I'm not sure about the melting point of jet engine alloys, but there is a significant safety factor built in. For example, the GE90-115B for the 777LRs was run at triple-redline (maximum fan speed, maximum core speed, and maximum exhaust gas temp) for 60 straight hours with no problems. I'm not aware of any incidents caused by simple thermal failure of engine components - cracking due to metal fatigue seems to be more common.
An easy formula for kinetic heating is (speed in mph / 100) ^ 2, which strangely produces heat rise in degrees C. The formula shows that even subsonic jets experience approximately 25 degrees of kinetic heating. Traveling at 1340 mph, Concorde feels a 180-degree C rise in air temperature at the nose, which when added to the -57 C ambient temperature gives a stagnation temp of about 123 C.
Standard aircraft aluminum is actually quite intolerant of high temperatures. Flight above Mach 2.2 demands a stronger alloy, usually titanium or more rarely stainless steel, which can handle temperatures well beyond Mach 3. SST designers also had a surprisingly hard time finding paint that could survive such high temperatures; Concorde's paint is unusually reflective to keep skin temperatures down. Hypersonic flight would require extremely expensive thermal protection systems, perhaps cryogenic cooling or ceramic skin tiles.
On Concorde, temperatures above 127 C at the nose are not permitted and require deceleration. Max stagnation temperature on the primarily titanium Mach 2.7 Boeing SST was 500 degrees F (260 C). Both aircraft used fuel as a heat sink for the air conditioners and vented exhaust cabin air through equipment bays for cooling. Apparently the nose of the Mach 3.5 SR
-71 gets so hot in flight that the black paint turns a deep blue.
Edit: I found a diagram in Larry Reithmaier's Mach One and Beyond
(a great reference) giving the thermal profiles of common aircraft alloys. I hope it's useful.
[Edited 2003-10-19 09:44:48]
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