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I was just reading this story from Sky news and reported the following

Lockheed Martin says it is close to developing a military plane that flies at Mach 6 - six times the speed of sound, which travels at around 762mph (1,236km per hour).

Am I missing something, or is that a misprint?

6 * speed of sound at sea level = 4567.2423 miles per hour

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    $\begingroup$ The speed of sound changes with temperature (and thus usually with altitude), see: aviation.stackexchange.com/questions/3693/… $\endgroup$ – ROIMaison Mar 17 '16 at 8:40
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    $\begingroup$ They have quoted the speed of sound at sea level - but planes don't cruise that low! $\endgroup$ – Ben Mar 17 '16 at 8:41
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    $\begingroup$ No, that's not a misprint, that is a bad writing. The subordinate clause relates to “sound”, not to the plane, so the number would be right if speed of sound at room temperature was relevant. $\endgroup$ – Jan Hudec Mar 17 '16 at 8:52
  • $\begingroup$ I thought that the general unit of measure for true airspeed was knots, whereas Mach is measure in units relative to the speed of sound for a given temperature, pressure, and air density. ???? $\endgroup$ – KorvinStarmast Oct 3 '16 at 17:16
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is the speed reported in this story wrong?

No

No it is not a misprint that the SR-72 is intended to be capable of Mach 6

From the horses' mouth:

Envisioned as an unmanned aircraft, the SR-72 would fly at speeds up to Mach 6, or six times the speed of sound. At this speed, the aircraft would be so fast, an adversary would have no time to react or hide.

The SR-72’s design incorporates lessons learned from the HTV-2, which flew to a top speed of Mach 20, or 13,000 mph, with a surface temperature of 3500°F.


Yes

Yes it is not true that Mach 6 is necessarily 4567.2423 MPH

From elsewhere:

enter image description here
Cmglee. License:CC by SA 3

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    $\begingroup$ Although as noted in that Lockheed article, the SR-71 did 5,600 km London-New York in <2 hours. With the SR-72 meant to be twice as fast by the same metric (Mach 6 vs Mach 3), that would indeed suggest Lockheed are talking about a ground speed of at the very least 5,600km/h. This is on the assumption that Lockheed are using the same definition of "Mach" for both aircraft, and suggests they're talking about the speed of sound at sea level. $\endgroup$ – Jon Story Mar 17 '16 at 11:14
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    $\begingroup$ @JonStory: So far as I know, in Aviation, Mach is always defined in terms of the actual altitude of the aircraft. The local speed of sound at a specific altitude has a dramatic aerodynamic effect on the aircraft. Otherwise most ordinary airliners would be travelling subsonically at Mach > 1. $\endgroup$ – RedGrittyBrick Mar 17 '16 at 11:27
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    $\begingroup$ Yeah totally agreed... but when communicating with the public people expect the scale to be linear. So in the aircraft I doubt it will say "Mach 6" on the ASI, but when announcing it to the public it appears Lockheed are sticking with sea level, so they can compare with the SR-71, Concorde, F-15, Boeing 737 etc $\endgroup$ – Jon Story Mar 17 '16 at 11:29
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    $\begingroup$ @JonStory: Lockheed wrote "Mach 20, or 13,000 MPH". 13000/20=650 not 762. $\endgroup$ – RedGrittyBrick Mar 17 '16 at 11:31
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    $\begingroup$ @RedGrittyBrick: It's temperature, not density that determines the speed of sound. Colder air = lower speed of sound. The break at 36K feet is due to the tropopause above which air temp is relatively constant. Look at the correlation of the curves on the chart. $\endgroup$ – Gerry Oct 3 '16 at 17:47
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While I'd in general remain sceptical of anything that Sky News talks about the numbers do seem realistic. Aviation Week (amongst others) have mentioned similar speeds and they're not beyond the realm of likelihood. Hypersonic test vehicles have already reached above Mach 5 and many of the designs are capable of the speeds talked about in the article. It's also worth noting that the SR-72 wouldn't enter service for another 15 years.

It's impossible to say for definite, though, as we're discussing something that is: a) In the design stage and b) Very secretive.

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