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smeagol
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Question About Paint Color Of Concorde And SR-71

Sat Mar 22, 2014 10:11 pm

Pardon my silly question: I've learnt that for safety reason that Concorde was painted with white because of the high temperatue. But why SR-71, the Blackbird can be painted with all black given its even faster speed?
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Mir
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RE: Question About Paint Color Of Concorde And SR-71

Sat Mar 22, 2014 10:23 pm

Different metals. The Blackbird is made of titanium, while Concorde is conventional aluminum. Titanium is much more heat-resistant.

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Aesma
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RE: Question About Paint Color Of Concorde And SR-71

Sat Mar 22, 2014 10:23 pm

From wikipedia about the SR-71 : Finished aircraft were painted a dark blue, almost black, to increase the emission of internal heat and to act as camouflage against the night sky. The dark color led to the aircraft's call sign "Blackbird".

Concorde being a passenger airplane didn't have that much "internal heat" to dissipate so the main problem was friction with the air. And it flew during the day so black would have been bad.
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nomadd22
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RE: Question About Paint Color Of Concorde And SR-71

Sun Mar 23, 2014 1:16 am

The amount of heat absorbed through sunlight hitting black paint is insignificant compared to the heat from friction at mach 2 and above. The white paint on the Concorde didn't really have much to do with temperature. It might have very well have run cooler if it was black, since black would have radiated friction based heat better.
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WingedMigrator
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RE: Question About Paint Color Of Concorde And SR-71

Sun Mar 23, 2014 4:18 am

Quoting Aesma (Reply 2):
Finished aircraft were painted a dark blue, almost black, to increase the emission of internal heat and to act as camouflage against the night sky.

Dark paint colors generally do not emit (in the infrared) any more than light colors. They do absorb more in the visible spectrum, but that is irrelevant here. I'm pretty sure the color was purely for camouflage.

Quoting nomadd22 (Reply 3):
It might have very well have run cooler if it was black, since black would have radiated friction based heat better.

Nope. White radiates just fine. In fact it is the color used for spacecraft thermal radiators.
 
BMI727
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RE: Question About Paint Color Of Concorde And SR-71

Sun Mar 23, 2014 5:20 am

Quoting nomadd22 (Reply 3):
The white paint on the Concorde didn't really have much to do with temperature.

The blue Concorde sponsored by Pepsi was limited in speed and time because it lacked the white paint of regular versions. The military also use anti-flash white to minimize the thermal effects of a nearby nuclear blast.
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Tristarsteve
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RE: Question About Paint Color Of Concorde And SR-71

Sun Mar 23, 2014 10:39 am

Quoting BMI727 (Reply 5):
The blue Concorde sponsored by Pepsi was limited in speed and time because it lacked the white paint of regular versions.

But wasn't that because the Pepsi blue was ordinary paint?
Concorde white was very special flexible paint that could cope with the expansion and contraction of the fuselage as it went supersonic. The Pepsi Concorde was only shown on the ground so didn't need to go supersonic so was painted with standard paint.
 
nomadd22
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RE: Question About Paint Color Of Concorde And SR-71

Sun Mar 23, 2014 2:06 pm

Quoting WingedMigrator (Reply 4):
Nope. White radiates just fine. In fact it is the color used for spacecraft thermal radiators.

Those radiators are sometimes white because sunlight in space is a much higher factor compared to the heat radiated. Black radiating heat more efficiently is not cutting edge science. Ben Rich had the same trouble explaining that to cereal box experts on the SR-71.
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BMI727
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RE: Question About Paint Color Of Concorde And SR-71

Sun Mar 23, 2014 4:37 pm

Quoting TristarSteve (Reply 6):
The Pepsi Concorde was only shown on the ground so didn't need to go supersonic so was painted with standard paint.

It was flown, but was able to fly at Mach 1.7 for long periods and was limited to short periods Mach 2. I imagine that the white paint was similar to the anti-flash white. Mach 1.7 would probably produce enough heat to make the aircraft expand significantly.
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WingedMigrator
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RE: Question About Paint Color Of Concorde And SR-71

Sun Mar 23, 2014 11:48 pm

Quoting nomadd22 (Reply 7):
Black radiating heat more efficiently is not cutting edge science.

The infrared emissivity of black is no higher than that of white, and therefore it does not radiate heat more efficiently. That is indeed not cutting edge science.
 
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CALTECH
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RE: Question About Paint Color Of Concorde And SR-71

Mon Mar 24, 2014 12:29 am

There was another purpose to the 'black paint', or 'indigo blue.' As per Lockhed-Martin, the paint was infused with a element, micro-iron balls, that absorbed radar, and the paint also radiated excess heat and camouflaged the aircraft against dark skies
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nomadd22
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RE: Question About Paint Color Of Concorde And SR-71

Mon Mar 24, 2014 1:56 am

Quoting WingedMigrator (Reply 9):
The infrared emissivity of black is no higher than that of white, and therefore it does not radiate heat more efficiently. That is indeed not cutting edge science.

Black coatings make for more efficient radiators for the same reason you don't want them on objects subject to heating from sunlight. They absorb both visible and infrared better than white. That means they're more efficient at passing infrared from a hot object and re-radiating it to the outside. Your understanding only applies to two object of equal temperature radiating heat. A white coating is better at rejecting infrared in both directions.
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smeagol
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RE: Question About Paint Color Of Concorde And SR-71

Mon Mar 24, 2014 2:19 am

The Pepsi Concorde did flew, with the crusing speed limited within Marh 1.7.
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WingedMigrator
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RE: Question About Paint Color Of Concorde And SR-71

Mon Mar 24, 2014 3:51 am

Quoting nomadd22 (Reply 11):
Black coatings make for more efficient radiators for the same reason you don't want them on objects subject to heating from sunlight. They absorb both visible and infrared better than white.

That is only partially correct. In colloquial usage, absorptivity means absorptivity in the visible spectrum and emissivity means emissivity in the infrared spectrum. Black and white (and just about any other color, for that matter) have similar emissivity and are about equally effective at radiating in the infrared, which is what you care about for rejecting heat as long as your temperature isn't so high that things start to glow.

You can convince yourself of this by looking at some datasheets for some typical space-grade paints. Aeroglaze Z306 (flat black) and Aeroglaze A276 (gloss white) both have the same infrared emissivity of 0.9

Black paint does indeed absorb more in the visible spectrum, which is why coatings that are primarily designed to reject heat are white and not black. The SR-71 was the color it was for other reasons mentioned up-thread.
 
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dlednicer
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RE: Question About Paint Color Of Concorde And SR-71

Mon Mar 24, 2014 6:09 pm

The Lockheed A-12 (predecessor to the SR-71) first flew in bare betal:


Then, after it was flow to high Mach number, the portions that got the highest frictional heating were painted black:


Finally, the whole aircraft was painted black:

View Large View Medium
Click here for bigger photo!

Photo © David Lednicer

 
keta
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RE: Question About Paint Color Of Concorde And SR-71

Tue Mar 25, 2014 9:06 pm

Quoting Aesma (Reply 2):
From wikipedia about the SR-71 : Finished aircraft were painted a dark blue, almost black, to increase the emission of internal heat and to act as camouflage against the night sky. The dark color led to the aircraft's call sign "Blackbird".

Concorde being a passenger airplane didn't have that much "internal heat" to dissipate so the main problem was friction with the air. And it flew during the day so black would have been bad.

The internal heat there refers to the airframe heat, caused by skin friction, as opposed to the heat by the environment. Nothing to do with internal systems of the aircraft.

Quoting WingedMigrator (Reply 13):
Quoting nomadd22 (Reply 11):
Black coatings make for more efficient radiators for the same reason you don't want them on objects subject to heating from sunlight. They absorb both visible and infrared better than white.

That is only partially correct. In colloquial usage, absorptivity means absorptivity in the visible spectrum and emissivity means emissivity in the infrared spectrum. Black and white (and just about any other color, for that matter) have similar emissivity and are about equally effective at radiating in the infrared, which is what you care about for rejecting heat as long as your temperature isn't so high that things start to glow.

You can convince yourself of this by looking at some datasheets for some typical space-grade paints. Aeroglaze Z306 (flat black) and Aeroglaze A276 (gloss white) both have the same infrared emissivity of 0.9

Black paint does indeed absorb more in the visible spectrum, which is why coatings that are primarily designed to reject heat are white and not black. The SR-71 was the color it was for other reasons mentioned up-thread.

Since you are, in a sense, both right, I thought I might step in to give more information to other people... or add more noise, who knows  

In general, materials which are good at absorbing external light are also good at emitting. Conversely, good reflectors are bad emitters. Hence, black is good at absorbing and emitting, and white paint, reflecting most light, would also be bad at emitting light... in the visible range.
In the infrared range, white paint might very well be "black". At the usual temperatures, the emitted radiation is mostly in the infrared spectrum, thus you want to look at the emissivity in this range. As WingedMigrator notes, white paints can very well achieve a good emissivity in the infrared.

Interestingly, the SR-71 was not that far away from glowing! I see in internet figures of 300 ºC, up to 500 ºC! BTW, since we started comparing Concorde and SR-71, Mach 3 generates much more heat than Mach 2, compare that to the 127 ºC maximum of Concorde (figures from internet).

Now, even though very hot, it did not glow and the peak of the spectrum is in the infrared. My bet is that they could have achieved similar heat dissipation capabilities had they used a different paint, and that the choice was mostly driven by camouflage, and perhaps as some sources quote, radar absorption.

In the end, the XB-70 was painted white and also flew at Mach 3, and the U-2 is usually black, and I wouldn't say it's because of speed...  
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HaveBlue
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RE: Question About Paint Color Of Concorde And SR-71

Wed Mar 26, 2014 12:15 am

Quoting keta (Reply 15):

In the end, the XB-70 was painted white and also flew at Mach 3, and the U-2 is usually black, and I wouldn't say it's because of speed...

Read this whole thread and the further down I got was pleasantly surprised that no one had mentioned one of my favorite planes, the XB-70, as I had it in my head to bring it up.. but alas, I get down the the last sentence!  

Anyhow, the XB-70 Valkyrie shed its white paint on several flights. I don't have the time right now to look it up to give a link, but it did happen.
 
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CALTECH
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RE: Question About Paint Color Of Concorde And SR-71

Wed Mar 26, 2014 3:46 pm

http://www.strangecosmos.com/content/item/126481.html

"Upon returning to Palmdale, engineers puzzled over why the Valkyrie had been shedding herself of paint on all but her first flight. Although not overly concerned, no one was especially happy with unexplainable things on a plane that was to test the limits of jet-powered flight! Although some observers wondered if the Air Force was being entirely truthful about the XB-70's top speed, it was finally determined that too-thick paint caused by several re-paintings (in order to pretty the plane up for various VIPs) was being cracked as the Valkyrie flexed in flight, and was then torn away by the airstream. During her winter stay at Plant 42, the XB-70 was carefully repainted with just a single, thin coat of white paint. "

http://www.strangecosmos.com/images/content/126481.jpg
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