[Cooter]

Mach 9 is a velocity. The Space Shuttle hits Mach 25 after launch. The astronauts on Apollo 10 reached Mach 36.

Perhaps you're thinking of 9 G's.

[Gunrunner2]

9 G's is not bad at all.

For me G's are darned comfortable and feel good.
7G's and up, that is when the HELMET becomes uncomfortable.
No big deal.

Fighter pilots in the F-15/16 hit 9 G's all the time. Heck, it's FUN to pull G's. Not pulling G's drives you nuts, and if I didn't hit at least 5 G's on a sortie, at least once, any type of sortie, I'd feel cheated. 4 G's in the traffic pattern is standard.
I've over-G'd the F-15E a few times, hitting 11 G's once and never really felt all that much different than 9 G's. It was only that pesky "bitching betty" auto-warn system that gave me a clue I was over-G'ing. Bitch.
Cheers