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The maximum and cruise speeds listed for North American XB-70 Valkyrie are:

  • Maximum speed: Mach 3.1 (2,056 mph, 3,309 km/h)
  • Cruise speed: Mach 3.0 (2,000 mph, 3,200 km/h)

Why is there such a small difference?

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    $\begingroup$ Designed in the era just before anti-aircraft missiles, it was designed to fly high and fast. It was designed to fly at it's nominal maximum speed at all time, so as to remain uncatchable and mostly untargetable by radar. My guess is the fastest it ever flew was the max speed, and they used an average for the cruise. It's cruise would be at full throttle in any case. $\endgroup$ – CGCampbell Mar 13 '15 at 19:44
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    $\begingroup$ @CGCampbell I think you are correct, the SR-71 was similar. Max 3.3, cruise 3.2, they get better when they fly faster but are limited by temperature. $\endgroup$ – fooot Mar 13 '15 at 19:54
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    $\begingroup$ Note that the situation here isn't at all unusual. For example, the Boeing 777 has a cruise speed of 560mph and a top speed of 590mph. $\endgroup$ – David Richerby Mar 13 '15 at 22:24
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    $\begingroup$ @DavidRicherby True, though that's for a different reason (supersonic flow over portions of the wings/control surfaces.) I'd hope that XB-70 and SR-71 were designed to not need to be concerned with such things. $\endgroup$ – reirab Apr 27 '15 at 5:38
  • $\begingroup$ Co-workers are former SR-71 pilots (actually most are now retired) and they are clear that the thrust headroom for the SR-71 was substantial. The book said 3.3 but there was enough thrust headroom for higher speed. The limitations were heat, as is extensively mentioned elsewhere. So it did not run full throttle, except perhaps in TO and climbs. I understand the XB-70 had similar heat issues. $\endgroup$ – mongo Dec 18 '18 at 1:00
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From the excellent point in the comments, the XB-70 was designed for one thing; speed. It was, until the Blackbird, the fastest manned air-breathing aircraft. It was intended to outrun Soviet interceptors like the MiG-25 and fly higher than ground-based anti-aircraft weapons of the day, making it a possible first-strike weapon. As such, it wasn't going to loiter around at 80% throttle.

A second point is that since the B-70 project was cancelled for several reasons (cost projections, development of ICBMs and of high-altitude fast-tracking SAMs), the two prototypes were used primarily as testbeds to push the envelope of air-breathing craft. As such, data regarding their fuel efficiency curve was secondary to more basic problems of aircraft design for hypersonic aerodynamic flight.

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    $\begingroup$ Actually, the MiG-25 was developed as an answer to the threat of the B-70 and the SR-71. And it worked: The SR-71 never overflew Soviet territory. $\endgroup$ – Peter Kämpf Mar 14 '15 at 18:22
  • $\begingroup$ Actaully, Oxcart, the A-12 preceded the XB-70 in time. The A-12 is a variant of the SR-71. $\endgroup$ – mongo Dec 18 '18 at 0:50

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