Scott Ferrin wrote in message . ..
owever, the airframe of the SR-71 is stressed for Mach 4.0
flight.


Where did you hear that?

From a credible Beale AFB source. I should mention that Mach 4.0 is
beyond the Blackbird's "stability limit" of Mach 3.5.
A typical mission for the Blackbird would be Mach 3.2 at 85,000 ft.
The aircraft could safely raise the speed from Mach 3.2 to 3.3 and
accellerate up to Mach 3.5 (the stability limit)for a limited time.
Pushing the aircraft up past 3.5 in an emergency was possible but not
recommended. The airframe could handle limited endurance up to Mach
4.0 with the penalty of damaging the aircraft/engines at sustained
flight at that speed.
The MiG-25 Foxbat had a similar stability limit of Mach 2.83. The lone
Foxbat chased by the Israelis up to Mach 3.2 suffered severe damage to
the engines as a result of exceeding the stability limit. MiG-25
pilots were instructed not to attempt speeds over Mach 2.5-2.6, with
special permission required for emergency dash at Mach 2.80. After
that the aircraft could easily still accelerate but the engines would
begin to suffer damage.
The radar absorbing paint on the SR-71 also acted as a heat sink,
lowering the airframe temperature by nearly 100 degrees. It would have
been better, however, if the aircraft was painted white overall
instead of blue-black. This was proposed at least once (based on the
X-15 research) but rejected.

Rob