A Question for the Scientifically Versed.

[rh]

personally, i'm wondering why foil has such excellent shielding properties against both government AND extraterrestrial mind-control apparati.

truly amazing stuff.

[emmline]

Many years ago, I saw a NASA guy do a demonstration at a high school. One part of the demo I'll never forget. He had a cube, maybe 2" on a side, made out of the material used to make the tiles on the undersurface of the Space Shuttles. He put bunsen burners on it until it was glowing. Then he turned off the heat, paused for MAYBE 2 seconds, and then picked it up with his bare hands. Cool. Literally and figuratively.

Well, that then is part 2, for which I did not have the explanation.
Why does it not burn hands?

[gonzo914]

Aluminium (note correct spelling, Merkins ) . . .

Actually, Humphry Davy, the guy who was trying to invent something to wrap his meat pie in, originally called it alumium (1808), but he decided that sounded too much like the name for some kind of automobile, so in 1812 he changed to aluminum. At that time, of course, the United States and Britain were at war, fighting over whether there should be a u i in words like color and humor , and some anonymous Brit, thinking that aluminum didn't sound pretentious enough, proposed aluminium. The US won the war, and thus the right to spell gray, jail and advertize the right way, and the British, in a fit of pique, decided aluminium it would be.

Andrew Jackson, who whipped the British handily at New Orleans several weeks after the war ended, due to no one remembering to tell General Packenham it was over, everyone being so obsessed with this aluminum/aluminium thing, said, "Fine. Spell it however you want. You'll be begging for it in 100 years or so, and we're going to go over there and seduce all your women with cigarettes and Hershey bars."

[Coffee]

Aluminium (note correct spelling, Merkins ) . . .

Actually, Humphry Davy, the guy who was trying to invent something to wrap his meat pie in, originally called it alumium (1808), but he decided that sounded too much like the name for some kind of automobile, so in 1812 he changed to aluminum. At that time, of course, the United States and Britain were at war, fighting over whether there should be a u i in words like color and humor , and some anonymous Brit, thinking that aluminum didn't sound pretentious enough, proposed aluminium. The US won the war, and thus the right to spell gray, jail and advertize the right way, and the British, in a fit of pique, decided aluminium it would be.

Andrew Jackson, who whipped the British handily at New Orleans several weeks after the war ended, due to no one remembering to tell General Packenham it was over, everyone being so obsessed with this aluminum/aluminium thing, said, "Fine. Spell it however you want. You'll be begging for it in 100 years or so, and we're going to go over there and seduce all your women with cigarettes and Hershey bars."

Just remember that 'colour' and 'humour' are simply frenchified variations of 'color' and 'humor.' As for aluminium; I have no idea.

[jsluder]

Many years ago, I saw a NASA guy do a demonstration at a high school. One part of the demo I'll never forget. He had a cube, maybe 2" on a side, made out of the material used to make the tiles on the undersurface of the Space Shuttles. He put bunsen burners on it until it was glowing. Then he turned off the heat, paused for MAYBE 2 seconds, and then picked it up with his bare hands. Cool. Literally and figuratively.

Well, that then is part 2, for which I did not have the explanation.
Why does it not burn hands?

Rapid heat dissipation.

From that link I posted earlier:

The thermal properties of the tiles are dependent on their very high purity. The manufacture of both types of tiles begins with fibers of pure white silica refined from common sand. The fibers are mixed with deionized water and other chemicals and poured into a plastic mold where excess liquid is squeezed out of the mixture.

The damp blocks are dried in the nation's largest microwave oven at the Sunnyvale, Calif., plant of Lockheed Space Operations Co. Then, they are sintered in a 2,350 degrees F oven. Sintering fuses the fibers without melting them.

...

The two types of tiles are the same except for their coating, which is primarily borosilicate glass. Chemicals are added to the coating to give the tiles different colors and heat rejection capabilities.

Surface heat dissipates so quickly that a tile can be held by its corners with a bare hand only seconds after removal from a 2,300 degrees F oven, while the center of the tile still glows red with heat.

[emmline]

Ok, well I figured it was along those lines. I now have
all the facts lined up to straighten the kid out.

This is, however, the most stubborn kid in this part of the galaxy, so we'll have to wait and see how it goes.
Right now I'm beginning to broach the conundrum of whether he's college material and, if so, where.
If not, I'm willing to sell him as an indentured servant, but in "as is" condition. No complaints or returns.
(right now, I hear him in the shower singing(?) New York, New York. This could also happen during
his indentured servitude. No guarantees.)

[avanutria]

If he's got any engineering tendencies, tell him to Google "heat sink" and inform him that heat sinks are part of why his computer works for more than a few minutes at a time.

Interestingly, Wikipedia says aluminum foil is used both for thermal insulation and for heat exchange (which we already know from this thread) but also says (http://en.wikipedia.org/wiki/Thermal_insulation) that aluminum will lose its effectiveness if it's touching anything else that is also a very good thermal conductor.

This suggests an experiment to me. Two foodstuffs wrapped in foil, perhaps hot dogs, one on a baking tray and one with some highly insulating material between the foil and the baking tray. Cook them at the same time and take the temperatures of the two hot dogs afterward. The insulated one will heat from convection but not from conduction, so it should be cooler.

Here's a list of thermal conductivities of some materials: http://en.wikipedia.org/wiki/Thermal_conductivity

[SteveShaw]

The tiles are made of a highly-refractory material and are extremely poor conductors of heat (the whole point of them is keep the spacecraft underneath from heating up), so even a good roasting in a bunsen flame will heat only the outermost layer. Even after a long time in the flame the amount of heat energy absorbed will be minimal and confined almost entirely to the surface, therefore when the tile is removed from the flame it actually has very little gained heat to radiate away, so it cools rapidly.

[SteveShaw]

Aluminium (note correct spelling, Merkins ) . . .

Actually, Humphry Davy, the guy who was trying to invent something to wrap his meat pie in, originally called it alumium (1808), but he decided that sounded too much like the name for some kind of automobile, so in 1812 he changed to aluminum.

I actually taught science to a direct descendant of Humphrey Davy a few years ago (he looked just like his ancestor's pictures and was even called Davy!) and he was a lousy speller too! And my story's true!

[gonzo914]

And my story's true!

Hey, there are at least three facts in that post, maybe upwards of five if you count generals Jackson and Packenham, and two actual dates.

Packenham, of course, was killed at New Orleans, and his body was packed in a cask of rum for the sea voyage home. He was known for being a man of surly temper, and on his return, a relative remarked "The General has returned home in better spirits than he left." Packenham's last words were recorded as either "Lost for the lack of courage," or "Two is, dammit, two is," although some military historians think they may have been "Lieutenant, do something about those sharpshoo . . ."

[Brian Lee]

Spending a decade at the local planetarium, and demonstrating shuttle tiles many times daily I may be able to shed a bit more insight for the masses here.

First off, I believe what your son may have been thinking of Emm, could be the older "ablative coatings" applied to the bottom of the Mercury, Gemini and Apollo capsules. These were coatings applied to the outer skin of space capsules (generally only the bottoms that were exposed to the highest heat) that were designed to burn away, and in so doing, carry away almost all of the energy of reentry in the process.

The shuttle tiles are a newer construct of course. They are indeed made primarily of silica and air. They feel almost like styrofoam in the hand. Similar texture and weight. You can easily dent and even crush a tile with hand strength alone if you desired. DON'T though, cause they be spendy!

We would demonstrate a tile (black underbelly variety) by holding the tile in our bare hand, and then applying a direct flame from a five gallon propane torch to the opposite side. This was no sissy BBQ flame mind you - it roared and shot a flame a good two feet. In the words of Darth Vader: MOST impressive!

The tiles do in fact heat in their cores, but they are excellent insulators and extremely poor conductors, so you could hold it in your hand still glowing white hot, and not be burned on the side opposite the flame.

We used the same tile for about 6 years, subjecting it to that flame probably an average of 8-10 times daily, six days a week. It never lost it's "cool" thank goodness!

Aluminum is a very good conductor of heat as others have stated. It's used quite frequently as a favorite material for heat sinks on the computer chips we use every day. It's also common for engine blocks, radiators and other high heat applications where an efficient means of dissipating heat is useful. Aluminum foil being so thin, will not hold heat long at all. That's why the fins on a CPU heat sink, or radiator are so thin as well. They move heat - and lots of it - away from the heat's source. R-A-P-I-D-L-Y.

Hence - no burny da fingers when you cook your pizza.

[SteveShaw]

Spending a decade at the local planetarium, and demonstrating shuttle tiles many times daily I may be able to shed a bit more insight for the masses here.

First off, I believe what your son may have been thinking of Emm, could be the older "ablative coatings" applied to the bottom of the Mercury, Gemini and Apollo capsules. These were coatings applied to the outer skin of space capsules (generally only the bottoms that were exposed to the highest heat) that were designed to burn away, and in so doing, carry away almost all of the energy of reentry in the process.

The shuttle tiles are a newer construct of course. They are indeed made primarily of silica and air. They feel almost like styrofoam in the hand. Similar texture and weight. You can easily dent and even crush a tile with hand strength alone if you desired. DON'T though, cause they be spendy!

We would demonstrate a tile (black underbelly variety) by holding the tile in our bare hand, and then applying a direct flame from a five gallon propane torch to the opposite side. This was no sissy BBQ flame mind you - it roared and shot a flame a good two feet. In the words of Darth Vader: MOST impressive!

The tiles do in fact heat in their cores, but they are excellent insulators and extremely poor conductors, so you could hold it in your hand still glowing white hot, and not be burned on the side opposite the flame.

We used the same tile for about 6 years, subjecting it to that flame probably an average of 8-10 times daily, six days a week. It never lost it's "cool" thank goodness!

Aluminum is a very good conductor of heat as others have stated. It's used quite frequently as a favorite material for heat sinks on the computer chips we use every day. It's also common for engine blocks, radiators and other high heat applications where an efficient means of dissipating heat is useful. Aluminum foil being so thin, will not hold heat long at all. That's why the fins on a CPU heat sink, or radiator are so thin as well. They move heat - and lots of it - away from the heat's source. R-A-P-I-D-L-Y.

Hence - no burny da fingers when you cook your pizza.

In their application on the outside of spaceships hurtling through the upper atmosphere it is not the function of the tiles to heat to the core. Were this to be the case the heat gained (from friction) would dissipate both ways, both back into space and also, fatally, into the ship. The whole point is that they heat up on their outer surface only, their cores remaining relatively cool, as a result of their being very poor conductors.

As for the thinness of aluminium foil and radiator fins, I'm at a loss as to why you omit the most crucial part of the explanation - in these instances, as with the elephant's ears, it is the very large surface area to volume ratio that ensures that heat can be dissipated rapidly. Sometimes it takes a biologist to get the physicists to the heart of the matter.

[jsluder]

Sometimes it takes a biologist to get the physicists to the heart of the matter.

Have you considered helping the physicists develop and refine their Theory of Everything?

[Denny]

ears, heart...the man nose

[Brian Lee]

In their application on the outside of spaceships hurtling through the upper atmosphere it is not the function of the tiles to heat to the core. Were this to be the case the heat gained (from friction) would dissipate both ways, both back into space and also, fatally, into the ship. The whole point is that they heat up on their outer surface only, their cores remaining relatively cool, as a result of their being very poor conductors.

As for the thinness of aluminium foil and radiator fins, I'm at a loss as to why you omit the most crucial part of the explanation - in these instances, as with the elephant's ears, it is the very large surface area to volume ratio that ensures that heat can be dissipated rapidly. Sometimes it takes a biologist to get the physicists to the heart of the matter.

Actually, no - that really isn't correct Steve. The tiles DO in fact heat up in their cores, and not just superficially on the surface. You would be perfectly correct however stating that the outer surface (the one directly in contact with the ionized atmosphere during re-entry, or the one directly in contact with the intense flame from the torch we used) heats up more than the core. But we did in fact heat the core of those tiles to incandescence daily, as well as the exterior surfaces.

As to the surface area issue - this is a property of any matter that is heated. Spread it out, and the increased surface area will cool it faster than if the material is more dense and compact. You can spread the coals and embers of a campfire to help put it out right? My point if you check, was that the thinner the heat sink (aluminum in this case as that was the original question) the faster it will radiate it's stored heat energy. A heat sink made with very thick vanes, but the same surface area will not cool quite as quickly or efficiently in most cases - as a general rule.

Blasting off to supper! All the best...