4 ms·

This proposal seems to ignore so much. Like the intense friction, and heating, supersonic flight produces. Concorde's service speed was limited by heat limits

This proposal seems to ignore so much. Like the intense friction, and heating, supersonic flight produces. Concorde's service speed was limited by heat limits place on the aluminium alloy to ensure a decent service life. They were always white as that was part of the spec - dark colours would have taken heat absorption out of limit! Wing roots varied by >100C each flight if I remember right, and fuel was used as a heat sink. Are we going to use the batteries? If the outer surfaces are conductors, what are we doing about icing conditions? Concorde, or likely most SST, engines put out a lot of thrust in a very fighter-like profile (they were a continuation of a fighter engine, and had reheat). Electric fans aren't going to work efficiently in that profile and would probably be large diameter, which isn't very supersonic friendly. How are we now handling taxiing and subsonic? Edit: To expand the last point, Concordes were horribly inefficient subsonic, and burnt something like 2t of fuel to get to the runway. Reheat was used at takeoff and going transonic. They pumped tonnes of fuel after reaching supersonic - for weight and balance and reduced drag. They were basically really big fighters.

msl 10y ago
> They were always white as that was part of the spec - dark colours would have taken heat absorption out of limit! Interesting. The SR-71 Blackbird was dark (almost black) in order to radiate heat efficiently [1]. Then again, the Blackbird flew much higher (at around 26 km compared to Concorde's just over 18 km) so maybe convection plays a bigger part in Concorde's thermal management. [1] https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird (Although my primary source is the book "Skunk Works" by Ben R. Ritch and Leo Janos.)
- chadgeidel 10y ago
  Zing! I, too remember this from "Skunk Works". What a great book!
- anexprogrammer 10y ago
  Fascinating. Now I want to know why! Maybe at mach 3 more heat's coming from inside than being absorbed. Concordes were specced with Anti-flash White (high reflectivity as used by US and UK nuclear bombers), and the main reason I remember is there was some publicity when France painted one Pepsi blue (not wings) for a sponsorship. It had to have speed restrictions. Wikipedia has a little more (not much), but says the white finish reduced the skin temperature by 6 to 11 degrees Celsius. https://en.wikipedia.org/wiki/Concorde https://en.wikipedia.org/wiki/Concorde
  - msandford 10y ago
    The sun adds 1000 watts per square meter, and black body radiation is the fourth power of temperature difference. If you're heating the skin say 5000 watts per square meter due to friction or compression, then the 1000 watts from the sun isn't bad. But if it's only say 500 watts, well, then the sun dominates. They operated at different speeds and altitudes so it's not entirely surprising to me that they had different strategies.
- hga 10y ago
  Note that they were using titanium, and while I'm not sure what the fuel cells were made of, until they heated up it leaked quite a bit. One of the first orders of business after takeoff was getting a refuel, which included changing the type of fuel, see e.g. http://iliketowastemytime.com/facts-you-didnt-know-about-sr71-blackbird http://iliketowastemytime.com/facts-you-didnt-know-about-sr7...
chadgeidel 10y ago
re: "... dark colours would have taken heat absorption out of limit!" The SR-71 Blackbird was black because that color dissipates heat. [1] The plane was also made from titanium, not aluminum so there may be other factors but black dissipates heat as well as it absorbs it. [1] http://www.lockheedmartin.com/us/100years/stories/blackbird.html http://www.lockheedmartin.com/us/100years/stories/blackbird....
- dghughes 10y ago
  I thought the black was paint impregnated with tiny iron oxide balls to absorb radar energy? edit: they say that farther down the page "...and a radar-absorbing element was added to the paint."
mrfusion 10y ago
It couldn't an electric fly way higher and avoid that friction because it has no need for oxygen? That's the other benefit of electric planes.
- anexprogrammer 10y ago
  Um. If we reach a height of no friction, what are the electric jets compressing? If there's air there's friction, if there's no air there's no thrust. Rockets take their own oxidant.
- ThrustVectoring 10y ago
  The height limit isn't because engines need oxygen. It's because in order to fly in less dense air, you need to have higher air speed to generate the same amount of lift, and above typical jet cruising altitudes you hit the speed of sound and run into issues. Supersonic aircraft fly at higher altitudes in general - the Concorde cruised at 56000 feet.
- gozur88 10y ago
  Even if it doesn't need oxygen it will still need reaction mass. The engine won't produce thrust if there's nothing to shoot out the back.
ars 10y ago
> They were always white as that was part of the spec - dark colours would have taken heat absorption out of limit! Since radiant (photons) heat emission follows radiant heat absorption: Black absorbs better, but also emits it better. Maybe they didn't care about emitting heat, they just didn't want the sun adding heat?
WalterBright 10y ago
> Like the intense friction, and heating The heating does not come from friction. If a gas is compressed, it heats up. Moving through air compresses it in front. There's a point on the leading edge where the air is actually still. That's the point of maximum compression, and maximum heat (and minimum friction!). (Pitot tubes work on this principle, they are simply air pressure gauges calibrated to read as speed rather than PSI.) This misattribution is one of my pet peeves. <grump>
- marmaduke 10y ago
  Thanks for your on point technical replies on aircraft engineering here. Really fascinating stuff.
tzs 10y ago
> They were always white as that was part of the spec - dark colours would have taken heat absorption out of limit! Wouldn't the color only matter when it comes to heating due to absorption of electromagnetic radiation? For heat transfer by direct contact with the heat source (in this case gases that were heated when compressed by the plane's motion), wouldn't color be irrelevant?
PhasmaFelis 10y ago
Does the math work out if you ditch the "supersonic" part? Supersonic is pretty niche anyway, since you can't do it anywhere near populated areas; but substantially reducing per-passenger costs for subsonic planes could change a lot.
- lodi 10y ago
  I think the math doesn't work out with subsonic since those huge delta wings are only efficient for supersonic flight. For subsonic a conventional swept wing (like on a 747) is much more efficient.

msl 10y ago

> They were always white as that was part of the spec - dark colours would have taken heat absorption out of limit! Interesting. The SR-71 Blackbird was dark (almost black) in order to radiate heat efficiently [1]. Then again, the Blackbird flew much higher (at around 26 km compared to Concorde's just over 18 km) so maybe convection plays a bigger part in Concorde's thermal management. [1] https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird (Although my primary source is the book "Skunk Works" by Ben R. Ritch and Leo Janos.)

chadgeidel 10y ago
Zing! I, too remember this from "Skunk Works". What a great book!
anexprogrammer 10y ago
Fascinating. Now I want to know why! Maybe at mach 3 more heat's coming from inside than being absorbed. Concordes were specced with Anti-flash White (high reflectivity as used by US and UK nuclear bombers), and the main reason I remember is there was some publicity when France painted one Pepsi blue (not wings) for a sponsorship. It had to have speed restrictions. Wikipedia has a little more (not much), but says the white finish reduced the skin temperature by 6 to 11 degrees Celsius. https://en.wikipedia.org/wiki/Concorde https://en.wikipedia.org/wiki/Concorde
- msandford 10y ago
  The sun adds 1000 watts per square meter, and black body radiation is the fourth power of temperature difference. If you're heating the skin say 5000 watts per square meter due to friction or compression, then the 1000 watts from the sun isn't bad. But if it's only say 500 watts, well, then the sun dominates. They operated at different speeds and altitudes so it's not entirely surprising to me that they had different strategies.
hga 10y ago
Note that they were using titanium, and while I'm not sure what the fuel cells were made of, until they heated up it leaked quite a bit. One of the first orders of business after takeoff was getting a refuel, which included changing the type of fuel, see e.g. http://iliketowastemytime.com/facts-you-didnt-know-about-sr71-blackbird http://iliketowastemytime.com/facts-you-didnt-know-about-sr7...

chadgeidel 10y ago

re: "... dark colours would have taken heat absorption out of limit!" The SR-71 Blackbird was black because that color dissipates heat. [1] The plane was also made from titanium, not aluminum so there may be other factors but black dissipates heat as well as it absorbs it. [1] http://www.lockheedmartin.com/us/100years/stories/blackbird.html http://www.lockheedmartin.com/us/100years/stories/blackbird....

dghughes 10y ago
I thought the black was paint impregnated with tiny iron oxide balls to absorb radar energy? edit: they say that farther down the page "...and a radar-absorbing element was added to the paint."

mrfusion 10y ago

It couldn't an electric fly way higher and avoid that friction because it has no need for oxygen? That's the other benefit of electric planes.

anexprogrammer 10y ago
Um. If we reach a height of no friction, what are the electric jets compressing? If there's air there's friction, if there's no air there's no thrust. Rockets take their own oxidant.
ThrustVectoring 10y ago
The height limit isn't because engines need oxygen. It's because in order to fly in less dense air, you need to have higher air speed to generate the same amount of lift, and above typical jet cruising altitudes you hit the speed of sound and run into issues. Supersonic aircraft fly at higher altitudes in general - the Concorde cruised at 56000 feet.
gozur88 10y ago
Even if it doesn't need oxygen it will still need reaction mass. The engine won't produce thrust if there's nothing to shoot out the back.

ars 10y ago

> They were always white as that was part of the spec - dark colours would have taken heat absorption out of limit! Since radiant (photons) heat emission follows radiant heat absorption: Black absorbs better, but also emits it better. Maybe they didn't care about emitting heat, they just didn't want the sun adding heat?

WalterBright 10y ago

> Like the intense friction, and heating The heating does not come from friction. If a gas is compressed, it heats up. Moving through air compresses it in front. There's a point on the leading edge where the air is actually still. That's the point of maximum compression, and maximum heat (and minimum friction!). (Pitot tubes work on this principle, they are simply air pressure gauges calibrated to read as speed rather than PSI.) This misattribution is one of my pet peeves. <grump>

marmaduke 10y ago
Thanks for your on point technical replies on aircraft engineering here. Really fascinating stuff.

tzs 10y ago

> They were always white as that was part of the spec - dark colours would have taken heat absorption out of limit! Wouldn't the color only matter when it comes to heating due to absorption of electromagnetic radiation? For heat transfer by direct contact with the heat source (in this case gases that were heated when compressed by the plane's motion), wouldn't color be irrelevant?

PhasmaFelis 10y ago

Does the math work out if you ditch the "supersonic" part? Supersonic is pretty niche anyway, since you can't do it anywhere near populated areas; but substantially reducing per-passenger costs for subsonic planes could change a lot.

lodi 10y ago
I think the math doesn't work out with subsonic since those huge delta wings are only efficient for supersonic flight. For subsonic a conventional swept wing (like on a 747) is much more efficient.