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By "true" top speed I mean the absolute fastest horizontal airspeed the aircraft can achieve if damage to the engines is disregarded (similar to how the MiG-25 apparently can reach mach 3.2 but may damage its engines beyond repair). Catastrophic structural failure (of anything beyond the engines) is not permissible in this scenario.

In "The Untouchables" Brian Shul claims that he reached mach 3.5 while evading a missile. This is also stated in the Wikipedia article on the SR-71 with a citation to the same book.

Are there any other sources than his own book that can confirm or reject this claim? Can it be argued for or against this top speed claim on technical merit or physical limitations?

On Wikipedia it is also stated "Maximum flight speed was limited by the temperature of the air entering the engine compressor, which was not certified for temperatures above 800 °F (430 °C).[57]"

However, it is not clear how much abuse beyond this the compressor could take. It is possible to give some estimate or educated guess based on the materials used?

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    $\begingroup$ I suspect you won't get a definitive answer to this. The information is almost certainly classified. $\endgroup$
    – user31680
    Jul 22, 2018 at 4:01
  • $\begingroup$ I also suspect that, but the question is written such that it is possible to answer it with arguments regarding what would physically be possible based on what is known about the SR-71. Perhaps you can argue based on the construction of the materials? The limitations in the variable geometry of the intake? Something else? If there was some simple source available that officially states "this is the actual top speed", obviously Wikipedia and others would have since long started to cite it. $\endgroup$
    – AlphaCentauri
    Jul 22, 2018 at 4:22
  • $\begingroup$ This is actually a good question, but it's beyond my knowledge of the plane to calculate. So, not an answer, but an estimate based on similar overspeeds for other aircraft would place the zero safety margin point for horizontal speed somewhere between Mach 3.4 and 3.55. Something a bit above that could be possible if you count momentary airspeed in a dive (before the air gets dense and slows it down again). $\endgroup$
    – Therac - Peace for Palestine
    Jul 22, 2018 at 9:57
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    $\begingroup$ I suspect the „fastest speed the aircraft can achieve“ in your scenario will be achieved diving towards the ground at full thrust. This is certainly faster than anyone ever cared to test, and I strongly suspect that any available engine performance tables for the aircraft would not cover a speed range fast enough to accurately model this... also, I don’t know whether structural failure is permissible in your scenario, but I‘m sure that it quickly becomes a limiting factor in a full thrust dive. $\endgroup$
    – Cpt Reynolds
    Jul 22, 2018 at 14:47
  • $\begingroup$ @CptReynolds Very good point. I edited the post to make it horizontal airspeed. Destruction of engines is permissible, not catastrophic structural failure. $\endgroup$
    – AlphaCentauri
    Jul 23, 2018 at 2:33

3 Answers 3

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Based on the SR-71A operational envelope, the top Mach number was Mach 3.3 when authorized by the commander and Mach 3.2 otherwise. The top airspeed was 400 kts equivalent airspeed when supersonic and 450 kts equivalent airspeed when subsonic.enter image description here

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    $\begingroup$ The graph says Vmo (Vh) 450 KEAS, not 400 KEAS. $\endgroup$
    – Jan Hudec
    Jul 22, 2018 at 16:14
  • $\begingroup$ @JanHudec Thanks! Posted too quickly. Corrected with subsonic/supersonic values. $\endgroup$
    – Cpt Reynolds
    Jul 22, 2018 at 16:33
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    $\begingroup$ Mind that this is max allowed operational speed, not the max speed the aircraft is capable of surviving. There will be some safety margin there, making a max emergency speed of 3.5 believable. $\endgroup$
    – jwenting
    Jul 23, 2018 at 4:58
  • $\begingroup$ @jwenting So it's to PIC discretion if he wanted to take his chances with the missile or out running it $\endgroup$
    – jean
    Jul 23, 2018 at 20:36
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    $\begingroup$ @jean of course. In emergencies, stated limits and procedures go out the window and pilot judgment takes over. Better to burn out an engine or cause some excess structural stress on the airframe than have the aircraft and crew blown out of the sky over hostile territory. $\endgroup$
    – jwenting
    Jul 24, 2018 at 4:50
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The SR-71 Pilot's Operating Handbook has been declassified and is available online. The operating limitations section states that Mach 3.17 is the recommended maximum cruise speed for normal operations. Speeds of up to Mach 3.3 could be authorised by the commander as long as the maximum CIT (compressor inlet temperature) of 427 degrees C was not exceeded. So while it might be possible to exceed the maximum Mach, doing so probably carried with it the risk of Bad Things(TM) happening.

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    $\begingroup$ Welcome to Aviation SE! $\endgroup$
    – Super
    Jul 23, 2018 at 2:05
  • $\begingroup$ Of course, in the referenced example it was a question of choosing between two Bad Things. $\endgroup$
    – T.J.L.
    Oct 21, 2019 at 17:11
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The USAF never did release just how fast an SR-71 CAN fly, preferring to state that it DID fly at certain speeds on certain dates and set certain records.

Maj. Brian Shul made references in his books to the jet attaining Mach 3.5+ during flights he flew over Libya evading SAMs. I believe him. There are also credible rumors that a Blackbird was capable of Mach 4 in ideal atmospheric conditions. If USAF or CIA Sled Drivers did attain that high of a speed on certain occasions, they’re not confirming it.

Most Blackbird pilots were pretty coy about a top speed. Shul liked to say that the muzzle velocity of a high powered rifle bullet was about 3000 ft/s. A Sled comfortably cruised at 3,300 ft/s and the Pilot still had a couple of inches of throttle left in case he REALLY needed to haul ass!

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