.

But CATAPULTS REALLY intrigued me and still do. All your power is
Post by giveitawhirl2008
on the ground so no propellant wasted lifting other propellant. Never
built any.
I made a small torsion one once (a model of a Roman "Onagar)
It used nylon rope to serve as the winding material, and had pretty good
range for its size - it was around 18 inches long and could hurl a small
pebble around 40 feet.
If you are looking for the math on how they work and how to design one
for optimal performance, latch on to a copy of J. G Landel's
"Engineering In The Ancient World" which has a whole chapter devoted to
them and the principles they work by.
Post by giveitawhirl2008
One day, driving down a road in High Point-Greensboro area, NC, a
white object, maybe the size and appearnce of a washcloth or small
bathtowel, came out of the sky, crossing the road and landing in some
woods to my left. (it had the aerodynamics of a somewhat weighty
object.) I came close to going back to look for it but decided not
to.. I had learned that Southerners have an obsession with catapults
and that this probably has something to do with the War Between the
States. (Dave Barry). So I pretty well figured what it was I saw and
how it got airborne.
Yeah, I heard about that fixation they have with hurling watermelons
around via catapults and trebuchets.
Well sir, when my giant catapult - "Abe's Revenge" is someday built, it
shall hurl Yankee cracker barrels full of Greek Fire upon their damnable
secessionist heads. ;-)

Pat "Little Round Belly" Flannery.

trigonometry1972@gmail.com |
2009-09-18 06:04:18 UTC
Permalink
Post by jonathan
Post by Joe Pfeiffer
Trollin' trollin' trollin'
More like reminiscing, didn't everyone like blowing things up
when they were kids?
Post by Joe Pfeiffer
--
As we enjoy great advantages from the inventions of others, we should
be glad of an opportunity to serve others by any invention of ours;
and this we should do freely and generously. (Benjamin Franklin)
Gawd..............I can just see a small town police chief
crowing about arresting a terrorist who was obviously planning
an attack.

Throw away that key...................Trig
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