The top speed and altitude capabilities of the Blackbirds have long since been declassified. The aircraft had a design cruise speed of Mach 3.2 or approximately 2,100 mph. It would cruise a little faster or slower depending on outside air temperature because it was limited by structural heating factors and compressor inlet temperature (CIT) limitations.
SR-71 pilots and engineers have stated that the aircraft would sometimes slightly exceed Mach 3.2 if the air temperature was cool enough. In higher
temperatures, the aircraft was unable to attain design cruise speed due to structural temperature limits.
In 1991, NASA and Lockheed engineers studied the possibility of extending the Mach number capability of the SR-71. They examined the advantages and
disadvantages of making the aircraft capable of flying at speeds from Mach 3.3 to as much as Mach 3.8 (the maximum potential of the J58 engine with an
extensively modified inlet).
They determined that an enlarged inlet with a water-injection system could provide a large thrust margin increase at Mach 3.5, but there were
relatively low benefits and relatively high risks. There were thermal (structural) concerns at speeds of Mach 3.5 and above. Engine compressor inlet
temperature was predicted to be marginal at Mach 3.4 and virtually all engine parameters were marginal at Mach 3.5 and unacceptable beyond that speed.
Ultimately the Mach extension modifications were not recommended due to the low benefit/cost ratio.
Fastest known Blackbird flights:
YF-12A (60-6936), Mach 3.14 (2,070 mph), 1 May 1965
A-12 (60-6928), Mach 3.29 (2,171 mph), 8 May 1965
SR-71A (61-7972), Mach 3.32 (2,193 mph), 27 July 1976
According to SR-71 pilot Richard Graham: "The design Mach number of the SR-71 is 3.2 Mach. When authorized by the Commander, speeds up to Mach 3.3
may be flown if the CIT limit of 427 degrees C is not exceeded. I have heard of crews reaching 3.5 Mach inadvertently, but that is the absolute
maximum I am aware of."
The Blackbirds were designed to fly as high as 90,000 feet, but typically operated between 70,000 and 85,000 feet.
Highest known flights:
YF-12A (60-6936) – 80,257 feet, USAF, official, 1 May 1965
SR-71A (61-7972) – 85,068 feet, USAF, official, 27 July 1976
SR-71A (61-7953) – 86,700 feet, USAF, unofficial, date unknown (1968) during developmental testing
A-12 (60-6932) – 90,000 feet, CIA, unofficial, 14 August 1965
There was no direct replacement for the SR-71 after it was retired. Strategic and tactical reconnaissance missions are currently conducted by the U-2,
satellites, and various unmanned vehicles.