Yes--rocket powered but before jets were really perfected and combat operational.
This is true.
The survival rates of experienced pilots was so low
Yes, but wasn't the bulk of deaths, or at least a good portion, upon landing, what were they thinking using skids. I know alot were shot down on approach to landing too, easy pickings.
I was thinking about planes that might have passed mach 1 before Yeager, and i'm actually surprised that the F-86 didn't achieve it, or come close. I know both the F-86 and the X-1 were flown about the same time, late '47 and with the 86's speed, ~600mph IIRC, it's not impossible that in high speed testing at altitude, they may have approached transition.
I know the F-86 did break the sound barrier in a dive after the X-1 had done it.
In any case the sound barrier had been well broke by 1947, in 1942 actually.