[SunkenCiv]

Also, the SR-71 was the only plane (so far) capable of sustained flight above Mach 2. The X-15 was a test bed that burned through all its fuel in a few minutes.

[T.B. Yoits]

Also, the SR-71 was the only plane (so far) capable of sustained flight above Mach 2. The X-15 was a test bed that burned through all its fuel in a few minutes.

Not to argue but Mach 3 was the threshold separating the SR-71 as there are/were several Mach 2 aircraft:
-XB-70 Valkyrie prototype
-the early B-1 prototypes
-the B-58 Hustler
-the F-105 Thunderchief
-the Russian Tu-144
-the Russian Tu-160

Now the Mach 5+ pulse jet aircraft that officially doesn't exist...