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After reading the answers to this question: After what speed air friction starts to heat up an object?

I understood that the compression of gas is what generates heat that we see when a spacecraft or Meteorite is entering the atmosphere.

And that it is only possible when the object is moving above the speed of sound

But I wonder why a Jet Fighter does not heat up? even with supersonic speed?

Is it because the Jet Fighter has better aerodynamics? if so, why not create spacecrafts with such aerodynamics?

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  • $\begingroup$ Who says they don't? en.wikipedia.org/wiki/… $\endgroup$ – Solomon Slow Apr 12 '17 at 13:36
  • $\begingroup$ P.S., The heating is not due to friction. It's because the aircraft/spacecraft compresses the air that it passes through. $\endgroup$ – Solomon Slow Apr 12 '17 at 13:37
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Every object moving through a fluid will heat up, both by friction and by compression of that fluid at and around the stagnation point. Jet fighters heat up enough already that it is not necessary to put de-icing devices on their wings. The temperature at the stagnation point goes up with the square of velocity, therefore the heating becomes quickly problematic for common aerospace materials like aluminium once the top speed grows above Mach 2. The MiG-25 used steel and the SR-71 titanium to enable their structure to suffer the heating from Mach 3+ speeds.

In supersonic flow, the air has no indication of the approaching vehicle. The first contact will cause a sudden change in direction, called a shock. For drag reduction it is important to make the shock as weak as possible, which means that the change in direction should be as small as possible. This can best be achieved by a slender, pointy tip.

In hypersonic flow (Mach > 5) a blunt nose might again be preferable if the nose's materials put an upper limit on aerodynamic heating. A pointed tip will produce an attached shock which will heat the tip to something close to the stagnation temperature of the flow. A blunt nose, however, will cause a separated shock. This creates more drag and higher heat loads overall, but allows to spread the heat over a larger area and produces lower peak loads. The Space Shuttle had such a blunt nose, since drag minimization is not a priority for a re-entry vehicle.

Jet fighters have indeed better aerodynamics because their heating is still low enough to afford pointed tips, and the poorer aerodynamics of reentry vehicles is caused by the need to limit peak heat loads. When the Shuttle was designed, the military desired to maximize the distance the craft could glide during reentry (called cross range), and sufficiently good aerodynamics were only possible with the newly developed thermal tiles.

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...Is it because the Jet Fighter has better aerodynamics? if so, why not create spacecrafts with such aerodynamics?

It's because they want the spacecraft to slow down. All of that heat comes from the spacecraft's own kinetic energy. If it didn't lose all of that KE before it touched the ground, then the landing site would resemble a meteor crater.

Reentry vehicles are designed to use atmospheric drag to slow down from orbital speed to a safe speed for landing.

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One obvious reason is spacecraft typically have much higher velocity than airplanes (say, the orbital velocity of about 8 km/s vs. about 1 km/s for SR-71).

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    $\begingroup$ But even so, the SR-71 had to accelerate to Mach 3 to generate enough heat to expand the body panels to close gaps in them (there so they could expand when heated) and stop fuel from leaking. So there's still a lot of heat generated! $\endgroup$ – tpg2114 Apr 12 '17 at 14:15
  • $\begingroup$ @tpg2114 : Maybe we read the question differently (and English is not my native language). I don't think "Jet fighters don't heat up like spacecrafts" implies "Jet fighters don't heat up", but I may be mistaken. $\endgroup$ – akhmeteli Apr 12 '17 at 14:59
  • $\begingroup$ I read it the same way -- I'm pointing out an example of a jet aircraft that does heat up, just like a spacecraft, and it is used as an important part of the design. So even at 1km/s, the heating is significant enough to be important and essential in the design of the SR-71. $\endgroup$ – tpg2114 Apr 12 '17 at 15:33
  • $\begingroup$ @tpg2114 not that I want to diminish the heating experienced by some spacecraft, but the space shuttle heated up to over 1500C during reentry (the layer of air just around it heats to 5500C). And the shuttle has one of the lowest reentry temperatures around. The thermal loads on the SR-71 peaked at just under 430C. Not very comparable (except as numbers. You can always compare two numbers) $\endgroup$ – Jim Apr 12 '17 at 17:11
  • $\begingroup$ I believe the Concorde also dealt with thermal expansion. $\endgroup$ – Mike Dunlavey Apr 12 '17 at 18:09

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