Black or White?

[MMEMatty]

I really should know the answer to this, having done a course in heat transfer and basic thermodynamics, but i cant for the life of me derive a satisfactory answer. The question, in a nutshell, is this:

How come the SR-71 blackbird was painted black, yet Concorde was painted white?

now they are both designed to deal with high heat loads (i recall reading on the "Concorde Coffin Corner" thread its limit was 127 deg. C; the SR-71 presumably being somewhat higher), but i remember reading that black absorbs heat energy, so therefore presumably this would cause the SR-71 to overheat? surely if it was painted white (neglecting operational "stealth" reasons) , white reflecting heat, its thermal limit would be that much higher?

I get the feeling i've got this all ar$e over t!t, but i cant fathom it out. :confused:


Matty

Retiring to the pub

[Genghis the Engineer]

I've not worked on either, but am guessing that it's related to

(1) Concorde was made from aluminium alloys, whilst SR71 was from titanium alloys.

(2) At the time, black was considered stealthy - SR71 was likely to be shot at (however futile this gesture may have been), whilst this was never seriously anticipated for Concorde.

G

[Mad (Flt) Scientist]

For the purposes of kinetic heating, which is what you're discussing in terms of providing a restriction on upper speeds, the colour of the body isn't going to matter.

The colour of an object will affect its temperature as a result of radiative heating i.e. a black car parked in desert heat will get hotter faster than a white car in the same conditions, because the black absorbs more of the solar radiation, while white reflects better.

But the effect of a jet of hot (or cold) air on both bodies will be essentially identical. Once the boidy is hot it will radiate itself slightly differently, but for the kinds of temperatures you're discussing I don't think it matters.

[MMEMatty]

But i thought the "Pepsi" concorde couldnt fly supersonicaly because it wouldm overheat? or would that simply mean that the paint would degrade, spoiling the graphic designers pretty little logo?

Matty

[Mad (Flt) Scientist]

I'd be suspicious that it was the thermal characteristics of the paint itself, since it was a temporary job, right? I doubt it was a concern for the aircraft's structure, unless it were VERY marginal. If it was a speed reduction from M2.0+ to <N1.0, that seems unlikely.

[Intruder]

I really should know the answer to this, having done a course in heat transfer and basic thermodynamics, but i cant for the life of me derive a satisfactory answer. The question, in a nutshell, is this:

How come the SR-71 blackbird was painted black, yet Concorde was painted white?

MARKETING!

There may have been a relatively insignificant gain to be realized by painting the Concorde black, but passengers would rather fly a bright white bird.

OTOH, the Blackbird was designed to press the absolute edge of the envelope at the time, without marketing concerns...

[lomapaseo]

Gee and all this time I thought that some military planes were painted black because it was harder to see at high altitudes.

[Old Smokey]

Gee and all this time I thought that the heat generated in Hugh Hefner's black DC9 came from the inside.

[Loose rivets]

Did I not once hear that the polish--or lack of it--had slightly more effect on the radiation and absorption than the colour? So, matt black becomes the optimum for shedding heat from friction. Without specific figures, I would guess that this was far more important than the heat from the Sun being absorbed.

Having said this, try sitting on a dark green plastic chair v white at the side of pool on a hot day. Or simply ask a Saudi male and female for a comparison of their running temps.

[MkVIII]

The SR71 was painted in black, but just no ordinary black!

The paint contains, amongst other STILL TOP SECRET ingredients, carbon. All the ingredients together actually work at radar return minimisation, ergo stealth.

The F117 Nighthawk is painted the same for the exact same reasons.

Problem I believe the paint is VERY matt, and as such is NOT a good aerodynamic paint as it is not gloss. That is not a factor in the SR71 because it darned well can outrun and outfly just about anything anyway, and the F117 is slow anyway, and does not need speed as it's defence, instead flying at night (usually), where visual non-radar tracking of it is next to impossible.

There was a NASA SR71 painted gloss white at one stage, and I believe it had a reasonable speed margin over the military examples.

[Pub User]

lomapaseo

These days a lot of military aircraft (and police/HEMS) are painted black because they are easier to see at most altitudes and against most backgrounds - by day, obviously.

[brain fade]

There are many 'wedges' of RAM in the leading edges of the SR-71 wing. Others on other parts of the airframe also. Even on early A-12's (which were mostly left natural titanium colour) these areas were always black. This made it harder for a casual observer to spot the wedges. The contrast twixt the RAM & the natural metal would have been obvious. These aircraft though were basically silver, so the black is plainly not significant in any aerodynamic or heat dissipating characteristic that the a/c has. It's for tactical reasons and to enhance 'stealth' ie reduction of radar cross section. I believe the paint (which may have Iron ferrites in it) helps to attenuate and dissipate radar energy that strikes it.

[Onan the Clumsy]

Gee and all this time I thought that some military planes were painted black because it was harder to see at high altitudes I'd heard the exact opposite, namely that white was the hardest to see and that was one reason parachutes were made in that colour.

I'm not saying I believe it mind, just that I "read it somewhere".

[hercboy]

MkVIII is correct, the SR-71 had a radar obsobing paint, and it was painted black because its harder to see at high altatudes and at night u cant see it

[Buitenzorg]

From my high school physics classes:

The effect of a surface consistency is the same for absorbing ardiation as for emitting radiation. In other words, a black surface is good in absorbing radiation energy (this is why tarmac gets hot in the sun) and also in losing that heat (energy) by emitting radiation. This is why tarmac cools markedly when the sun goes down.

The heat build-up in the skin of the Blackbird is purely due to friction, not radiation. So if you want to get rid of as much of this heat (energy) as possible, which you do, then choose the most efficient way of emitting radiation possible, i.e. paint the thing black.