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Old January 17th 10, 04:30 PM posted to sci.astro
Mike Dworetsky
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Posts: 715
Default Heat in front of moving object

Androcles wrote:
"Mike Dworetsky" wrote in message
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Androcles wrote:
"Mike Dworetsky" wrote in message
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Androcles wrote:
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Richard Fangnail wrote:
A fast-moving object like a meteorite compresses the air in
front of it, causing heat. Does an ordinary plane have a
problem with this or is it not moving fast enough?

Others have confirmed that supersonic aircraft need to have an
allowance for expansion built into their designs. Passenger
airliners fly at speeds of c 400-500 mph and there is certainly
some frictional heating.
The Siberian meteor of 1908 exploded in midair. Why do you
suppose it did, as opposed to hitting the earth?

It was moving at several km/sec and at that sort of speed, air
resistance generates a shockwave that will propagate through the
rock/ice/whatever and if the tensile strength is not great enough
it will break up. I don't think there was time for frictional
heating to cause an explosion via ice evaporation--the passage
through the air lasted only a few seconds, and heating propagates
very slowly through rock.


Let me see if I have this right...
If it moves slowly through air then it doesn't heat up, but if it
moves quickly through air it doesn't have time to heat up. Is that
what you are saying?
I rather think the heat shield tiles on the shuttle will not agree
with you. E = m . 1/2v^2 only if v goes to zero, otherwise it
remains kinetic energy.

Shuttle reentry lasts several minutes.

Heat does not have time to get from the outside to the inside of
the large object falling to Earth at several km/sec.

Yeah, you've already said that. Repeating it won't make it true
either.
Obviously you have a reading comprehension problem.

Not at all, I fully comprehend your ridiculous bull****.
If it moves slowly through air then it doesn't heat up, but if it
moves quickly through air it doesn't have time to heat up.
So it doesn't heat up.
Obviously you have a illogical problem.


An asteroid is not a supersonic aircraft, which is hollow and flies
for hours at a time, hence of course its skin heats up during flight.


One favourite ploy of the illogical twerp is to say
"A {object/expression} is not a {different object/expression} "
as in
"An asteroid is not a supersonic aircraft!"
or
"A sausage is not a dog's breakfast!"
The dog may conclude otherwise and prove it by eating the sausage.
Obviously you have a illogicality problem.


For a large solid asteroid entering the atmosphere, the surface layer
heats to incandescence and breaks up or evaporates (carrying off a
large amount of the heat via ablation). The bulk of the interior
remains at the temperature it had in interplanetary space. The
reason is, that the rate at which heat is transmitted through a
solid body is rather slow, and the passage through the atmosphere
only lasts a few seconds. If the asteroid has a weak structure, as many
comet nuclei and
asteroids appear to have from in-situ investigations, it is likely
to be disrupted by shock waves from the hypersonic entry speed into
the atmosphere. I have no illogical problems with these concepts from
basic
thermodynamics, but you have several. I think


That's not the first time you've made that claim.
Do you have any empirical evidence to support the ludicrous
assertion that you are capable of actually thinking?


everyone else who has read the thread understands this point.


What, slow and fast heat transfer?

Let's see... object moves through air at v1 mph.
surface temperate rises 1 degree a second.
core temperature rises 1 degree a second,
delayed by 100 seconds for heat transfer.

After 10 seconds the surface temperate is 10 degrees.
After 10 seconds the core temperate is 0 degrees.

After 100 seconds the surface temperate is 100 degrees.
After 100 seconds the core temperate is 0 degrees.

After 200 seconds the surface temperate is 200 degrees.
After 200 seconds the core temperate is 100 degrees.

Water boils at 100 degrees, object explodes after 200 seconds.

Object moves through air at v2 mph.
surface temperate rises 10 degrees a second.

After 50 seconds the surface temperate is 500 degrees.
core temperature rises 10 degrees a second,
delayed by 50 seconds for heat transfer.
After 50 seconds the core temperate is 0 degrees.

After 60 seconds the surface temperate is 600 degrees.
After 60 seconds the core temperate is 100 degrees.

Water boils at 100 degrees, object explodes after 60 seconds.

object moves through air at v3 mph.
surface temperature rises at 100 degrees / second.

After 10 seconds the surface temperate is 1000 degrees.
core temperature rises 100 degrees a second,
delayed by 10 seconds for heat transfer.
After 10 seconds the core temperate is 0 degrees, it
takes 10 seconds for the heat to transfer

After 20 seconds the surface temperate is 2000 degrees.
After 11 seconds the core temperate is 100 degrees.
After 12 seconds the core temperate is 200 degrees.
After 13 seconds nothing, the core exploded.
The surface temperature never got to 2000 degrees,
there is no surface.


What the hell are you mumbling about?

The difference between core and surface temperatures is almost immaterial
for an asteroid entering the atmosphere at a typical 10/km/s speed, in terms
of causing a breakup. The surface gets very hot, but there is no time for
the heat to transfer to the interior The entire atmospheric entry takes
about 3-10 sec. Rock is a very good insulator.

What does get transmitted to the interior is a shockwave which can disrupt a
very large meteoroid once the shock builds up. The energy does not all go
into heating the asteroid, but into disrupting it. The small fragments will
then either turn to dust or burn up (if they are moving fast enough).

The shockwave energy can cause internal compressions and heating which
enhance the explosive disassembly of the asteroid. It has nothing to do
with conduction of heat from the surface.

Imagine the same experiment done on an iron meteorite, which would not be
disrupted because it has a very high tensile strength. It will reach the
surface intact, as actual observations of iron meteorites show.



Are you aware that fresh meteorites have been found that have frost
on the outside, because the interiors were at very low temperatures
in space and only the outer crust was heated by atmospheric entry.

Are you aware that energy and heat can be exchanged, we've only
been making steam trains running on coal fires and capable of
100 mph for 200 years?

E = m . v^2/2 only if v goes to zero, otherwise it remains kinetic
energy. That means deceleration is converted to heat, no matter how
long it takes. Obviously you have a physics problem.


I don't think


At least you admit that now.

I am the one with the physics problem here. An asteroid entering
the atmosphere at 10 km/s is not a steam train.


One favourite ploy of the illogical twerp is to say
"A {object/expression} is not a {different object/expression} "
as in
"An asteroid travelling at 10 km/s is not a steam train!"
or
"Eight hotdogs are not a dog's breakfast!"
The dog may conclude otherwise and prove it by eating all the hotdogs.
Obviously you have a thermal transfer problem, but you are correct
in one respect: the boiler of the steam train would definitely explode
within seconds if it travelled in air at 10 km/s, as was so tragically
demonstrated by the shuttle Columbia.
Learn about heat transfer which is faster if there is a greater
temperature gradient, as anyone with a lick of common sense would
know.


Columbia was, regrettably, a well designed hypersonic glider with a hole in
one wing. The heat transfer via hot gases penetrating still took over a
minute to cause fatal damage to the craft. It wasn't a "boiler explosion"
but the wings falling off.


If it was a stony object, like some asteroids that have been
studied close up, it may have had a fairly loose structure so it
is not hard to see why it would break up. I understand this is
what is generally postulated. In contrast, consider an iron
meteorite, which would have held
together. They never found any indications of iron in the
composition. --
Mike Dworetsky


--
Mike Dworetsky

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