Originally Posted by
Pugilistic Animus
From what I know there's a few of them hidden that are airworthy. I bought the POH for the BlackBird and it's really interesting, very complex airplane. There is still some pages classified. And also the actual flight envelope is likewise, classified. my guess is FL 90 and M 3.6
I recall ages ago reading some work someone had done to calculate the top speed, came out at M3.4 so far as I can recall. The limiting factor is the compressor inlet air temperature, which increases with speed. Any faster than that and the air temperature profile throughout the compressor results in the latter stages getting hot enough to melt.
Of course, Reaction Engines’ helium cooled heat exchanger on their Sabre concept overcomes that general limitation. I wouldn’t be entirely surprised if that pitches up in something hypersonic in the US.