# SR-72 — is the speed reported in this story wrong?

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

• The speed of sound changes with temperature (and thus usually with altitude), see: aviation.stackexchange.com/questions/3693/… – ROIMaison Mar 17 '16 at 8:40
• They have quoted the speed of sound at sea level - but planes don't cruise that low! – Ben Mar 17 '16 at 8:41
• 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. – Jan Hudec Mar 17 '16 at 8:52
• 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. ???? – KorvinStarmast Oct 3 '16 at 17:16

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: