Why not "Combat" Model Rocketry?

David Spain wrote:
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?

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.



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.

[...]

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.

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
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).


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



Dave

On Sep 18, 1:06*am, "Polyp" wrote:
"jonathan" wrote in message

...



"Joe Pfeiffer" wrote in message
t...
Trollin' trollin' trollin'

More like reminiscing, didn't everyone like blowing things up
when they were kids?

--

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.

giveitawhirl2008 wrote:
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
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.


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.