At 42,000' in approximately level flight, a third cylinder was turned on. Acceleration was rapid and speed increased to .98 Mach. The needle of the machmeter fluctuated at this reading momentarily, then passed off the scale. Assuming that the off-scale reading remained linear, it is estimated that 1.05 Mach was attained at this time.
Chuck YeagerI ran the astronaut school for six years, and I was the commandant and when I finished in '65, 26 of my guys went into space as NASA astronauts that I trained.
Chuck YeagerLeveling off at 42,000 feet, I had thirty percent of my fuel, so I turned on rocket chamber three and immediately reached .96 Mach. I noticed that the faster I got, the smoother the ride. Suddenly the Mach needle began to fluctuate. It went up to .965 Mach - then tipped right off the scale ... We were flying supersonic. And it was a smooth as a baby's bottom; Grandma could be sitting up there sipping lemonade.
Chuck Yeager