A riddle

[onix]

We often think of a perfect radiator of energy as being black, e.g. black body radiation, so why are stealth fighters and spy planes like the SR71 black?

Added 3/25/2005 01:37PM (PST):

It seems that for superfast planes like the SR71 the frictional heat they generate should be visible under infrared. Background infrared under a clear sky should only be 4K.


Added 3/29/2005 08:40AM (PST):

I found the following links pretty informative:

http://www.abovetopsecret.com/forum/thread127850/pg1

http://www.answers.com/topic/stealth-technology

Clearly, there are multiple aspects to stealth, and being invisible depends on to what.

 

[Gibsons]

The SR-71 was black because it emits the heat better. The SR-71s generated lots and lots of heat from friction obviously. (source is Ben Rich - Skunk Works. He won a bet with Kelly Johnson on this topic).

Stealth fighter is black for low visual observability in dark skies. But (again according to Rich) it should probably be a dark grey instead.

 

[CycloWizard]

Stealth planes were designed to rely more heavily on their intricate shaping to be stealthy than the paints. Newer paints go a long way in decreasing their detectability in a variety of ways. The USAF has a whole research group that works just on developing new paints for this application. The work is pretty much all classified.

 

[Jon855]

Originally posted by: onix

We often think of a perfect radiator of energy as being black, e.g. black body radiation, so why are stealth fighters and spy planes like the SR71 black?

Straight up your facts before saying SR-71 is black, it's only black due to the extreme high temp/friction in the latter. It's painted in dark blue and turns Black after one use I guess, depending on the altitiude and speed.

 

[91TTZ]

Originally posted by: onix

We often think of a perfect radiator of energy as being black, e.g. black body radiation, so why are stealth fighters and spy planes like the SR71 black?

The key is to not radiate the energy back in the direction that it came from, and not in the same wavelength it was originally.

A boxy, polished aluminum aircraft will reflect that radar right back. A stealthily angled jet with special paint will absorb the radar and turn it into heat.

That's one of the qualities about that paint- it absorbs radar and dissipates it as heat.

 

[TuxDave]

Originally posted by: onix

We often think of a perfect radiator of energy as being black, e.g. black body radiation, so why are stealth fighters and spy planes like the SR71 black?

Why should I be believing that black objects radiate more energy than any other color again?

 

[Gibsons]

Originally posted by: onix
We often think of a perfect radiator of energy as being black, e.g. black body radiation, so why are stealth fighters and spy planes like the SR71 black?

Added 3/25/2005 01:37PM (PST):

You guys are smart, but 91TTZ wins. Although it seems that for superfast planes like the SR71 the frictional heat they generate should be visible under infrared. Background infrared under a clear sky should only be 4K.

Uh, the color of the airplanes has nothing to do with the radar asorbtion or reflection. AFAIK, the iron-ball paint (radar absorbent) can be most any color. The answer to your question about both planes is in this book Skunk Works.
It's written by Ben Rich, who was head of the Skunk Works when the F-117 was designed and built.


He states pretty clearly that the SR-71 is black in order to help radiate heat. It generates loads of it flying at Mach 3.2 of course. As I said, the author had a bet with Kelly Johnson about the plane, Johnson thought the paint would weigh too much to be worth the extra heat dissipation, Rich thought the heat dissipation would prove to be too important. Rich was right, so he won the bet, and SR-71s are black. Its paint was not radar absorbent afaik. And yes, if you looked at the SR-71 in flight with an IR imager, it would be extremely bright and visible from about as far away as the atmosphere and your optics allowed.

The F-117 was originally painted gray as that's what Lockheed (or someones) research showed had the lowest visibility in a night sky. The Air force wanted in black. So it was black. The color had/has nothing to do with radar absorbance - note that they're painting some of them different colors now as black stands out too much (optically) for daytime missions link. Heat radiation isn't that big of a problem for the F-117 (except for the engines, but that's another story) since it flies at subsonic speeds.

 

[91TTZ]

Originally posted by: Gibsons

He states pretty clearly that the SR-71 is black in order to help radiate heat. It generates loads of it flying at Mach 3.2 of course. As I said, the author had a bet with Kelly Johnson about the plane, Johnson thought the paint would weigh too much to be worth the extra heat dissipation, Rich thought the heat dissipation would prove to be too important. Rich was right, so he won the bet, and SR-71s are black. Its paint was not radar absorbent afaik.

That's false. The story about Kelly Johnson is an probably an old wive's tale talking about the good ol' days. The XB-70 Valkyrie bomber flew at mach 3+ and was white. Meanwhile, the other main spyplane, the U2, was painted with the same radar absorbing "iron ball" black paint that the SR-71 used. The U-2 was slow and certainly didn't use it because of its heat dissipation qualities.

And like I mentioned before, the SR-71's paint was radar absorbing.

SR-71

"After the Air Force began to operate the SR-71, it acquired the official name Blackbird-- for the special black paint that covered the airplane. This paint was formulated to absorb radar signals, to radiate some of the tremendous airframe heat generated by air friction, and to camouflage the aircraft against the dark sky at high altitudes."

 

[onix]

Technically black is one that absorbs all colors, but what is "black" at radar frequencies may not be optically black. I think that with the right mix of nanoparticles (for example) the paint could be made black for one set of frequencies, and white for another set. Therefore it is remotely possible that the paint color could be gray (part white, part black) and yet absorb at lower frequencies.

So in its more general form, this statement "Uh, the color of the airplanes has nothing to do with the radar asorbtion or reflection." is (and I have to agree with 91TTZ) false.

 

[Gibsons]

Originally posted by: 91TTZ

Originally posted by: Gibsons

He states pretty clearly that the SR-71 is black in order to help radiate heat. It generates loads of it flying at Mach 3.2 of course. As I said, the author had a bet with Kelly Johnson about the plane, Johnson thought the paint would weigh too much to be worth the extra heat dissipation, Rich thought the heat dissipation would prove to be too important. Rich was right, so he won the bet, and SR-71s are black. Its paint was not radar absorbent afaik.

That's false. The story about Kelly Johnson is an probably an old wive's tale talking about the good ol' days. The XB-70 Valkyrie bomber flew at mach 3+ and was white. Meanwhile, the other main spyplane, the U2, was painted with the same radar absorbing "iron ball" black paint that the SR-71 used. The U-2 was slow and certainly didn't use it because of its heat dissipation qualities.

And like I mentioned before, the SR-71's paint was radar absorbing.

SR-71

"After the Air Force began to operate the SR-71, it acquired the official name Blackbird-- for the special black paint that covered the airplane. This paint was formulated to absorb radar signals, to radiate some of the tremendous airframe heat generated by air friction, and to camouflage the aircraft against the dark sky at high altitudes."

Here are some passages from Skunk Works, reprinted without permission.

advance apologies for any errors due to my poor typing:

Only one small U.S. company milled titanium, but it sold it in sheets of wildly varying quality. We had no idea how to extrude it, push it though into various shapes, or weld or rivet or drill it. Drilling bits used for aluminum simply broke into pieces trying to pierce titaniums unyielding hide. This exotic alloy would undoubtedly break our tools as well as our spirits. At one of our daily 7am planning sessions in Kelly's office, I volunteered some unsolicited advice about how we could use a softer titanium that began to lose its strength at 550 degrees. My idea was to paint the airplane black. From my college days I remembered that a good heat absorber was also a good heat emitter and would actually radiate away more heat than it would absorb through friction. I calculated that black paint would lower the wing temperature 35 degrees by radiation. But Kelly snorted impatiently and shook his head. "Goddam it Rich, you're asking me to add weight - at least a hundred pounds of black paint - when I'm desperately struggling to lose even an extra ounce. The weight of your black paint will cost me about eighty pounds of fuel." I said, "But, Kelly, think of how much easier it will be to build the airplane using a softer titanium which we can co if we lower the heat friction temperatures on the surface. Adding a hundred pounds is nothing compared to that."

"Well, I'm not betting this airplane on on any damned textbook theories you've dredged up. Unless I got wax buildup, I'm only hearing you suggest a way to add weight."

Overnight however, he apparently had second thoughts or did some textbook reading on his own, and at the next meeting he turned to me as the first order of business. "on the black paint," he said, "you were right about the advantages and I was wrong." He handed me a quarter. It was a rare win. So Kelly approved my idea of painting the airplane black, and by the time our first prototype rolled out the airplane became known as the Blackbird.

edit pic of a light grey F-117 (not modem friendly).

 

[CycloWizard]

Originally posted by: onix
Technically black is one that absorbs all colors, but what is "black" at radar frequencies may not be optically black. I think that with the right mix of nanoparticles (for example) the paint could be made black for one set of frequencies, and white for another set. Therefore it is remotely possible that the paint color could be gray (part white, part black) and yet absorb at lower frequencies.

So in its more general form, this statement "Uh, the color of the airplanes has nothing to do with the radar asorbtion or reflection." is (and I have to agree with 91TTZ) false.

Actually, the color of the plane doesn't necessarily have anything to do with radar. Coloration is granted by the micro/nanostructure of the paint. The manner in which the paint interacts with a given EM range depends on the distribution of these structures in a given size range. If the dimension of a micro/nanostructure is comparable to that of the EM wavelength, then the two will interact.

 

[JTWill]

As far as the paint being used for radar reduced signature, bear in mind that was not a concern with the SR-71, You cant shoot it down due to its altitude and speed . Its shape does reduce a radar profile. The paint is for heat. F117 was developed in consideration of mass battle with the soviets, and would only have been used at night. It does not have the capability of engaging multiple enemy fighters faster than it is.

 

[unipidity]

Blackbird is at least partly stealthy thanks to RAM. Anything that puts off detection by a few seconds is good. Im pretty sure that a SAM could down a blackbird it very favourable conditions.

Incidentally, how the hell DID the Valkerie not melt? Was it a titanium fuselage? Cool looknig plane, shame it died. At least the Fantastic Four got one....

 

[Gibsons]

Incidentally, how the hell DID the Valkerie not melt? Was it a titanium fuselage? Cool looknig plane, shame it died. At least the Fantastic Four got one....

AFAIK, it made use of the fuel to cool the hotter spots, it was just routed through there on the way to the engine. Or maybe they used a coolant on the hot spot and the fuel to remove heat from the coolant. I read somewhere that they had some method to only use warmest portions of the fuel, so the whole gas reserves wouldn't heat up as the flight went on. Dunno how true any of this really is... seems reasonable enough though, I'm sure it carried huge amounts of fuel. I've no idea what it was made of though.

 

[CycloWizard]

Originally posted by: JTWill
As far as the paint being used for radar reduced signature, bear in mind that was not a concern with the SR-71, You cant shoot it down due to its altitude and speed . Its shape does reduce a radar profile. The paint is for heat. F117 was developed in consideration of mass battle with the soviets, and would only have been used at night. It does not have the capability of engaging multiple enemy fighters faster than it is.

As you say, we weren't worried about anyone shooting down the SR-71. However, I'm guessing that we didn't exactly want it to be seen every time it made a run.

Originally posted by: Gibsons
AFAIK, it made use of the fuel to cool the hotter spots, it was just routed through there on the way to the engine. Or maybe they used a coolant on the hot spot and the fuel to remove heat from the coolant. I read somewhere that they had some method to only use warmest portions of the fuel, so the whole gas reserves wouldn't heat up as the flight went on. Dunno how true any of this really is... seems reasonable enough though, I'm sure it carried huge amounts of fuel. I've no idea what it was made of though.

I'm not sure about this particular plane, but typically heat is carried through the hydraulic fluid, which is subsequently cooled by the fuel as you said. Hydraulic lines run throughout planes, so it's the easiest way to do it and pretty effective. Of course, this lead to a completely different set of problems as planes ran hotter and hotter, resulting in the development of fire-resistant hydraulic fluids and so on. Lots of fun stuff.

 

[Calin]

Originally posted by: TuxDave

Originally posted by: onix

We often think of a perfect radiator of energy as being black, e.g. black body radiation, so why are stealth fighters and spy planes like the SR71 black?

Why should I be believing that black objects radiate more energy than any other color again?

Why the radiators on the back of some audio amplifiers are black?

 

[The Boston Dangler]

The next big thing in stealth aircraft will be a color changing paint job. Kind of like in "Predator", only not nearly that good. That and low-temp exhaust.

Btw, I believe the Blackbird and Valkyrie used fuel to cool hot spots. The preheated fuel is used to get combustion in difficult cicumstances. Remember, these planes use special fuel, JP-7 I think. Like diesel, you can put out a small fire with it.

 

[CycloWizard]

Originally posted by: The Boston Dangler
The next big thing in stealth aircraft will be a color changing paint job. Kind of like in "Predator", only not nearly that good. That and low-temp exhaust.

There won't be a low-temp exhaust unless the laws of thermodynamics change. Jets work by combusting fuel, thereby causing a large increase in the specific volume of the input gas stream due to temperature increase and a stoichiometric increase in the number of moles of gas. For jets to work, the reaction must occur very, very quickly, necessitating high temperatures. Combustion releases additional energy, which raise the temperatures even more.

That said, the B-2 already has some pretty nifty engine exhaust ports that largely mitigate the problems with thermal detection of exhaust. By increasing the area of the exhaust, the heat output of the engines is decentralized, effectively decreasing the thermal signature. Maybe this is what you meant by 'low-temp exhuast', but that's a misnomer.

Btw, I believe the Blackbird and Valkyrie used fuel to cool hot spots. The preheated fuel is used to get combustion in difficult cicumstances. Remember, these planes use special fuel, JP-7 I think. Like diesel, you can put out a small fire with it.

Maybe the Blackbird and Valkyrie use fuel directly to cool the body, but I seriously doubt it. The USAF had more than enough problems running hydraulic fluid through planes to do the job (pic). Running older fuels that extensively through a plane is not a trivial task (I have a pic of hydraulic lines in an F-22 somewhere around here, I'll see if I can find it), nor were jet fuels of the time as fire-resistant as they are now. JP-7 can be used to put out a fire, depending on a few conditions - its temperature, the flame temperature, and the duration of contact time. Newer jet fuels (JP-8 and higher) are much more fire-resistant, but they are still not used directly to cool the plane. Instead, the hydraulic fluid retrieves heat from the plane and returns it to central heat exchangers. On the other side of the heat exchanger is the fuel. Thus, the fuel is indirectly used to cool the plane. Again, maybe this is what you meant, but clarification was in order.

 

[CycloWizard]

Originally posted by: Calin
Why the radiators on the back of some audio amplifiers are black?

To match the equipment. The rate of energy radiated from any solid body is given by a constant multiplied by a function of temperature. It's independent of color.

 

[onix]

Originally posted by: CycloWizard

Originally posted by: Calin
Why the radiators on the back of some audio amplifiers are black?

To match the equipment. The rate of energy radiated from any solid body is given by a constant multiplied by a function of temperature. It's independent of color.

This comment is highly dismissive.

The constant is the emissivity of the material, and equals (1-absorptivity), by definition. Emissivity is a function of electromagnetic frequency.

"Color" does matter, but in a general sense. Think not just in our visible spectrum of red to violet, but think radio waves to high-energy gamma rays. Also painting a highly polished aluminum surface black can significantally alter its heating rate (or cooling rate).

Check out these links:

http://www.usenet.com/newsgroups/sci.aeronautics/msg00085.html

http://www.electro-optical.com/bb_rad/emissivity/emisivty.htm

 

[CycloWizard]

Originally posted by: onix
This comment is highly dismissive.

The constant is the emissivity of the material, and equals (1-absorptivity), by definition. Emissivity is a function of electromagnetic frequency.

"Color" does matter, but in a general sense. Think not just in our visible spectrum of red to violet, but think radio waves to high-energy gamma rays. Also painting a highly polished aluminum surface black can significantally alter its heating rate (or cooling rate).

Check out these links:

http://www.usenet.com/newsgroups/sci.aeronautics/msg00085.html

http://www.electro-optical.com/bb_rad/emissivity/emisivty.htm

The comment was 'highly dismissive?' I believe he was referring to radiative heat flux, in which case what I said is the long and short of it (the Stefan-Boltzmann law of radiative heat transfer). The constant accounts for a summation over all wavelengths.

 

[onix]

Originally posted by: CycloWizard
The comment was 'highly dismissive?' I believe he was referring to radiative heat flux, in which case what I said is the long and short of it (the Stefan-Boltzmann law of radiative heat transfer). The constant accounts for a summation over all wavelengths.

Yes, I also believe he was referring to radiative heat transfer, as opposed to convection or condution. My comments on emissivity and dependence of color remain, as they would affect emissivity, i.e. EM/radiative transport - the only form of heat transfer remaining in pure vacuum.

 

[CycloWizard]

Originally posted by: onix
Yes, I also believe he was referring to radiative heat transfer, as opposed to convection or condution. My comments on emissivity and dependence of color remain, as they would affect emissivity, i.e. EM/radiative transport - the only form of heat transfer remaining in pure vacuum.

Yeah, I guess I should read the actual Stefan-Boltzmann law rather than trying to recall it off the top of my head... I was thinking of the black body version, in which emissivity = 1. This is what we use for almost any engineering application, since it's within 1-2% in almost every case. Obviously, if you're designing a stealth plane, every little bit helps so you would consider this. However, I sincerely doubt it played much of a role in selecting black for the color of an amplifier radiator.