Why Don't Jet Airplanes Heat Up Like Probes Re-entering Earth's Atmosphere?

[cj]

If a probe entering the Earth's atmosphere heats up because it encounters the atmosphere, then why don't jet airplanes (for example) also get really hot? They are flying around in the same atmosphere that heats up the probe when it re-enters the Earth's atmosphere.

 

[bapowell]

It's a matter of speed. Probes re-entering the Earth's atmosphere do so at several miles per second. Subsonic commercial airliners travel at around 0.1 miles/sec and don't heat up appreciably. However, supersonic jets, like the Blackbird which is capable of achieving Mach 3 (around 0.6 miles/sec), do have to deal with heat generation.

 

[Jonathan Scott]

If a probe entering the Earth's atmosphere heats up because it encounters the atmosphere, then why don't jet airplanes (for example) also get really hot? They are flying around in the same atmosphere that heats up the probe when it re-enters the Earth's atmosphere.

Any space probe will typically be going at least fast enough to have been in low Earth orbit, which is around 17,500mph, so it will be re-entering at least that fast, and possibly even faster if it has been further away. In contrast most jets only fly at subsonic speeds, say 550mph, so re-entry hits the air very many times faster than ordinary jets.

For supersonic jets, such as Concorde and the record-breaking SR-71 Blackbird, heating from atmospheric friction causes significant problems and requires careful design and selection of construction materials, but even the Blackbird only flew at something like 2,275mph.

At subsonic speeds, the air "knows that the plane is coming". The air pressure increases just in front of parts of the aircraft and pushes the air smoothly out of the way. At supersonic speeds, the aircraft is moving faster than the compression waves can move, so the plane "takes the air by surprise", and has to push it out of the way very abruptly, creating shock waves and a lot of heating.

 

[D H]

For supersonic jets, such as Concorde and the record-breaking SR-71 Blackbird, heating from atmospheric friction causes significant problems and requires careful design and selection of construction materials, but even the Blackbird only flew at something like 2,275mph.

Friction plays a very minor role in the heating caused by supersonic and hypersonic flight. The shock layer and the extremely high stagnation temperature are a much bigger problems. That the air is ionized and dissociated creates additional problems.

 

[DaveC426913]

At subsonic speeds, the air "knows that the plane is coming"...
At supersonic speeds... the plane "takes the air by surprise..."

 

[Eagle9]

If a probe entering the Earth's atmosphere heats up because it encounters the atmosphere, then why don't jet airplanes (for example) also get really hot? They are flying around in the same atmosphere that heats up the probe when it re-enters the Earth's atmosphere.

It depend on speed and altitude you can see this online-calculator and try to compute the heating temperature:
http://www.grc.nasa.gov/WWW/BGH/stagtmp.html

 

[Jonathan Scott]

Friction plays a very minor role in the heating caused by supersonic and hypersonic flight. The shock layer and the extremely high stagnation temperature are a much bigger problems. That the air is ionized and dissociated creates additional problems.

True - I used the word "friction" loosely (and inaccurately) to mean all the forms of heating associated with moving fast through the atmosphere, but I assume that heating is mostly due to the compression within shock waves, as I mentioned at the end of my post.

 

[Drakkith]

So the air is compressed and it is this compression causes heating of the air which is the major source of heat on an aircraft/ spacecraft ?

 

[qraal]

So the air is compressed and it is this compression causes heating of the air which is the major source of heat on an aircraft/ spacecraft ?

Exactly. Air being compressed in the shockwaves is having work done on it, thus it heats up. The energy for this heating is coming from the kinetic energy of the moving vehicle ramming into the air, requiring either engine thrust to counteract it or creating a braking force.

 

[qraal]

It depend on speed and altitude you can see this online-calculator and try to compute the heating temperature:
http://www.grc.nasa.gov/WWW/BGH/stagtmp.html

Thanks for the link! Excellent resource. I have looked around for something explaining the processes with more clarity than technical papers and university lecture notes. Now shockwaves physics might make sense!

 

[Eagle9]

Thanks for the link! Excellent resource

You are welcome that calculator is really very good, however take into consideration that actually it works up to 76200 meters altitude, for example if you write in "Altitude" field 90 000 meters it will be lowered to 76200 meters, perhaps above than 76200 m altitude other laws (and calculator) are needed for calculating the temperature in rarified atmosphere