Why not "Combat" Model Rocketry?

Rick Jones wrote:
In sci.space.history Pat Flannery 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

David Spain wrote:
'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" wrote in message
...
Pat Flannery writes:

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!





;-)

Dave

"Jim" wrote in message
m...
Pat Flannery wrote:
Jim wrote:

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_g...to.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_g...to.asp?pid=853







Jim

Pat Flannery writes:

David Spain wrote:
'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=pZL...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_subscri...ction=detai l

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-P...&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....

On Sep 18, 7:17*pm, "jonathan" wrote:
"David Spain" wrote in message

...

Pat Flannery writes:

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.

David Spain wrote:
Pat Flannery writes:

Scott M. Kozel wrote:
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

I'm beg to differ with this link on two points:

/quote
[...]The scientific community defines
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
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
difference here.

Not necessarily. All high explosions are detonations, but not all
detonations are high explosions.

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.

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.



'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?'

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

Peter Fairbrother 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 writes:

Peter Fairbrother writes:

(etc, see below)

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

David Spain wrote:
Peter Fairbrother 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.

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.



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.


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.

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.

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

Dave