Answer: machone

AnswerCrossword Clue
MACHONENumber that Yeager famously exceeded
MACHONESpeed of sound
MACHONESonic boom speed
MACHONENumber indicating supersonic speed.
MACHONESpeed equaling sound
MACHONESupersonic threshold
MACHONEThe speed of sound
MACHONEChuck Yeager was the first to reach it