The SR-71 ran on two J58 turbojet engines and cruised at Mach 3.2.

What I want to know is, what is the actual exhaust velocity (relative to the airplane itself) of the jet exhaust at those conditions?

  • $\begingroup$ The is a good question, I cannot reply to you but as mach speed depend on temperature it might be a lower mach number. $\endgroup$
    – ant Bldel
    Aug 26, 2016 at 11:50

1 Answer 1


It's not a cut and dried single figure but this SR-71 PROPULSION SYSTEM - P&W J58 ENGINE (JT11D-20) seems to have enough information to allow you to arrive at a reasonable figure with 'some work'. If I had to choose one information source to get a feel for the answer this would be on the short short list (so far :-) ). Unlike most other sources, the result will be at least somewhat meaningful, if not overly accurate.

It also seems to make it 'clear enough' that if someone gives you a simple unqualified numerical answer it will be meaningless, or wrong, or both :-). It's a 44 page PDF slideshow - but has enough detail to show how different the J58 is to what you may expect.

He also supplies these references which will almost certainly be interesting, and possibly also useful.

  1. Connors, Jack, The Engines of Pratt & Whitney: A Technical History, AIAA (American Institute of Aeronautics and Astronautics, Inc.), Reston, VA, 2010, pp. 321-332.
  2. St. Peter, J., The History of Aircraft Gas Turbine Engine Development in the United States: A Tradition of Excellence, ASME, New York, 1999, pp.264-275.
  3. Campbell, David H., F-12 Series Aircraft Propulsion System Performance and Development, AIAA No. 73821, 1973. Also found in AIAA Journal of Aircraft, Vol. 11, No. 11, November 1974, pp. 670-676. Note: This paper presents information only up to Mach 3.0 due to security restrictions at that time.
  4. California Institute of Technology, Course Ae107, Case Studies in Engineering: The Lockheed SR-71 Blackbird, Spring 1991. Published by Cal Tech for course students and presenters only.
  5. Anderson, J. Thomas, unpublished training course syllabus, How Supersonic Inlets Work: Details of The Geometry And Operation Of The SR-71 Mixed Compression Inlet, 2013.
    If you do want a single misleading and probably wrong figure :-) then this book say 18,587 m/s at Mach 3.2 (table 1 page 175)

More M&PW answers:

Page 39-40

and here

Somewhere in here probably :-).


Comment in passing bottom of page 1



  • 1
    $\begingroup$ In the book by St. Peter, I presumed the word Tradition doesn't appear twice in the title, so I fixed that. Excellence could appear twice, but probably doesn't so I tweaked that, too. Please feel free to revert if I've overstepped bounds. Also, +1 for providing a "single misleading and probably wrong" figure to answer the question - most excellent! ;) $\endgroup$
    – FreeMan
    Aug 26, 2016 at 12:38
  • $\begingroup$ @FReeman You're very welcome, Sir, he said. Thanks I looked at the original and it had no duplicates. Also ref 5 had 2 duplicates!. I tried re cccccopying and ppasting and it dddddoubled up again the same way ! Something is fooling its system. So I hand corrected ref 5 and have now added an Amazon link for ref 2. (Yessss the repeated characters in this comment are intentional :-) ). $\endgroup$ Aug 26, 2016 at 12:49
  • $\begingroup$ 18.6 km/s? That sounds more like an effective exhaust velocity (considering only fuel burn and not the mass of oxygen mixed with it) rather than the actual exhaust velocity. $\endgroup$
    – DrZ214
    Aug 27, 2016 at 2:25
  • $\begingroup$ @DrZ214 Yep - that's what you get from a "... single misleading and probably wrong figure..." :-). The first reference and some figuring may be as good as you will easily get. $\endgroup$ Aug 27, 2016 at 13:54

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