Blackbird SR-71 did not survive through concealing but through escape. Lockheed had constructed the Skunk Works of the SR-71 in the wake of the 1960 U-2 shootdown in the belief that, once spotted, the only thing that was safe was being in a different place as soon as possible. The published cruise of the aircraft -Mach 3.2- became the figure most in the minds of people, yet became the number which spawned decades of debate on what the jet could actually achieve when crews needed all the knots they could get.
There is no disagreement regarding the baseline performance. The SR-71 regularly flew at altitudes greater than 80,000 feet and was designed to fly long-range at Mach 3-capability instead of short-range dash missions. During service, the tactic of the jet, when it received a warning about a missile, to leave the plane, was to accelerate, which was the most famous. It was even in the plain-language doctrine of the program, that should any launch be detected the usual response was to boost and out accelerate the missile. That concept can only work when the airframe, engines and inlets are constructed to survive in a thermal environment that can punish any shortcut.
At Mach 3 the battle with the Blackbird was heat. Airframe temperatures were so great that the aircraft was virtually a thermal machine rather than an aerodynamic one hence titanium was the material of the airplane approximately 85 per cent. of the airframe. It is also why the skin of the aircraft was deliberately slack on the ground before expanding in a flight and why the fuel seepage on the ramp became a given trade-off in a plane that could expand to a match after it took off. The signature black finish itself proved useful, too: it was more effective at radicating heat than bare metal and alleviated thermal stress on long supersonic legs.
The other half of the equation was in the nacelles. Pratt & Whitney J58 is most commonly characterized as acting like a turbojet that is stealing ramjet tendencies at velocity, but the additional disclosing reality is that the propulsion system of the SR-71 was uncontrollable as a detached engine. North of Mach 3 the inlet itself was doing a significant portion of the job; in full afterburner at such velocities, it was doing 54 percent of the work. This was the reason why inlet stability was so important and why the phenomenon of “unstarts” with their violent yaw and compressor disturbance became a headache of engineering engineering text-book status until control-system engineering brought the frequency down.
Legend and engineering are apt to fuse, however, when it comes to speed. The official, safe operating figures of the SR-71 were low by design, partly due to the fact that long-term overheating might reduce its service life, and also because the release of performance information had some strategic ramifications. Nonetheless, the record book of the aircraft depicts the amount of margin the program could get. It made an absolute speed record in July 1976 of 2,193.167 miles per hour, about 3.3 times light speed, on a straight course. And former pilot Brian Shul has long asserted an operational sortie of more than “Mach 3.5” in evading a missile over Libya in 1986, which remains the basis of the Mach 3.5 debate ever since it puts additional speed in the context of an emergency maneuver not a cruise environment.
The end result of the numbers is not an individual top-speed performance but an indication of design intent. The SR-71 was designed to render Mach 3 viable, both thermally and mechanically and to transform speed into survivability. In that regard, it does not matter whether the jet momentarily reached Mach 3.5, but the fact of engineering that it was capable of surviving at Mach 3 to move some of the rules of what a crewed aircraft was capable of doing.
