After separating from the booster, the scramjet engine was ignited, accelerating the aircraft to Mach 5.1 at 60, 000 feet.

NPR: Experimental Air Force Aircraft Goes Hypersonic

The X-51A WaveRider flew for more than three minutes under power from its exotic scramjet engine and hit a speed of Mach 5.1, or more than five times the speed of sound.

NPR: Experimental Air Force Aircraft Goes Hypersonic