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Virtually every time I read about the Lockheed SR-71 (including other threads on this forum), I see that its speed record is listed as approximately 2193 mph. The same blurb will invariably say this is "about Mach 3.3".

But when I search for how fast Mach 1 is, the number that almost always comes up is 767 mph.

Now 767 * 3.3 comes out to 2531, well above 2193. If I take 2193 and divide it by 767, I get Mach 2.85

I doubt I've stumbled on something new here. So what am I missing? How can 2193 mph be Mach 3.3 when 767 mph is Mach 1?

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    $\begingroup$ Did you factor in temperature? The figure you found is likely for a standard temperature at sea level. The speed of sound changes with temperature, and it is much colder at the altitudes where the Blackbird flew. $\endgroup$
    – Michael Hall
    May 6, 2020 at 20:23
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    $\begingroup$ The graph in this answer provides a good overview. $\endgroup$
    – Manu H
    May 6, 2020 at 20:29
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    $\begingroup$ related What is the reason for changing the speed reference (IAS or Mach number) with altitude? $\endgroup$
    – Manu H
    May 6, 2020 at 20:36
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    $\begingroup$ Ah, thanks all. I apologize for not picking better search terms. I should have been able to answer this on my own $\endgroup$
    – Joe
    May 6, 2020 at 20:41
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    $\begingroup$ Don't apologize for that. Other may use the same search terms as yours and thus find this question that can help them. $\endgroup$
    – Manu H
    May 6, 2020 at 21:00

1 Answer 1

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The SR-71 was only capable of Mach 3.3 flight at altitude. You can see the limitations in the pilot's operating handbook(POH) for the aircraft:

enter image description here

At higher altitudes the relative speed of sound is lower. If we use this handy NASA calculator in a standard atmosphere the relative speed of sound at 70,000 ft. is 660 MPH making your 2193 MPH equal to Mach 3.321

Its also important to note that the word "about" is often used as the SR-71 was flown to a maximum compressor inlet temperature(CIT) not a maximum Mach when doing a 3.3 run. According to the POH

Mach 3.2 is the design Mach number. Mach 3.17 is the maximum scheduled cruise speed recommended for normal operations. However, when authorized by the Commander, Speeds up to Mach 3.3 may be flown if the limit CIT of 427°C is not exceeded.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Federico ♦
    May 10, 2020 at 7:10

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